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Clinical efficacy and safety of okra (Abelmoschus esculentus (L.) Moench ) in type 2 diabetic patients: a randomized, double-blind, placebo-controlled, clinical trial

• Original Article
• Published: 28 July 2023
• Volume 60 , pages 1685–1695, ( 2023 )

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• Mahdi Tavakolizadeh 1 ,
• Saeid Peyrovi 2 ,
• Hussein Ghasemi-Moghaddam 3 ,
• Ali Bahadori 4 ,
• Zeynab Mohkami 5 ,
• Mahtab Sotoudeh 1 &
• Mojtaba Ziaee   ORCID: orcid.org/0000-0002-9725-3033 2  

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A Correction to this article was published on 26 August 2023

This article has been updated

The recent trend toward the use of natural functional and medical supplements has motivated the focus on the search and revival of traditional medicinal plant applications for many years. As a valuable dietary crop, okra fruit ( Abelmoschus esculentus (L.) Moench) has been used for thousands of years as a medicinal food. This clinical trial aimed to assess the efficacy and safety of the okra pod capsule as an adjuvant treatment in controlling type 2 diabetes mellitus and provide clinical trial-based evidence about its anti-inflammatory effects.

A total of 100 type II diabetic patients, aged between 40 and 60 years, were randomly assigned into two groups of okra and placebo. The first group was administered 1000 mg of powdered okra fruit three times a day for 3 months, while the other group received a placebo capsule with the same dosage. Both groups continued the standard antidiabetic therapy (consisting of metformin and gliclazide, as well as a nutritional regimen). At the start and three months later, various factors were measured, including FBG, insulin, HbA 1c , cholesterol, triglycerides, HDL, LDL, CRP, liver and renal function tests, blood pressure, and BMI changes.

According to the results, patients who received okra treatment exhibited a significant decrease in FBG, HbA 1c , total cholesterol, and triglyceride levels when compared to both the baseline and the placebo group. Patients in the okra group have lower levels of hs-CRP compared with the placebo group after 3 months of treatment. No liver, kidney, and blood pressure or other side effects were observed in the groups associated with okra treatment.

Conclusions

The present study demonstrated that adjunctive consumption of okra, in type 2 diabetic patients with 1000 mg three times a day for three months, improves lipid profile, glycemic control, and chronic inflammation without any tangible adverse effects.

Clinical Trial Registry : IRCT.Ir (IRCT20120112008712N2). https://www.irct.ir/trial/42042 .

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Change history

26 august 2023.

A Correction to this paper has been published: https://doi.org/10.1007/s00592-023-02170-4

Abbreviations

Alanine aminotransferase

Serum aspartate aminotransferase

Alkaline phosphatase

Blood urea nitrogen

Body mass index

Cardiovascular disease

Enzyme-linked immunosorbent assay

Fasting plasma glucose

Hemoglobin A 1c

High-density lipoprotein

Homeostasis model assessment-insulin resistance

International Diabetes Federation

Low-density lipoprotein

Metabolic syndrome

Oral glucose tolerance test

Type 2 diabetes melitus

Total cholesterol

Triglyceride

World Health Organization

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Acknowledgements

This research was financially supported by a Grant from the Research Vice-Chancellor of Maragheh University of Medical Sciences, Maragheh, Iran (Grant No. 68504). The authors also would like to sincerely appreciate the vice president of research of the Faculty of Pharmacy, Zanjan University of Medical Sciences for support.

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Department of Pharmacognosy, School of Pharmacy, Zanjan University of Medical Sciences, Zanjan, Iran

Mahdi Tavakolizadeh & Mahtab Sotoudeh

Medicinal Plants Research Center, Maragheh University of Medical Sciences, Maragheh, Iran

Saeid Peyrovi & Mojtaba Ziaee

Hematology and Oncology Research Center, Tabriz University of Medical Sciences, Tabriz, Iran

Hussein Ghasemi-Moghaddam

Department of Medical Microbiology, Sarab Faculty of Medical Sciences, Sarab, Iran

Ali Bahadori

Department of Agriculture and Plant Breeding, Agriculture Institute, Research Institute of Zabol, Zabol, Iran

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Contributions

M.Z. and M.T. contributed to conceptualization; M.Z, M.T, and A.B. helped in methodology; Z.M. and A.B. contributed to formal analysis; M.S. and M.T. helped in preparing of dosage forms; S.P, H. GM., A.A., and S.M. contributed to patient visiting; M.Z, A.B., and M.T helped in writing—original draft preparation; M.Z, M.T., S.P, and H. GM. contributed to writing—review and editing; M.Z. helped in supervision. All authors have read and agreed to the published version of the manuscript.

Corresponding author

Correspondence to Mojtaba Ziaee .

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This study was approved by the Ethics of Committees of Zanjan University of Medical Sciences and in accordance with the 1964 Declaration of Helsinki and its later amendments or comparable ethical standards.

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Informed consent was waived for this retrospective study.

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Tavakolizadeh, M., Peyrovi, S., Ghasemi-Moghaddam, H. et al. Clinical efficacy and safety of okra (Abelmoschus esculentus (L.) Moench ) in type 2 diabetic patients: a randomized, double-blind, placebo-controlled, clinical trial. Acta Diabetol 60 , 1685–1695 (2023). https://doi.org/10.1007/s00592-023-02149-1

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Received : 21 March 2023

Accepted : 27 June 2023

Published : 28 July 2023

Issue Date : December 2023

DOI : https://doi.org/10.1007/s00592-023-02149-1

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A REVIEW ON: ABELMOSCHUS ESCULENTUS (OKRA)

Okra (Abelmoschus esculentus L.) is the only vegetable crop of significance in the Malvaceae family and is very popular in the Indo-Pak subcontinent. In India, it ranks number one in its consumption but its original home is Ethiopia and Sudan, and North-eastern African countries. Medicinal plants are the nature's gift to human being to have disease-free healthy life. It plays a vital role to preserve our health. In recent times, the use of herbal products has increased tremendously in the western world as well as developed countries. India is one of the most medico-culturally diverse countries in the world where the medicinal plant sector is part of a time-honoured tradition that is respected even today. Medicinal plants are believed to be safer and proved elixir in the treatment of various ailments. Abelmoschus esculentus (Okra) is an important medicinal plant of tropical and subtropical India. Its medicinal usage has been reported in the traditional systems of medicine such as Ayurveda, Siddha and Unani.

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This review aimed at summarizing phytochemical reports and biological activities of Abelmoschus esculentus L. from the database sources. For this, an up-to-date search was made in the PubMed, Science Direct, MedLine, Scopus, and Google Scholar. The findings suggest that A. esculentus contains various nutrients and important phytochemicals. It possesses a number of important biological activities, including antioxidant, anti-inflammatory and immunomodulatory, antibacterial, anticancer, antidiabetic, organ protective, and neuropharmacological activities. Moreover, the plant also has lipid-lowering, trypsin inhibitory, hemagglutinating, antiadhesive, and antifatigue activities. The fruit and seeds are well tolerated in humans and other animals. In conclusion, A. esculentus may be one of the best sources of pharmacologically active lead compounds. More research is necessary on its toxicogenetic effects in animals.

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Herbal medicine is playing an essential role in health care, with about 75–80% of the world’s population relying mainly on the use of traditional or alternative systems of medicines for their primary health care. Abelmoschus moschatus Medik., commonly known as musk okra belonging to the family Malvaceae, is used traditionally in the treatment of various health ailments throughout the world. The plant has been extensively studied by various researchers for its biological activities and therapeutic potentials. The present review summarizes information published in various academic journals and books, covering folkloric uses, chemical compositions, pharmacological activities of the extracts and isolated compounds, and safety profile of A. moschatus for further research studies.

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Recently, there has been a paradigm shift from conventional therapies to relatively safer phytotherapies. This divergence is crucial for the management of various chronic diseases. Okra (Abelmoschus esculentus L.) is a popular vegetable crop with good nutritional significance, along with certain therapeutic values, which makes it a potential candidate in the use of a variety of nutraceuticals. Different parts of the okra fruit (mucilage, seed, and pods) contain certain important bioactive components, which confer its medicinal properties. The phytochemicals of okra have been studied for their potential therapeutic activities on various chronic diseases, such as type-2 diabetes, cardiovascular, and digestive diseases, as well as the antifatigue effect, liver detoxification, antibacterial, and chemo-preventive activities. Moreover, okra mucilage has been widely used in medicinal applications such as a plasma replacement or blood volume expanders. Overall, okra is considered to be an e...

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Peer-reviewed

Research Article

Seed production system and adaptability of okra ( Abelmoschus esculentus L.) cultivars in Buea, Cameroon

Roles Conceptualization, Data curation, Formal analysis, Funding acquisition, Investigation, Methodology, Supervision, Validation, Writing – review & editing

* E-mail: [email protected]

Affiliations Department of Agronomic and Applied Molecular Sciences, Faculty of Agriculture and Veterinary Medicine, University of Buea, Buea, Cameroon, Department of Food Science and Technology, Faculty of Agriculture and Veterinary Medicine, University of Buea, Buea, Cameroon

Roles Data curation, Formal analysis, Writing – original draft

Affiliation Department of Agronomic and Applied Molecular Sciences, Faculty of Agriculture and Veterinary Medicine, University of Buea, Buea, Cameroon

Roles Conceptualization, Data curation, Formal analysis, Funding acquisition, Supervision, Writing – review & editing

Roles Data curation, Formal analysis, Investigation, Writing – original draft

Roles Conceptualization, Data curation, Formal analysis, Writing – review & editing

Affiliations Department of Agronomic and Applied Molecular Sciences, Faculty of Agriculture and Veterinary Medicine, University of Buea, Buea, Cameroon, Department of Agrochemistry and Soil Science, Faculty of Agronomy, Agricultural University Plovdiv, Plovdiv, Bulgaria

• Raymond Ndip Nkongho, 
• Jill Tracy Efouba-Mbong, 
• Lawrence Monah Ndam, 
• Gwendoline Ashie Ketchem, 
• Ebob Glacilia Etchu-Takang, 
• David Tavi Agbor

• Published: December 14, 2022
• https://doi.org/10.1371/journal.pone.0278771
• Reader Comments

Okra is grown globally for its nutritional and economic benefits. Okra seeds ensure continuous production of the crop but challenges of poor production, adaptability and management may not allow the seeds to express their full potential. There are two seed production systems in Cameroon; the informal and formal. In Buea, the informal seed system is used by most farmers for seed production/utilization and farmers are reluctant to use hybrid seeds. This study aimed to assess the informal seed system of okra and evaluate the adaptability of seed produce from informal and formal systems in Buea. A survey and a field experiment were carried out. The designs for the survey and field experiment were stratified random sampling and randomized complete block design respectively. Data collection for the survey was done using questionnaires and other data collection instruments, while for the field experiment, data was collected on germination, vegetative growth parameters, incidence and severity of pests / diseases and yield. Data analysis for the survey was done using descriptive statistics, while data from the field experiment was done using a two-way ANOVA test and treatment means compared using the Tukey test at 5% probability. Results from the survey showed that women (60%) were mostly involved in seed production by mass selection from two landraces identified. Preservation of seeds was mostly done with the use of wood ash (58%) and insects were the major postharvest pest (76%). For the field experiment, at 66 DAP, Yellen recorded the highest significant number of leaves (13.417), leaf area (771.4 cm 2 ) and the number of branches (5.64). Clemson spineless recorded the highest significant incidence (89.9%) and severity for pests / diseases while Kirikou and Landrace recorded the least incidence (0.0%) and severity. Kirikou recorded the highest significant yield (6.0 tons/ha), followed by Landrace (5.3 tons/ha). These findings reveal the performance of the landrace and provide reasons why farmers in Buea are reluctant to use hybrid okra seeds. The Landrace should certainly have adaptable genes, coupled with the autogamous nature of okra which encourages inbreeding for homozygous traits, which are dominant in expression compared to heterozygous traits.

Citation: Nkongho RN, Efouba-Mbong JT, Ndam LM, Ketchem GA, Etchu-Takang EG, Agbor DT (2022) Seed production system and adaptability of okra ( Abelmoschus esculentus L.) cultivars in Buea, Cameroon. PLoS ONE 17(12): e0278771. https://doi.org/10.1371/journal.pone.0278771

Editor: Muhammad Faisal Shahzad, Gomal University, PAKISTAN

Received: July 18, 2022; Accepted: November 23, 2022; Published: December 14, 2022

Copyright: © 2022 Nkongho et al. This is an open access article distributed under the terms of the Creative Commons Attribution License , which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

Data Availability: All relevant data are within the paper and its Supporting Information files.

Funding: We extend gratitude to the Ministry of Higher Education of Cameroon and the Faculty of Agriculture and Veterinary Medicine (FAVM) of the University of Buea, Cameroon, for the research modernization and Faculty research grants, respectively. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.

Competing interests: The authors have declared that no competing interests exist.

Introduction

Okra ( Abelmoschus esculentus (L.) Moench), is an economically important annual vegetable grown from seed in tropical, subtropical and warm temperate regions [ 1 , 2 ]. Vavilov [ 3 ], Murdock [ 4 ], Dantas, et al . [ 2 ], proposed the origin of okra in the Abyssinian centre. Okra later spread to the Caribbean and the U.S.A in the 1700s and was introduced to Western Europe soon after.

It is an annual flowering plant in the Malvaceae or mallow family which is cultivated for its edible green capsules rich in vitamins, carbohydrates, phosphorus, magnesium and potassium [ 5 ]. Economically, okra is sold to generate income for farmers [ 6 ]. Industrially, okra seed flour is used to fortify and improve the nutrient composition and functional properties of foods. Okra gum is used as a film coating agent for drug tablets [ 7 ] and is also used in the production of okra oil which is high in unsaturated fatty acids such as oleic and linoleic acids which may be suitable for use as biofuel [ 8 , 9 ].

The major okra-producing countries in the world are India, Nigeria, Sudan, Mali, Pakistan, Cameroon, Ivory Coast, Ghana, and Benin. World okra production in 2018 was 9.87 million tons for a total cultivated area of 2.02 million hectares with India recording the highest production of 6 million tons and accounting for 62.0%. Cameroon produced 104,200 tons of okra in the same year [ 10 ].

The okra plant is propagated through seeds. The seed is one of the most crucial element in the livelihood of agricultural communities. It is the repository of the genetic potential of crop species and their varieties, resulting from continuous improvement and selection over time [ 11 ]. Seed systems are the ’vehicles’ through which farmers get good quality seeds for the crop varieties they need. An effective seed system has the potential to increase production quickly and economically [ 12 ]. Hence, more attention directed towards increasing seed yield with good quality for the successful production of okra is expected to drive the global market of okra [ 13 , 14 ]. There are two seed production systems, the informal and formal seed production systems. The informal system comprises seeds derived from previous harvests, trade-by-barter and friends while the formal system is organized with a set of planned activities leading to the production of certified seeds. Both have a role to play in making seeds available for planting [ 11 ].

The growth and yield of okra depend upon various parameters including seed quality, agro-climatic and soil conditions, as well as agronomic practices [ 13 ]. Okra requires a long, warm and humid growing climate for better yield. It is sensitive to frost and extremely low temperatures [ 15 ], with an optimal temperature range of 21–30°C, maximum temperature of 35°C and a minimum of 18°C [ 15 ]. Okra requires well-drained sandy loam soil, with a pH between 5.8–6.5. Though drought-tolerant, okra requires a certain amount of water through irrigation. The adaptability of okra cultivars is greatly influenced by these biophysical conditions. That is why a given cultivar that is adaptable in agro-ecological zone A may not be adaptable in agro-ecological zone B. This explains the relationship between genes and the environment, which influences the phenotypic characteristics of crops.

It has been recognised that seeds more than any other farm input is the key to enhanced food production and increased income generation. Despite efforts to develop a national seed programme in Cameroon, the seed situation remains dismal. Hence, a proper seed production and distribution system is required to conveniently make available quality seeds for crop production.

In sub-Saharan Africa with Cameroon inclusive, the dominant technique used in producing okra seeds is the informal method. Moreover, okra has been considered a “minor” crop and very little attention has been paid to its improvement in research programmes in the past. Despite the presence of commercial okra hybrids in the market, farmers are reluctant to use them for various reasons.

The main objective of the study was to assess the informal seed production system of okra and evaluate the adaptability of seed produced from informal and formal systems under the agro-climatic and edaphic conditions of Buea. While the specific objectives were (i) to document the informal seed system of okra and (ii) to evaluate the adaptability of the okra seeds cultivated in Buea.

Materials and methods

Description of the study site.

The study was carried out in Buea which is the headquarter of the South-West Region, in the Republic of Cameroon. Buea is located at 4°10’0” N of the Equator and 9°14’0” E of Greenwich Meridian and has an elevation of 870 m above sea level. Buea is geographically bounded to the North by the tropical rainforest at the foot of Mount Cameroon, to the South-west by Limbe, to the South-east by Tiko, to the East by Muyuka and to the West by Idenau. Buea falls within the humid forest agroecological zone, with mono-modal rainfall pattern. The average rainfall ranges from 3000–5000 mm annually, while the temperature ranges between 20–28°C. The area is composed mainly of volcanic rocks which range from massive basaltic lava flows around the upper slopes of Mount Cameroon to pyroclastic flows further down the slope [ 16 ]. The pH of the soil is between 5.58 and 5.74 [ 17 ]. The topography of Buea is composed of gentle slopes, with the sloping nature fairly stable throughout.

Assessment of the informal seed system of okra in Buea

Reconnaissance survey and site selection..

A reconnaissance survey was carried out to the sub-delegation of Agriculture in Buea to identify the major okra-producing areas in Buea municipality and the stakeholders involved in the okra sector. The areas identified were villages, which included; Small Soppo, Bwitingi, Molyko, Muea, Mile 16 (Bolifamba) and Wotutu.

Research design, field layout and application of treatments.

A stratified random sampling method was used to select the informants needed to get information on seed production, cultivation, post-harvest management and marketing of okra in Buea. This sampling technique consists of partitioning the okra sector into sub-populations (Okra farmers, farmers shop owners, agricultural officers, marketers, consumers etc,)and randomly collecting data from each sub-population.

The field layout of the survey consisted of the various villages sampled which included Small Soppo, Bwitingi, Molyko, Muea, Mile 16, and Wotutu.

Test questionnaires were administered in the field and some corrections were effected before final administration to farmers. The different informants (stakeholders) sampled included; farmers, farmers’ shops, extension officers, marketers (retailers), consumers and some staff at the sub and regional delegation of agriculture and rural development. This triangulation helped to adequately collect relevant information on the informal seed system of okra in Buea.

Evaluation of the adaptability of okra cultivars produced informally and formally

Land preparation, field layout, experimental design and application of treatments..

A piece of land with a surface area of 420 m 2 was cleared and gotten rid of stones and other unwanted materials with the help of a cutlass and rake. The experimental layout consisted of 3 replicates parallel to each other. Each replicate within the layout had 8 plots, making 24 experimental units in total. Plots of 2 m by 3 m (6 m 2 ) each were tilled with the help of a hoe and raised to 30 cm height for easy root proliferation. A furrow of 1 m between each plot, 2 m between replicates and 2 m border around the experimental field were established. The experimental design used is a Randomised Complete Block Design (RCBD), with 8 treatments replicated 3 times ( Fig 1 ). Randomisation of treatments on the experimental plots was done with the aid of a random number table. There were 6 intra-rows and 3 inter-rows giving a total of 18 stands per plot of 3 m x 2 m. Three (03) seeds were seeded per stand at a depth of 3 cm and later thinned to two plants per stand after two weeks, to give a total of 36 plants per plot. The seeds were sown at a planting distance of 50 cm by 75 cm, giving a plant density of 53,333 plants per hectare. The treatments for the experiment consisted of different cultivars of okra ( Table 1 ). The seeds were purchased from local farmer shops in Buea and Douala, Cameroon.

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Crop maintenance operations

Cultural control practices..

Weeding . Weekly removal of weeds with a hoe was done from plots to avoid competition for nutrients, water and sunlight with the okra plants, as well as to remove potential hosts of pests.

Thinning . This is removing the less vigorous plant from a stand of three to reduce competition for nutrients and encourage seedling establishment and development.

Replanting . Replanting was done to replace seeds that did not germinate.

Watering . The plants relied on water from rain-fed and supplementary water, with the use of watering cans, during drought-prone conditions.

Mulching . This was done by placing dry and weeded grass around the root zone of crops to create a moist environment, which enhances the absorption of water and nutrients for the plants.

Pruning . Pruning was done with the use of a knife, to remove senesced and pests / disease infested leaves to avoid the build-up of pests and diseases.

Earthing—up . This is the mounting of top soil on the root zone of the crop, as the need arises to prevent root exposure and subsequent damage.

Rogueing . Diseased and genetically abnormal plants were removed and burnt to prevent further disease spread and other anomaly.

Chemical control practices.

Fertiliser application . NPK (20:10:10) was applied at the rate of 10 g per stand under blanket application to promote vegetative growth and yield. This was done on the 3 rd and the 6 th week after planting (WAP), in split application with 5 g at each session.

Pesticide application . At the 5 th week after planting, the insecticide Inca with active ingredient Fipronil was applied, with 4 g dissolved in a 16 L knapsack sprayer. From the 8 th week after planting, the insecticide Acarius, with the active ingredient Abamectin was applied weekly at a rate of 50 ml/16L due to the build-up of whiteflies, ants and ladybird beetle. The fungicide Mancostar with active ingredient Mancozeb was applied weekly at the rate of 100 g/16L from the 8 th week after planting to control fungi.

Data collection

The informal seed system of okra in buea..

Data collection instruments included the following: questionnaires, semi-guided discussion, focus group discussion, literature (secondary data), farm and market observation visits. One hundred (100) questionnaires were administered on a one-on-one basis to the farmers. Questions were properly explained before getting the answers from the respondents. In the course of the questionnaire administration, key informants were identified, who served as a backup for clarification as the need arose and were also invited to participate in the focus group discussion. The purpose of designing the questionnaire and administration was to find out; why the farmers plant okra (how beneficial it is to them), how they get their seeds (from the formal or informal seed sector) and why they choose to get them from there, how they produce their seeds (for those involved in seed production), the challenges faced in seed production (pests and diseases, lack of inputs, etc.), how they handle the seeds at postharvest, the challenges faced in storing / maintaining seed quality and how they market the seeds.

Semi-guided discussions with wholesalers and retailers of okra, as well as consumers and staff at the sub/regional delegation of agriculture and rural development in Buea, were held. Focus group discussions were held with key informants in the okra sector selected during the field survey. Secondary data from the literature review were used to complement knowledge already gathered from other data collection instruments. Field visits to okra farms as well as local markets were additional sources of data.

Evaluation of adaptability of okra cultivars produced informally and formally.

Germination percentage . Data for germination was collected one week after seeding. This was done by getting the fraction of germinated seeds from the total and expressed in percentage.

Vegetative growth parameters . Data for vegetative growth was collected at 21, 36, 51 and 66 days after planting (DAP). Six stands in the middle of each plot were tagged, giving a total of 12 sampled plants, with a sampling intensity of 37.5%. Stands in the center were taken to minimize errors due to border effect. The following parameters were recorded:

Plant height (cm): This was done with the use of a measuring tape, from the base of the plant to the tip of the shoot.

Number of leaves: The number of fully opened leaves from the lower stem to the top of the plant was counted and recorded.

Stem circumference (cm): The stem circumference of tagged plants was recorded using a piece of thread round the stem of the plant and placing it on a meter rule to get the value.

Leaf area (cm 2 ): The estimated leaf area of tagged plants was gotten with the use of a meter rule, positioned at three different points in the middle of the broadest leaf to the edge, to get an average and using the formula, estimated leaf area = Πr 2 .

Number of branches: Number of branches of tagged plants from the lower stem to the top of the plant was gotten by counting and recording on the data collection sheet.

Common pests and diseases . Data on pest and disease incidence and severity were collected at 51DAP, 66DAP and 81DAP.

Incidence: This is the percentage of diseased plants or parts in the sample or population of plants [ 18 ]. It was done by a visual counting of infected plants, divided by the total number of sampled plants and multiplied by 100.

Incidence = Number of infected plants X 100

Total number of sampled plants

Severity: This is the percentage of relevant host organs or tissues covered by symptoms or lesions or damaged by pest / disease. Severity results from the number and size of lesions.

Severity = Sum (Observed grade x number of plants in the same grade) X 100 [ 19 ]

Total number of plants evaluated x maximum grade on the scale

Aphid and whitefly severity . Assessment of severity by insect pests was done visually on a 0-5scale, where grade 0 = absence of aphids, grade 1 = scattered appearance of few aphids/whiteflies, grade 2 = few isolated colonies, grade 3 = several small colonies, grade 4 = large isolated colonies, grade 5 = large continuous colonies [ 20 ].

Grasshopper severity . Severity was estimated using a visual rating scale of 1–5 where 1 = 1–15 perforations on leaf (very mild damage), 2 = 16–30 perforations (mild damage), 3 = 31–45 perforations (moderately severe damage), 4 = 46–60 perforations (very severe damage), 5 = more than 60 perforations (extremely severe damage) [ 21 ].

Wilt severity . A visual scale of 0–4 was used to measure disease severity where 0 = plants with no symptoms, 1 = plant with no symptoms of wilt or yellowing, but with dark vascular bundles, 2 = plants with intensely darkened vascular bundles and with incipient wilt or yellowing of leaves, 3 = plants with severe wilt, associated with yellowing and premature leaf drop, 4 = dead plants [ 22 ].

Powdery mildew severity . The percentage of foliage disease severity was recorded by using the disease severity scale from 0–4 where 0 = no leaf lesions, 1 = 25% or less infected area of the leaf, 2 = 26–50% infected area of the leaf, 3 = 51–75% infected area of the leaf, 4 = 76–100% infected area of the leaf [ 23 ].

Mosaic disease severity . The assessment was carried out using a severity scale of 0–5 where 0 = no symptoms were observed, 1 = mild chlorosis, mottle or mosaic on leaves, 2 = mild chlorosis, mottle or mosaic without significant leaf distortion, 3 = moderate chlorosis, mottle or mosaic with leaf malformation, 4 = severe chlorosis, mottle or mosaic plus stunting or dwarfing of the whole plant, 5 = score 4 plus drying and leaf drop [ 24 ].

Yield components and yield . Data was collected at 66 DAP, 74 DAP and 82DAP for yield components and yield.

Number of okra capsules: The number of okra capsules was gotten by visually counting fully formed capsules and recording.

Length and width of okra capsules: The length and width were taken using a meter rule where:

Length of capsule/plant/treatment = Length of capsules harvested

Total number of capsules

Width of capsule/plant/treatment = Width of capsules harvested

Fresh weight of okra capsule : Fresh capsules were harvested and weighed using a scale balance.

Yield: This was gotten by multiplying the capsule weight per plant per treatment by the planting density.

Data analysis

Data for the field survey was analysed by descriptive statistics using Microsoft Excel® 2016 version and SPSS version 25. A descriptive statistical method including percentage and frequency was used to summarize the ethnobotanical data collected through interviews and focus group discussions. Field experiment data were analysed by inferential statistics using a two-way ANOVA to test for significance and comparison of treatment means by Tukey’s Least Significance test at 5% probability level.

The informal seed system of okra in Buea

Demographic information of respondents of the okra seed sector..

Table 2 shows the personal information of the various respondents tending okra in Buea. Most okra farmers in Buea were women (60%) and the majority of farmers who cultivated okra fall within the age group of 40–49 years (42%). A great majority of the farmers were married (62%) and most of the respondents were primary school leavers (40%). In terms of farmers’ occupation, 70 respondents (70%) relied solely on farming for their livelihood.

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Seed production and okra cultivation

Most farmers obtained planting materials from mass selection (choosing from the healthiest capsules of the previous harvests), representing 42 respondents (42%) relative to 20 farmers (20%) using certified seeds bought from farmers’ shops, 19 (19%) using seeds bought from the market, 12 (12%) using seeds from family and 7(7%) using seeds gotten from friends ( Table 3 ). During cultivation, the majority of the farmers 78 (78%) adopted a mixed cropping system while 22 respondents (22%) adopted monoculture. In terms of the performance of the landrace and certified seeds, 74 (74%) of the respondents reported the landrace to perform better than the certified seeds. The plot sizes used in okra cultivation in Buea ranged between 25–20,000 m 2 and majority of the farmers either used personal land (inherited or purchased) while others used rented land. Experience in okra cultivation among the respondent ranged from one to fifty years.

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Facilities used for okra preservation and storage in Buea

Farmers use different preservatives to improve the shelf-life of okra seeds during storage. Fifty-eight of the respondents (58%) used wood ash as a form of preservative, while 27 respondents (27%) reported the use of assorted preservatives (excluding chemical options) before storage. Just 15 respondents (15%) used chemicals like Mocap 10G as a preservative for okra seeds.

Farmers in Buea used different media for the storage of okra seeds. While 70 (70%) of the respondents used plastic containers for okra seed storage, 16 (16%) of them allowed the seeds in the capsules and 14 (14%) used cloth bags as their storage medium. In terms of the cause of seed deterioration during storage, 69 (69%) of the respondents attest that weevils were the major insect pest. Concerning challenges during storage,76 (76%) of the respondents opined that attack by insect pests were the most prominent compared to fungal and animal pests as shown in Table 4 .

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Evaluation of adaptability of okra cultivars produced informally and formally

Germination percentage of okra cultivars..

Germination percentage is an indicator of seedling vigour and plays a role in seed establishment. While F1 Comet recorded the highest germination percentage (93.46%), the cultivar with the least germination percentage was Nain with 33.53% ( Table 5 ).

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Vegetative growth parameters of okra cultivars

Plant height . For the plant height, F1 Comet recorded the highest value (64.36 cm) which was consistently significant across the different sampling periods ( Table 6 ). At 21 days after planting (DAP), the height of F1 Comet (8.43 cm) was not significantly different from Clemson spineless, the Landrace, Kirikou and Yellen but was significantly different from Longbec, Nain and Cafeier.

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At 36 DAP, the height of the F1 Comet cultivar (15.16 cm) was also not significantly different from Clemson spineless, the Landrace, Kirikou, Yellen, Longbec but was significantly taller than Nain and Cafeier.

The height of F1 Comet (42.50 cm) was not significantly different from the Landrace, Kirikou, Clemson spineless, Yellen, Longbec, but was significantly different from Cafeier and Nain at 51DAP.

F1 Comet was significantly taller (64.36 cm) than the other cultivars at 66 DAP. The Landrace was second in terms of height (42.25 cm), which was not significantly different from Clemson spineless, Kirikou, Yellen, Longbec, but significantly different from Cafeier and Nain.

Stem circumference . All the okra cultivars recorded increasing values for stem circumference across the different weeks of measurement ( Table 7 ). Cultivars did not record any significant results at the 21, 36 and 51 DAP respectively. At 66 DAP, Kirikou recorded the highest value (5.00 cm) for stem circumference, which was significantly different from Longbec, but not significantly different from the other cultivars examined.

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Number of leaves . Okra cultivars showed robust growth throughout the sampling period for the number of leaves ( Table 8 ). At 21 DAP, the Landrace and F1 Comet recorded the highest values (4.67), which was not significantly different from the other cultivars but was significantly different from Longbec. Cultivars did not record any significant results at 36 and 51DAP respectively. At 66DAP, Yellen recorded the highest value (13.42), which was not significantly different from the other cultivars but was significantly different from Clemson spineless.

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Leaf area . Analysis of variance revealed that okra cultivars recorded significant results at 21, 36 and 51 DAP respectively ( Table 9 ). At 21 DAP, F1 Comet recorded the highest leaf area (30.93 cm 2 ), which was not significantly different from Kirikou, Yellen, Landrace, and Clemson spineless but significantly different from Longbec, Nain and Cafeier. At 36 DAP, F1 Comet recorded the highest leaf area (181.8 cm 2 ), which was not significantly different from the Landrace and Kirikou, but was significantly higher than Clemson spineless, Yellen, Longbec, Nain and Cafeier. At 51 DAP, Kirikou recorded the highest leaf area (447.7 cm 2 ) which was not significantly different from Yellen but was significantly different from F1 Comet, Clemson spineless, Landrace, Cafeier, Longbec and Nain. At 66 DAP, Yellen recorded the highest value for leaf area (771.4 cm 2 ), which was significantly different from Kirikou, F1 Comet, Clemson spineless, Landrace, Longbec, Nain and Cafeier.

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Number of branches . At 36DAP, Kirikou and Yellen recorded the highest value (2.47) for the number of branches per plant, which was not significantly different from F1 Comet and Cafeier, but significantly out-numbered those of other cultivars ( Table 10 ). At 51DAP, Kirikou also recorded the highest value for the number of branches (3.97), which was not significantly different from Yellen and Cafeier, but was significantly different from the other cultivars. At 66DAP, Yellen recorded the highest value (5.33), which was not significantly different from Kirikou and Cafeier, but was significantly higher than the other cultivars.

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Incidence and severity of common pests and diseases on okra cultivars in Buea

Incidence of pests and diseases on okra cultivars . Okra cultivars recorded differences with respect to the incidence of pests and diseases across weeks. At 51DAP, Clemson spineless recorded the highest incidence (80.87%), which was significantly different from the other cultivars ( Table 11 ). This was closely followed by Longbec (50.67%), which was also significantly different from the other cultivars and lastly, F1 Comet recorded the third-highest value (8.17%), which was not significantly different from the other cultivars. At 66 and 81 DAP, Clemson spineless recorded the highest incidence (81.27% and 89.90% respectively), which were significantly different from the other cultivars.

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Severity of common pests on okra cultivars in Buea : The severity of aphids and whiteflies on okra cultivars . The severity of infestation of okra cultivars to aphids and whiteflies varied among cultivars within and across the sampling periods ( Table 12 ). At 51DAP, Clemson spineless recorded the highest value (3.33), which was not significantly different from Longbec, Yellen and F1 Comet but was significantly different from the other cultivars. At 66DAP, Clemson spineless still recorded the highest value (3.67), which was not significantly different from Longbec, Yellen and Kirikou, but was significantly different from the other cultivars. At 81DAP, Clemson spineless again recorded the highest value (3.67), which was not significantly different from Longbec and F1 Comet, but was significantly different from the other cultivars.

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The severity of grasshoppers on okra cultivars . At 51, 66 and 81 DAP, Clemson spineless recorded the highest severity (3.67,4.17 and 3.50 respectively) for grasshoppers which significantly out-numbered that of the other cultivars ( Table 13 ).

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The severity of wilt disease infestation on okra cultivars . Table 14 presents the results of the severity of infestation of wilt disease on okra cultivars in Buea. Apart from Longbec and F1 Comet that were not significantly different from Clemson spineless that registered the highest severity of wilt disease infestation of 4.33 at 51 and 81 DAP respectively, it (Clemson spineless) was significantly different from the other cultivars and all the cultivars at 66 DAP with a peak infestation value of 4.67.

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The severity of powdery mildew infestation on okra cultivars . The results of the severity of powdery mildew infestation on okra cultivars are presented in Table 15 . Clemson spineless registered the greatest severity of infestation to powdery mildew at all sampling dates (1.30, 1.63 and 2.00 at 51, 66 and 81 DAP respectively), which were significantly different from the values recorded by the other cultivars except for Longbec at 66 and 81 DAP.

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The severity of mosaic infestation on okra cultivars . At 51DAP, Clemson spineless recorded the highest severity of infestation (2.83), which was significantly different from the other cultivars. At 66DAP, Clemson spineless still recorded the highest severity of infestation (2.67), which was not significantly different from the other cultivars. Likewise, at 81DAP, Clemson spineless again recorded the highest severity of infestation (4.50), which was significantly different from the other cultivars, as shown in Table 16 .

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Yield components of okra cultivars grown in Buea.

Concerning the yield components of okra cultivars, F1 Comet recorded the highest value for capsule length (19.72 cm), which was significantly different from the other cultivars. The Landrace recorded the highest value for capsule width (6.19 cm), which was also significantly different from the other cultivars. Kirikou recorded the highest value for the number of capsules (14.03), which was not significantly different from Yellen, Cafeier, Landrace and F1 Comet, but was significantly different from the other cultivars. With respect to capsule weight and yield, Kirikou again recorded the highest values (0.11 kg/plant/treatment and 6.05t/ha) respectively, which was significantly different from the other cultivars ( Table 17 and Fig 2 ).

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Correlation of vegetative growth parameters and pest / disease severity with okra yield . A positive correlation was noticed for all vegetative growth parameters compared with the yield with stronger correlation values for plant height (r = 0.6) and stem circumference (r = 0.7) ( Fig 3 ). While pest and disease was inversely proportional to the yield and registered very weak correlation values ( Fig 4 ).

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Most of the okra farmers involved in informal seed production and cultivation were women of the middle age group (40–49 years) who are married and had at least a primary school education. This conforms with research carried out by Tata Ngome et al . [ 25 ] in 3 towns of Cameroon (Bafoussam, Buea and Ebolowa) and reported that a greater percentage of vegetable farmers in Buea and Ebolowa were women while those in Bafoussam were men. On the other hand, Farinde et al . [ 26 ], showed that the males in Egbedore Local Government Area, Osun State of Nigeria, were the ones who cultivated okra and the women participated at the level of processing, preservation and marketing. The disparity in gender with more female farmers can have an influence on the cropping system used for seed / okra production, consumption and marketing prerogatives. This study also shows that it was the middle-age group who were mostly concerned with okra farming as the youths showed little interest, which ties with studies by Farinde et al . [ 26 ], who pointed out that only 4% of respondents in their study were youths. The system of seed production and okra cultivation are the informal seed system and small-scale production system respectively, characterize with insufficient capital, low level of technology and intensive labour, thus making it not friendly and appetizing for the youths.

Farmers mostly used mass selection to produce planting materials. This conforms to a study carried out by Phippen [ 27 ] on the evaluation of nine okra varieties for seed production where he concluded that even hybrid okra varieties still have a tremendous amount of genetic diversity between them to support a breeding program for increasing seed yield. Ahiakpa et al . [ 28 ] also presented similar findings and said that traditional farmers often engage in selection for different purposes, facilitating its diversification into a vast number of landraces adapted to various agroecological systems. Massucato et al . [ 29 ] in a study conducted on 30 accessions of okra in Brazil, noted that small farmers usually store some of the seeds to sow during the next cropping season, generation after generation. The dominant cropping system for both seed production and okra cultivation was mixed cropping, followed by monoculture. The mixed cropping system used by okra farmers has also been highlighted previously by Tata Ngome et al . [ 25 ]. Farmers attributed the landrace to favourably compete with the commercial okra hybrids in terms of vigour, pest and disease resistance, as well as yield. In a related study by Tata Ngome et al . [ 25 ], it was noticed that while farmers in Buea and Ebolowa used seeds from mass selection, on the contrary, farmers in Bafoussam used commercial hybrids as their planting materials. The informal system of okra seed production essentially by mass selection is questionable with respect to seed quality due to the absence of temporary and spatial isolation conditions to avoid the introgression of foreign genes which could result in ’contamination’ of the progeny’s genetic integrity and the expression of phenotypic traits. The situation is checked by the crops autogamous nature and coupled to the fact that the percentage of allogamy is greatly reduced, since dehiscence of the anthers / pollination at anthesis has a restricted time interval.

The cultivar preferred by most of the farmers according to the results was the landrace ( Abelmoschus caillei (A. Chev.) Stevels). This is similar to findings by Farinde et al . [ 26 ], who found that farmers preferred the landrace compared to the commercial hybrids for their planting materials. These landraces may have relevant traits for vigour, resistance, tolerance and improved yield, coupled with the autogamous nature of the crop [ 30 , 31 ].

Farmers used indigenous postharvest handling methods, processing techniques, preservatives and storage media to improve the viability and shelf-life of okra seeds [ 14 ]. Amongst the preservatives and storage media used, wood ash and plastic containers were dominantly used by farmers to preserve and store seed, respectively, in accordance with [ 25 ]. Meanwhile, wood ash is relevant in absorbing water and rendering the storage medium constantly arid, plastic containers are good in providing an air-tight environment, which goes a long way in reducing the deterioration rate of the seeds. Farmers were faced with limitations in terms of the utilization of preservatives and storage technology for okra seeds and common postharvest pests that infested okra seeds were insects.

Evaluation of the adaptability of okra cultivars

Seed germination and vegetative growth of okra cultivars..

Seed germination constitutes a major parameter of vigour and adaptability. Meanwhile, orthodox seeds have a better shelf life, viability and germination percentage, this is the reverse for recalcitrant seeds. Seed quality attributes and environmental factors also greatly influences the breaking down of seed dormancy and germination percentage.

For plant height, F1 Comet recorded significant results within cultivars and across periods of data collection. Other cultivars which recorded better results for plant height in descending order were the Landrace, Clemson spineless, Kirikou, Yellen, Longbec, Cafeier and Nain. Plant height is important in sunlight interception, especially in interspecific competition conditions, since it exposes the leaves of heliophytic plants to sunlight radiation for food production. The variation in plant height among the different okra cultivars may also be due to the fact that some varieties are early maturing (such as Clemson spineless, F1 Comet), medium maturing (Longbec, Kirikou, Landrace, Yellen) and late maturing (such as Cafeier) and thus, have different growth rates. Farooq et al . [ 32 ] also opined that there were significant variations in growth habits for all the five varieties of okra under experimentation (Penta green, Pusa sawani, Local cultivar, Pusa green and Clemson spineless) in India. The result also coincided with the findings of Omonhinmin and Osawaru [ 33 ], who stated that different okra cultivars showed varied plant heights at different growth stages. Nsimi et al . [ 34 ] corroborated that the highest plant height was registered from a local okra cultivar while an imported cultivar (Clemson spineless) had the least height.

For stem circumference, Kirikou recorded the most significant results within cultivars and across the data collection schedule while the other cultivars with better results for stem circumferemce in descending order were F1 Comet, Yellen, Clemson spineless, the Landrace, Cafeier, Nain and Longbec. The stem is relevant in maintaining an upright position, especially for plants with erect growth habits. It also acts as a pathway for the transportation and translocation of water/nutrients and food substances respectively. The larger the stem girth, together with the type of tissues, the lesser the possibility of lodging especially during stormy climatic conditions. This is in tandem with studies by Temam et al . [ 35 ] who reported differences in stem girths of various okra cultivars ranging from 14.5–30.5 cm in Ethiopia. These results contradict the findings of Azeem et al ., [ 36 ] who reported an insignificant difference in stem girth among the various okra cultivars under his study.

In terms of the number of leaves, the cultivar that produced the most significant results was F1 Comet. While leaves are important for sunlight interception, the more the number of leaves on a plant, the greater the possibility for the plant to produce more food. The increased production in the number of leaves, across the developmental stages of the crop, may be attributed to the vigour of the cultivar and increased absorption of nutrients which resulted in increased synthesis of photosynthates. Similar findings by Saleem et al . [ 37 ] revealed differences in the number of leaves recorded for okra cultivars under test due to high phenotypic and genotypic variance. Alam and Hossain [ 38 ] also found significant differences in number of leaves for the different accessions of okra tested. Chadha et al ., [ 39 ] documented significant differences in the number of leaves and observed that okra cultivars with more leaves per plant produced higher fruit yields.

With regard to the leaf area, the cultivar that recorded the most significant results was Yellen. The other cultivars in descending order of importance were Kirikou, F1 Comet, Clemson spineless, the Landrace, Longbec, Cafeier and Nain. Broader leaves have the tendency of intercepting more sunlight radiation and thus, the possibility of producing more food. Leaf radius is directly proportional to the leaf area. So, a larger leaf radius means a larger leaf area [ 37 ].

For number of branches, the cultivar with an outstanding result is Yellen and was closely followed by Kirikou, Cafeier, the Landrace, Nain, F1 Comet, Longbec and Clemson spineless. Under favourable growing conditions, plants with more branches will contain more leaves and have greater chances of photo assimilation. Branches are important as they are the sites for leaf attachment and growth and add some support to the plant. The discrepancy in the number of branches per cultivar might be due to their genetic make-up and environmental factors. Similar findings by Alam and Hossain [ 38 ] showed significant variation in the number of branches of okra accessions under study. Also, they stated that little differences were found between the genotypic and phenotypic variance as well as between the genotypic and phenotypic coefficients of variation, indicating low environmental influence.

Cultivar specificity in performance with respect to germination percentage and vegetative growth parameters are important predictions of vigorous growth and adaptability in a given environment.

Incidence and severity of pests and diseases on okra cultivars.

The cultivar which recorded a significant incidence of pests and diseases was Clemson spineless, which was closely followed by Gombo Longbec and F1 Comet. Likewise, for the severity of pests and diseases, the most susceptible cultivar was Clemson Spineless, closely followed by Gombo Longbec. Meanwhile, the Landrace was the cultivar that recorded the least incidence and severity of pests and diseases.

The degree of incidence and severity of a cultivar to common pests and diseases is another important tool to determine its genetic potential (vigour) and adaptability. Meanwhile, Clemson spineless recorded significantly higher values for incidence and severity of pests and diseases, the Landrace on the contrary was more resistant to pests and diseases. This conforms with the findings of Dimkpa et al . [ 40 ] where Clemson spineless was the most susceptible cultivar to attack by Podagrica spp (okra flea beetle) and out of 7 cultivars under study, the least affected by pest and disease were the two landraces. Lamont [ 30 ] reported that Abelmoschus caillei (landrace) is prized for its yield, vigorous growth, and tolerant to negative environmental factors, serving as a source of many desirable characteristics” [ 41 ].

Yield components and yield of okra.

F1 Comet and the Landrace recorded significant results for capsule length and capsule width respectively, while Kirikou recorded significant results for the number of capsules. Concerning yield, Kirikou recorded the most significant results and was closely followed by the Landrace. Rajesh et al . [ 42 ] found some variations in capsule length, width, number of capsules, capsule weight and yield of 14 okra accessions under study.

Crop yield is greatly influenced by the genetic potential of the cultivar, its adaptability to agroecological specificity and the adoption and implementation of best agronomic practices. The study recorded significant variability in terms of responses of the different cultivars to site-specificity vegetative growth parameters, incidence and severity of pests and diseases. Cultivars that recorded better results for vegetative growth parameters, as well as pest and disease resistance, also recorded better yields (i.e. Kirikou and the Landrace).

The Landrace due to its genetic potential and autogamous nature recorded optimal vigour and yield and was the most adaptable cultivar in terms of resistance to incidence and severity of pests and diseases. This implies that it could have genes of interest that can be useful in crop improvement, especially for those cultivars that were heavily infested with pests and diseases (i.e. Clemson spineless and Longbec). Tata Ngome et al . [ 25 ] had similar results in that some farmers preferred the “tall” (landrace) varieties of okra although they were late-maturing because they have a prolonged fruiting period spanning several seasons. Their study also reported that variability in varietal preferences could also be linked to taste. This is in agreement with the findings of this study as most farmers partly preferred the landrace for cooking because it tastes better, produces more mucilage (“draws” well) and a small quantity is required for cooking. In addition, since okra is autogamous, it is possible that there is little or no change (genetic modification) from one generation to another and this could play an important role in having more homozygous traits [ 43 , 44 ]. This is possibly why farmers depend on the landrace since they have more or less 100% maintenance of these characters or traits, giving rise to a pure breed, as dominant or recessive characters are easily expressed in pure breeds than in heterozygous conditions.

Correlation of vegetative growth parameters and pest / disease severity with the yield of okra.

Positive correlation values were recorded for the vegetative growth parameters, with stronger correlation for plant height and stem circumference with the yield. On the other hand pest and disease severity were inversely proportional to the yield and gave very low correlation values. Cultivars with significant vegetative growth and low infestation of pest and disease also recorded better adaptability. Pest and disease destroy important organs and tissues of the crop which in turn upsets the normal physiological functioning of the crop. For example Aphids and white flies are sucking pests of the leaves, while the Wilt destroys the vascular tissues of the crop rendering them inefficient in the uptake water and nutrients.

The farmers’ system of okra seed supply and crop development in Buea which forms by far the most important source of seed in most farming systems in the world has been documented. Despite the efforts of large seed programmes to replace the farmers’ seed system, the major part of okra agricultural land in Buea is still sown with seeds that are informally gotten through mass selection by choosing the best capsules from two landraces identified as the 3-months (annual) and the 6-months (biennial) types. The storage methods used by farmers encouraged rapid deterioration in the quality, the reason why they had very high seeding rates.

Based on the adaptability test, Kirikou is the cultivar that was the most adaptable in Buea with a yield of 6.0 t/ha, followed by the Landrace (5.3t/ha). Early maturing cultivars included Clemson spineless and F1 Comet, medium maturing cultivars included Kirikou, Longbec, Landrace, Yellen and late-maturing cultivars were Cafeier and Nain.

This study has highlighted the need for coexistence of the formal and farmers’ seed systems and their improvement should be embraced, not only because they are mutually beneficial, but also since farmers and their communities cannot depend on one system only. The fundamental principle of the integrated seed sector development concept is the need to develop a twin-track approach, where the effectiveness of both the informal and the formal seed systems can be improved and through a concerted effort at every component of the seed value chain.

Recommendation

For higher yields, farmers should plant cultivars like Kirikou, Landrace or F1 Comet. For farmers wanting quick returns, an early maturing cultivar such as F1 Comet or Clemson spineless can be planted.

Supporting information

S1 file. vigor and yield..

https://doi.org/10.1371/journal.pone.0278771.s001

S2 File. Pest and disease severity.

https://doi.org/10.1371/journal.pone.0278771.s002

S3 File. Field survey.

https://doi.org/10.1371/journal.pone.0278771.s003

Acknowledgments

The Authors thank the okra stakeholders of Buea, Cameroon for provision of information. Authors are also thankful to the Ministry of Higher Education for the research modernization allowance.

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• Published: 20 May 2021

Review on the “Biological Applications of Okra Polysaccharides and Prospective Research”

• Ali A. A. Al-Shawi   ORCID: orcid.org/0000-0002-0690-4612 1 ,
• Mustafa F. Hameed 2 ,
• Kawkab A. Hussein   ORCID: orcid.org/0000-0001-9796-0929 1 &
• Haneen K. Thawini   ORCID: orcid.org/0000-0002-8709-4198 1 , 3  

Future Journal of Pharmaceutical Sciences volume  7 , Article number:  102 ( 2021 ) Cite this article

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Vegetables with edible parts like flowers, fruits, stems, leaves, fibers, roots, and seeds are rich sources of essential vitamins, minerals, and trace elements with various medical functions. Many diseases such as osteoporosis, diabetes, high cholesterol, obesity, heart diseases, and stroke are caused by poor, healthy lifestyle or nutrition. Therefore, generation of new biological functions from vegetables will increase the interests of scientific research and applications.

Okra is an edible vegetable which contains vitamins, fiber, carbohydrates, protein, and minerals. The bioactive compounds of okra possess various biological activities such as anti-inflammation, antibacterial, anticancer, and antifungal. Polysaccharides from vegetables or medicinal plants are important large molecules with various biological applications. In this review, we will focus on the biological properties and nanoparticle uses of polysaccharides isolated from okra and the extraction methods of polysaccharides.

This review will enhance the scientific research findings of okra polysaccharides and recommend future prospective of polysaccharides for biological uses.

Edible plants are one of the important sources of proteins, carbohydrates, vitamins, amino acids, minerals, and lipids that enhance the immune system, bones, muscles, and other parts of the human body to fight diseases [ 1 , 2 ]. Edible vegetables have common benefits for the human body and animals due to the chemical components of primary metabolism, which depends on the type of soil, used water, and environment changes [ 3 , 4 ].

Okra is one of the delicious edible vegetable in North America, West Africa, South Asia, and Arab countries; it has few common names like lady fingers (English-speaking countries), Bamya (common name in Iraq), and father of musk (some Arabic countries) [ 5 ]. Okra belongs to the Malvaceae family, genus Abelmoschus, species Esculentus and contains edible green seeds, pods, and fibers (Fig. 1 ) [ 6 ].

Okra vegetable

Fresh okra contains energy, 90% water, 7% carbohydrates, 2% protein, fibers (contains alpha-cellulose, hemicellulose, lignin, pectin, fat, and wax matter), some important soluble vitamins in water and fat, and minerals like calcium, iron, magnesium, phosphorus, potassium, and zinc [ 7 , 8 ]. Therefore, okra is an important edible vegetable for human health. Okra mucilage is used in industrial as turbidity from wastewater [ 9 , 10 ], and also under investigations as biodegradable food packaging [ 11 , 12 ]. The biological studies of okra bioactive compounds were investigated as antioxidant, neuroprotective, anti-diabetic, anti-hyperlipidemia, and anti-fatigue activities [ 13 ]. Okra polysaccharides have not yet pharmacology extensively been investigated. In this review, we will present the extraction methods, chemical structure, nanoparticles, and the biological activities of polysaccharides extracted from okra vegetable, which presents a wide understanding of okra polysaccharides’ importance and further uses.

Okra polysaccharide (OP)

Isolation and purification of polysaccharides from plants depend on the extraction methods and purification solutions, which may keep or break down the structure of polysaccharides and may reduce the biological properties. Okra polysaccharides have been isolated and identified. Liu et al. used hot water method for extraction and isolation of okra polysaccharides; it consisted of the main four monosaccharides (arabinose, galactose, rhamnose, and galacturonic acid). The isolated polysaccharides showed good hyperglycemia activity (Fig. 2 ) [ 14 ].

An example of the chemical structure of polysaccharides isolated from okra vegetable via hot water extraction method

Chandra et al. extracted polysaccharides from the okra head waste which contains a high ratio of mucilage and found it to lower thermal degradation properties [ 15 ]. Kunli et al. used ultrasound-assisted extraction method to extract polysaccharides from okra vegetable which contains monosaccharides (glucose, mannose, galactose, arabinose, xylose, fructose, and rhamnose). It exhibited high antioxidant activity versus superoxide radicals and DPPH, and weak antioxidant activity versus hydroxyl radicals was revealed [ 16 ]. Xi et al. used hot water method to isolate polysaccharides from various five cultivated okra in China. The polysaccharide structure consists of similar monosaccharides (rhamnose, galacturonic acid, galactose, and arabinose); they suggested that the identified polysaccharides could be used as functional food ingredients for industrial application prospects [ 17 ]. Huricha et al. used macerated method and identified three fractions from okra polysaccharides with different molecular weight (600, 990, and 1300 kDa) and two groups of monosaccharide composition (group 1 galactose, rhamnose, galacturonic acid, and glucuronic acid; and group 2, galactose, rhamnose, galacturonic acid, glucuronic acid, and glucose). They found that those okra polysaccharides may potentially serve as novel immunomodulators supported by future studies [ 18 ]. Qin et al. used three extraction methods to evaluate the efficiency of okra polysaccharide extract; the three extraction methods were hot water extraction (HWE), pressurized water extraction (PWE), and microwave-assisted extraction (MAE). They found that the method PWE was a good extraction technique for okra polysaccharide with high biological activity for industrial applications [ 19 ]. Xi et al. used an ultrasonic method to extract okra polysaccharides (obtained pectic polysaccharides, composed of rhamnose, galacturonic acid, and galactose) which promised to be a potential functional food and pharmaceutical industries [ 20 ].

Table 1 showed the type of extraction method and structure analysis of okra polysaccharides. Ultrasound extraction method of okra polysaccharides showed only two similar monosaccharides (galactose and rhamnose). These differences in monosaccharide types cause variations in the biological activities, therefore, needed extensive applications to compare and target the function of structure on the efficiency of okra polysaccharides.

Antioxidant and biological activities of OP

Natural antioxidant plays a role in our life because it can keep and protect the human health rather than an industrial antioxidant. Several studies showed the antioxidant activity of okra chemical components and related to the phenolic and flavonoid contents in okra seeds, flowers, and fruits [ 21 , 22 , 23 , 24 ]. Gemede et al. found that okra pod mucilage is a good source as a natural antioxidant [ 25 ]. Okra polysaccharides have been investigated for its antioxidant activity; Kunli et al. found that okra polysaccharides extracted by the ultrasound method exhibited significant in vitro antioxidant activity [ 16 ]. Weijie et al. extracted polysaccharides from okra flowers using the hot water extraction method. The composition of isolated polysaccharides was [2)-α-D-Rhap-(1 → 4)-α-D-GalpA-(1 → 2,4)-α-D-Rhap-(1 → 4)-α-D-GalpA-(1] with a branch of terminal T-α-D-Galp pointed at C4 of 1,2,4-α-D-Rhap), and it was found that it exhibited a significant antioxidant activity and could be used in nutritional food and material application [ 26 ].

There are several biological applications of okra polysaccharides. Wang et al. found that those okra polysaccharides extracted by the cold water extraction method exhibited antioxidant, α-amylase, and α-glucosidase inhibitory activities in vitro [ 27 ]. Li et al. found that the neutral saccharide side chains of the OP could induce different secondary conformation change of gelatin during complex coacervation [ 28 ]. Gao et al. used fractions of okra polysaccharides, as anti-fatigue and observed it may be the main anti-fatigue remedy among A. esculentus substances [ 29 ]. Wahyuningsih et al. found that crude okra polysaccharides could play a role in enhancing the immune response, including phagocytic activity, spleen index, splenocytes proliferation, and control immune responses through cytokine production [ 30 ]. Liu et al. found that polysaccharides isolated from okra named (rhamnogalacturonan) possess hypoglycemic activity and are responsible for the hypoglycemia function in OP [ 31 ]. Fan et al. those okra polysaccharides possess therapeutic functions on metabolic diseases by the inhibition of LXR and PPAR signaling [ 32 ]. Deters et al. found that pectin-like polysaccharides reduced the proliferation significantly, but improved the cell viability [ 33 ]. Table 2 summarized the historical research of OP.

Anticancer properties of OP

Anticancer properties of okra extracts have been little investigated [ 35 , 36 ]. Thus, the anticancer activity of polysaccharides isolated from okra has not yet been reported, and this point will enhance to explore the anticancer properties of okra polysaccharides using different extraction methods.

Nanoparticles of OP

Gold nanoparticles of aqueous extract of okra have been synthesized and exhibited antibacterial activity against Bacillus subtilis , Bacillus cereus , E. coli , Micrococcus luteus , and P. aeruginosa and act as an effective antifungal agent [ 37 , 38 ] . Silver nanoparticles of the okra aqueous extract have been synthesized by Jassim et al. and showed different antibacterial and enzyme activities [ 39 ]. Hamid et al. synthesized ZnO nanostructure film that contains okra mucilage that showed high antibacterial activity against S. aureus than E. coli [ 34 ]. Agi et al. used cost-effective and easier method to synthesize cellulose nanoparticles from okra mucilage using an ultrasonic method [ 40 ]. Bhavani et al. used okra extract to synthesize ZnAl 2 O 4 nano-catalysts and found that microwave method is better than conventional heating in conversion of alcohol to carbonyl group [ 41 ]. Thus, further investigation of gold or silver or other metal nanoparticles of okra polysaccharides is of importance in discovering new biological functions and mechanism of actions.

Okra is an important vegetable for human health because of its functional bioactive compounds as antioxidant. A polysaccharide of okra had some biological functions such as anti-fatigue, hypoglycemia, and phagocytic activities. Therefore, needed extensive studies of the biological research to identify the anticancer and antimicrobial properties of okra polysaccharide and nanoparticles forms to target the main purposes of polysaccharide uses, and develop its functions in the medical applications.

Availability of data and materials

Data and material are available upon request.

Abbreviations

Phosphoinositide 3-kinase

Protein kinase B

Glycogen synthase kinase 3 beta

Nuclear factor erythroid 2-related factor 2

Liver X receptor

Peroxisome proliferator-activated receptors

2,2-diphenyl-1-picrylhydrazyl

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This review is supported by the Chemistry Department, College of Education for Pure Sciences, University of Basrah, Basrah, Iraq.

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Al-Shawi, A.A.A., Hameed, M.F., Hussein, K.A. et al. Review on the “Biological Applications of Okra Polysaccharides and Prospective Research”. Futur J Pharm Sci 7 , 102 (2021). https://doi.org/10.1186/s43094-021-00244-0

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• Antioxidant
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ISSN (Online): 0975-8232, ISSN (Print): 2320-5148

International Journal Of Pharmaceutical Sciences And Research

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A brief review on abelmoschus esculentus linn. okra.

Okra is a cultigen (a plant that has been altered by humans through a process of selective breeding). The exact origin of okra is unknown, but it is thought to have come from Africa, where it has been grown as a crop for centuries. Evidence suggests it was grown in Egypt as long ago as 2,000 BC. Today it is widely cultivated for its edible green fruits, which are harvested when immature (after 3 – 5 days of development), and are infamous for their slimy mucilage. It plays a vital role to preserve our health. In recent times, the use of herbal products has increased tremendously in the western world as well as developed countries. India is one of the most medico-culturally diverse countries in the world where the medicinal plant sector is part of a time-honoured tradition that is respected even today. Medicinal plants are believed to be safer and proved elixir in the treatment of various ailments. Abelmoschus esculentus (Okra) is an important medicinal plant of tropical and subtropical India. Its medicinal usage has been reported in the traditional systems of medicine such as Ayurveda, Siddha and Unani.

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Page No: 58-66

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Cited By: 24

Language: English

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Authors: D. K. Chanchal*, S. Alok, M. Kumar, R. K. Bijauliya, S. Rashi and S. Gupta

Authors Address: Department of Pharmacognosy, Institute of Pharmacy, Bundelkhand University, Jhansi, Uttar Pradesh, India.

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Received: 06 May, 2017

Revised: 16 November, 2017

Accepted: 22 November, 2017

DOI: 10.13040/IJPSR.0975-8232.9(1).58-66

Published: 01 January, 2018

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Performance and quality attributes of okra ( Abelmoschus esculentus (L.) Moench) fruits grown under soil applied Zn-fertilizer, green biomass and poultry manure

Christopher muyiwa aboyeji.

1 College of Agricultural Sciences, Landmark University, Omu-Aran, Kwara State Nigeria

4 Landmark University SDG 1 (No Poverty Group), Omu-Aran, Kwara State Nigeria

5 Landmark University SDG 2 (Zero Hunger Group), Omu-Aran, Kwara State Nigeria

6 Landmark University SDG 3 (Good Health and Well-Being Group), Omu-Aran, Kwara State Nigeria

9 Landmark University SDG 15 (Life on Land Group), Omu-Aran, Kwara State Nigeria

Samuel Olatunde Dahunsi

2 Microbiology Programme, College of Agriculture, Engineering and Science, Bowen University, Iwo, Osun State Nigeria

8 Landmark University SDG 12 (Responsible Consumption and Production Group), Omu-Aran, Kwara State Nigeria

Deborah Oluwatosin Olaniyan

Oluwagbenga dunsin, aruna olasekan adekiya, adeniyi olayanju.

3 Department of Agricultural and Biosystems Engineering, Landmark University, Omu-Aran, Kwara State Nigeria

7 Landmark University SDG 9 (Industry, Innovation and Infrastructure Group), Omu-Aran, Kwara State Nigeria

Field experiments were carried out in 2018 and 2019 cropping seasons at Landmark University Teaching and Research farm, Omu-Aran Kwara state, Nigeria, to determine the effect of soil applied Zn-fertilizer, Tithonia diversifolia (Ti), Chromolaena odorata (Ch) and poultry manure (PM) on the performance, yield, minerals and vitamins composition of okra fruits. Treatments were combined and tested as follows:—Control (T 1 ), Control + 10 kg ha −1 Zn (T 2 ), 5 t ha −1 Ti + 5 t ha −1 PM (T 3 ), 5 t ha −1 Ch + 5 t ha −1 PM (T 4 ), 5 t ha −1 Ch + 5 t ha −1 Ti (T 5 ), 10 t ha −1 Ti + 0 kg ha −1 Zn (T 6 ), 10 t ha −1 Ti + 10 kg ha −1 Zn (T 7 ), 10 t ha −1 Ch + 0 kg ha −1 Zn (T 8 ), 10 t ha −1 Ch + 10 kg ha −1 Zn (T 9 ), 10 t ha −1 PM + 0 kg ha −1 Zn (T 10 ), and 10 t ha −1 PM + 10 kg ha −1 Zn (T 11 ). The experiment was laid out in a randomized complete block design with four repetitions. Vegetative, yield and quality parameters of okra were taken. Data were subjected to analysis of variance and means were compared using Duncan Multiple Range Test (DMRT) at p ≤ 0.05. Variations were observed on the vegetative parameters, yield, minerals and vitamin composition of okra among the applied amendments. The combined application of green biomass, poultry manure, and Zn-fertilizer improved all the variables tested as compared to when they were applied singly. Application of Zn-fertilizer to some selected plots significantly increased yield, Zn, Mg and vitamins concentration of okra. Application of 5 t ha −1 Ti + 5 t ha −1 PM + 10 kg ha −1 Zn (T 3 ) and 5 t ha −1 Ch + 5 t ha −1 PM + 10 kg ha −1 Zn (T 4 ) significantly improved all the parameters tested but the use of 5 t ha −1 Ti + 5 t ha −1 PM + 10 kg ha −1 Zn (T 3 ) resulted in optimum yield and at the same time increase minerals and vitamin concentration of okra. The results of this study therefore showed that Tithonia diversifolia (Ti) as green biomass contained and released more and quality nutrients than Chromolaena odorata (Ch) when combined with equal rate of Zn fertilizer and poultry manure.

Introduction

Vegetable crops play an important role in human nutrition and health management in which they are reliable sources of vitamins, minerals and income 1 , 2 . Fresh fruit is a good source of vitamins, minerals, and plant protein 3 . Rehn and Espig 4 stated that okra contains about 20% edible oil and protein, while its mucilage is utilized for medicinal purposes. Okra fruit is a traditional valuable nutrient vegetable that provides a good source of vitamin C and other vitamins such as vitamin A, B6, K, pyridoxine, and folates, it contains minerals such as calcium, iron, phosphorus that help in regulating the physiological activities in the body.

Okra has a huge potential for enhancing livelihoods in urban and rural areas 5 and it is also regarded as the powerhouse of valuable nutrients that is low in calories and is fat-free 6 . Okra fruit also provides significant antioxidant properties, mostly due to their high content in vitamin C, carotenoids, and flavonoids 7 , as well as therapeutic properties against diabetes, hyperlipidemia, microbes, ulcers and neurodegenerative diseases 8 .

Green biomass 9 and poultry manure 10 are both organic materials that have been discovered to contain high mineral content such as nitrogen, phosphorus, potassium, calcium, and magnesium and some micronutrients that are capable of improving yields and quality of vegetable crops. Organic manures are also known to be environmentally friendly and have been found to be a good source of nutrients in rejuvenating poor soils by improving crop quality, organic matter and the physical and chemical properties of the soil 11 .

Zinc is a micronutrient that has a great influence on the growth, yield and quality of vegetable crops. Zinc is a constituent element that accelerates the performance of many plant enzymes and chlorophyll synthesis. The mechanism of action of carbonic anhydrase and a number of dehydrogenases which are some of the important enzymes in plants are activated by the zinc element 12 . Deficiency of zinc restrain protein integration as a result of restriction of RNA synthesis 13 . Tryptophan, a precursor of auxin which is accelerated by the application of zinc, is important in flower bud formation, fruit setting and fruit yield 14 . Uptake of soil nitrogen and potash is also known to be increased by the application of zinc. Presence of zinc in the soil avert excessive absorption of P by the roots which may be transported to the leaves 15 . Deficiency of zinc produces changes in leaf morphology and cell histology, which causes several-known disorders “little leaf” or “rosette mottled leaf” etc. its deficiency also causes interveinal chlorosis, reduce root growth, blossoming and fruiting.

Similarly, shortened internodes and chlorotic areas on the older leaves and yellowing of younger leaves due to its deficiency were reported by 16 .

Chromolaena odorata originated from North America and belongs to the Asteraceae family. C. odorata is very common in the southwest Nigeria where it grows in abundance and restore soil fertility 17 . Tithonia diversifolia , a shrub also of the Asteraceae family which originated from Mexico and Central America can now be found in the inter-tropical belts of Asia and Africa 18 . The high quantity of some macronutrients (N, P and K) present in T. diversifolia and its ability to extract some other nutrient needed for plant growth in the soil has evoked many research interest 19 . Gachengo et al. 20 noted that high amount of N element in T. diversifolia biomass facilitated its mineralization thereby making N readily available to crops. In Kenya 19 , Malawi 21 , Rwanda 22 and Zimbabwe 23 , leaves of T. diversifolia has been found to improve soil fertility and structure leading to increased crop yield.

Despite the nutritional quality of okra, there are few published articles about using organic materials 24 and Zn-based fertilizer 25 in improving the quality of crop. The study was therefore carried out to determine the performance of T. diversifolia and C. odorata (green biomass) when combined with Zn fertilizer and poultry manure on the yield and nutritional quality of okra fruits.

Initial soil properties

The pre-planting soil analysis is as shown in Table ​ Table1. 1 . The pH of the soil was strongly acidic, the available phosphorus and nitrogen were relatively low, and the exchangeable K, Ca, and Mg was moderate. The organic matter was adequate. There was a high quantity of sand, with a relatively low quantity of silt and clay in the soil, therefore, the textural class sandy loam.

Soil physical and chemical characteristics of the experimental sites at 0 -15 cm layer.

Chemical composition of amendments

Table ​ Table2 2 shows the chemical composition of the organic materials used as amendment. Poultry manure had a lower C: N ratio but higher nitrogen, phosphorus, magnesium, organic matter, and organic carbon concentration.

Chemical composition of the amendments used.

O.M organic matter, O.C organic carbon, ND not detected.

Effects of soil-applied Zn-fertilizer, green biomass, and poultry manure on the vegetative parameters of okra

Vegetative variables of okra were influenced by various types and doses of soil amendments (Table ​ (Table3). 3 ). In 2018 and 2019, when compared with the control (T 1 ) which gave statistically lower values for vegetative variables but similar to the application of the control + 10 kg ha −1 Zn (T 2 ), plots treated with 5 t ha −1 Ti + 5 t ha −1 PM + 10 kg ha −1 Zn (T 3 ) produced higher values for plant height, the number of leaves and stem girth though the values were not significant with the application of 5 t ha −1 Ch + 5 t ha −1 PM + 10 kg ha −1 Zn (T 4 ). Plots treated with 10 t ha −1 PM + 0 kg ha −1 Zn (T 10 ) and 10 t ha −1 PM + 10 kg ha −1 Zn (T 11 ) resulted in statistically higher values when compared with 5 t ha −1 Ch + 5 t ha −1 Ti + 10 kg ha −1 Zn (T 5 ), 10 t ha −1 Ti + 0 kg ha −1 Zn (T 6 ), 10 t ha −1 Ti + 10 kg ha −1 Zn (T 7 ), 10 t ha −1 Ch + 0 kg ha −1 Zn (T 8 ) and 10 t ha −1 Ch + 10 kg ha −1 Zn (T 9 ).

Effects of soil applied Zn-fertilizer, green biomass and poultry manure on vegetative parameters of okra at 7 weeks after sowing (WAS) in 2018 and 2019 cropping seasons.

Means in a column under any given treatment followed by the same letter do not differ significantly at p ≤ 0.05 by Duncan multiple range test.

Ti tithonia, Ch chromolaena, PM poultry manure.

Effects of soil-applied Zn-fertilizer, green biomass, and poultry manure on the yield of okra

Data on the yield of okra as influenced by soil-applied Zn-fertilizer, green biomass and poultry manure is as shown in Table ​ Table4. 4 . There was a significant response of okra yield to different types and rates of soil amendments. The two years study showed that number of fruits were significantly higher with plots treated with 5 t ha −1 Ti + 5 t ha −1 PM + 10 kg ha −1 Zn (T 3 ), 5 t ha −1 Ch + 5 t ha −1 PM + 10 kg ha −1 Zn (T 4 ), 10 t ha −1 PM + 0 kg ha −1 Zn (T 10 ) and 10 t ha −1 PM + 10 kg ha −1 Zn (T 11 ). Other treatments gave similar but varying yield values which were statistically higher than the control (T 1 ) and control + 10 kg ha −1 Zn (T 2 ).

Effects of soil applied Zn-fertilizer, green biomass and poultry manure on yield of okra in 2018 and 2019 cropping seasons.

Effects of soil-applied Zn-fertilizer, green biomass, and poultry manure on micronutrients and vitamins composition of okra fruits

The concentration of the micronutrients (Zn, Cu, Fe, and Mg) and vitamins (B6, B9, A, and C) in okra fruits are presented in Table ​ Table5. 5 . Plots treated with 5 t ha −1 Ti + 5 t ha −1 PM + 10 kg ha −1 Zn (T 3 ) significantly increased values for all the micronutrients, though the values were statistically with plots treated with 5 t ha −1 Ch + 5 t ha −1 PM + 10 kg ha −1 Zn (T 4 ) and 10 t ha −1 PM + 10 kg ha −1 Zn (T 11 ) except for Cu at 10 t ha −1 PM + 10 kg ha −1 Zn (T 11 ). Compared with the control (T 1 ) which gave the least value for Zn and Mg, other plots treated with Zn either sole or in combination with other treatments significantly increased Zn and Mg composition of okra. In a similar vein, application of 5 t ha −1 Ch + 5 t ha −1 Ti + 10 kg ha −1 Zn (T 5 ), 10 t ha −1 Ti + 0 kg ha −1 Zn (T 6 ), 10 t ha −1 Ti + 10 kg ha −1 Zn (T 7 ), 10 t ha −1 Ch + 0 kg ha −1 Zn (T 8 ), 10 t ha −1 Ch + 10 kg ha −1 Zn (T 9 ) resulted in similar but varying values for Cu and Fe that are statistically higher than the control plots and plots treated with Zn only.

Effects of soil applied Zn-fertilizer, green biomass and poultry manure on micronutrients and vitamins concentration of okra fruits (pooled analysis of 2018 and 2019 cropping seasons).

Application of all the amendments either as sole or combined had effects on the availability of vitamins in okra fruits. Control plots gave significantly lower values for all the vitamins though similar to the application of Control + 10 kg ha −1 Zn (T 2 ) except for vitamin A which was significantly higher than the control. Combination of lower rates of green biomass, poultry manure and Zn enhanced the vitamins composition of okra fruits though the values were only similar when 10 t ha −1 PM + 0 kg ha −1 Zn (T 10 ) and 10 t ha −1 PM + 10 kg ha −1 Zn (T 11 ) were applied except for vitamins B6 and A. Varying but similar values for all vitamins were observed with other treatments except for vitamin A where addition of Zn to the biomass significantly increased its value.

Laboratory analysis of the chemical composition of T. diversifolia, C. odorata, and poultry manure used for the study revealed that they all contained macro and micronutrients in varying proportions suitable for the cultivation of okra. The pre-planting analysis of the experimental soil also showed that the nutrient status of the soil was low in major macronutrients. The low fertility status of the soil could be as a result of inherent or continuous cropping on the same piece of land over a period of time without amending the soil with either organic manure or inorganic fertilizer.

That combined application of green biomass with poultry manure increased vegetative parameters could be a result of integrating two or more different sources of nutrients. Adekiya 26 found that adequate application of poultry manure to mulches should be encouraged in the cultivation of tomatoes, especially where farmers use Acacia legume materials as mulch to maximize their contribution to soil and crop productivity. Nitrogen is an important element in plant growth and development. It plays a key role in chlorophyll, protein, nucleic acid, hormone and vitamin synthesis and also helps in cell division and cell elongation. The positive interaction of the combined mineralization of N present in the amendments could also be an indication of the increase in the vegetative parameters. Higher N mineralization occurs where organic amendments were combined, compared to their individual application, indicating a synergistic relationship between the inputs 27 .

Poultry manure is an effective natural source of nutrients 28 , it is also known to be a better soil amendment compared with some animal manure and chemical fertilizers because of its greater capability to preserve its N 29 . The increase in vegetative parameters of okra could also be a result of the presence of both macro and micronutrients in the PM and their ability to improve the water holding capacity of the soil. Animal manures have been shown to supply required plant nutrients, improve soil structure and water holding capacity, increase microbial population, and promote plant growth 30 . Higher yield was also recorded in all plots containing PM. The superior N supply by poultry manure may have contributed to being the better growth and yield of okra in plots with poultry manure. The results corroborated the finding of 31 where they observed that poultry manure increased the performance of okra relative to other amendments. The poor vegetative and yield parameters observed at the control and Control + 10 kg ha −1 Zn (T 2 ) plots might be a result of the low fertility status of the experimental soil as determined by the laboratory analysis carried out before the start of the experiment. Application of organic manure increases organic elements’ availability in soil, thereby improving the nutrient use efficiency (NUE) of crops and alleviating the harmful impact of climate change on crop production 32 .

Zn is an important trace element required for plant growth and development. According to the Food and Agriculture Organization (FAO), about 30% of the cultivable soils of the world contain low levels of plant-available Zn 33 . The increase in the vegetative parameters of okra with the application of Zn-fertilizer could be a result of the role of Zn in photosynthesis by increasing the amount of chlorophyll in the leaves and making Mg available to plant. Aboyeji 34 concluded in their study that combined application of Zn fertilizer with PM and NPK fertilizer resulted in a significant increase in the vegetative parameters of tomato. Increase in the vegetative performance of okra plant could again be attributed to the effect of application of Zn fertilizer which assisted in the nitrogen use efficiency of the plant. Zinc application also helps in increasing the uptake of nitrogen and potash 35 .

The relative increase in okra yield grown under combined application of green biomass, poultry manure, and Zn-fertilizer could be attributed to the applied amendments which supplied the needed nutrient required by the crop for its metabolic functions or as a result of the complementary roles of organic materials and inorganic Zn-fertilizers in improving crop yield. Zinc might have increased the efficiency of added organic materials in the soil and increase the rate of humification of zinc thereby enhancing the availability of both native and added nutrients in the soil and thus increased the yield of okra. The high yield response of okra could also be ascribed to the effect of Zn in making some nutrients available in the soil. Gurmani et al. 36 found that application of Zn significantly increased the dry biomass, fruit yield, fruit fresh weight, and numbers of fruits per plant in the tomato, the highest increase was found with 10 kg ha −1 Zn. The results are also in accordance with the findings of 37 where they found that the application of zinc in soil increased the availability of zinc in the rhizosphere.

The addition of organic materials to soil leads to a significant increase in macro and micronutrient availability in soil compared to the control 34 . Combined incorporation of green biomass, poultry manure and Zn-fertilizer increased availability of micronutrients in okra fruits than when applied singly. Increased availability of Zn and Mg in okra fruits could be due to the application of Zn which in turn made Mg be available in okra fruits. It could also be attributed to the effect of Zn in increasing the uptake of some nutrients. Singh and Singh 38 reported that Zn application increased chlorophyll content and raised the concentration of Zn, Ca, Mg, K, and P in tissues.

Results also indicated that combined application of green manure, poultry manure, and Zn-fertilizer increased Fe and Cu concentration of okra fruits similar to the application of sole PM and PM + 10 kg ha −1 Zn. This could be attributed to the initially high content of total trace elements in chicken manure. The effect of organic materials on Fe and Cu uptake by okra could be due to the influence of organic carbon which acts as a source of energy for soil microorganisms, which upon mineralization releases organic acids that decreased soil pH and improve the availability of Fe and Cu 39 .

The increased mineral and vitamin contents of okra fruits as a result of combined application of green biomass, poultry manure, and Zn-fertilizer or PM alone can be adduced to increase soil chemical properties which led to greater metabolic activities and hence higher minerals and vitamins in 5 t ha −1 Ti + 5 t ha −1 PM + 10 kg ha −1 Zn (T 3 ) and 5 t ha −1 Ch + 5 t ha −1 PM + 10 kg ha −1 Zn (T 4 ) plots compared with their sole forms.

Different organic wastes influence the nutritional quality of crops differently. The study revealed that the application of 5 t ha −1 Ti + 5 t ha −1 PM + 10 kg ha −1 Zn (T 3 ) and 5 t ha −1 Ch + 5 t ha −1 PM + 10 kg ha −1 Zn (T 4 ) significantly improved the minerals and vitamin composition of okra. An increase in the nutritional quality of okra could be attributed to the combined application of green biomass and poultry manure. Integrated application of green biomass, poultry manure, and Zn-fertilizer not only influenced the micronutrient composition of okra but also helped in enhancing its vitamin contents.

Wolf and Snyder 40 reported that C: N ratio of organic materials markedly influences the decomposition rate and the mineralization of N because N determines the growth and turnover of the microorganisms that mineralize organic carbon. The result also revealed that T. diversifolia and PM contained higher N values and lower C: N ratio than C. odorata . This could be attributed to the faster mineralization and nutrient release thereby leading to a consistent increase in the agronomic and quality parameters of okra 41 . In their experiments found that the rate of litter decomposition depends on the C: N ratio and/or nitrogen content of the leaf litter.

Application of 5 t ha −1 Ti + 5 t ha −1 PM + 10 kg ha −1 Zn (T 3 ) and 5 t ha −1 Ch + 5 t ha −1 PM + 10 kg ha −1 Zn (T 4 ) significantly improved all the variables tested though having similar values with 10 t ha −1 PM + 0 kg ha −1 Zn (T 10 ) and 10 t ha −1 PM + 10 kg ha −1 Zn (T 11 ) in some cases. T diversifolia contained higher values for N, P, K, Ca. and Mg with lower C: N which may have facilitated the rate of decomposition and release of nutrients than C. odorata , then the optimum dose for better productivity and crop quality in the study area is 5 t ha −1 Ti + 5 t ha −1 PM + 10 kg ha −1 Zn (T 3 ).

Materials and methods

Description of the experimental site.

The experiment was conducted at the Teaching and Research farm of Landmark University, Omu-Aran Kwara State (latitude 8 o 8 ′ N and longitude 5 o 6 ′ E) with an altitude of 495 m elevation above the sea level in the derived savannah zone of Nigeria. It has an annual rainfall ranging from 600 mm to 1, 200 mm and an annual average temperature of 24.9 °C. The months of December and January coincide with the cold and dry harmattan period. The study area is classified as Aw; which means tropical wet and dry or savanna climate; with the driest month having precipitation less than 60 mm (2.4 inches) and less than 4% of the total annual precipitation.

Soil sampling and analysis and laboratory determination of poultry manure, T. diversifolia and odorata

Soil samples were randomly taken from each plot before the commencement of the field experiment. All samples were bulked together to obtain the composite samples for laboratory analysis . Poultry manure used was analysed for nutrient composition after being air-dried using warm air (18–21 °C) in the poultry house for 7 days. The dried poultry manure was crushed and passed through a 2-mm sieve. The analysis was done for organic carbon (OC), total N, P, K, Ca, Mg, Cu, Mn, Zn, and Na 42 . Leaf samples of T. diversifolia and C. odorata were collected fresh, oven-dried for 24 h at 80 °C and grinded in a Willey mill. N was determined by the micro-Kjeldahl digestion method. Ground samples were digested with the nitric-perchloric-sulphuric acid mixture for the determination of P, K, Ca and Mg. Phosphorus was determined colorimetrically using the vanadomolybdate method, K was determined using a flame photometer and Ca and Mg were determined by the EDTA titration method 42 . The percentage of organic carbon in the sample was determined by the Walkley and Black procedure using the dichromate wet oxidation method 43 . Sample pH was determined by using a soil–water medium at a ratio of 1:2 using Jenway digital electronic pH meter model 3520 44 . Contents of Zn, Cu, Mg, and Fe, were determined by an atomic absorption spectrophotometer.

Treatment and experimental design

Treatments consists of Tithonia diversifolia leaves (Ti), Chromolaena odorata leaves (Ch), poultry manure (PM) and zinc sulphate (Zn). Treatments were combined and tested as follows:—Control (T 1 ), Control + 10 kg ha −1 Zn (T 2 ), 5 t ha −1 Ti + 5 t ha −1 PM (T 3 ), 5 t ha −1 Ch + 5 t ha −1 PM (T 4 ), 5 t ha −1 Ch + 5 t ha −1 Ti (T 5 ), 10 t ha −1 Ti + 0 kg ha −1 Zn (T 6 ), 10 t ha −1 Ti + 10 kg ha −1 Zn (T 7 ), 10 t ha −1 Ch + 0 kg ha −1 Zn (T 8 ), 10 t ha −1 Ch + 10 kg ha −1 Zn (T 9 ), 10 t ha −1 PM + 0 kg ha −1 Zn (T 10 ) and 10 t ha −1 PM + 10 kg ha −1 Zn (T 11 ). The experiment was laid out in a Randomized Complete Block Design (RCBD) replicated four times. Vegetative, yield and quality parameters of okra were taken.

Sources of materials

Fresh and tender leaves of T. diversifolia and C. odorata were collected from a fallow field of Landmark University Teaching and Research farm. Poultry manure was collected from the poultry house of Landmark University farms while zinc sulphate fertilizer was purchased from a reputable agro-allied farm store.

Land preparation and plot size

The land was ploughed and harrowed to pulverize and loosen the soil. Thereafter the field layout was carried out to mark out the appropriate number of treatment plots. The size of each experimental plot was 2 m × 2 m = 4 m 2 and there were 11 plots per replicate (4 m × 11 m = 44 m 2 ) which were replicated four times. The size of the whole experimental plot was 44 m 2  × 4 m 2  = 176 m 2 .

Seed variety and sowing

The variety used for the experiment was the local variety that is commonly grown and eaten in Omu-Aran, Kwara State, Nigeria. Okra seeds were subjected to a viability test before sowing. The seeds were poured inside a bowl containing clean water and were allowed to stay for 2 min. The floated seeds were discarded while the ones that sank were sown. The seeds were sowed two weeks after the incorporation of the green biomass and poultry manure. The sowing spacing was 60 cm by 30 cm inter and intra row respectively. Two seeds were sown per hole and were later thinned to one plant per stand two weeks after sowing.

Application of organic materials and Zn-fertilizer

Fresh and tender leaves of both T. diversifolia and C. odorata were chopped into pieces and incorporated into the soil immediately after land preparation and field layout and were allowed to decompose for two weeks before sowing. Zn-fertilizer was applied to designated plots 2 weeks after sowing by side placement 8 cm away from the base of the plant.

Weed control

Weed control was manually done using the local hand-held hoe and rouging. Weed control started 2 weeks after germination.

Mature okra pods were harvested at three days’ intervals, counted and weighed using a weighing balance based on each treatment.

Data collection

The following parameters were taken during the experiment:—plant height, number of leaves, stem girth, yield per plot/hectare, micronutrients and vitamins composition of okra fruits.

Laboratory determination of micronutrients (minerals) in okra fruits

Representative okra fruit samples were taken per plot and per replicate to analyze for the levels of mineral elements contained in the fruit at the crop and soil laboratory of Landmark University, Omu-Aran, Nigeria. Mature fresh okra fruits were collected, oven-dried for 24 h at 80 °C, and ground in a Willey mill. Mineral elements were determined according to the methods as recommended by the Association of Official Analytical Chemists 45 . One gram of each sample was digested using 12 cm −3 of the mix of HNO 3 , H 2 SO 4 , and HClO 4 (7:2:1 v/v/v). Contents of Zn, Cu, Mg, and Fe, were determined by an atomic absorption spectrophotometer.

Laboratory analysis of vitamins in okra fruits

The concentrations of the vitamins (vitamins B6, B9, A, and C) in water extracts of okra samples were determined as previously described by 46 . All analyses were carried out in triplicate.

Statistical analysis

Data collected were subjected to analyses of variance using Statistical Analysis Software 47 (SAS, Inc. 2002). The treatment means were compared using the Duncan Multiple Range Test at p ≤ 0.05.

Ethical declaration

I confirm that all the research meets ethical guidelines and adheres to the legal requirements of the study country.

Acknowledgements

I would like to thank the management of Landmark University who provided enabling environment in carrying out the experiment.

Author contributions

C.M.A., developed the concept and wrote the manuscript, S.O.D. did all the laboratory analysis, D.O.O. took all the field parameters, O.D. did the statistical analysis, A.O.A. prepared the figures while T.M.A.O. designed the field experiment and reviewed the manuscript.

Competing interests

The authors declare no competing interests.

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