Enhancing nutritional content, phytochemical levels, growth and yield of okra (Abelmoschus esculentus L.) using the organic medium enclosed trough system

Abstract

Okra is an indigenous vegetable consumed in Southern Africa. Its growth and yield are negatively affected by water and nutrient deficit. There is insufficient scientific information on the growth and yield attributes of underutilised indigenous vegetables such as okra. The information on enhancement and evaluation of growth, yield, nutrients, and phytochemical compositions of okra using the climate-smart OMET growing technique has not been documented. The Organic Medium Enclosed Trough (OMET) system was developed as a non-drainable growing technique which improves crop yield by reducing water and nutrient seepage. The aim of this study was to develop scientific information on the effects of the OMET growing technique and growing environment on growth and yield, nutritional composition and phytochemical composition in okra. The objectives of this study were to (1) investigate the effect of the OMET growing technique and growing environment on growth and yield attributes, (2) nutritional composition and (3) the phytochemical composition of okra grown under greenhouse and micro-plots conditions. To achieve the objectives, four-week-old okra seedlings were transplanted on the established OMET and non-OMET growing technique concurrently in both the growing environment (greenhouse and micro-plot) following a randomised complete block design (RCBD) for a period of 110 days, with three replications and twelve plants per replicate. The amount of irrigation water used in both experiments was recorded until harvest and computed as cumulative irrigation water. The mean separation was done using a parametric T-test at the significance level of 5% using the Genstat version 18.0 statistical package. The growth attributes which included plant height and stem diameter (n=9) were taken on a weekly basis. At harvest, yield components including the number of branches per plant, plant biomass, fresh pod weight, number of pods per plant, fresh pod length and fresh pod diameter width were recorded. The harvested leaves and pods were then used for nutritional and phytochemical composition analysis. The OMET growing technique significantly (p≤0.05) affected the growth and yield attributes of okra regardless of the growing condition. At termination (110 days after transplanting), the OMET growing technique had significantly increased the stem diameter by 40 and 37%, while the plant height was increased by 68 and 48% under greenhouse and micro-plot experiments respectively. When evaluating the yield attributes, a similar trend was observed where the OMET system significantly increased the yield attributes of okra as follows: biomass by 64 and 50%, number of branches by 67 and 50%, number of pods per plant by 60 and 49%, fresh pod weight by 75 and 53%, pod length by 64 and 51% in both the growing environment, while the pod diameter width was increased by 68% in the greenhouse environment and there was no significant difference on the micro-plot trial. The OMET growing technique significantly affected the nutritional composition of okra leaves and pods (p≤0.05). Both the essential and non-essential amino acids were determined and quantified in the leaves and pods of okra grown under OMET and Non-OMET growing techniques. The OMET growing technique significantly improved the essential amino acid composition of okra leaves with Thr (0.57 mg/kg), Val (0.70 mg/kg) Leu (0.90 mg/kg) and Phe (1.03 mg/kg) being higher than the non-OMET grown okra leaves. The pods showed that the OMET growing technique also significantly improved the accumulation of all the tested non-essential amino acids with Lys being the highest (p≤0.05). It was observed that the micro-plot experiment resulted in the OMET growing technique significantly enhancing the accumulation of all the tested essential amino acids in both the leaves and pods with Phe and Lys (1.53 and 0.70 mg/kg) being the highest in the leaves and pods respectively. The non-essential amino acid composition was also significantly improved using the OMET growing technique in both the leaves and pod with Glu.. The non-essential amino acid composition was also significantly improved using the OMET growing technique in both the leaves and pod of okra grown under micro-plot with Glu (2.73 mg/kg and 4.05 mg/kg) being the highest respectively. The OMET growing technique showed the ability to maintain a daily recommended amino acids ratio coefficient equal to 1 by consuming 100g of the tested okra. The nutritional composition which includes proteins, mineral elements and amino acids of the tested okra leaves and pods grown using the OMET system regardless of the growing condition has resulted in an increase in the % protein content compared to the non-OMET grown okra. Nutritional elements Ca, K, P, Mg and Na were predominant in okra leaves and pods irrespective of the growing environment and growing technique (OMET). The results generated showed that the OMET system significantly (p≤0.05) improved the nutritional composition in okra leaves and pods regardless of the growing condition, though the micro-plot experiment resulted in higher nutritional composition as compared to the greenhouse experiment. Untargeted metabolites, phenolic acids and antioxidant activity were also determined and compared for both the okra leaves and pods extract grown using the OMET and non-OMET growing techniques in both the growing conditions. For untargeted metabolites, methanol extracts were analysed using UPLC-ESI-QTOF-MS. The UPLC-MS untargeted metabolites profile detected 161 polar analytes classified within the glucuronic acid, tricarboxylic acids, O-glycosyl derivatives, flavonoid-O-glycosides, iridoid o-glycosides and terpene glycosides. Explorative principle component analysis demonstrated three main clusters according to metabolites heterogeneity in plant tissue (pods and leaves) and growing conditions (greenhouse or mirco-plot). There was major heterogeneity in the metabolome profile of leaves tissue along the vertical PC1 suggesting their metabolic moiety. Okra leaves grown under micro-plot were highly predominated by the 2-O-caffeoylglucaric acid (286.13 mg/kg) and 2-(E)-O-feruloyl-D-galactaric acid (111.69 mg/kg). Leaves samples grown in non-OMET were predominated by citroside A (412.04 mg/kg). Okra pods grown under OMET micro-plot enhanced the accumulation of quercetin 3-galactoside (87.83 mg/kg) and quercetin 3-galactoside-7-glucoside (150.00 mg/kg). The OMET under greenhouse conditions encouraged the accumulation of icariside F2 and benzyl beta-D-apiofuranosy (49.21 mg/kg). The results generated showed that flavonoids were the major contributors to the total antioxidant activity and OMET enhanced the accumulation of the majority of the metabolites. Phytochemical analysis showed that the OMET growing technique significantly increased the concentration of total phenolics and flavonoids in both the growing environment (p≤0.05). The OMET growing technique significantly affected the antioxidant activity in both growing conditions.