Nutritional and biochemical evaluation of momordica balsamina leaf powder at different harvesting stages

Abstract

Momordica balsamina Linn., is one of the African leafy vegetables (ALVs) that holds a great deal of nutraceutical and pharmaceutical properties, attributed to the presence of micro nutrients and secondary metabolites. These compounds form an important part of the human diet to reduce malnutrition and offer a wide range of remarkable preventive and therapeutic properties. Generally, the levels of nutritive and non-nutritive substances in ALVs are influenced by the plant’s developmental stages and hence the stages in which the edible plant parts are harvested. A greenhouse study was undertaken, (1) to determine the effect of harvesting at different growth stages on the nutritional quality of M. balsamina and (2) to evaluate the effect of harvesting at different growth stages on the biochemical composition of M. balsamina leafy vegetable. Seeds of M. balsamina were planted in seedling trays and then when the seedlings were bearing three (3) leaves, they were transplanted into 20-cm diameter plastic bags, containing a mixture of steam pasteurised loam soil and Hygromix at 3:1 v/v. Six harvesting stages, namely: vegetative, bud development, flower initiation, fruit set, fruit ripening, and physiological maturity, represented treatments, and were replicated ten (10) times. The experiment was laid out in a randomised complete block design (RCBD). At the vegetative stage, harvesting of M. balsamina leaves commenced and was continued at each at each growth stage, with plant variables being measured and recorded. The harvested leaves were freeze-dried and ground into fine powder for nutritive and biochemical analysis. Data were subjected to analysis of variance (ANOVA) using Statistics 10.0 software. In the first objective, data on the plant variables, namely, stem diameter (SD), vine length (VL), number of leaves (NL), leaf area (LA), chlorophyll content (CC), and leaf colour [brightness (L*), redness (a*), greenness (b*) and hue (h)], were measured and recorded, whereas data on the selected nutrients, namely, Potassium (K), Calcium (Ca), Magnesium (Mg), Zinc (Zn), Iron (Fe), and Phosphorus (P) were determined and recorded. The growth stages had highly significant (P≤ 0.00) effect on the measured plant variables, contributing 92, 87, 85 and 81% to the total treatment variation (TTV) in VL, NS, NL and LA, respectively. No significant (P ≥ 0.5) effect was observed in SD. Similarly, treatments had a highly significant effect in CC and leaf colour, contributing 28, 33, 68, 71 and 63% to TTV in CC, brightness (L*), redness (a*), greenness (b*) and hue (h), respectively. Treatments highly increased VL (42-234%), NS (10-970%), NL (70-140%) and LA (53-86%) in M. balsamina leafy vegetable at all growth stages. Chlorophyll content increased by 10-37% at all growth stages, but experienced a decline (30%) at maturity stage. With regard to the leaf colour, the brightness of the leaves increased by 1% at the bud development stage, then followed by a decline of 1-11% at all the other growth stages. In contrast, the redness and the greenness of M. balsamina leaves were reduced at early developmental stages by 14-22% and 10- 18%, respectively, but when the plants started setting fruits, the redness and the greenness of the leaves increased by 16-27% and 16-73%, respectively, up until physiological maturity. Harvesting of M. balsamina leaves at different growth stages again significantly influenced the nutritional quality of the leaf powder, contributing 46, 93, 68, 93, 95, and 96% to the TTV in K, Ca, Mg, Zn, Fe and P, respectively. The accumulation of K (56-95%), P (5-12%) and Zn (10-12%) in M. balsamina leaves were found to be high at early growth stages, but were reduced from fruit maturity stage up to physiological maturity by 84-91%, 154-173% and 81- 229%, respectively. Notably, Fe was increased by 5% only, when the leaves were harvested at the bud development stage, but was reduced (6-508%) in all the other growth stages. Contrarily, Ca and Mg, were increased by 41-12033% and 101-2129%, respectively, from early vegetative stage throughout to physiological maturity. In the succeeding objective, the biochemical compounds, namely, total phenolic content (TPC), ascorbic acid (AA), and radical scavenging activity (DPPH) were determined. A highly significant (P≤ 0.01) effect was observed on the biochemical quality of M. balsamina, contributing 96, 98, and 97% to TTV in TPC, AA and DPPH, respectively. Relative to the vegetative stage, treatments increased TPC by 17% at the physiological maturity stage. However, in the rest of the tested growth stages, treatments decreased TPC by 25-40%. Harvesting at different growth stages also decreased AA concentration decreased by 32-58% at all the growth stages, when compared to the standard control. In contrast, the same treatments increased DPPH by 23- 42% from the bud development, throughout to physiological maturity stage. In conclusion, harvesting at different growth stages influenced the nutritional and biochemical quality of M. balsamina. The nutritional and biochemical components evaluated where all adequately available at the vegetative and bud development stages.