Biochemical responses of Moringa oleifera Lam. plants to graded moisture deficit

Abstract

Continual water loss through transpiration and evaporation as a result of drought leads to stomatal closure and subsequently the overproduction of reactive oxygen species (ROS) which causes oxidative stress in plants. A set of biochemical mechanisms are concurrently triggered to set in motion a defence line, which, among others, involves the synthesis of antioxidant and osmoprotectants compounds. These play a protective role to overcome stress. The objective of this study was to evaluate the effect of varying moisture regimes on the accumulation patterns and concentration of phenolic compounds, antioxidant activity and osmolyte proline in Moringa oleifera. The study was conducted in a greenhouse (25 ± 3°C) at ARC, Roodeplaat in Pretoria, South Africa. Moringa oleifera seeds were individually planted in pots using hygro-mix as the growth media. The experiment was laid out in a completely randomised design with four watering regimes of 100%, 75%, 50% and 35% field water capacity (FWC) each with 50 replicate plants. Proline content increased with the severity of moisture stress in all plant parts (root, stem and leaf), with the leaves under severe stress (35% FWC) recording the highest concentration. Folin C, aluminium chloride and butanol-HCL assays were used to determine total phenolic, flavonoid and condensed tannin concentrations, respectively, in the different plant parts at different growth stages and harvests. The concentrations of these secondary metabolites were consistently higher in leaves and increased with an increase in moisture stress. The highest concentrations were recorded in the second harvest under severe stress (35% FWC) for total phenolics and proanthocyanidins at 50% FWC for flavonoids . Leaves showed a better antioxidant ability and antioxidant activities increased with drought progression as determined by FRAP while DPPH radical scavenging and the β-carotene-linoleic acid model system did not show a distinct trend in antioxidant activities in relation to the different moisture levels. Based on the results of this study, antioxidant and osmoprotectant compounds as well as antioxidant activity appears to increase corresponding to severity of moisture stress in all plant parts at both harvests. This trend suggests that M. oleifera employs these mechanisms in adapting to moisture stress.