Effect of Post-Flowering Drought On Flavonoid Composition and Antioxidant Activity in Vigna Unguiculata.

Bioactive compounds with potential health benefits are affected by biotic and abiotic stresses altering their levels in grains. Effect of post-flowering drought stress on flavonoid composition and antioxidant activity was evaluated in thirteen diverse varieties of Vigna unguiculata (Cowpea), grown in 2008, 2009 and 2010 under irrigated and limited water conditions at Coachella Valley Agriculture Research Station, CA. Seeds were harvested at maturity and evaluated for yield, 100 seed weight (seed size), moisture, protein (high temperature combustion method), polyphenol content (Folin-Ciocalteu method) and antioxidant activity (ABTS antiradical method). Condensed tannins and flavonoid composition was determined using High Performance Liquid Chromatography (HPLC) & Ultra Pressure Liquid Chromatography-Mass Spectroscopy (UPLC-MS). Drought stress significantly decreased yields in cowpeas (p=0.0339). Treatment (irrigated & non-irrigated)*variety interaction was highly significant (p<0.0001). There were no significant differences in 100 seed weights, total phenols, antioxidant activity and total condensed tannin, in stressed and irrigated plants. However, tannin content showed significant treatment*variety interaction (p<0.001). Stressed plants showed an average 4% increase in protein content. This increase in protein content did not correlate with yields. HPLC quantifications of seed extracts indicate no significant difference in the phenolic acids, flavonols and proanthocyanidin content in stressed and non-stressed treatments. However, there is a significant variety*treatment interaction (p<0.05). Of all the 13 varieties, a 49 % average increase in flavonols was observed under drought stress in 5 varieties whereas there was a decrease of 32 % in the other 8 varieties. Anthocyanins quantified in IT95K1105-5 (black seed) indicated an overall 14 % decrease in drought stressed plants. Compositional changes in flavan-3-ols were observed under drought stress in some varieties with some compounds being synthesized only under stressed conditions. Further studies are needed to determine the effect of genotype on accumulation of bioactive flavonoids under limited water availability.