Tolerance of Cowpea (Vigna unguiculata (L.) Walp.) Genotypes to Drought Stress During Vegetative and Reproductive Stages

Cowpea is a valuable legume known for its high-quality protein and multiple uses, but it often suffers from sub-optimal moisture during different growth stages. While the effects of drought at certain stages are studied, comprehensive studies across all growth stages are limited. This study evaluated drought tolerance of two cowpea genotypes by examining morphophysiological, pigments, biomass, yield and quality traits under contrasting soil moisture conditions: control and drought during critical vegetative (V2 and V4) and reproductive (R1 and R4) stages. Gas exchange traits were more affected by drought during the vegetative stages than the reproductive stage, with photosynthesis declining by 86% at the vegetative stage, compared to a 68% reduction at the reproductive stage. The chlorophyll index declined by 54% in the R4 stage, while the V2 stage showed no significant changes under drought. Drought at the R1 stage led to a substantial decrease in seed yield by 46%, followed by 35% in R4, 33% in V2 and 27% in V4. The yield reduction at R1 correlated with an 82%, 68% and 44% drop in stomatal conductance, photosynthesis rate and chlorophyll index, respectively, followed by a 3.4°C increase in leaf temperature. Drought stress during reproductive stages led to an increase in hundred-seed weight and seed protein, while starch content decreased, indicating a shift in resource allocation under stress conditions. Based on seed number and weight, the order of cowpea growth stage resilience to drought stress followed the order of R1 < R4 < V2 < V4. These findings suggest that breeding for a multi-stage drought-resilient genotype is needed to sustain yield under rainfed conditions.