Phenotyping cowpea (Vigna unguiculata L. Walp) for seedling drought stress tolerance using leaf wilting index, delayed senescence and tolerance indices

Botswana's semi-arid climate and variable rainfall patterns make early-season drought a threat to cowpea production. Selecting cowpea for drought tolerance at seedling stage is critical for maintaining survival and maximising yield potential in water-limited areas. This study evaluated 30 cowpea genotypes for drought tolerance at the seedling stage using a wooden box technique. Drought stressed cowpea showed a gradual increase in leaf wilting and a decrease in relative water content and chlorophyll content. Genotypes such as Mahutohuto and Wild cowpea demonstrated resilience to stress, showing regrowth and recovery, while ER7, B500 and B138 did not. Calculations for Stress Susceptibility Index (SSI), Stress Tolerance Index (STI), Tolerance (TOL), Mean Productivity (MP), Biomass Drought Resistance (BDR), Yield Reduction (YR) and Relative Drought Index (RDI) were based on yields under drought (Ys) and irrigated (Yp) conditions. These tolerance indices exhibited genotypic variation and correlated with yield under both water conditions. Principal component analysis indicated significant relationships between yield and both STI and BDR, yielding coefficients of determination of 0.93 and 0.97 as well as 0.69, and 0.44 under drought-stressed and non-stressed conditions, respectively. This suggests that STI and BDR are reliable indicators for selecting drought-tolerant cowpea. Mahutohuto, a landrace from Kalahari sandveld, was recognized as highly tolerant, along with other tolerant types, which include a mutant and a wild cowpea. These genotypes are valuable resources for drought tolerance breeding. Further investigations involving transcriptome profiling in Mahutohuto may reveal the genetic factors contributing to drought tolerance by identifying transcriptional regulators of the relevant genes.