Effects of Water Stress on Chlorophyll Content and Yield Performance of Selected Cowpea (Vigna Unguiculata) Genotypes Under Controlled Conditions

Cowpea (Vigna unguiculata (L.) Walp) is a drought-tolerant legume vital for food security in Zambia, but its production is constrained by water stress. This study evaluated the physiological and agronomic responses of three cowpea genotypes (Msandile, Bubebe, and Namuseba) to different water regimes. A completely randomized design with three replications was used. Plants were subjected to three weekly water levels: 1000 mL (control, simulating Agro-Ecological Zone III), 800 mL (moderate stress, AEZ II), and 600 mL (severe stress, AEZ I). Chlorophyll content (SPAD), pod number, grain weight, and biomass were measured. Water stress significantly (p < 0.05) reduced all measured parameters. Chlorophyll content declined with increasing stress; Msandile and Namuseba showed higher values under control conditions (56.9 and 54.43, respectively) but Msandile experienced the sharpest decline under severe stress. Pod number and grain weight followed similar trends, with Msandile showing the highest yield under control (41.67 pods, 0.27 g/plant) but poor performance under stress. Namuseba demonstrated the most stable yield performance across stress levels. A strong positive correlation was found between chlorophyll content and pod count (r = 0.82), pod weight (r = 0.76), and biomass (r = 0.70). The genotypes exhibited differential tolerance to water stress. Namuseba is recommended for drought-prone environments due to its stability, while Msandile is suitable for high-potential areas. Chlorophyll content is a reliable physiological marker for screening drought tolerance in cowpea.