Sensitivity to increase in leaf endogenous ABA is responsible for stomatal closure under drought stress in cowpea ( Vigna unguiculata (L.) Walp.)

Drought stress is a major environmental factor limiting crop productivity worldwide. Plants respond to drought through various physiological mechanisms, including stomatal closure mediated by abscisic acid (ABA). This study investigated the relationship between leaf ABA content and stomatal closure in drought-tolerant cowpea (Vigna unguiculata) and drought-sensitive soybean (Glycine max). Under drought conditions, stomatal conductance decreased faster in cowpea than in soybean, significantly by day 2. Leaf ABA content increased earlier in cowpea, suggesting a strong correlation between ABA accumulation and stomatal closure. In contrast, both stomatal conductance and ABA accumulation were delayed in soybean. A lower ABA concentration was required to induce stomatal closure than in soybean, indicating that stomatal sensitivity to ABA was higher in cowpea. These findings suggest that cowpea's superior drought tolerance is due to its rapid and more sensitive ABA-mediated stomatal response and provide insights for improving drought resilience in soybean through targeted breeding or biotechnological approaches.