Leaf Expansion and Transpiration Response to Soil Drying and Recovery among Cowpea Genotypes

Sensitivity of leaf expansion to water-defcit conditions could have a major infuence on C assimilation rate and water loss rate under developing drought conditions. While cowpea (Vigna unguiculata L.) is commonly grown in more arid regions, there is no information on the sensitivity of its leaf expansion with drying soil. Three experiments were undertaken in controlled environments to document leaf expansion during increasing soil drying (11–13 d). Eight cultivars of cowpea were studied. It was found that the initiation of the decrease in leaf expansion occurred earlier in the soil drying cycle than the decrease in transpiration rate in all genotypes. Also, the soil water content at which leaf expansion completely stopped was slightly greater than the termination of transpiration. Therefore, both measures of leaf expansion sensitivity to soil water showed greater sensitivity to soil drying than plant gas exchange as measured by transpiration rate. Genotypic differences were observed among the genotypes in their sensitivity to soil drying. In one experiment, the severely stressed plants were rewatered and recovery in leaf expansion rate occurred very rapidly. Leaf expansion rates of all genotypes following rewatering returned to the rates of the well-watered plants within ~1 d.