Leaf emergence (phyllochron index) and leaf expansion response to soil drying in cowpea genotypes

Drought can result in severely decreased leaf area development, which impacts plant growth and yield. However, rarely is leaf emergence or leaf expansion separated to resolve the relative sensitivity to water-deficit of these two processes. Experiments were undertaken to impose drought over approximately 2 weeks for eight cowpea (Vigna unguiculata) genotypes grown in pots under controlled environmental conditions. Daily measures of phyllochron index (PI, leaf emergence) and leaf area increase (leaf expansion) were obtained. Each of these measures was referenced against volumetric soil water content, i.e. fraction transpirable soil water. Although there was no clear difference between leaf emergence and leaf expansion in sensitivity to drying soil, both processes were more sensitive to soil drying than plant transpiration rate. Genotypic differences in the soil water content at the initiation of the decline in PI were identified. However, no consistent difference in sensitivity to water-deficit in leaf expansion was found. The difference in leaf emergence among genotypes in sensitivity to soil drying can now be exploited to provide guidance for plant improvement and crop yield increase.