Drought Adaptation of Cowpea. II. Influence of Drought on Plant Water Status and Relations with Seed Yield

Concurrent studies described the drought resistance of cowpeas [Vigna unguiculata (L.) Walp.] during the vegetative stage, and susceptibility to drought during flowering and pod filling. The present field studies were conducted to examine possible mechanisms of cowpea adaptation to drought and relations between yield and plant water status to guide programs for the development of improved cultivars and irrigation practices. Two cowpea cultivars were subjected to different levels of irrigation at the vegetative, flowering, and pod filling stages using a line-source sprinkler system during 2 years having contrasting weather patterns. The soil is an Arlington fine sandy loam (coarse-loamy, mixed, thermic Haplic Durixeralf) with a high bulk density. Soil water content was determined with a neutron scattering device and plant water status measurements were made with a pressure chamber and thermocouple psychrometers. The plants exhibited extreme drought avoidance in the presence of soil water deficits or high evaporative demands with xylem pressure potential remaining above −18 bar. Drought avoidance by cowpeas appeared to be mainly due to several mechanisms for regulating water loss. Predawn xylem pressure potential was more consistently related to soil water deficits than afternoon values. Leaf solute potential was correlated with xylem pressure potential, becoming more negative in the afternoon than predawn. Solute potential also was more negative during periods of high evaporative demands but was only slightly reduced by soil water deficits. Seed yield was linearly correlated with seasonal cumulative xylem pressure potential measured either just before sunrise or in the afternoon. Consequently, the pressure chamber may provide guidelines for research on irrigation management and the selection of cultivars with improved drought tolerance. However, because of the large differences in seed yield associated with small differences in cumulative xylem pressure potential the pressure chamber may not be useful as an indicator for commercial irrigation of cowpeas.