Drought Adaptation of Cowpea. III. Influence of Drought on Plant Growth and Relations with Seed Yield

Concurrent studies described drought effects on seed yield, plant water status, and water use of cowpea (Vigna unguiculata L. Walp.). In the present studies, drought effects on the growth of cowpea were determined to analyze possible relationships between seed yield, plant growth, and drought, and to evaluate possible mechanisms for the extreme drought avoidance exhibited by cowpea. Two cowpea cultivars were subjected to different intensities of drought at the vegetative, flowering, and pod filling stages in the field during 2 years having contrasting weather patterns. Plant growth analyses were conducted at the end of the vegetative and flowering stages. Increase in percent ground cover with time during the vegetative and flowering stages was either slowed down or delayed by drought. Increasing levels of drought, as determined by cumulative xylem pressure chamber values, resulted in progressively less leaf area, shoot dry matter, number of leaflets, and average leaflet area with total leaf area having the greatest sensitivity, and leaflet size the least sensitivity to drought. These responses were partially responsible for the extreme drought avoidance by this crop, but they also may have caused yield reductions. Seed yields were correlated with the leaf area index at the end of flowering and with the percent cover duration from the beginning of flowering to pod filling, but they were not correlated with the amount of leaf area present during the vegetative stage. Earliness, determined by the dry weight in pods at the end of flowering, increased with moderate drought and decreased with more severe drought. Specific leaf weight increased with increasing levels of drought and may provide a measure of the long term water status experience of plants due to its correlation with cumulative pressure chamber values. A multiple correlation of seed yield with specific leaf weight and degree days above 35 C at flowering accounted for most of the variation in seed yield due to progressive drought and for three growing seasons with contrasting temperature regimes.