Drought Adaptation of Cowpea. IV. Influence of Drought on Water Use, and Relations with Growth and Seed Yield

In concurrent studies it was demonstrated that the drought rsistance of cowpeas [Vigna unguiculata (L.) Walp.] is associated with extreme drought avoidance resulting from drought-induced regulation of water loss. The present studies were conducted to quantify drought effects on crop water use, and relations with growth and seed yield. Two cowpea cultivars were subjected to different drought intensities at the vegetative, flowering, and pod filling stages in the field during 2 years having contrasting weather patterns. Soil moisture was measured weekly throughout the season using the neutron scattering method. Water use (ET) and its ratio with pan evaporation (ET/Eₚₐₙ) were calculated both weekly and for the entire season. The seasonal patterns of ET/Eₚₐₙ were similar for the 2 years which had contrasting patterns of potential evapomtion. Under well-watered conditions, ET/Eₚₐₙ increased linearly with ground cover and had an average value of 0.94 with complete ground cover. ET was restricted well below the potential level by different levels of drought, and at different growth stages, but showed rapid recovery following a vegetative-stage drought. When plant cover was not limiting, ET/Eₚₐₙ was extremely sensitive to percent available water in the rooting zone with a high threshold of 90% in contrast to results from earlier work with other crops. A more general description of the factors influencing ET of cowpeas under varying types and levels of drought was provided by a linear relationship between ET/Eₚₐₙ and fractional, ground cover × available water. Root growth increased linearly throughout the season to a depth of 135 cm under well-watered conditions, but was restricted greatly under dry conditions. A linear relationship was observed between relative seed yield and relative water use from early flowering to harvest. Water-use efficiency (WUE) generally increased with vegetative-stage drought and was reduced by flowering-stage and late-season droughts. WUE increased slightly with moderate reductions in the level of weekly irrigation and decreased with further reductions in irrigation level.