ADAPTATION TO MIDSEASON DROUGHT OF COWPEA GENOTYPES WITH CONTRASTING SENESCENCE TRAITS

Monocarpic senescence traits may influence adaptation to midseason drought by their effects on drought escape and dehydration avoidance. Adaptive attributes of two related cowpea [Vigna unguiculata (L.) Walp.] genotYPes, contrasting in monocarpic leaf senescence, were evaluated under well-watered and drought treatments in the field. Drought stress was imposed during flowering and pod filling for 19 consecutive days before irrigation was resumed. Delayed leaf senescence was expressed by genotype 8517 under both water regimes. Drought and monocarpic senescence concurrently reduced leaf area in both genotypes, but 8517 maintained greater leaf area and plant viabilitY under either water regime than did the senescent cultivar, CB5. Both genotYPes showed similar C-13 discrimination under complete irrigation but 8517 discriminated significantly less against C-13 than did CB5 under drought stress. Based on discrimination data, transpirational water-use efficiency of droughted 8517 was estimated to be 10 to 14% higher than that of CB5. Plant survival was not significantly affected by the drought treatment, but a higher percent. age of 8517 than CB5 plants survived the first podset and subsequently produced a large second podset. Total seed yields did not differ significantly between genotypes, but seasonal yield distribution differed markedly. Total reproductive duration of drought-stressed 8517 was 50% greater than that of CB5, and 60% of its pod production escaped the drought. The delayed leaf senescence traits may enhance adaptation of cowpea to semiarid zones by improving the capacity to survive and recover from midseason drought.