Yield stability in cowpea as influenced by five drought resistance mechanisms

Abstract

Cowpeas, Vigna unguiculata (L.) Walp., are considered relatively drought resistant as compared to other grain crops, yet substantial seed yield reductions in response to drought stress, especially during reproductive development, are commonly observed. Among the many mechanisms studied in grain crops, 5 drought adaptive or resistance mechanisms have been hypothesized to play key roles in maintaining seed yield in grain legumes under drought including, earliness of reproductive development, greater partitioning of dry matter to developing seeds, longer pod-filling period, longer duration of viable (green) canopy during pod-filling, and maintenance of moderate to high levels of N₂ fixation during late pod-filling. The purpose of this investigation was to identify high-yielding cowpea genotypes with superior seed yield stability under both dryland and irrigated conditions, and to determine whether genotypes which expressed superior seed yield stability utilized one or more of the 5 drought-adaptive mechanisms associated with leguminous species. Field experiments were conducted using either 20 or 14 genetically diverse, indeterminate, cowpea genotypes under both dryland and irrigated conditions. Intraspecific variability for maintenance of seed yield potential during drought was observed. Genotypes identified as drought resistant initiated reproductive growth earlier, and partitioned larger quantities of dry matter earlier and more rapidly than drought-sensitive genotypes, but the duration of the pod-filling period was similar in all genotypes. Furthermore, resistant genotypes produced smaller green and total canopy areas as compared to susceptible genotypes, but maintained a higher percentage of green leaf area late in the pod-filling period. Moderate drought stress was sufficient to inhibit N₂ fixation activity in all genotypes. Separate greenhouse experiments were conducted in two successive years to determine whether 2 F₇ lines, produced by repeatedly selecting for differential maintenance of larger green canopy area, expressed higher pod yield under warm, dry conditions as compared to plants which displayed normal leaf senescence. Nonsignificant differences between selected and normally senescing genotypes suggested that maintenance of green canopy did not improve pod yield response in any treatment combination.