INHERITANCE OF CARBON-ISOTOPE DISCRIMINATION AND WATER-USE EFFICIENCYIN COWPEA

Theory has been developed predicting an association between water-use efficiency (WUE = total biomass/transpiration) and leaf discrimination against C-13 (DELTA), which could be used to indirectly select for WU E in C3 plants. Previous studies indicated variation in WUE and DELTA among genotypes of cowpea [Vigna unguiculata (L.) Walp.] and due to dr ought. Moreover, a highly significant negative correlation between WUE and DELTA was observed for both genotypic and drought effects, as exp ected based on theory. Present studies were conducted to investigate w hether the inheritance of WUE and DELTA is nuclear or maternal, and wh ether any dominance is present. Contrasting cowpea accessions and hybr ids were grown over 2 yr in two outdoor pot experiments, subjected to wet or dry treatments, and under full irrigation in natural soil condi tions in 1 yr. Highly significant differences in WUE were observed amo ng cowpea parents and hybrids, and due to drought, which were strongly and negatively correlated with DELTA, as expected based on theory. Da ta from reciprocal crosses indicated that both WUE and DELTA are contr olled by nuclear genes. High WUE and low DELTA exhibited partial domin ance under pot conditions. In contrast, high DELTA was partially domin ant for plants grown under natural soil conditions but in a similar ae rial environment as in the pot studies. We speculate that differences in rooting conditions were responsible for the differences in extent o f dominance for DELTA of plants growing under pot conditions compared with natural soil conditions in a similar field aerial environment.