INHERITANCE OF CARBON-ISOTOPE DISCRIMINATION AND WATER-USE EFFICIENCY IN 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 WUE in C3 plants. Previous studies indicated variation in WUE and DELTA among genotypes of cowpea [Vigna unguiculata (L.) Walp.] and due to drought. Moreover, a highly significant negative correlation between WUE and DELTA was observed for both genotypic and drought effects, as expected based on theory. Present studies were conducted to investigate whether the inheritance of WUE and DELTA is nuclear or maternal, and whether any dominance is present. Contrasting cowpea accessions and hybrids were grown over 2 yr in two outdoor pot experiments, subjected to wet or dry treatments, and under full irrigation in natural soil conditions in 1 yr. Highly significant differences in WUE were observed among cowpea parents and hybrids, and due to drought, which were strongly and negatively correlated with DELTA, as expected based on theory. Data from reciprocal crosses indicated that both WUE and DELTA are controlled by nuclear genes. High WUE and low DELTA exhibited partial dominance under pot conditions. In contrast, high DELTA was partially dominant for plants grown under natural soil conditions but in a similar aerial environment as in the pot studies. We speculate that differences in rooting conditions were responsible for the differences in extent of dominance for DELTA of plants growing under pot conditions compared with natural soil conditions in a similar field aerial environment.