HERITABILITY OF CARBON-ISOTOPE DISCRIMINATION AND CORRELATIONS WITH EARLINESS IN COWPEA

Drought is a major constraint limiting rainfed crop production in semiarid regions. Selection criteria for increasing transpiration efficiency (TE) may be useful for enhancing drought adaptation. Carbon isotope discrimination (Delta) has been negatively correlated with TE in cowpea [Vigna unguiculata (L.) Walp.] and several other C-3 species. However, the realized heritability of Delta is not known and Delta has shown negative associations with days to flowering, which could limit its usefulness as a selection criterion for enhancing drought adaptation. Field studies were undertaken to estimate the heritability of Delta, estimate the genetic correlation between Delta and earliness, and determine the response to selection for high and low Delta and for earliness and lateness in two cowpea populations. Broad-sense heritabilities were low to intermediate (0.27 and 0.58) for Delta and high (0.72 and 0.80) for days to first flower (DFF). Selections were made among individual F-2 plants, and selected progenies were evaluated as F-3 and F-4 families. Realized heritabilities were low for Delta (0.25-0.31) but significant and high for DFF (0.89-0.96). Selecting individual F-2 plants for high and low Delta was effective in identifying F-3 and F-4 families with high and low Delta only in one of the crosses. Genetic correlations between Delta and earliness were negative and low to intermediate in magnitude (-0.14 to -0.66). Selection for earliness can be effective in the F-2 generation, but it may result in some indirect selection for high Delta and, therefore, low TE. Selection for low Delta may be most efficient with advanced families.