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

Drought is a major constraint limiting rainfed crop production in semi arid regions. Selection criteria for increasing transpiration efficien cy (TE) may be useful for enhancing drought adaptation. Carbon isotope discrimination (Delta) has been negatively correlated with TE in cowp ea [Vigna unguiculata (L.) Walp.] and several other C-3 species. Howev er, the realized heritability of Delta is not known and Delta has show n 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, e stimate the genetic correlation between Delta and earliness, and deter mine the response to selection for high and low Delta and for earlines s and lateness in two cowpea populations. Broad-sense heritabilities w ere 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 indivi dual 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 s ignificant and high for DFF (0.89-0.96). Selecting individual F-2 plan ts for high and low Delta was effective in identifying F-3 and F-4 fam ilies with high and low Delta only in one of the crosses. Genetic corr elations between Delta and earliness were negative and low to intermed iate in magnitude (-0.14 to -0.66). Selection for earliness can be eff ective in the F-2 generation, but it may result in some indirect selec tion for high Delta and, therefore, low TE. Selection for low Delta ma y be most efficient with advanced families.