Heritability of carbon isotope discrimination and correlations with harvest index in cowpea

Breeding for drought adaptation by selecting only for grain yield is slow, Carbon isotope discrimination (Delta) may be a useful selection criterion because of its correlation with transpiration efficiency. However, effectiveness of indirect selection will depend on the realized heritability of Delta and genetic correlations with other traits contributing to grain yield, Heritabilities and genetic correlations between Delta and harvest index (III), grain yield, and biomass were estimated in two cowpea [Vigna unguiculata (L.) Walp.] crosses, Broad-sense heritability estimates were intermediate for Delta (0.33 and 0.47) and HI (0.58 and 0.30), Selection based on F-2 single-plant values was performed for high and Low Delta, and high and low HI and selection responses were evaluated in the F-3 and F-4 generations. Realized heritabilities were low for Delta (0.06 and 0.19) and low to intermediate for HI (0.14 and 0.42) and were higher for upward than downward selection for both traits. Heritabilities estimated from parent-offspring regression with 50 random lines were significant for Delta only in one of the crosses, Heritabilities of HI were significant only from F-2:F-3 regression suggesting that nonadditive effects were present in the expression of that trait. Low to moderate positive correlations existed between Delta and HI, indicating possible genetic association, No consistent correlations were found between grain yield and Delta, but a positive genetic correlation was present between HI and grain yield. Selection would be more efficient for low Delta in families in advanced generations rather than with single F-2 plants and could result in some indirect selection for low HI.