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 (Δ) 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 Δ and genetic correlations with other traits contributing to grain yield. Heritabilities and genetic correlations between Δ and harvest index (HI), grain yield, and biomass were estimated in two cowpea [Vigna unguiculata (L.) Walp.] crosses. Broad‐sense heritability estimates were intermediate for Δ (0.33 and 0.47) and HI (0.58 and 0.30). Selection based on F2> single‐plant values was performed for high and low Δ, and high and low HI and selection responses were evaluated in the F3 and F4 generations. Realized heritabilities were low for Δ (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 Δ only in one of the crosses. Heritabilities of HI were significant only from F2:F3 regression suggesting that nonadditive effects were present in the expression of that trait. Low to moderate positive correlations existed between Δ and HI, indicating possible genetic association. No consistent correlations were found between grain yield and Δ, but a positive genetic correlation was present between HI and grain yield. Selection would be more efficient for low Δ in families in advanced generations rather than with single F2 plants and could result in some indirect selection for low HI.