HERITABILITY OF CARBON-ISOTOPE DISCRIMINATION AND CORRELATIONS WITH HARVEST INDEX IN COWPEA

Breeding for drought adaptation by selecting only for grain yield is s low, Carbon isotope discrimination (Delta) may be a useful selection c riterion because of its correlation with transpiration efficiency. How ever, effectiveness of indirect selection will depend on the realized heritability of Delta and genetic correlations with other traits contr ibuting to grain yield, Heritabilities and genetic correlations betwee n Delta and harvest index (III), grain yield, and biomass were estimat ed in two cowpea [Vigna unguiculata (L.) Walp.] crosses, Broad-sense h eritability 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 per formed for high and Low Delta, and high and low HI and selection respo nses were evaluated in the F-3 and F-4 generations. Realized heritabil ities were low for Delta (0.06 and 0.19) and low to intermediate for H I (0.14 and 0.42) and were higher for upward than downward selection f or both traits. Heritabilities estimated from parent-offspring regress ion 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 regr ession suggesting that nonadditive effects were present in the express ion of that trait. Low to moderate positive correlations existed betwe en Delta and HI, indicating possible genetic association, No consisten t correlations were found between grain yield and Delta, but a positiv e genetic correlation was present between HI and grain yield. Selectio n would be more efficient for low Delta in families in advanced genera tions rather than with single F-2 plants and could result in some indi rect selection for low HI.