Correlation between Water‐Use Efficiency and Carbon Isotope Discrimination in Diverse Cowpea Genotypes and Isogenic Lines

Breeding cultivars with improved water use efficiency (WUE = total biomass production/water use) has been limited by the lack of suitable screening criteria. Theory has predicted an association between WUE and Δ (leaf discrimination against 13C) that could be used in indirect selection for WUE in C3 plants. Several studies have provided empirical evidence supporting this theory but with some exceptions. Earlier studies showed genotypic and drought‐induced variation in Δ in cowpea (Vigna unguiculata (L.) Walp.) and expected associations with leaf gas exchange for drought‐induced effects but not for genotypic effects. Studies were conducted to evaluate directly the association between seasonal WUE and Δ in cowpea. Five contrasting genotypes and two isogenic lines were grown in the field either in pots or under natural soil conditions and subjected to well‐watered or drought treatments. Genotypic rankings for Δ in leaves were consistent in the wet and dry pot and natural soil conditions. Seasonal WUE was measured in the pot‐experiments. Significant genotypic and drought‐induced effects on WUE were found, and drought increased WUE by 29% due to biomass being reduced less than water use. Total genotypic variation in WUE was 19 and 23% under wet and dry conditions, respectively, and was not associated with differences in plant vigor. Significant genotypic and drought‐induced effects on Δ were present which were strongly correlated with WUE in a manner expected based on theory; Δ should therefore be useful for indirectly selecting genotypes with greater WUE.