Reliable interpretation of water-use efficiency in chickpea from d13C of leaf tissue

Barbour MM and Lockhart E

The University of Sydney, 380 Werombi Road, Brownlow Hill.

The carbon isotope composition of leaf tissue (δ13C) has been widely used for many years as a proxy for both whole plant water-use efficiency (WUE; biomass produced per unit water transpired) and leaf-intrinsic water-use efficiency (WUEi; photosynthetic rate divided by stomatal conductance), and has been successful in many crop species, notably wheat. However, there is conflicting evidence for the utility of δ13C in chickpea as a WUE proxy. Chickpea are somewhat unusual in having high levels of organic acids excreted by leaf hairs, so we tested the hypothesis that surface organic acids are responsible for the inconsistency in δ13C-WUE relationships either directly or indirectly. Across 20 chickpea genotypes, removal of surface acids altered whole leaf tissue δ13C in 3 genotypes, suggesting that acids may contribute to the lack of consistency in δ13C-WUE relationships between different genotypes. Water-soluble CHO samples were 2‰ more enriched, on average, than whole leaf tissue samples. For 2 chickpea genotypes grown in well-watered and droughted conditions, the strongest correlations were found between δ13C of CHO and WUE. Weaker correlations were found for δ13C of CHO and WUEi, and whole leaf δ13C with either WUE or WUEi. Removal of surface acids and extraction of water-soluble CHO is recommended when using leaf δ13C as a proxy for WUE and WUEi in chickpea. Previous studies describing genotypic variability and heritability of δ13C in chickpea may have identified variation in leaf acid presence, or differences in leaf chemical composition, rather than WUE.