δ13C of leaf-respired CO2 reflects intrinsic water-use efficiency in barley

Abstract

Leaf intrinsic water-use efficiency (WUE), the ratio of photosynthetic rate to stomatal conductance (A/gs), is a key plant trait linking terrestrial carbon and water cycles. A rapid, integrative proxy for A/gs is of benefit to crop breeding programmes aiming to improve WUE, but also for ecologists interested in plant carbon-water balance in natural systems. We hypothesize that the carbon isotope composition of leaf-respired CO2 (δ13CRl), two hours after leaves are transferred to the dark, records photosynthetic carbon isotope discrimination and so provides a proxy for A/gs. To test this hypothesis, δ13CRl was measured in four barley cultivars grown in the field at two levels of water availability and compared to leaf-level gas exchange (the ratio of leaf intercellular to ambient CO2 partial pressure, Ci/Ca, and A/gs). Leaf-respired CO2 was more 13C-depleted in plants grown at higher water availability, varied between days as environmental conditions changed, and was significantly different between cultivars. A strong relationship between δ13CRl and δ13C of sucrose was observed. δ13CRl was converted into apparent photosynthetic discrimination (Δ13CRl) revealing strong relationships between Δ13CRl and Ci/Ca and A/gs during the vegetative stage of growth. We therefore conclude that δ13CRl may provide a rapid, integrative proxy for A/gs in barley. © 2011 Blackwell Publishing Ltd.