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

Leaf intrinsic water-use efficiency (WUE), the ratio of photosynthetic rate to stomatal conductance (A/g_s), is a key plant trait linking terrestrial carbon and water cycles. A rapid, integrative proxy for A/g_s 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 (δ¹³C_Rl), two hours after leaves are transferred to the dark, records photosynthetic carbon isotope discrimination and so provides a proxy for A/g_s. To test this hypothesis, δ¹³C_Rl 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, C_i/C_a, and A/g_s). 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 δ¹³C_Rl and δ¹³C of sucrose was observed. δ¹³C_Rl was converted into apparent photosynthetic discrimination (Δ¹³C_Rl) revealing strong relationships between Δ¹³C_Rl and C_i/C_a and A/g_s during the vegetative stage of growth. We therefore conclude that δ¹³C_Rl may provide a rapid, integrative proxy for A/g_s in barley.