Relationship of leaf oxygen and carbon isotopic composition with transpiration efficiency in the C(4) grasses Setaria viridis and Setaria italica

Leaf carbon and oxygen isotope ratios can potentially provide a time-integrated proxy for stomatal conductance (g(s)) and transpiration rate (E), and can be used to estimate transpiration efficiency (TE). In this study, we found significant relationships of bulk leaf carbon isotopic signature (δ(13)C(BL)) and bulk leaf oxygen enrichment above source water (Δ(18)O(BL)) with gas exchange and TE in the model C(4) grasses Setaria viridis and S. italica. Leaf δ(13)C had strong relationships with E, g(s), water use, biomass, and TE. Additionally, the consistent difference in δ(13)C(BL) between well-watered and water-limited plants suggests that δ(13)C(BL) is effective in separating C(4) plants with different availability of water. Alternatively, the use of Δ(18)O(BL) as a proxy for E and TE in S. viridis and S. italica was problematic. First, the oxygen isotopic composition of source water, used to calculate leaf water enrichment (Δ(18)O(LW)), was variable with time and differed across water treatments. Second, water limitations changed leaf size and masked the relationship of Δ(18)O(LW) and Δ(18)O(BL) with E. Therefore, the data collected here suggest that δ(13)C(BL) but not Δ(18)O(BL) may be an effective proxy for TE in C(4) grasses.