Determination of leaf carbon isotope discrimination in C4 plants under variable N and water supply

Understanding the mechanisms underlying variations in carbon isotope discrimination (Delta) in C4 plants is critical for predicting the C3/C4 ratio in C3/C4 mixed grassland. The value of Delta is determined by bundle sheath leakiness (Phi) and the ratio of intercellular to ambient CO2 concentration (Ci/Ca). Leaf nitrogen concentration (N-leaf) is considered a driver of Delta in C4 plants. However, little is known about how N-leaf affects Phi and Ci/Ca, and subsequently.Delta Here leaf carbon isotope composition, N-leaf,Phi, and leaf gas exchange were measured in Cleistogenes squarrosa, a dominant C4 species in the Inner Mongolia grassland.Delta remained relatively stable under variable N and water supply. Higher N supply and lower water supply increased N-leaf, stimulated photosynthesis and further decreased Ci/Ca. High N supply increased Phi, which responded weakly to water supply. N-leaf exerted similar effects on Ci/Ca and on Phi in the field and pot experiments. Pooling all the data, N-leaf explained 73% of the variation in Ci/Ca. Overall, both Phi and Ci/Ca determined Delta; however, the contribution of Phi was stronger. N-leaf influenced Delta primarily though Ci/Ca, rather than Phi.Phi should be considered in estimating Delta of C4 endmember.