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

Understanding the mechanisms underlying variations in carbon isotope discrimination (Δ) in C4 plants is critical for predicting the C3/C4 ratio in C3/C4 mixed grassland. The value of Δ is determined by bundle sheath leakiness (Ф) and the ratio of intercellular to ambient CO(2) concentration (C (i)/C (a)). Leaf nitrogen concentration (N (leaf)) is considered a driver of Δ in C4 plants. However, little is known about how N (leaf) affects Ф and C (i)/C (a), and subsequently Δ. Here leaf carbon isotope composition, N (leaf), Ф, and leaf gas exchange were measured in Cleistogenes squarrosa, a dominant C4 species in the Inner Mongolia grassland. Δ remained relatively stable under variable N and water supply. Higher N supply and lower water supply increased N (leaf), stimulated photosynthesis and further decreased C (i)/C (a). High N supply increased Ф, which responded weakly to water supply. N (leaf) exerted similar effects on C (i)/C (a) and on Ф in the field and pot experiments. Pooling all the data, N (leaf) explained 73% of the variation in C (i)/C (a). Overall, both Ф and C (i)/C (a) determined Δ; however, the contribution of Ф was stronger. N (leaf) influenced Δ primarily though C (i)/C (a), rather than Ф. Ф should be considered in estimating Δ of C4 endmember.