C₄ photosynthetic isotope exchange in NAD-ME- and NADP-ME-type grasses

Monitoring photosynthetic isotope exchange is an important tool for predicting the influence of plant communities on the global carbon cycle in response to climate change. C₄ grasses play an important role in the global carbon cycle, but their contribution to the isotopic composition of atmospheric CO₂ is not well understood. Instantaneous measurements of ¹³CO₂ (Δ¹³C) and C¹⁸OO (Δ¹⁸O) isotope exchange in five NAD-ME and seven NADP-ME C ₄ grasses have been conducted to investigate the difference in photosynthetic CO₂ isotopic fractionation in these subgroups. As previously reported, the isotope composition of the leaf material (δ¹³C) was depleted in ¹³C in the NAD-ME compared with the NADP-ME grasses. However, Δ¹³C was not different between subtypes at high light, and, although Δ¹³C increased at low light, it did so similarly in both subtypes. This suggests that differences in leaf δ¹³C between the C₄ subtypes are not caused by photosynthetic isotope fractionation and leaf δ¹³C is not a good indicator of bundle sheath leakiness. Additionally, low carbonic anhydrase (CA) in C₄ grasses may influences Δ¹³C and should be considered when estimating the contribution of C₄ grasses to the global isotopic signature of atmospheric CO₂. It was found that measured Δ¹⁸O values were lower than those predicted from leaf CA activities and Δ¹⁸O was similar in all species measured. The Δ¹⁸O in these C₄ grasses is similar to low Δ¹⁸O previously measured in C₄ dicots which contain 2.5 times the leaf CA activity, suggesting that leaf CA activity is not a predictor of Δ¹⁸O in C₄ plants.