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

Abstract

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. Show more