title: 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. C(4) grasses play an important role in the global carbon cycle, but their contribution to the isotopic composition of atmospheric CO(2) is not well understood. Instantaneous measurements of (13)CO(2) (Delta(13)C) and C(18)OO (Delta(18)O) isotope exchange in five NAD-ME and seven NADP-ME C(4) grasses have been conducted to investigate the difference in photosynthetic CO(2) isotopic fractionation in these subgroups. As previously reported, the isotope composition of the leaf material (delta(13)C) was depleted in (13)C in the NAD-ME compared with the NADP-ME grasses. However, Delta(13)C was not different between subtypes at high light, and, although Delta(13)C increased at low light, it did so similarly in both subtypes. This suggests that differences in leaf delta(13)C between the C(4) subtypes are not caused by photosynthetic isotope fractionation and leaf delta(13)C is not a good indicator of bundle sheath leakiness. Additionally, low carbonic anhydrase (CA) in C(4) grasses may influences Delta(13)C and should be considered when estimating the contribution of C(4) grasses to the global isotopic signature of atmospheric CO(2). It was found that measured Delta(18)O values were lower than those predicted from leaf CA activities and Delta(18)O was similar in all species measured. The Delta(18)O in these C(4) grasses is similar to low Delta(18)O previously measured in C(4) dicots which contain 2.5 times the leaf CA activity, suggesting that leaf CA activity is not a predictor of Delta(18)O in C(4) plants.
authors: Molecular Plant Physiology Group, Research School of Biological Sciences, Australian National University, Canberra, Australian Capital Territory, 2601 Australia. a.cousins@wsu.edu
authors: Asaph B
authors: AB
authors: Cousins
authors: Murray R
authors: MR
authors: Badger
authors: Susanne
authors: S
authors: von Caemmerer
mesh_terms: D002245:Carbon Dioxide / Q000378:metabolism*; D002247:Carbon Isotopes; D002256:Carbonic Anhydrases / Q000378:metabolism; D008291:Malate Dehydrogenase / Q000378:metabolism*; D010788:Photosynthesis / Q000502:physiology*; D018515:Plant Leaves / Q000201:enzymology; D006109:Poaceae / Q000378:metabolism*
chemical_list: D002247:Carbon Isotopes; D002245:Carbon Dioxide; D008291:Malate Dehydrogenase; C046446:malate dehydrogenase-(oxaloacetate-decarboxylating) (NAD+); C031917:malate dehydrogenase (oxaloacetate-decarboxylating) (NADP+); D002256:Carbonic Anhydrases