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JXB Advance Access originally published online on March 28, 2008
Journal of Experimental Botany 2008 59(7):1695-1703; doi:10.1093/jxb/ern001
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© The Author [2008]. Published by Oxford University Press [on behalf of the Society for Experimental Biology]. All rights reserved. For Permissions, please e-mail: journals.permissions@oxfordjournals.org

RESEARCH PAPER

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

Asaph B. Cousins1,2,*, Murray R. Badger1,2 and Susanne von Caemmerer1

1Molecular Plant Physiology Group, Research School of Biological Sciences, Australian National University, Canberra, Australian Capital Territory, 2601 Australia
2ARC CoE, Plant Energy Biology, Research School of Biological Sciences, Australian National University, Canberra, Australian Capital Territory, 2601 Australia

* Present address and to whom correspondence should be sent: School of Biological Sciences, Washington State University, Pullman, WA 99164-4236, USA. E-mail: a.cousins{at}wsu.edu

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 ({Delta}13C) and C18OO ({Delta}18O) isotope exchange in five NAD-ME and seven NADP-ME C4 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 ({delta}13C) was depleted in 13C in the NAD-ME compared with the NADP-ME grasses. However, {Delta}13C was not different between subtypes at high light, and, although {Delta}13C increased at low light, it did so similarly in both subtypes. This suggests that differences in leaf {delta}13C between the C4 subtypes are not caused by photosynthetic isotope fractionation and leaf {delta}13C is not a good indicator of bundle sheath leakiness. Additionally, low carbonic anhydrase (CA) in C4 grasses may influences {Delta}13C and should be considered when estimating the contribution of C4 grasses to the global isotopic signature of atmospheric CO2. It was found that measured {Delta}18O values were lower than those predicted from leaf CA activities and {Delta}18O was similar in all species measured. The {Delta}18O in these C4 grasses is similar to low {Delta}18O previously measured in C4 dicots which contain 2.5 times the leaf CA activity, suggesting that leaf CA activity is not a predictor of {Delta}18O in C4 plants.

Key words: C4 grasses, carbonic anhydrase, isotope discrimination, leakiness

Received 27 September 2007; Revised 19 December 2007 Accepted 21 December 2007


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