Plant and Cell Physiology Advance Access published online on November 20, 2007
Plant and Cell Physiology, doi:10.1093/pcp/pcm160
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Relationships between quantum yield for CO2 assimilation, activity of key enzymes and CO2 leakiness in Amaranthus cruentus, a C4 dicot, grown in high or low light
1 Department of Biological Sciences, Graduate School of Science, The University of Tokyo, Bunkyo-ku, Tokyo, 113-0033, Japan
3 Center for Bioresource Field Science, Kyoto Institute of Technology, Ukyo-ku, Kyoto, 616-8354, Japan
4 Department of Biological Science, Graduate School of Science, Hiroshima University, Higashihiroshima, Hiroshima, 739-8526, Japan
Corresponding author: Dr. Youshi Tazoe, GPO box 475, Canberra, ACT, 2601, Australia, Tel: +61 (0)2 6125 8144; Fax: +61 (0)2 6125 4919, e-mail: youshi.tazoe{at}anu.edu.au
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Abstract |
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In C4 photosynthesis, a part of CO2 fixed by phosphoenolpyruvate carboxylase (PEPC) leaks from the bundle-sheath cells. Because the CO2 leak wastes ATP consumed in the C4 cycle, the leak may decrease the efficiency of CO2 assimilation. To examine this possibility, we studied the light dependence of CO2 leakiness (
), estimated by the concurrent measurements of gas exchange and carbon isotope discrimination, initial activities of ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco) and pyruvate, orthophosphate dikinase (PPDK), the phosphorylation state of PEPC and the CO2 assimilation rate using leaves of Amaranthus cruentus (NAD-malic enzyme subtype, dicot) plants grown in high-light (HL) and low-light (LL). was constant at photon flux densities (PFDs) above 200 µmol m-2 s-1 and around 0.3. At PFDs less than 150 µmol m-2 s-1, increased markedly as PFD decreased. At 40 µmol m-2 s-1, was 0.76 in HL and 0.55 in LL leaves indicating that the efficiency of CO2 assimilation at low PFD was greater in LL leaves. The activities of Rubisco and PPDK and the phosphorylated state of PEPC all decreased as PFD decreased. Theoretical calculations with a mathematical model clearly showed that the increase in with decreasing PFD contributed to the decrease in the CO2 assimilation rate. It was also shown that the 'conventional' quantum yield of photosynthesis obtained by fitting the straight line to the light response curve of the CO2 assimilation rate at the low PFD region is seriously overestimated. Ecological implications of the increase in in low light are discussed.
Keywords: Carbon isotope discrimination - Phosphoenolpyruvate carboxylase (PEPC) - Pyruvate, orthophosphate dikinase (PPDK) - Ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco)
2 Present address: Research School of Biological Sciences, Australian National University, Canberra, Australian Capital Territory 2601, Australia
(Received July 17, 2007; Accepted November 15, 2007)
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