Journal Article

Relationships Between Quantum Yield for CO<sub>2</sub> Assimilation, Activity of Key Enzymes and CO<sub>2</sub> Leakiness in <i>Amaranthus cruentus</i>, a C<sub>4</sub> Dicot, Grown in High or Low Light

Youshi Tazoe, Yuko T. Hanba, Tsuyoshi Furumoto, Ko Noguchi and Ichiro Terashima

in Plant and Cell Physiology

Published on behalf of Japanese Society of Plant Physiologists

Volume 49, issue 1, pages 19-29
Published in print January 2008 | ISSN: 0032-0781
Published online January 2008 | e-ISSN: 1471-9053 | DOI:
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

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  • Biochemistry
  • Molecular and Cell Biology
  • Plant Sciences and Forestry


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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 (φ), 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). φ was constant at photon flux densities (PFDs) >200 μmol m−2 s−1 and was around 0.3. At PFDs <150 μmol m−2 s−1, φ increased markedly as PFD decreased. At 40 μmol m−2 s−1, φ 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 φ 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 φ in LL are discussed.

Keywords: Carbon isotope discrimination; Phosphoenolpyruvate carboxylase (PEPC); Pyruvate, orthophosphate dikinase (PPDK); Ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco)

Journal Article.  7920 words.  Illustrated.

Subjects: Biochemistry ; Molecular and Cell Biology ; Plant Sciences and Forestry

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