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.
In C(4) photosynthesis, a part of CO(2) fixed by phosphoenolpyruvate carboxylase (PEPC) leaks from the bundle-sheath cells. Because the CO(2) leak wastes ATP consumed in the C(4) cycle, the leak may decrease the efficiency of CO(2) assimilation. To examine this possibility, we studied the light dependence of CO(2) leakiness (phi), 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 CO(2) assimilation rate using leaves of Amaranthus cruentus (NAD-malic enzyme subtype, dicot) plants grown in high light (HL) and low light (LL). phi was constant at photon flux densities (PFDs) >200 micromol m(-2) s(-1) and was around 0.3. At PFDs <150 micromol m(-2) s(-1), phi increased markedly as PFD decreased. At 40 micromol m(-2) s(-1), phi was 0.76 in HL and 0.55 in LL leaves, indicating that the efficiency of CO(2) 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 phi with decreasing PFD contributed to the decrease in the CO(2) 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 CO(2) assimilation rate at the low PFD region is seriously overestimated. Ecological implications of the increase in phi in LL are discussed.
Tazoe
Youshi
Y
Department of Biological Sciences, Graduate School of Science, The University of Tokyo, Bunkyo-ku, Tokyo, 113-0033 Japan. youshi.tazoe@anu.edu.au
Hanba
Yuko T
YT
Furumoto
Tsuyoshi
T
Noguchi
Ko
K
Terashima
Ichiro
I
D027721:Amaranthus / Q000378:metabolism* / Q000528:radiation effects*; D002245:Carbon Dioxide / Q000378:metabolism*; D002734:Chlorophyll / Q000378:metabolism; D008027:Light; D010766:Phosphorylation; D010788:Photosynthesis; D017346:Protein Serine-Threonine Kinases / Q000378:metabolism; D011772:Pyruvate, Orthophosphate Dikinase / Q000378:metabolism; D012273:Ribulose-Bisphosphate Carboxylase / Q000378:metabolism
D002734:Chlorophyll; D002245:Carbon Dioxide; C069767:phosphoenolpyruvate carboxylase kinase; D017346:Protein Serine-Threonine Kinases; D011772:Pyruvate, Orthophosphate Dikinase; D012273:Ribulose-Bisphosphate Carboxylase