Parameters of the Reductive Pentose Phosphate Cycle and of the Glycolate Pathway under Different Concentrations of Oxygen

Abstract

Summary Various CO 2 fluxes were measured in Phaseolus vulgaris and Nicotiana tabacum leaves under 0.03 % CO 2 , 1.5 or 21 % O 2 , and limiting or saturating light(5 or 35 mW cm -2 , respectively). A combined radio- and gasometric method was applied. From the components of the CO 2 exchange, the rate of the turnover of ribulose-1,5-diphosphate(RuDP) and of 3-phosphoglycerate(PGA) was calculated. In the saturating light, the consumption of RuDP in oxygenation was compensated by its enhanced synthesis in 21 % O 2 . Therefore the inhibition of carboxylation accounted for the Warburg effect only in the limiting light while in the saturating light the effect was due only to photorespiration and the intrafoliar re-assimilation. At the light saturation, the PGA turnover proceeded also faster in 21 % O 2 . A quantitative model of the reductive pentose phosphate(RPP) cycle and the associated glycolate pathway has been proposed for the two oxygen concentrations. From the model it follows that in the saturating light the RPP cycle works in 21 % O 2 twice as fast as in 1.5 % O 2 . Energetic requirements per mole CO 2 bound at carboxylation and per ng atom carbon incorporated into end products are in 21 % O 2 nearly two fold higher than in 1.5 % O 2 . In the air, the re-entry of carbon from the glycolate pathway into the RPP cycle is obligatory for the supporting of the steady state photosynthesis.