Effect of Temperature on Partitioning of Photosynthetic Electron Flow Between CO 2 Assimilation and O 2 Reduction and on the CO 2/O 2 Specificity of RubisCo

Abstract

Summary Photosynthetic electron transport rate and partitioning of electrons between CO 2 assimilation and O 2 reduction were estimated in vivo at different temperatures using simultaneous measurements of leaf gas exchange and chlorophyll fluorescence emission on intact leaves of both hairy-willow herb ( Epilobium hirsutum L.) and French bean ( Phaseolus vulgaris L.). In E. hirsutum leaves at low temperatures (below 15 °C), leaf net CO 2 assimilation, A, was stimulated by normal O 2 compared with air containing 2% O 2 , while at high temperatures the inhibition of A by normal O 2 via stimulation of the oxygenase function of RubisCo masked the stimulatory effect of O 2 on A. As a consequence of stimulation of A, the non-cyclic electron flow rate always remained higher in normal air than in air with 2% O 2 . In P. vulgaris , switching to non-photorespiratory conditions by increasing CO 2 concentration stimulated A but did not change electron transport rates, indicating that only the partitioning of electrons between CO 2 and O 2 was changed. Two methods used to estimate the total photosynthetic electron transport rate gave similar results. The validity of the technique used was also tested by estimation of the CO 2 /O 2 specificity factor of RubisCo, S. In both species, the estimated values of S agreed with that of the literature, obtained using spinach enzyme in vitro within a wide range of temperature (18 to 32 °C). However, in E. hirsutum S was considerably higher at low temperatures (below 18 °C). Overall, our results suggest that in both species, at temperatures above 18 °C, carboxylation and photorespiration are the main processes consuming photosynthetic electrons, the processes of O 2 reduction other than photorespiration being negligible.