EFFECT OF TEMPERATURE ON PARTITIONING OF PHOTOSYNTHETIC ELECTRON FLOW BETWEEN CO2 ASSIMILATION AND O-2 REDUCTION AND ON THE CO2/O-2 SPECIFICITY OF RUBISCO

Photosynthetic electron transport rate and partitioning of electrons between CO2 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 degrees C), leaf net CO2 assimilation, A, was stimulated by normal O-2 compared with air containing 2% Or, 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 CO2 concentration stimulated A but did not change electron transport rates, indicating that only the partitioning of electrons between CO2 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 CO2/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 degrees C). However, in E. hirsutum S was considerably higher at low temperatures (below 18 degrees C). Overall, our results suggest that in both species, at temperatures above 18 degrees C, carboxylation and photorespiration are the main processes consuming photosynthetic electrons, the processes of O-2 reduction other than photorespiration being negligible.