PHOTORESPIRATION AND RELATED ENZYMES IN PEA (PISUM-SATIVUM) GROWN IN HIGH CO2

The adaptation of pea (Pisum sativum L. cv. Douce Provence) to the low photorespiratory conditions imposed by high CO2 was investigated at t he level of enzymes and gas exchange. Seedlings were CO2-enriched (100 0 and 4800 mu L CO(2)L(-1)) during most of the vegetative period, yiel ding <<acclimated leaves>>. Alternatively, young plants were pre-grown in ambient CO2 and then CO2-enriched, yielding <<transferred leaves>> . The level of nutrient supply was high. High CO2 did not significantl y alter the specific activities of the photorespiratory enzymes glycol ate oxidase, NADH- and NADPH-hydroxypyruvate reductase and glutamine s ynthetase in either of the experiments. Moreover, no significant effec t of high CO2 on specific carboxylase activity and relative abundance of ribulose bisphosphate carboxylase-oxygenase (Rubisco) was observed. In contrast, high CO2 markedly affected the photorespiratory enzymes catalase (Cat) and phosphoglycolate phosphatase, the activity of the l atter being increased. Decline of Cat activity was detected 1 day afte r transfer to high CO2 and in the course of 7 days, the inhibition rea ched values of 33% (1000 mu L CO(2)L(-1)) and 50% (4800 mu L CO(2)L(-1 )). The relative abundance of Cat protein also declined, but no change in the isoform pattern was observed. Photorespiratory O-2 uptake, det ermined with O-18(2), decreased by 54% in an atmosphere containing 100 0 mu L CO(2)L(-1). This suggests that Rubisco-oxygenase activity occur red at a substantial rate at threefold that of the current atmospheric CO2 concentration. CO2 enrichment to 4000 mu L CO(2)L(-1) further inh ibited photorespiratory O-2, uptake. The decline of Cat was thus posit ively correlated with the inhibition of light O-2 uptake. In <<acclima ted leaves>>, Cat inhibition was slight or absent, depending on the le vel of CO2 enrichment. This suggests that Cat inhibition in <<transfer red leaves>> is a transient response that can be overcome by yet unide ntified adaptative mechanisms.