Carbon fixation in Pinus halepensis submitted to ozone. Opposite response of ribulose‐1,5‐bisphosphate carboxylase/oxygenase and phosphoenolpyruvate carboxylase

The effects of ozone exposure on carbon‐fixation‐related processes in Pinus halepensis Mill. needles were assessed over 3 months under controlled conditions. Ozone fumigation (200 ppb) did not induce a modification of either net CO 2 assimilation or stomatal conductance in 1‐year‐old needles, whereas ribulose‐1,5‐bisphosphate carboxylase/oxygenase (Rubisco, EC 4.1.1.39) activity was shown to be reduced by a half. Moreover, this ozone‐induced reduction in Rubisco activity was associated with a decrease in the quantity of Rubisco, as determined by the decrease in the large subunit (LSU). On the other hand, 200‐ppb ozone fumigation induced a strong increase in both activity and quantity of another carboxylating enzyme, phospho enol pyruvate carboxylase (PEPC, EC 4.1.1.31), generally considered in C 3 plants to participate in carbon catabolism processes. Ozone induced a significant decrease in the Rubisco/PEPC activity ratio which promotes the role of PEPC in trees under ozone stress. The role of this carboxylase will be discussed.