Reduced ascorbate pool and its maintenance are important determinants of O damage to net photosynthetic rate in Fagus crenata under elevated CO and soil N supply.

Many studies have reported modification in the degree of O damage to photosynthesis by elevated CO and soil N supply. However, the mechanism underlying the modification is unclear. To clarify the important determinants in the degree of O damage to net photosynthetic rate (A) in the leaves of Fagus crenata (Siebold's beech) under elevated CO and with different soil N supply, F. crenata seedlings were grown for two growing seasons under combinations of two O levels (low concentration at approximately 4 nmol mol and two times the ambient concentration), two CO levels (ambient and 700 μmol mol), and three levels of soil N supply (0, 50 and 100 kg N ha year). During the second growing season, we determined A, stomatal conductance for calculating phytotoxic O dose (POD), antioxidant concentrations, and antioxidative enzyme activities in the leaves for evaluating O detoxification capacity. We calculated the O-induced reduction in mean A (ΔA) during the second growing season using the data reported in our previous study and plotted it against mean daily POD without flux threshold (POD). There was no significant linear nor non-linear relationship, suggesting that not only POD but also O detoxification capacity are important determinants of ΔA under elevated CO and N supply. We found significant negative linear relationships of ΔA per unit POD (ΔA/POD) with reduced ascorbate concentration in the low O treatment, and with percentage of O-induced change in activity of monodehydroascorbate reductase (MDAR). In addition, the ΔA/POD was positively and significantly correlated with the activity ratio of ascorbate peroxidase to MDAR. These results suggest that reduced ascorbate pool and its maintenance through the action of MDAR could be important determinants in the degree of O damage to net photosynthesis under elevated CO and soil N supply.