## 4 Discussion
Because it has been suggested that the degree of O\({}_{3}\) damage is determined by the balance between stomatal O\({}_{3}\) flux and leaf cellular detoxification [14; 44; 45], the higher POD-based O\({}_{3}\) sensitivity of the net photosynthesis in the leaves of Japanese deciduous broad-leaved tree species could be attributed to the lower detoxification capacity of O\({}_{3}\) in the leaves. Among the four tree species used in the present study, there were obvious differences in the POD-based O\({}_{3}\) sensitivities of cumulative photosynthetic CO\({}_{2}\) uptake (Figure 2e-h). At a lower POD\({}_{0}\) up to around 20-30 mmol O\({}_{3}\) m\({}^{-2}\), \(\Sigma P_{\rm n,est}\) of _F. crenata_, _Q. serrata_ and _Q. mongolica_ var. _crispula_ were less responsive to increasing POD\({}_{0}\) as compared with that of _B. plattyphylla_ var. _japonica_ which showed an almost constant reduction as POD\({}_{0}\) increased (Figure 2e-h). The extent of the pronounced plateau at a lower POD\({}_{0}\) suggests a tolerance to the O\({}_{3}\) taken into the leaves and is represented by the variable \(\lambda\) in the Weibull function [37], which were 3.07, 3.44, 2.80 and 1.78 in _F. crenata_, _Q. serrata_, _Q. mongolica_ var. _crispula_ and _B. plattyphylla_ var. _japonica_, respectively (Figure 2e-h). [PERSON] et al. (2007) [46] supposed that leaf mass per area (LMA) reflects the depth of the biochemical defenses against O\({}_{3}\) taken into the leaves, and thus it could be used as an indicator of the leaf cellular detoxification capacity of O\({}_{3}\). In the present study, we measured LMA at the end of each growing season (data not shown). Although there was considerable variation in the LMA between the years and among the treatments within one species, there was a significant positive correlation between the \(\lambda\) and median LMA of each species (\(R\) = 0.970, \(n\) = 4, \(p\) < 0.05). This result suggests that, among the four tree species, different responses at a lower POD\({}_{0}\) might result from differences in detoxification capacity in the leaves. Among the three tree species showing \(\lambda\) of around 3, the response of relative \(\Sigma P_{\rm n,est}\) to increasing POD\({}_{0}\) of _Q. serrata_ become drastically sensitive above the POD\({}_{0}\) of around 30 mmol O\({}_{3}\) m\({}^{-2}\). Because the activity of antioxidative enzymes and antioxidant concentration increases in response to O\({}_{3}\) exposure [15; 47; 48], the different responses of \(\Sigma P_{\rm n,est}\) to increasing POD\({}_{0}\) above the POD\({}_{0}\) of around 30 mmol O\({}_{3}\) m\({}^{-2}\) might be caused by the difference in the inducible antioxidative capacity. On the other hand, differences in the flux threshold of POD\({}_{\rm Y}\) appropriate for the linear regression could also suggest differences in the detoxification capacity in the leaves among the tree species, because the flux threshold could focus the POD to the portion of the O\({}_{3}\) flux that is thought to be not detoxified in the leaves. However, the rank in descending order of the flux threshold was _Q.__mongolica_ var. _crispula_, _F. creata_, _Q. serrata_ and _B. platyphyllla_ var. _japonica_ (Table 4) and did not match with that of the \(\lambda\) (Figure 2). The flux threshold of Y is the cut-off O\({}_{3}\) flux at a constant value throughout the growing season, even though the detoxification capacity in the leaves changes not only owing to O\({}_{3}\) exposure but also owing to leaf aging. Thus, the flux threshold of Y might not be biologically relevant. To make quantification of O\({}_{3}\) damage on photosynthetic CO\({}_{2}\) uptake more mechanistic and biologically relevant, therefore, further study is required to clarifying the mechanisms underlying the different response of \(\Sigma P_{\rm n\_est}\) to POD\({}_{0}\) among the species with emphasis not only on constitutive but also on inducible and phenological changes in antioxidative capacity in the leaves [14; 15; 44]. Although [PERSON] et al.