Interactive effect of leaf age and ozone on mesophyll conductance in Siebold's beech

Mesophyll conductance (G(m)) is one of the most important factors determining photosynthesis. Tropospheric ozone (O-3) is known to accelerate leaf senescence and causes a decline of photosynthetic activity in leaves. However, the effects of age-related variation of O-3 on G(m) have not been well investigated, and we, therefore, analysed leaf gas exchange data in a free-air O-3 exposure experiment on Siebold's beech with two levels (ambient and elevated O-3: 28 and 62 nmol mol(-1) as daylight average, respectively). In addition, we examined whether O-3-induced changes on leaf morphology (leaf mass per area, leaf density and leaf thickness) may affect CO2 diffusion inside leaves. We found that O-3 damaged the photosynthetic biochemistry progressively during the growing season. The G(m) was associated with a reduced photosynthesis in O-3-fumigated Siebold's beech in August. The O-3-induced reduction of G(m) was negatively correlated with leaf density, which was increased by elevated O-3, suggesting that the reduction of G(m) was accompanied by changes in the physical structure of mesophyll cells. On the other hand, in October, the O-3-induced decrease of G(m) was diminished because G(m) decreased due to leaf senescence regardless of O-3 treatment. The reduction of photosynthesis in senescent leaves after O-3 exposure was mainly due to a decrease of maximum carboxylation rate (V-cmax) and/or maximum electron transport rate (J(max)) rather than diffusive limitations to CO2 transport such as G(m). A leaf agexO(3) interaction of photosynthetic response will be a key for modelling photosynthesis in O-3-polluted environments.