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

Mesophyll conductance (Gm) is one of the most important factors determining photosynthesis. Tropospheric ozone (O3) is known to accelerate leaf senescence and causes a decline of photosynthetic activity in leaves. However, the effects of age-related variation of O3 on Gm have not been well investigated, and we, therefore, analysed leaf gas exchange data in a free-air O3 exposure experiment on Siebold's beech with two levels (ambient and elevated O3: 28 and 62 nmol mol-1 as daylight average, respectively). In addition, we examined whether O3-induced changes on leaf morphology (leaf mass per area, leaf density and leaf thickness) may affect CO2 diffusion inside leaves. We found that O3 damaged the photosynthetic biochemistry progressively during the growing season. The Gm was associated with a reduced photosynthesis in O3-fumigated Siebold's beech in August. The O3-induced reduction of Gm was negatively correlated with leaf density, which was increased by elevated O3, suggesting that the reduction of Gm was accompanied by changes in the physical structure of mesophyll cells. On the other hand, in October, the O3-induced decrease of Gm was diminished because Gm decreased due to leaf senescence regardless of O3 treatment. The reduction of photosynthesis in senescent leaves after O3 exposure was mainly due to a decrease of maximum carboxylation rate (Vcmax) and/or maximum electron transport rate (Jmax) rather than diffusive limitations to CO2 transport such as Gm. A leaf age×O3 interaction of photosynthetic response will be a key for modelling photosynthesis in O3-polluted environments.