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 (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 G_m 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 G_m was associated with a reduced photosynthesis in O₃ -fumigated Siebold's beech in August. The O₃ -induced reduction of G_m was negatively correlated with leaf density, which was increased by elevated O₃ , 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 O₃ -induced decrease of G_m was diminished because G_m 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 (V_cmax ) and/or maximum electron transport rate (J_max ) rather than diffusive limitations to CO₂ transport such as G_m . A leaf age×O₃ interaction of photosynthetic response will be a key for modelling photosynthesis in O₃ -polluted environments.