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

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