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 CO 2 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 CO 2 transport such as G m . A leaf age×O 3 interaction of photosynthetic response will be a key for modelling photosynthesis in O 3 ‐polluted environments.