Photosynthetic response of early and late leaves of white birch (Betula platyphylla var. japonica) grown under free-air ozone exposure

•We examined the effect of ozone on photosynthetic traits of heterophyllous birch.•Measurements were carried out in a free-air ozone exposure experiment on white birch.•Ozone induced the decline of photosynthesis not in early leaves but in late leaves.•This was due to lower stomatal conductance resulting in avoidance of ozone stress.•Also this may be due to ozone-related increase in nitrogen content of early leaves.

Betula platyphylla var. japonica (white birch) has heterophyllous leaves (i.e., early and late leaves) and is a typical pioneer tree species in northern Japan. Seedlings of white birch were exposed to ozone during two growing seasons, and measurements were carried out in the second year. Early leaves did not show an ozone-induced reduction in photosynthesis because of lower stomatal conductance resulting in higher avoidance capacity for ozone-induced stress. Also, an ozone-related increase in leaf nitrogen content may partly contribute to maintain the photosynthetic capacity in early leaves under elevated ozone in autumn. On the other hand, late leaves showed an ozone-induced decline of photosynthesis and early defoliation of leaves occurred. Also, smaller leaf size and higher stomatal density in late leaves were observed under elevated ozone. Differences in stress resistance to ozone may be related to differing functional roles of early and late leaves for birch species.