Photosynthetic responses of Monarch birch seedlings to differing timings of free air ozone fumigation

To study the effects of different periods of ozone (O-3) fumigation on photosynthesis in leaves of the Monarch birch (Betula maximowicziana), we undertook free air O-3 fumigation to Monarch birch seedlings at a concentration of 60 nmol mol(-1) during daytime. Plants were exposed to O-3 at early, late or both periods in the growing season. The light-saturated net photosynthetic rate (A (sat)) in July and August was reduced by O-3 exposure through a reduction in the maximum rate of carboxylation (V (c,max)). In early September, on the other hand, despite a reduction in V (c,max), A (sat) was not reduced by O-3 due to a counteracting increase in the stomatal conductance. Through the experiment, there was no difference in sensitivity to O-3 between maturing and matured leaves. We analyzed the relationship between A (sat), V (c,max) and accumulated stomatal O-3 flux (AF(st)). Whereas V (c,max) decreased with increasing AF(st), the correlation between A (sat) and AF(st) was weak because the response of stomatal conductance to O-3 was affected by season. We conclude photosynthetic response of Monarch birch to O-3 exposure changes with season. This is due to the inconstant stomatal response to O-3 but not due to the respose of biochemical assimilation capacity in chloroplasts.