Determinants of stomatal sluggishness in ozone-exposed deciduous tree species

Our knowledge of ozone effects on dynamic stomatal response is still limited, especially in Asian tree species. We thus examined ozone effects on steady-state leaf gas exchange and stomata] dynamics in three common tree species of China (Ailanthus altissima, Fraxinus chinensis and Platanus orientalis). Seedlings were grown and were exposed to three levels of ozone in open-top chambers (42, 69, 100 nmol mol(-1) daylight average, from 09:00 to 18:00). At steady-state, ozone exposure induced an uncoupling of photosynthesis and stomatal conductance, as the former decreased while the latter did not. Dynamic stomatal response was investigated by cutting the leaf petiole after a steady-state stomatal conductance was reached. Ozone exposure increased stomata! sluggishness, i.e., slowed stomatal response after leaf cutting, in the following order of sensitivity, F. chinensis > A. altissima > P. orientalis. A restriction of stomatal ozone flux reduced the ozone-induced sluggishness in P. orientalis. The ozone-induced impairment of stomatal control was better explained by stomatal ozone flux per net photosynthesis rather than by stomatal ozone flux only. This suggests that ozone injury to stomatal control depends both on the amount of ozone entering a leaf and on the capacity for biochemical detoxification or repair. Leaf mass per area and the density of stomata did not affect stomata! sluggishness.