Hybrid poplars grown from cuttings were exposed to 80% and 30% relative humidity respectivly in controlled
environment chambers. Half of the trees were fumigated with 150 ug/m3 ozone during 18 hours daily. The other half was kept in filtered air. Ozone caused leaf chlorosis, yellowing and abscission. These symptoms occurred much earlier and were more pronounced in the trees kept in humid air than in those kept in dry air. Ozone caused a substantial increase of stomata! conductance in the trees in humid air but not in dry air. Therefore, it is very likely that ozone in flux into the leaves was higher in humid air, which could explain the
higher ozone sensitivity of the plants in the humid environment. Ozone sensitivity thus seems to be strongly
dependent on air humidity, which should be taken into account in field and laboratory studies of ozone effects on trees.
Ozone had no influence on chlorophyll fluorescence as long as the leaves remained green. It seems therefore unlikely that ozone acts directly on the chloroplast and thus on photosynthesis itself.