Photosynthesis of two poplar clones contrasting in O3 sensitivity

Two clones of poplar known for their phenomenological difference in response to ozone were fumigated with 150 nl l(-1) of ozone for 5 h. In both clones the treatment significantly reduced the light-saturated rate of CO2 uptake of recently mature leaves and this was accompanied by a decrease in stomatal conductance. Intercellular CO2 concentration of the resistant clone increased following the fumigation. After 20 h of recovery, photosynthesis recovered completely only in the resistant clone. Electrolyte leakage of leaf disks increased in both clones to indicate damage to membranes; after the recovery time this parameter only reached values of the control in the resistant clone. The photochemical efficiency of PSII slightly decreased in the resistant clone. In the other clone, the treatment caused a decline of all chlorophyll fluorescence parameters and only some of them returned to normal values after the recovery time. The physiological response appears to be different in the two clones. In the resistant one, the most probable mechanism involved in the photosynthetic reduction was a regulatory reduction in CO2 fixation. Also data obtained by the solute leakage indicate that in the resistant clone repair mechanisms play a role. The reduction of photosynthesis observed in the sensitive clone is related both to strong stomatal closure and to an impairment in fluorescence parameters. These alterations can indicate a general disruption at the membrane level as confirmed by the solute leakage data.