PHOTOSYNTHESIS OF 2 POPLAR CLONES CONTRASTING IN O-3 SENSITIVITY

Two clones of poplar known for their phenomenological difference in re sponse to ozone were fumigated with 150 nl l(-1) of ozone for 5 h. In both clones the treatment significantly reduced the light-saturated ra te of CO2 uptake of recently mature leaves and this was accompanied by a decrease in stomatal conductance. Intercellular CO2 concentration o f the resistant clone increased following the fumigation. After 20 h o f recovery, photosynthesis recovered completely only in the resistant clone. Electrolyte leakage of leaf disks increased in both clones to i ndicate damage to membranes; after the recovery time this parameter on ly reached values of the control in the resistant clone. The photochem ical efficiency of PSII slightly decreased in the resistant clone. In the other clone, the treatment caused a decline of all chlorophyll flu orescence parameters and only some of them returned to normal values a fter the recovery time. The physiological response appears to be diffe rent in the two clones. In the resistant one, the most probable mechan ism involved in the photosynthetic reduction was a regulatory reductio n in CO2 fixation. Also data obtained by the solute leakage indicate t hat in the resistant clone repair mechanisms play a role. The reductio n of photosynthesis observed in the sensitive clone is related both to strong stomatal closure and to an impairment in fluorescence paramete rs. These alterations can indicate a general disruption at the membran e level as confirmed by the solute leakage data.