Similar-aged leaflets of kidney beán plants grown either in soii or sand culture but otherwise under identical laboratory conditions, were exposed to acute O3 concentrations of 0.5 to 0.7 pmol mol"l for 1-1.5 h with a 15 to 20 min respite interval in the middle of the ozonation period. The CO2 exchange versus leaf intemal CO2 concentration relationships were determined just before ozonation, during respite period and immediately after the treatment. Under comparable absorbed O3 fluxes the calculated mesophyll conductance (gn,) and measured C02-saturated photosynthesis (Pj) in leaflets of sand-grown plants declined more rapidly than in leaflets of soil-grown plants. A rise in the CO2 compensation concentration in the sand-grown plants was also more pronounced. About 20 % greater thickness of leaves of the sand-grown plants was probably not the cause for their higher susceptíbility to absorbed O3. From the proportíonal decline in ^nd Pg, it is concluded that possible differences in patchiness of stomatal closure under O3 were also not the cause. A weaker O3 defence systém in the mesophyll cells of the sand- grown plants may be the cause of their greater sensitivity to absorbed O3.
Using an air-conditioned single-plant chamber, shoots of two dwarf beán cultivars were exposed for 5 oř 6 h to O3 concentrations, ranging from 0.3 to 0.8 pmol mol**. Diuing exposure water vapour, CO2, and O3 exchange rates of shoots were monitored with the aim to quantify possible differences between cultivars in sensitivity of stomata and mesophyll to O3 uptake. Aíter exposure changes in the water holding capacity of the treated leaves were also estůnated, combining pressure bomb, 3-gauge, and leaf chamber techniques. Rates of change of stomatal (Vg) and mesophyll (Vg,) conductances increased linearily with increase in the absorbed O3 flux via the stomata (Qg), the direshold for stomatal response being lower than that for mesophyll response. Above the threshold values of Qg, sensitivity of V„ to Qg was lower than ďiat of Vg. The water holding capacity of leaves decreased with increase in Qg, although no distinct threshold Qg was found. Cultivar differences in O3 sensitivities of Vg were statístically distinguishable on the stomatal segment of CO2 uptake route ordy.