RESPONSE OF LEAF DIFFUSIVE RESISTANCE OF RADISH, CUCUMBER AND SOYBEAN TO O3 AND SO2 SINGLY OR IN COMBINATION

The effect of O3 and SO2 singly or in combination on leaf diffusive resistance was determined for two plant species (radish and cucumber) exhibiting visible foliar injury consisting of a greater-than-additive (synergistic) response to a SO2 O3 mixture and for soybean which exhibits visible injury consisting of a lessthan-additive (antagonistic) response to the pollutant mixture. Leaf diffusive resistances were measured on the upper and lower surfaces of the following leaves: the two primary leaves of soybean, the first leaf in cucumber, and the first two leaves of radish. Changes in leaf diffusive resistance of the three species were similar for each pollutant singly or for the combination. Sulphur dioxide (0.15 ppm) tended to stimulate stomatal opening while O3 (0.15 ppm) tended to cause stomatal closure. A SO2 O3 mixture (0.15/0.15 ppm) caused an increase in resistance much greater than O3 alone. The similarity in response of the stomata of different species to a SO2 O3 mixture does not explain why species respond differently when SO2 is added to ozone during exposure. © 1979.