Antagonistic effects of simultaneous exposure to sulfur-dioxide gas and vapor of the herbicide 2,4-d iso-octyl on the carbon-dioxide exchange of tomato and field bean-plants

Tomato and field bean plants were exposed simultaneously to SO2 gas (in the range 0.07-0.57-mu-g l-1) and to vapour of the herbicide [C-14]2,4-D iso-octyl (0.8-2.7 ng l-1), for periods of 6-24 h in an air-flow system at a water vapour pressure deficit of > 2 kPa. Greater response was shown by the plants to 2,4-D vapour than to SO2. Herbicide vapour alone decreased CO2 exchange during the 3 d experimental period in both species, but the combination of herbicide plus SO2 had less effect. The response by tomato was both greater and more rapid than by field bean (which showed an initial stimulation of CO2 exchange due to 2,4-D), and the two species differed in the effect on transpiration. SO2 did not affect the uptake and distribution of herbicide within the plants. It appeared that 2,4-D iso-octyl vapour and SO2 were antagonistic in effect on the CO2 exchange of the two species under the experimental conditions, and that in tomato this was largely due to changes in stomatal conductance.