CO2 EXCHANGE-RATES AND STOMATAL DIFFUSIVE RESISTANCE IN SOYBEAN EXPOSED TO O3 AND SO2

Soybean (Glycine max (L.) Merr. ''Essex'') plants, 21-24 days old, were exposed to 400 .mu.g/m3 (0.20 .mu.l/l) O3 and 1865 .mu.g/m3 (0.70 .mu.l/l) SO2 in various combinations. Fumigation was administered for 2 h either as single pollutants (O3 and SO2), simultaneously (O3 + SO2), or in overlapping pollutant combinations (O3 for 1 h followed by O3 + SO2 or SO2 for 1 h followed by SO2 + O3). CO2 exchange rates (CER) of trifoliolate leaves were measured during the fumigations, and stomatal resistance to H2O was determined immediately before and after pollutant exposure. At the end of a 2-h exposure, O3 and SO2, administered separately, did not significantly affect CER. Exposure to O3 followed by O3 + SO2, SO2 followed by SO2 + O3, and continuous O3 + SO2 significantly reduced CER to 62, 41 and 33% of preexposure rates, respectively. Stomatal resistance was not significantly altered by pollutant fumigation except in the simultaneous application of O3 + SO2 where an 11% decrease occurred. Reductions in CER in response to fumigation were not attributed to changes in stomatal resistance, but appeared to result from changes in mesophyll resistance. A proposed mechanism for pollutant-induced reduction in CER is discussed.