INHIBITION OF PHOTOSYNTHESIS AND LEAF CONDUCTANCE INTERACTIONS INDUCED BY SO2, NO2 AND SO2+NO2

Effects of 2-h exposures to 0-1 μmol mol-1 SO2, NO2 and (1:1) SO2 + NO2 on CO2 uptake by standardized snap bean leaves were studied. Interactions resulting from pollutant-induced changes in leaf conductance were evaluated. Minimum exposure concentrations required to depress CO2 exchange rates (CER) under the test conditions were:0.17 μmol mol-1 SO2, 0.38 μmol mol-1 NO2, and 0.08 μmol mol-1 of each pollutant in the 1:1 mixture. Treatments with 1 μmol mol-1 NO2 reduced CER 10% without affecting leaf conductance. One μmol mol-1 SO2 depressed CER by 50%. Leaf conductances increased in SO2-treated leaves showing 30% inhibition of CER. Greater inhibition led to subsequent stomatal closure. Inhibition caused by the individual pollutants (applied singly) was linear over the range of concentrations investigated. The dual-pollutant mixture produced a synergistic response that was most pronounced at the lower pollutant concentrations. The potentiated effect was correlated with marked stomatal closure. Experimental plants for this study were grown under low moisture stress conditions to enhance stomatal opening in the plant stock material and reduce (damp) the potential for further SO2-induced stimulation of stomatal opening. The experiments were designed to obtain limiting data for the test conditions. © 1990.