Experiments were performed on an evergreen (Heteromeles arbutifolia) and a drought deciduous shrub (Diplacus aurantiacus) to determine, 1) whether approaches for evaluating SO2 absorption by leaves in laboratory studies could be extended to field studies, 2) the effects of irrigation on metabolism and SO2 responses of the study species during a season when water was limiting, 3) to interpret SO2 responses on the basis of SO2 flux rates. Laboratory-developed approaches for evaluating SO2 absorption by leaves were found to be suitable for use with field plants, despite field plants having lower gas exchange rates. Supplementing water during times of deficit did not override all the biological and environmental factors that limited photosynthesis (A). Irrigation increased leaf longevity of D. aurantiacus, and stomatal conductance to water vapour (g); g was also shown to increase with H. arbutifolia on irrigation. Irrigation profoundly influenced plant response to SO2. Unwatered D. aurantiacus had only a small g and therefore a reduced capacity to absorb SO2 and respond to SO2; which resulted in apparent SO2 avoidance. Water availability and SO2 both affect g and therefore, SO2 flux rates into the mesophyll. Different ambient SO2 concentrations of 8.3 and 26.2 μmol m-3 (0.2 and 0.6 ppm) were both found to result in similar SO2 flux rates into the leaf, due to variations in g in response to water availability. Changes in g did not always result in changes in A, implying that carbon fixation may be little affected by some SO2 exposures, although still potentially affecting such processes as maintenance of leaf water potential, transpirational cooling and nutrient uptake.
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