The objectives of this study were to determine the gas exchange responses of Heteromeles arbutifolia (Ait.) Roem. to a range of sulphur dioxide (SO2) concentrations applied in various sequences including possible ambient levels. Carbon dioxide and light-response curves were used to determine the mechanisms of SO2 responses. These experiments were performed with greenhouse-grown plants.
Net photosynthesis (A) of H. arbutifolia was shown to be depressed by SO2 concentrations ranging from 1.5 to 43.8 μmol m−3 (0.04 to 1 μl l−1). In most cases on first exposure to SO2 a larger decrease in stomatal conductance to water vapour (gH) occurred than on subsequent exposures to either the same or greater SO2 concentrations. Acclimation of gH to SO2 appeared to occur with repeated exposure.
Light-response curves determined during exposure to 25 μmol SO2 m−3 (0.6 μl l−1) for a leaf repeatedly exposed to SO2 indicated that inefficiency of photon utilization in the primary reaction of energy transduction or reduction in photosynthetic capacity was unlikely. The maximum rates of A (Amax) determined from these curves were similar in both SO2 and SO2− free air. The limitations of gH to carbon acquisition were removed using CO2−response curves [photosynthesis expressed relative to the internal CO2 concentration (Ci)]. This analysis shows that A may have been limited during repeated exposures by reduced carboxylation efficiency. It appeared that at similar values of gH, plants exposed to SO2 would fix less carbon and water use efficiency (WUE) would decrease in polluted plants.