Response of gas-exchange characteristics and chlorophyll fluorescence to acute sulfur dioxide exposure in landscape plants.

To explore the toxicity and action mechanism of acute sulfur dioxide (SO2) on urban landscape plants, a simulated SO2 stress environment by using fumigation chamber involving increasing SO2 concentration (0, 25, 50, 100, 200 mg m-3) was carried out among three species. After 72 h of exposure, SO2-induced oxidative damage indicated by electrolyte leakage increased with higher dose of SO2. Meanwhile, SO2 decreased the contents of chlorophyll a, chlorophyll b and carotenoid and increased the contents of sulfur. Net photosynthetic rate (Pn) decreased as a result of stomatal closure when SO2 dose was lower than 50 mg m-3, out of this range, non-stomatal limitation play a dominant role in the decline of Pn. Simultaneous measurements of chlorophyll fluorescence imaging (CFI) also revealed that the maximal quantum efficiency of PSII photochemistry in dark-adapted state (Fv/Fm) and the realized operating efficiency of PSII photochemistry (Fq'/Fm') was reduced by SO2 in a dose-dependent manner. In addition, the maximum quantum efficiency of PSII photochemistry in light-adapted state (Fv'/Fm') and the PSII efficiency factor (Fq'/Fv') decreased when exposure to SO2. These results implied that acute SO2 exposure induced photoinhibition of PSII reaction centers in landscape plants. Our study also indicated that different urban landscape plant species resist differently to SO2: Euonymus kiautschovicus > Ligustrum vicaryi > Syringa oblata according to gas-exchange characteristics and chlorophyll fluorescence responses.