Oxidation and reduction of sulfite contribute to susceptibility and detoxification of SO2 in Populus x canescens leaves

The critical level for SO (2) susceptibility of Populus x canescens is approximately 1.2 mu L L (-1) SO (2) . Both sulfite oxidation and sulfite reduction and assimilation contribute to SO (2) detoxification. In the present study, uptake, susceptibility and metabolism of SO2 were analyzed in the deciduous tree species poplar (Populus x canescens). A particular focus was on the significance of sulfite oxidase (SO) for sulfite detoxification, as SO has been characterized as a safety valve for SO2 detoxification in herbaceous plants. For this purpose, poplar plants were exposed to different levels of SO2 (0.65, 0.8, 1.0, 1.2 mu L L-1) and were characterized by visible injuries and at the physiological level. Gas exchange parameters (stomatal conductance for water vapor, CO2 assimilation, SO2 uptake) of the shoots were compared with metabolite levels (sulfate, thiols) and enzyme activities [SO, adenosine 5'-phosphosulfate reductase (APR)] in expanding leaves (80-90 % expanded). The critical dosage of SO2 that confers injury to the leaves was 1.2 mu L L-1 SO2. The observed increase in sulfur containing compounds (sulfate and thiols) in the expanding leaves strongly correlated with total SO2 uptake of the plant shoot, whereas SO2 uptake rate was strongly correlated with stomatal conductance for water vapor. Furthermore, exposure to high concentration of SO2 revealed channeling of sulfite through assimilatory sulfate reduction that contributes in addition to SO-mediated sulfite oxidation to sulfite detoxification in expanding leaves of this woody plant species.