Sulfur dioxide: Relevance of toxic and nutritional effects for Chinese cabbage

Shoots of Chinese cabbage formed a sink for atmospheric SO2 and there was a linear relation between the rate of uptake and the atmospheric SO2 level (0.03–1.4 μl l−1). Chinese cabbage appeared to be rather susceptible to the toxic effects of SO2. Shoot biomass production was reduced upon prolonged exposure to ≥0.1 μl l−1 SO2. SO2 exposure resulted in an increase in the content of SO42−, water-soluble non-protein thiols and total S in the shoot with atmospheric levels of ≥0.1 μl l−1, however, the ratio of organic S to total S remained unaffected. The organic N content was not affected by SO2 exposure. The impact of SO2 on Chinese cabbage seemed to be ambiguous, since the SO2 taken up by the shoot also could be used as S source for growth and was even beneficial when SO42− supply to the root was deprived. A 5-day exposure of plants to 0.06–0.18 μl l−1 SO2 resulted in an alleviation of the development of S deficiency symptoms upon SO42− deprivation. An atmospheric SO2 level as low as 0.06 μl l−1 appeared to be sufficient to cover the plants’ S requirement for growth. The N/S ratio was much lower in SO42-sufficient plants than in SO42-deprived plants in both shoot and root. Exposure of SO42−-deprived plants to SO2 resulted in a decrease in the N/S ratio of the shoot but did not affect that of the root. The N/S ratio of the shoot decreased with increasing SO2 levels as a consequence of the increase in total S and SO42− content. In contrast, the N/S ratio of shoot and root of SO42−-sufficient plants was not significantly affected upon exposure to 0.06–0.18 μl l−1 SO2. SO42− deprivation resulted in a shift in shoot to root biomass partitioning during growth in favor of root production, which was not rapidly alleviated when SO2 was used as S source for growth. Its possible consequences for Chinese cabbage growing under field conditions are evaluated.