Impact of sulfate nutrition on the utilization of atmospheric SO2 as sulfur source for Chinese cabbage

The ability of Chinese cabbage (Brassica pekinensis) to utilize atmospheric sulfur dioxide (SO2) as sulfur (S) source for growth was investigated in relation to root sulfate (SO42-) nutrition. If seedlings of Chinese cabbage were transferred to a sulfate-deprived nutrient solution directly after germination, plants became rapidly S-deficient. Plant-biomass production was decreased and the shoot-to-root ratio decreased. Sulfate deprivation resulted in a substantial decrease in total S, sulfate, organic-S, and water-soluble nonprotein thiol contents and in an increase in amino-acid content of both shoot and root. The sulfate-uptake rate of the root was strongly increased, whereas nitrate-uptake rate was decreased. Upon resupply of sulfate, the onset of S-deficiency symptoms was prevented, and growth was restored, whereas sulfate and nitrate-uptake rates were quite similar to those of the sulfate-sufficient plants. A 6-day exposure to 0.12 mu L L-1 SO2 Of Sulfate-sufficient plants did not affect plant-biomass production, shoot-to-root ratio, S and nitrogen (N) compounds of shoot and root, or sulfate and nitrate uptake by the root. Exposure of sulfate-deprived plants to SO2 resulted in enhanced total S, organic-S, and water-soluble nonprotein thiol contents of the shoot. The contribution of SO2 as S source for biomass production depended on the duration of the sulfate deprivation. If Chinese cabbage was transferred to a sulfate-deprived nutrient solution and simultaneously exposed to SO2, then plants benefited optimally from the foliarly absorbed S. The development of S-deficiency symptoms was prevented, and shoot-biomass production was quite similar to that of sulfate-sufficient plants. However, upon SO2 exposure root-biomass production was even higher than that of sulfate-sufficient plants, whereas sulfate uptake was still enhanced. Evidently, upon SO2 exposure there was no strict and direct shoot-to-root signaling in tuning sulfate uptake by the root and its transport to the shoot to the need for growth, via down-regulation of sulfate uptake and normalizing shoot-to-root biomass partitioning.