Brassica juncea and Brassica rapa were able to utilize foliarly absorbed H2S and SO2 as sulfur source for growth and resulted in a decreased sink capacity of the shoot for sulfur supplied by the root and subsequently in a partial decrease in sulfate uptake capacity of the roots. Sulfate-deprived plants were able to utilize foliarly absorbed H2S and SO2 as sole source for growth. However, the decrease in shoot to root partitioning and the upregulated sulfate uptake capacity upon sulfate-deprivation was hardly affected by H2S or SO2 exposure, indicating a poor shoot to root interaction between sulfur assimilation in the shoot and the regulation of the sulfate transporter activity in the root. Root transporter activity was regulated by the in situ sulfate rather than the water-soluble non-protein thiols (glutathione) concentration. There was also no direct relation between the thiol levels and the transcript levels of the enzymes involved in the sulfate reduction pathway in either roots or shoots of B. juncea and B. rapa. In sulfate-sufficient plants, both H2S and SO2 exposure resulted in strongly decreased transcript levels of APS reductase in the shoot of both species. Sulfate deprivation resulted not only in strongly enhanced transcript levels of APS reductase in both shoots and roots of the plants, but also in enhanced transcript levels of ATP sulfurylase and APS kinase in the shoots. H2S and SO2 exposure also resulted in a decrease in transcript levels of APS reductase in the roots and shoots of sulfate-deprived plants, but remained significantly higher than that in sulfate-sufficient plants. The transcript levels of ATP sulfurylase, APK kinase and sulfite reductase were not affected in both sulfate-sufficient and sulfate-deprived plants upon H2S and SO2 exposure.
