Metabolism of atmospheric hydrogen sulfide in onion

Amongst other elements sulfur is present in plant tissue in minor quantities only. The predominant proportion of the sulfur is present in proteins, as cysteine and methionine residues. Sulfur is also required for the synthesis of various other compounds, as thiols, sulfolipids and secondary sulfur compounds (alliins, glucosinolates, phytochelatins), which play an important role in the physiology of plants and in the protection and adaptation of plants against stress and pests. Allium species viz. onion, garlic, leek and chive, contain a variety of secondary sulfur compounds: γ-glutamyl peptides and alliins. Plants generally utilize sulfate taken up by the roots as sulfur source for growth. Contrary to its detrimental effects, foliarly absorbed hydrogen sulfide (H2S) can also be utilized as sulfur source for plants and may even be beneficial when the sulfur supply to the roots is limited. The general aim of this study was to get a better understanding of the regulation of sulfur metabolism in plants and specifically to investigate the significance of secondary sulfur compounds as possible sink for foliarly absorbed sulfur in onion and how this may affect the metabolism of H2S and the uptake and assimilation of sulfate. Onion was able to use atmospheric H2S as sole sulfur source for growth. H2S exposure resulted in an accumulation of total sulfur in all species and cultivars of Allium. Determining the composition of the accumulated sulfur compounds proved to be a greater challenge. A large part consisted of sulfate, the origin of which was not clear. Accumulation of organic sulfur could be explained by increased contents of secondary sulfur compounds and thiols, but not protein sulfur and sulfolipids. Data were largely corroborated by X-ray absorption near edge structure (XANES) spectroscopy. Sulfate deprivation resulted in an increased sulfate uptake capacity, which was not accompanied by a clear increase in the expression of sulfate transporters. In contrast to Brassica, H2S exposure did not affect the net sulfate uptake in onion.