Impact of pedospheric and atmospheric sulphur nutrition on sulphur metabolism of Allium cepa L., a species with a potential sink capacity for secondary sulphur compounds

Onion (Afflum cepa L.) was able to use atmospheric H2S as sole sulphur source for growth. The foliarly absorbed H2S was rapidly metabolized into water-soluble, non-protein thiol compounds, including cysteine, and subsequently into other sulphur compounds in the shoots. In H2S-exposed plants, the accumulation of sulphur compounds in the shoots was nearly linear with the concentration (0.15-0.6 mul l(-1)) and duration of the exposure. Exposure of onion to H2S for up to 1 week did not affect the sulphur content of the roots. Secondary sulphur compounds formed a sink for the foliarly absorbed sulphide, and the sulphur accumulation upon H2S exposure could, for a great part, be ascribed to enhancement of the content of gamma-glutamyl peptides and/or alliins. Furthermore, there was a substantial increase in the sulphate content in the shoots upon H2S exposure. The accumulation of sulphate originated both from the pedosphere and from the oxidation of absorbed atmospheric sulphide, and/or from the degradation of accumulated secondary sulphur compounds. From studies on the interaction between atmospheric and pedospheric sulphur nutrition it was evident that H2S exposure did not result in a down-regulation of the sulphate uptake by the roots.