Low levels of H2S may replace sulfate as sulfur source in sulfate-deprived onion

Onion (Allium cepa L.) was exposed to low levels of H2S in order to investigate to what extent H2S could be used as a sulfur source for growth under sulfate-deprived conditions. Sulfate deprivation for a two-week period resulted in a decreased biomass production of the shoot, a subsequently decreased shoot to root ratio and an increased dry matter content in shoot and roots. Furthermore, it resulted in decreased contents of total sulfur, sulfate and organic sulfur and in a decreased sulfate to total sulfur ratio. Symptoms of sulfur deficiency disappeared upon simultaneous exposure to relatively low levels of H2S (0.05, 0.1 and 0.15 mu l l(-1)), which showed that H2S could be used as a sulfur source for growth. H2S exposure even resulted in a slightly increased biomass production in sulfate-sufficient plants. The observed accumulation of sulfate and organic sulfur upon H2S exposure in both sulfate-sufficient and sulfate-deprived plants is discussed.