Light-dependent Emission of Hydrogen Sulfide from Plants

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Abstract
With the aid of a sulfur-specific flame photometric detector, an emission of volatile S was detected from leaves of cucumber (Cucumis sativus L.), squash and pumpkin (Cucurbita pepo L.), cantaloupe (Cucumis melo L.), corn (Zea mays L.), soybean (Glycine max [L.] Merr.) and cotton (Gossypium hirsutum L.). The emission was studied in detail in squash and pumpkin. It occurred following treatment of the roots of plants with sulfate and was markedly higher from either detached leaves treated via the cut petiole, or whole plants treated via mechanically injured roots. Bisulfite elicited higher rates of emission than sulfate. The emission was completely light-dependent and increased with light intensity. The rate of emission rose to a maximum and then declined steadily toward zero in the course of a few hours. However, emission resumed after reinjury of roots, an increase in light intensity, an increase in S anion concentration, or a dark period of several hours. The emission was identified as H2S by the following criteria: it had the odor of H2S; it was not trapped by distilled H2O, but was trapped by acidic CdCl2 resulting in the formation of a yellow precipitate, CdS; it was also trapped by base and the contents of the trap formed methylene blue when reacted with N,N-dimethyl-p-phenylenediamine and Fe3+. H2S emission is not the cause of leaf injury by SO2, since bisulfite produced SO2 injury symptoms in dim light when H2S emission was low, while sulfate did not produce injury symptoms in bright light when H2S emission was high. The maximum rates of emission observed, about 8 nmol min-1 g fresh wt-1, are about the activity that would be expected for the sulfur assimilation pathway of a normal leaf. H2S emission may be a means by which the plant can rid itself of excess inorganic S when HS- acceptors are not available in sufficient quantity.