Light-dependent Emission of Hydrogen Sulfide from Plants

With the aid of a sulfur-specific flame photometric detector, an emission of volatile sulfur 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 sulfur 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 weight⁻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 sulfur when HS⁻ acceptors are not available in sufficient quantity.