Cysteine, γ‐glutamyl‐cysteine and glutathione contents of spinach leaves as affected by darkness and application of excess sulfur

In the light, glutathione was the major water‐soluble, non‐protein, sulfhydryl compound in leaves of spinach (Spinacia oleracea L. cv. Estivato). In the dark, another sulfhydryl compound accumulated, which proved to be γ‐glutamyl‐cysteine. In the light, exposure of leaves to excess sulfur in the form of atmospheric H2S (0.25 μl l−1) resulted in considerably increased levels of glutathione and cysteine. In the dark, in addition to these thiols, levels of γ‐glutamyl‐cysteine were also enhanced considerably. When leaves of plants exposed to H2S in the dark were illuminated, the dipeptide rapidly disappeared. At the same time, glutathione contents increased by approximately the same amount, indicating a light‐dependent conversion of γ‐glutamyl‐cysteine into glutathione. Possible mechanisms for these light‐induced changes in thiol metabolism are discussed.