THE POOL OF WATER-SOLUBLE, NONPROTEIN THIOLS IS NOT REGULATED WITHIN NARROW LIMITS IN SPINACH LEAVES EXPOSED TO ATMOSPHERIC H2S

In order to obtain more information on the regulation of the free thiol pool under conditions of excess sulfur, we studied the patterns of accumulation of various low molecular mass thiols in leaves of spinach (Spinacia oleracea L., cv. Estivato) in response to H2S fumigation. Exposure to 0.25 mu l l(-1) H2S in both light and darkness resulted in a rapid accumulation of thiols in the leaves. A maximum level was reached within 24 h. Different thiols accumulated simultaneously; in the light the levels of cysteine and glutathione (GSH) increased, in the dark cysteine and gamma-glutamylcysteine (gamma-GC). After cessation of the fumigation, the thiol levels rapidly decreased. In the light, the levels of cysteine and GSH decreased simultaneously. A dark-light transition immediately after cessation of the H2S fumigation resulted in a rapid decrease in the gamma-GC level and a simultaneous increase in the GSH content. Subsequently, the levels of GSH and cysteine decreased at similar rates. These results demonstrate that the composition of the thiol pool in leaves of H2S-exposed plants is not regulated within narrow limits.