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 thio l pool under conditions of excess sulfur, we studied the patterns of a ccumulation of various low molecular mass thiols in leaves of spinach (Spinacia oleracea L., cv. Estivato) in response to H2S fumigation. Ex posure 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 reach ed within 24 h. Different thiols accumulated simultaneously; in the li ght the levels of cysteine and glutathione (GSH) increased, in the dar k cysteine and gamma-glutamylcysteine (gamma-GC). After cessation of t he fumigation, the thiol levels rapidly decreased. In the light, the l evels of cysteine and GSH decreased simultaneously. A dark-light trans ition immediately after cessation of the H2S fumigation resulted in a rapid decrease in the gamma-GC level and a simultaneous increase in th e GSH content. Subsequently, the levels of GSH and cysteine decreased at similar rates. These results demonstrate that the composition of th e thiol pool in leaves of H2S-exposed plants is not regulated within n arrow limits.