The effect of various herbicides on glutathione levels in barley (Hordeum vulgare L.), tobacco (Nicotiana tabacum L.), soybean (Glycine max [L.] Merr.), and corn (Zea mays L.) was examined. Illumination of excised barley, tobacco, and soybean plants for 8 hours in solution containing 2 millimolar aminotriazole (a catalase inhibitor) resulted in an increase in leaf glutathione from 250 to 400 nanomoles per gram fresh weight to 600 to 1800 nanomoles per gram fresh weight, depending on the species tested. All of this increase could be accounted for as oxidized glutathione. Between 25 and 50% ofthis oxidized glutathione was reduced when plants were darkened for 16 hours, but there was no significant decline in total glutathione. Another catalase inhibitor, thiosemicarbazide, was as effective as aminotriazole in elevating glutathione in soybean but was less effective in barley and tobacco. Glyphosate, an inhibitor of aromatic amino acid biosynthesis, had no significant effect on glutathione levels in any ofthe plants examined. Whereas methyl viologen (paraquat), which is a sink for photosystem I electrons, caused oxidation of leaf glutathione in all of the plants but did not increase the total amount of glutathione present.Under photorespiratory conditions, a catalase-deficient mutant (RPr 79/4) of barley and wild-type barley treated with the catalase inhibitor aminotriazole, accumulated glutathione to 3-fold the level in nonphotorespining plants (14,15). Because a large fraction of this glutathione was oxidized, we proposed that H202, generated during photorespiration by the action of glycolate oxidase, directly or indirectly oxidized GSH to GSSG. A consequence of a lower GSH pool would be additional synthesis of glutathione, because GSH is a feedback inhibitor of glutathione biosynthetic enzymes ( 12).We have two short-term objectives in the present study. First, to determine whether glutathione accumulation is a general phenomenon in photorespiring plants treated with catalase inhibitors. Second, to determine the extent to which plants can reduce GSSG in the dark. A long-term goal is to determine the stoichiometric relationship between photorespiratory H202 production and GSSG formation, because the latter may be a more reliable measure of photorespiration rates than is currently available. 3 and 8 weeks. Usually, shoots were excised close to the ground, placed in 150 ml of solution in a 250 ml beaker, and the stem cut under liquid about 5.1 cm from the base to ensure adequate transpiration. Plants were exposed to 8 h of light, intensity 400 ,uE m-2 s-' at 27°C, unless specified otherwise. We previously showed that intact or excised shoots of barley behaved similarly 0.8
MATERIALS AND METHODS
Barley
