Influence of plant age on glutathione levels and glutathione transferases involved in herbicide detoxification in corn (Zea mays L.) and giant foxtail (Setaria faberi Herrm)

The influences of plant age on levels of extractable glutathione transferases (GSTs, EC 2.5.1.18) active in detoxifying atrazine, metolachlor, alachlor, and fluorodifen and on the availability of glutathione (GSH) have been determined in leaves of corn (Zea mays L. var. Artus) and the competing weed giant foxtail (Setaria faberi Herrm). Young corn plants, up to 30 days old, contained higher extractable GST activities toward atrazine, alachlor, and metolachlor than did giant foxtail plants of similar age. After 30 days growth, this difference was lost as the specific activities of the GSTs in corn declined while the GST activities in giant foxtail remained largely unchanged. Foliar applications of atrazine showed that the herbicide was only selectively phytotoxic to young giant foxtail plants, when the differences in GST activities between corn and giant foxtail were the greatest. The levels of GSH were highest in the foliage of younger plants of both species, though the levels of the thiol were always significantly higher in giant foxtail than those determined in corn. Dissection of the foliage of giant foxtail plants showed that there was an inverse correlation between the age of the leaves and the GST activity and GSH content. Increasing the GSH content of mature detached giant foxtail leaves by feeding with oxothiazolidine-2-carboxylate increased the rate of metabolism of [14C]atrazine in vivo but did not reduce steady-state chlorophyll fluorescence due to the presence of atrazine. Decreasing the GSH content in giant foxtail leaves by treatment with buthionine sulfoximine resulted in an unexpected increase in the rate of metabolism of [14C]atrazine and a reduction in the chlorophyll fluorescence caused by atrazine treatment. We conclude that GSH-mediated detoxification may be important in determining selectivity in seedlings but is less important in more mature plants.