BASIS OF DIFFERENTIAL TOLERANCE OF 2 CORN HYBRIDS (ZEA-MAYS) TO METOLACHLOR

Greenhouse and laboratory studies were conducted to determine the basis of differential response of two corn hybrids to the chloroacetanilide herbicide metolachlor. In greenhouse experiments, metolachlor at 6.7 kg ha-1 reduced the height of the susceptible 'Northrup-King 9283' corn by 53% relative to untreated controls and caused extensive visible injury 14 d after treatment. Under the same conditions, the height of metolachlor-treated 'Cargill 7567' corn seedlings was reduced by only 18% without any visible herbicide injury. The C-14-metolachlor was more rapidly absorbed by the emerging shoot of the metolachlor-susceptible hybrid, Northrup-King 9283. Thus, differential metolachlor tolerance may be due in part to processes at the level of herbicide uptake. Metabolism experiments revealed that both hybrids were able to conjugate C-14-metolachlor with glutathione at similar rates. However, glutathione S-transferase activity increased earlier during seedling development and reached higher activities in the metolachlor-tolerant hybrid, Cargill 7567.