The growth responses of six grain sorghum (Sorghum bicolor(L.) Moench) cultivars exposed to single and (or) combined treatments of the herbicide metolachlor (2-chloro-N-(2-ethyl-6-methylphenyl)-N-(2-methoxy-1-methlethyl)acetamide) and excess H+were investigated under growth chamber conditions. The sorghum cultivars tested were '38M', '58M', 'GP-10', 'SC574', 'SC283' (inbred lines), and 'Funk G522DR' (F1hybrid). Shoot lengths were measured on seedlings from each cultivar grown for 14 days in white quartz flintshot sand treated with metolachlor at 0, 0.25, 0.5, 1.0, and 2.0ensp;mg/kg. Pots were watered with 100ensp;mL of 0.1ensp;M acetate at pH 6.0, 5.5, 5.0, 4.5, or 4.0 on alternate days. A sorghum mutant ('58M') shown to have enriched gibberellin levels and '38M', a second sorghum mutant, were essentially immune to either metolachlor (2ensp;mg/kg) or excess H+concentration (pH 4.0). 'Funk G522DR' was sensitive to both excess H+concentrations and metolachlor, and interactions between these two stresses were evident. The remaining cultivars were intermediate in response to both excess H+concentration and metolachlor. Glutathione contents and glutathioneS-transferase activities of the metolachlor-resistant 'GP-10' and the metolachlor-sensitive 'Funk G522DR' were comparable. Thus, enriched gibberellin levels may be a major factor in the differential response of the tested sorghum cultivars to metolachlor and (or) excess H+stresses. Phenological response in relation to multiple stresses is discussed.Key words: pH, metolachlor, gibberellin, acid soil.
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