Oxabetrinil Reversal of Metolachlor and Acid Soil Stress*

M etolachlor and excess Mn:+ (acid soil stress) induce alterations in gibberellin precursor biosynthesis that can explain the morphological responses to these physiological stresses. Oxa­ betrinil protects sorghum [Sorghum bicolor (L.) Moench] from the influence o f metolachlor and excess Mn2+. Sorghum cultivar variations in response to excess M n2+ are explicable as dif­ ferential rates o f tvjf-kaurene biosynthesis between acid soil sensitive and tolerant cultivars. Concentrations o f Mn2+ present in vegetative leaves and reproductive stem tissues were not different. Therefore, cultivar differences in e«/-kaurene biosynthesis explain the acid soil toler­ ance differences rather than differential Mn2+ absorption, translocation, and/or compartmentation. Metolachlor and safener responses are found in cellular compartments and tissues that do not match a decreased herbicide concentration through absorption, transport, or degrada­ tion as a sole mode o f action for safeners.