Relationship between metolachlor sensitivity and Mn toxicity tolerance in sorghum cultivars

Abstract

Four sorghum [Sorghum bicolor (L.) Moench] cultivars were evaluated for metolachlor resistance and degradation. LD₅₀ for TAM 428 and SC 599 exceeded 3 ppmw metolachlor. This was a > 5 × increase in metolachlor resistance. SC 283 and GP 140 LD₅₀’s were 0.4 and 0.5 ppmw metolachlor which were ~ 2 × as sensitive to metolachlor as the reference cultivar Funk G-522 DR. Degradation rates did not parallel resistance. TAM 428 (LD₅₀ 3 ppm in bathing solution) had the same degradation rate as GP 140 (LD₅₀ 0.5 ppm). Sorghum genotypic selectivity to metolachlor is not explicable as a differential herbicide degradation rate.

When grown in soil at pH 4.4, SC 599 was highly susceptible to excess Mn²⁺ (and possibly Al) and height and fresh weight were greatly decreased (86%). Aluminum toxicity was apparently not important according to soil Al values. TAM 428 was intermediate in response to excess Mn²⁺ (77% decreased height); while SC 283 and GP 140 were resistant to excess available Mn²⁺ and their growth was decreased 13 and 40%, respectively, due to acid soil stress. Mg/Mn ratios indicate that SC 599 absorbed Mn²⁺ or Mg²⁺ to the extent that they were available in the soil solution. The soil solution Mg/Mn ratio at low pH was the same as for the plants. But TAM 428, SC 283, and GP 140 appeared to have a mechanism to exclude excess Mn²⁺ that also decreased the absorption of Mg²⁺ so that Mg/Mn ratios remained uniform at low soil pH.

Differences in metolachlor and Mn²⁺ susceptibilities may be due to differential responses of the terpenoid biosynthesis enzyme systems which are strongly modified by both metolachlor and Mn²⁺-Mg²⁺ concentrations.