MT-5950, a New Anilide Herbicide, Inhibits PSII at a Site that Slightly Overlaps the Triazine Binding Site

Abstract

The new selective acylanilide herbicide, MT-5950 [N-(3-chloro-4-isopropylphenyl)-2- methylpentanamide], was found to be a potent inhibitor of photosystem II (PS II) in photosynthesis. It was not as effective as atrazine in increasing variable fluorescence of wheat leaves dipped in herbicide solutions. However, in assays in which absorption across the leaf cuticle did not play a role, MT-5950 was about 10-fold more effective than atrazine in inhibiting the PSII activity of all the species tested. For example, the I₅₀ values of MT-5950 and atrazine for inhibiting linear electron transport in broken chloroplasts of spinach were 0.08 and 0.8 μm, respectively. Atrazine- resistant biotypes of Brassica napus and Solanum nigrum were about 10-fold and < 2-fold less susceptible, respectively, to MT-5950 than were susceptible biotypes of the same species as measured by DCPIP photoreduction and CO₂-dependent oxygen evolution. However, MT-5950 was equally as effective as atrazine in competing for ¹⁴C-labeled atrazine from thylakoid proteins. We conclude that MT-5950 inhibits PS II by binding the quinone-binding 32 kD protein at a site that overlaps the binding site of atrazine, but does not significantly overlap that portion of the atrazine binding site that, when modified, confers atrazine resistance.