Effects of sodium bisulphate, acidic buffers and ammonium sulphate on imazamethabenz phytotoxicity to wild oats

Abstract

Greenhouse studies determined the effects of sodium bisulphate (NaB), acidic buffers and ammonium sulphate on the phytotoxicity of imazamethabenz, (±)-2-[4,5-dihydro-4-methyl-4-(1-methylethyl)-5-oxo-1 H-imidazol-2-yl]-4(and 5)-methylbenzoic acid (3:2), applied to wild oat plants at the 2- to 3-leaf stage. At an equivalent dosage of imazamethabenz the liquid concentrate (LC) formulation had higher herbicidal activity and much lower pH than did the suspension concentrate (SC) formulation. NaB added to the SC formulation lowered solution pH by > 3 units to ∼ 2.2 and enhanced herbicidal activity at 200–300 g ha −1 active ingredient (a.i.), but not at 400 g a.i. ha −1. NaB did not improve the activity of the LC formulation. Increasing the spray solution pH from 5.6 to ∼ 7.3 with 1 m KOH antagonized phytotoxicity of both LC and SC formulations of imazamethabenz alone, and imazamethabenz-SC plus NaB. Citric acid-phosphate buffers at pH 3–6, or HCl-KCl buffers at pH 1 and 2, either had no effect or antagonized the activity of the SC formulation of imazamethabenz. However, the same HCl-KCl buffers plus 1% (v/v) ammonium sulphate enhanced herbicidal activity at 250 g a.i. ha −1, while 1% ammonium sulphate, alone, enhanced herbicidal activity at both 200 and 250 g ha −1. NaB provided both bisulphate ion and low pH concurrently. Acidic buffers did not improve the activity of the SC formulation except in the presence of bisulphate or sulphate ions. It is suggested that at low pH the herbicide becomes cationic, forms the bisulphate or sulphate salt of the methyl ester of imazamethabenz, and enters true solution. In this form it readily penetrates the cuticle. High solution pH acts against this process.