Studies of the mechanism of action of asulam in plants. Part I: Antagonistic interaction of asulam and 4‐aminobenzoic acid

Studies have been carried out on the herbicidal action of asulam [methyl (4‐aminophenylsulphonyl)carbamate] and sulphanilamide, alone or in association either with 4‐aminobenzoic acid (4ABA) or 4, 6‐diamino‐1‐(3, 4‐dichlorophenyl)‐1, 2‐dihydro‐2, 2‐dimethyl‐1,3,5‐triazine (DCDT). The soaking of wheat seeds (Triticum estivum L.) for 12 h at 30°C in asulam and DCDT in a 10:1 ratio doubled the inhibition of root growth produced by soaking in asulam alone; the addition of 4ABA partially reversed the activity of asulam. Foliar applications of a mixture of asulam + DCDT (1.1 + 0.55 kg ha−1) markedly increased the activity of asulam in susceptible wheat, wild oat (Avena fatua L.), tolerant flax (Linum usitatissimum L.), and in Stellaria media L. The activity of asulam at 1.1 kg ha−1 was reversed by 4ABA at 2.2 kg ha−1 by about 50% in wheat and wild oat, 82% in flax and 100% in S. media. The results indicate that asulam and sulphanilamide act by similar mechanisms in apparently inhibiting the biosynthesis of folic acid.