Substituent Effect on Photosynthesis of N²-α-Substituted Benzyl-N⁴-alkyl-2, 4-diamino-6-chloro-s-triazines and Their Phytotoxic Property

Abstract

The influence of substituents on the activities of a series of N²-α-substituted benzyl-N⁴-alkyl-2, 4-diamino-6-chloro-s-triazines as inhibitors of photosystem II (PSII) was examined, and the phytotoxic differences between them and atrazine, as to the photosynthesis in leaf disks, mesophyll cells, intact chloroplasts and broken chloroplasts of spinach, and as to seedling-growth, were discussed. The inhibitory activity of the N²-α, α-dimethylbenzyl-N⁴-ethyl derivative (6), which was comparable on that of atrazine, was lower than those of the N²-α-alkylbenzyl analogues (1-5). The N⁴-n-alkyl-N²α-methylbenzyl derivatives, in spite of the carbon length of the alkyl group, exhibited more potent activity than atrazine, but an α or β substitution of the N⁴-n-alkyl group caused a decrease in the activity with a few exceptions. These data may imply that the space of the binding site on PSII surrounding both the N² and N⁴ amino groups is relatively large. The binding between the receptor site and the N⁴ amino group, however, is easily influenced by a slight structural change in an inhibitor. The herbicidal compounds, N²-α-methylbenzyl-N⁴-ethyl (1), N²-α, α-dimethylbenzyl-N⁴-1-methylpropyI (30) and N²-α-methylbenzyl-N⁴, N⁴-diethyl (42) derivatives, exhibited potent inhibitory activity in the seedling growth test under dark/light conditions, whereas atrazine was very poor. The inhibitory activity of compound (1) toward photosynthesis was poor with leaf disks, compared to atrazine, whereas, the order of their activities was the reverse for plant preparations such as abaxial epidermis peeled leaf disks, mesophyll cells, intact chloroplasts and broken chloroplasts. It was indicated that a change in the phytotoxic symptom in the whole plant assay would be correlated to the permeability of the compound through the plant membrane(s).