Studies on the Mode of Action of s-Triazine Herbicides : 2. Influences of temperature on the phytotoxicity of methylmercapto substituted triazines for rice plant and barnyardgrass.

Abstract

As was reported in the previous paper, it was found that the methylmercapto triazines, simetryne, prometryne and ametryne were most suitable for weed control in paddy fields. In this experiment, the influence of temperature on the phytotoxicity of these herbicides for rice plant and barnyardgrass was examined both by the treatment of foliage under flooded conditions at the seedling stage and by the treatment of underground parts at the germinating stage at 16, 23 and 30°C, respectively. The results obtained were as follows: 1. The influence of temperature on the toxicity for rice plant and barnyardgrass was markedly different according to the herbicides in each of both treatments. Namely the effect of temperature on the phytotoxicity of simetryne was slight, but in the case of prometryne and ametryne it was severe. The latter two herbicides were much more toxic at 30°C than 16 or 23°C, especially for rice plant. 2. In both treatments, the selectivity of simetryne in toxicity, which was stronger for barnyardgrass than for rice plant, was very high with little influence of temperature. On the other hand the selectivity of prometryne decreased at higher temperature, and ametryne had extremely low selectivity at 30°C than at lower temperatures. 3. The influence of temperature on the toxicity for rice plant and barnyardgrass and on the selectivity between both plants was higher in the treatment of underground parts at the germinating stage than in the foliage treatment under flooded conditions at the seedling stage. 4. Judging from the above-stated results, simetryne was found to be more widely suited for weed control in paddy fields in view of the effect of temperature.