Abstract
In order to clarify the mechanism of differential phytotoxic activity of herbicides simetryn (2, 4-bis(ethylamino)-6-methylthio-s-triazine) and dimethametryn [2-(1, 2-dimethyl-propylamino)-4-ethylamino-6-methylthio-s-triazine], their absorption, translocation and metabolism were investigated in rice (Oryza sativa L.) cultivars Nihonbare and Choseng Tongil and barnyardgrass (Echinochloa oryzicola Vasing.). The plants were grown in water culture to the 3-leaf stage and their roots were treated with 10-5M of 14C-simetryn and dimethametryn. Rice cultivars absorbed both herbicides at similar rates, while barnyardgrass absorbed dimethametryn more quickly. Simetryn moved to shoots faster than dimethametryn in all plants. In Nihonbare, 14C concentration in shoots was significantly higher after simetryn treatment, while no difference was observed in Choseng Tongil and barnyardgrass. Nihonbare metabolized simetryn more quickly than dimethametryn. This may indicate that difference in metabolic activity determines tolerance to the herbicides. In Choseng Tongil and barnyardgrass, the rate of simetryn metabolism was much slower than Nihonbare, and no clear difference was observed in metabolic activity between the two herbicides. This may explain the similar phytotoxic effect of the herbicides on Choseng Tongil and Barnyardgrass. The results indicate that the slower metabolic rate of dimethametryn is one of factors in its greater phytotoxicity.
