Effect of temperature on absorption and translocation of simetryn [herbicide] in rice cultivars

Rice cultivars differ widely in their susceptibility to an herbicide simetryn [2, 4-bis (ethylamino)-6-methylthio-1, 3, 5-triazinel]. Japonica-type cultivars are tolerant and many indica-type and their hybrid are relatively susceptible. However, it was observed in growth chambers that high temperature caused severe phytotoxicity of simetryn even to the tolerant type. In order to clarify the physiological responses of rice cultivars under different temperature conditions, cultivars Nihonbare and Bluebell (japonica-type), IR-8 and CH-45 (indica-type) and Yushin and Tongil (hybrids) were used, and their absorption and translocation of 14C-simetryn was surveyed. Under both high and low temperature conditions, cultivars Nihonbare and Bluebell absorbed the substance at a higher rate. Although high temperature accelerated absorption in all cultivars, no difference in the rate of increase with increasing temperature was observed among them. Enhancement of translocation in Nihonbare and Bluebell was remarkable for the first several hours after treatment. Greater concentration of 14C was detected in shoots of these two cultivars under both temperature conditions, and their enhanced 14C accumulation with increasing temperature was also remarkable. The increase of phytotoxicity in tolerant cultivars was believed to be related to both increased rates of absorption and translocation. It was concluded that the physiological response of each cultivar to temperature variation differed and that cultivars Nihonbare and Bluebell were most sensitive to it among those tested.