Uptake, Translocation, and Metabolism of Thiobencarb in Two Lettuce,Lactuca sativa, Cultivars

Abstract

Two lettuce cultivars exhibiting differential levels of tolerance to thiobencarb in soil and nutrient solution assays were examined. Seedlings of ‘Dark Green Boston’ (BOS), a susceptible cultivar, were found to show significant inhibitions in foliar growth compared to the tolerant ‘Great Lakes 366’ (GLA). Reductions of 57% occurred in BOS leaf dry weights at rates of 3 μM thiobencarb as soon as 4 days after treatment. In addition, growth abnormalities including fused leaves were observed in the BOS cultivar, indicating inhibition early in leaf development at the meristem. Twenty-nine and 22% of applied14C-thiobencarb was absorbed from nutrient solution by BOS and GLA, respectively. This difference is probably due to BOS having a 50% greater root system at the time of treatment. Greater absorption and accumulation of radioactivity in the leaves, as well as significantly greater amounts of parent14C-thiobencarb in the foliage of BOS compared to GLA (30 and 19%, respectively) may account for the selectivity observed. Metabolism of14C-thiobencarb occurred within 1 day in both cultivars, with the apparent production of herbicide conjugates accounting for more than 90% of the extracted radiolabel 12 days after treatment. A thiobencarb sulfoxide metabolite was not identified in these studies, indicating sulfoxide production is not a mechanism of selectivity in lettuce.