Delayed phytotoxicity syndrome in Louisiana rice caused by the use of thiobencarb herbicide.

摘要

Thiobencarb (TB), widely used for the control of broadleaf weeds, grasses, and sedges in rice fields, is considered safe for rice plants when used at recommended rates. TB's reductive dechlorination product, dechlorinated thiobencarb (DTB), is highly toxic to rice. TB is naturally transformed into DTB in field soils in certain areas in Japan and the United States. The resultant syndrome is called delayed phytotoxicity syndrome (DPS). This research was conducted to characterize DPS in Louisiana, to compare the toxicity of TB and DTB to rice, to determine uptake and retention rates of TB and DTB by rice, to confirm that soil microorganisms convert TB to DTB, to determine factors affecting the dechlorination of TB, and to develop methods for isolating dechlorinating microorganisms.;An in vitro bioassay developed in this study showed that seedling heights were reduced as concentrations of TB and DTB in soil increased. The effective dosage for 50% reduction in height, using Lafitte rice, was 6.6 μg/ml for TB and 0.3 μg/ml for DTB. By developing and using a gas chromatography/mass spectrometry (GC/MS) method, it was shown that DTB was not taken up preferentially by rice plants. Rice plants absorbed and accumulated more TB than DTB when exposed at equal concentrations. The toxic effects of TB and DTB to rice seedlings was additive. When rice cultivars were evaluated for sensitivity to DTB, M201 was more tolerant than Bengal, Cocodrie, and Lafitte. The conditions affecting the transformation of TB into DTB in soil were studied using a special apparatus developed to measure the redox potential of soil columns at different depths. Reductive dechlorination of TB peaked after 14 days incubation, at a position in the soil column corresponding to an Eh of −230 mV. TB was converted to DTB in vitro in a conducive soil, but not after the soil was autoclaved. Bacterial and fungal isolates from conducive soil inoculated into sterile soil suspensions, or the soil column, all failed to dechlorinate TB. Repeated attempts to isolate the organisms responsible for dechlorination of TB in Louisiana rice field soils failed.