| Metsulfuron-methyl, a sulfonylurea herbicide, possesses many properties, such as high activity, wide control range, low toxicity, low use rate. But residue time of metsulfuron-methyl was affected utterly by environmental factors . It would resulted in damage to crop in this season or behind rotation if metsulfuron-methyl was not applied rationally .The trial was purposed to provide scientific basis for metsulfuron-methyl to apply rationally in paddy fields. It was bioassayed different tolerance of 48 rice varieties or combinations against metsulfuron-methyl with selecting rice height as parameter for measure. The result showed that there was different tolerance to metsulfuron-methyl among various rice varieties or combinations. The classificatory standard of rice tolerance to metsulfuron-methyl was put forward exploringly. According to the standard, five varieties possessed strong drug tolerance(IC50>1.0 mg ?L-1), 27 varieties had moderate drug tolerance ( IC50 0.1-1.0 mg ?L-1 ) , 16 varieties was sensitive to metsulfuron-methyl (IC50< 0.1 mg ?L-1) .The test was selected acetolactate synthase (ALS, the site of metsulfuron-methyl action) as the target for researching the mechanism of drug tolerance of rice and assayed effect of metsuifuron-methyl in vitro and in vivo on rice ALS activity. The result showed that there was no differenceamong ALS activities of various rice varieties in normal condition(120.2?134.6nmol acetoin -1?mg-1 protein), ALS I50 values to metsulfuron-methyl were from 12.6 to 20.8 u g ?L"1, but the difference was not accord with the variance of plant whole tolerance. So the sensitivity of ALS was not the mode of drug tolerance of rice varieties. With the same metsulfuron-methyl concentration, ALS acthvity in vivo was far higher than ALS activity in vitro, which was accord with rice tolerance. This could explain that metsulfuron-methyl was metabolized strongly into inactive compounds, which might be the mechanism of tolerance of rice species to metsulfuron-methyl.
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