Objective  Effects of the herbicide, tribenuron-methylon, in varied concentrations on the survival rate, chlorophyll content, and agronomic traits of Brassica napus L. were studied to determine the appropriate dosage for field application.

  Method  In a pot experiment, seedlings of B. napus polima cytoplasmic male sterility (polCMS) restorer lines, including 18Z363, M342, 2350C, and the hybrid (LDX95-58A×18Z363), at 4 to 6 leaves stage were sprayed with tribenuron-methylon solutions in varied concentrations. Chlorophyll contents in leaves of the plants were measured 7d after treatments using a SPAD-502 chlorophyll meter. Survival rates of the seedlings were determined 14d after the spraying. In the field, agronomic traits of the mature plants, which had been treated at 4 to 6 leaves stage, were observed.

  Result  Overall, the leaf SPAD decreased with increasing herbicide dosage applied on the plants. When the application dosage was greater than or equal to 18.0 g·hm⁻² , it declined significantly in the herbicide-susceptible line 2350C reaching a low level, but remained high in the resistant lines 18Z363, M342, and the hybrid. Meanwhile, all 2350C seedlings died in 2 weeks, whereas, more than 88.9% of 18Z363, M342 or the hybrid survived the treatments. Once the herbicide application was beyond 18.0 g·hm⁻², even the resistant species began to show ill-effects in varying degrees on the agronomic traits.

  Conclusion  The results appeared that the upper limit of tribenuron-methyl usage around the restorer line18Z363 and its hybrids was 18.0 g·hm⁻².