This study documents the first case of triazine resistance in wild radish and the resistance mechanism involved. The high survival (57 to 97%) of the resistant (R) biotype progeny plants treated at a rate four times higher than the commonly recommended rate of simazine or atrazine clearly established that the R biotype plants were resistant to triazines. All the plants of the susceptible (S) biotype plants were killed when treated at half the commonly recommended rate of atrazine (0.5 kg/ha) or simazine (0.25 kg/ha). The dry weight of the S biotype was reduced by 89 to 96% at the commonly recommended rate of atrazine or simazine, while the dry weight of the R biotype plants was reduced by only 36 to 54% even when treated at a rate four times higher than the commonly recommended rate of atrazine or simazine. The growth-reduction–ratio values indicated that the R biotype progeny plants were 105 and 159 times more resistant to atrazine and simazine, respectively, than the S biotype plants. Leaf chlorophyll fluorescence yield was reduced by 97% in the S biotype 24 h after application of triazine compared with only 9% reduction in the R biotype, indicating that the resistance mechanism involved is target-site based. The R biotype was effectively controlled by herbicides of different modes of action.
Nomenclature: Atrazine; simazine; triazine; wild radish, Raphanus raphanistrum L. #3 RAPRA.
Additional index words: Dry weight, leaf chlorophyll fluorescence.
Abbreviations: ALS, acetolactate synthase; DAE, days after emergence; GR50 ratio, the ratio of the herbicide rate required to inhibit the growth of resistant biotype progeny plants by 50% to the rate required to inhibit the growth of S biotype plants by 50%; PS II, photosystem II; R, resistant (biotype); S, susceptible (biotype); SE, standard error; TT, triazine tolerant; WA, Western Australia.