Mode of resistance of triazine-resistant annual bluegrass (Poa annua)

Two previously identified triazine-resistant annual bluegrass (Poa annua) b iotypes in Mississippi were evaluated to determine the level as well as the mode of resistance compared to a triazine-susceptible biotype. Annual blue grass seeds were collected at two sites (Meridian and Tupelo, MS) from fiel d plots previously treated with 11.2 kg ai/ha simazine. Seeds were planted on agar containing simazine (0, 0.001, 0.01, 0.1, 1, 10, or 100 mM) or diur on (0, 0.01, 0.1, 1, 10, 100, or 1,000 mu M), placed in the growth chamber with an 8 h daylength and alternating temperatures of 15 and 20 C, and allo wed to grow for 3 wk, after which fresh weight was determined. Data were su bjected to regression analyses and the amount of simazine or diuron require d to reduce fresh weight to 50% of the untreated plants was determined. The se data indicated a greater than 1,000-fold level of resistance to simazine with either biotype. Diuron concentrations of 5.0, 3.0, and 3.2 mu M were required to reduce fresh weight to 50% of the untreated for the triazine-su sceptible, Meridian, and Tupelo biotypes, respectively. Sequencing a portio n of the chloroplast psbA gene revealed a serine 264 to glycine mutation, w hich reduced the affinity of the triazine herbicides for the Q(B)-binding n iche on the D1 protein in photosystem II.