| Experiments were conducted on a biotype of johnsongrass [Sorghum halepense (L.) Pers.] from Washington County, Mississippi, to characterize resistance to the aryloxyphenoxy propionate herbicide fluazifop-P and cyclohexanedione herbicides clethodim and sethoxydim (acetyl coenzyme-A [ACCase, EC 6.4.1.2] inhibitors), determine if a fitness penalty exists, and evaluate the expression of ACCase resistance in pollen. Resistant/susceptible ratios (R/S) were 11.0, 5.7, and 5.5 for clethodim, fluazifop-P, and sethoxydim, respectively, for seedling plants. R/S ratios were 15.6, 22.7, and 22.3 for clethodim, fluazifop-P, and sethoxydim, respectively, for rhizome plants. ACCase from the susceptible biotype had an I50 value of 0.29 muM clethodim. ACCase from the resistant biotype had an I50 value of 1.32 muM clethodim. The resultant R/S ratio for clethodim was 4.5. Resistance to clethodim in this johnsongrass biotype resulted from an altered ACCase enzyme. When grown in noncompetitive conditions in growth chamber experiments, photosynthetic rate, net assimilation rate, leaf number, leaf area, specific leaf area, leaf dry biomass, and shoot dry biomass were similar for resistant and susceptible biotypes 21, 27, and 35 days after planting. The biotypes were similar for plant height and leaf number. Relative crowding coefficients for above ground dry biomass were similar, and a t-test indicated that the resistant and susceptible biotype did not differ for above ground dry biomass (t lof=0.54, 1.3; P=0.38, 0.23, respectively). Johnsongrass pollen was trinucleate, and in vitro tests for pollen viability indicated johnsongrass pollen was viable (92.6-98.4%). Germination was highest in a suspension culture with media containing 0.3 M sucrose, 2.43 mM boric acid, and 3 mM calcium nitrate. Pollen germination using the above media was 78.9% when harvested from flowers just before anthesis. Pollen from the susceptible biotype of johnsongrass was strongly inhibited by increasing concentrations of clethodim, with a GR50 of 25.8 muM and a GR50 of 16.4 muM clethodim by visual and spectrophotometric assessment, respectively. Minimum R/S values were >3.9 and >6.1 by visual and spectrophotometric assessment, respectively. ACCase target-site resistance is expressed in johnsongrass pollen. |
