Biology, Physiology, and Pollen Expression of ACCase Resistance in Johnsongrass (Sorghum halepense).

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Title: Biology, Physiology, and Pollen Expression of ACCase Resistance in Johnsongrass (Sorghum halepense).
Author: Burke, Ian Cristofer
Advisors: Dr. Alan C. York, Committee Member
Dr. James D. Burton, Committee Member
Dr. John W. Wilcut, Committee Chair
Dr. James Holland, Committee Member
Abstract: Greenhouse dose-response experiments were conducted on a biotype of johnsongrass from Washington County, Mississippi to determine the level of purported resistance to the aryloxyphenoxy propionate herbicide fluazifop-P and cyclohexanedione herbicides clethodim and sethoxydim. Both seedling and rhizome plants were evaluated. 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. There was no difference between the resistant and susceptible biotypes in the absorption, translocation, or metabolism of 14C-clethodim in the resistant and susceptible biotypes. Specific activity of acetyl Co-A carboxylase (ACCase) from the susceptible and resistant johnsongrass biotypes (means of 0.221 and 0.223 nmol/mg protein/min, respectively) were similar. ACCase from the susceptible biotype was sensitive to clethodim, with an I50 value of 0.29 mM clethodim. ACCase from the resistant biotype was less sensitive, with an I50 value of 1.32 mM clethodim. The resultant R/S ratio for clethodim was 4.5. These results indicate that resistance to clethodim in this johnsongrass biotype resulted from an altered ACCase enzyme. The relative competitiveness and non-competitive productivity of R and S johnsongrass were assessed in greenhouse and growth chamber experiments. 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 R and S biotypes 21, 27, and 35 days after planting. The biotypes were similar in terms of plant height and leaf number. Relative crowding coefficients for above ground dry biomass similar, and a combined t-test indicated that the resistant and susceptible biotype did not differ for above ground dry biomass (tlof=0.54, 1.3; P=0.38, 0.23, respectively). There does not appear to be a fitness penalty associated with the resistance. A seedling bioassay was developed for the determination of resistance to clethodim and fluazifop-P in johnsongrass. The assay was based on differences in the coleoptile length of R and S seedlings exposed to clethodim and fluazifop-P in petri dishes for 5 d. A bioassay concentration of 0.09 mg/L clethodim and 0.18 mg/L fluazifiop-P where chosen as discriminant based on rate responses of each biotype to increasing herbicide dose. At 5 DAT, the R:S ratio for clethodim was 18.7, and the R:S ratio for fluazifop was 35.4. A study was conducted to determine the nuclear state and develop a suitable medium and culture method for in vitro germination of johnsongrass pollen. Johnsongrass pollen was trinucleate, and in vitro tests for pollen viability using Alexander's stain and a fluorochromatic reaction method indicated johnsongrass pollen was viable (92.6-98.4%). A factorial treatment of four concentrations of sucrose, two concentrations of boric acid, and two concentrations of calcium nitrate was used to determine the optimum pollen germination media. The factorial study was conducted using three different cultural methods: suspension culture, agar culture, and cellophane membrane culture. Germination was highest in a suspension culture with media containing 0.3 M sucrose, 2.43 mM boric acid, and 3 mM calcium nitrate. In a second study, pollen germination using the above media was 78.9% when harvested from flowers just before anthesis. Three studies were conducted to develop pollen tests for the screening of ACCase target-site resistance in a biotype of johnsongrass using the developed germination media. Pollen from the susceptible biotype of johnsongrass was strongly inhibited by increasing concentrations of clethodim, with a GR50 of 25.8 (standard error of ±0.6) mM and GR50 of 16.4 (standard error of ±1.7) mM clethodim by visual assessment and spectrophotometric assessment, respectively. Minimum R/S values were >3.9 by visual assessment and >6.1 by spectrophotometric assessment. Both assessment methods differentiated the susceptible and resistant biotypes, and ACCase target-site resistance is expressed in johnsongrass pollen.
Date: 2005-10-20
Degree: PhD
Discipline: Crop Science

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