Two populations of Palmer amaranth from New Mexico have been confirmed to be resistant to glyphosate. In the present study, the molecular basis of resistance and the mode of inheritance of resistance in those populations were investigated. Quantitative real-time polymerase chain reaction analysis indicated up to an eightfold increase in genomicEPSPScopy number in glyphosate resistant plants compared with susceptible plants. The relative genomicEPSPScopy number of resistant plants was positively correlated with the relativeEPSPScDNA expression levels. Eight hours after treatment with glyphosate, the shikimate accumulation levels in resistant plants were negatively correlated with the genomicEPSPScopy numbers. Multiple sequencing of theEPSPScDNA of resistant plants did not reveal any glyphosate resistance-conferring mutations. The evaluation of F1, reciprocal F1, and F2Palmer amaranth families indicated that resistance to glyphosate does not follow a single-gene segregation pattern. Results suggest that theEPSPSamplification is the primary molecular basis of resistance in glyphosate resistant populations of Palmer amaranth from New Mexico.

Twelve populations of eastern black nightshade from different locations in Ontario are resistant to imazethapyr. This study aimed at determining the molecular basis of resistance in these populations and the activity of the resistant acetohydroxyacid synthase (AHAS) enzyme compared to that of the sensitive AHAS in response to different herbicides and branched-chain amino acid concentration. The results of partialAHASsequencing indicated that all resistant populations had a cytosine331to thymine substitution coding for an alanine205to valine substitution.In vitroAHAS enzyme assays of one resistant population showed that the specific activity of the resistant enzyme was 56% less than that of the susceptible enzyme. AHAS from the resistant population was 72-, 70-, and 64-fold less sensitive than that of the susceptible population to imazethapyr, imazamox, and primisulfuron, respectively. Furthermore, the resistant enzyme was less sensitive to feedback inhibition from branched-chain amino acids compared to the susceptible enzyme. Results confirmed that resistance in resistant populations of eastern black nightshade was conferred by target-site modification and that the Ala205Val substitution alters the kinetics and regulation of branched-chain amino acid biosynthesis.

Buckwheat (Fagropyrum esculentum Moench.), cabbage (Brassica oleracea L), and conventional and glyphosate-resistant varieties of canola (Brassica napus L.) were used to study the bases of saflufenacil and glyphosate interactions. Compared to the addition of Merge (surfactant), the addition of both Transorb (i.e., commercial product, Transorb formulation with glyphosate) and Merge increased the cuticular absorption of [(14)C] saflufenacil in cabbage plants with thick epicuticular wax layers. However, in all cases, the addition of glyphosate reduced the translocation of [(14)C]saflufenacil in glyphosate-susceptible plants, while translocation was not affected in glyphosate-resistant canola. Moreover, the phytotoxicity of saflufenacil reduced the activity of glyphosate, possibly by reducing its translocation in all plant species studied. Increased absorption of saflufenacil by the addition of Transorb (i.e., Transorb formulation with glyphosate) plus Merge appears to increase its contact activity, thus the interaction of saflufenacil and glyphosate involves two separate processes, absorption and translocation.

No abstract

The Ala 205 Val substitution in acetohydroxyacid synthase confers herbicide resistance in Solanum ptychanthum, but the effect on fitness is unknown. This study aimed to compare the germination, growth, seed production and competitiveness of resistant (R) and susceptible (S) populations under various light conditions in the field and in a controlled environment. Under optimal light conditions of high irradiance and high red ⁄ far-red, the S populations produced more seeds compared with R populations, despite no difference in germination and above-ground vegetative productivity. Any reduction in light intensity and red ⁄ far-red diminished the repro-ductive ability of both R and S populations. This effect was more pronounced for S, so that, under sub-optimal conditions, there were no differences between the two types. Replacement series with three ratios of R:S (16:0, 8:8 and 0:16) showed no competitive difference between the two types. Under optimal light conditions, the S populations always had a higher reproductive output and fitness than the R populations. This would likely cause S individuals to dominate in the absence of herbicide selection pressure, if light competition is reduced.