Glyphosate hormesis effects on the vegetative and reproductive development of glyphosate-susceptible and -resistant Conyza sumatrensis biotypes.

Low glyphosate doses that produce hormesis may alter the susceptibility to herbicides of weeds or enhance their propagation and dispersal. The objective of this work was to evaluate the hormetic effects of glyphosate on the vegetative, phenological and reproductive development in resistant (R) and susceptible (S) Conyza sumatrensis biotypes. The glyphosate resistance level of biotype R was 11.2-fold compared to the S biotype. Glyphosate doses <11.25 g ae ha-1 induced temporary and permanent hormetic effects for the number of leaves, plant height and dry mass accumulation up to 28 d after application in both R and S biotypes. The S biotype required 15-19% fewer thermal units at 1.4 and 2.8 g ae ha-1 glyphosate than untreated plants to reach the bolting stage. Also, this biotype had less thermal units associated with the appearance (1225 vs 1408 units) and opening (1520 vs 1765 units) of the first capitulum than the R biotype. In addition, glyphosate affected reproductive traits of both biotypes compared to their controls, increasing the number of capitulum's and seeds per plant up to 37 and 41% (at 2.8 and 0.7 g ae h-1, respectively) in the S biotype, and by 48 and 114% (both at 5.6 g ae ha-1) in the R biotype. Depending on environmental parameters, glyphosate may or may not cause hormetic effects on the vegetative and phenological development of C. sumatrenis biotypes; however, this herbicide increases the speed and fecundity of reproduction, regardless of the glyphosate susceptibility level, which can alter the population dynamics and glyphosate susceptibility of future generations.

The role of the antioxidant system and the photosynthetic behavior of paraquat-resistant Conyza sumatrensis in Brazil.

Greenhouse experiments were carried out aiming to characterize-morphologically and biochemically-resistant and susceptible plants of C. sumatrensis. Two experiments were carried out to evaluate the behavior of morphological variables such as leaf area, height, and dry biomass weight, without application of paraquat (1,1'-dimethyl-4,4'-bipyridinium dichloride). Other experiments were conducted with two rates of paraquat application (0 and 800 g a.i ha-1); physiological variables were assessed at 2, 4, and 24 h after application (HAA), and plants were collected at 4 HAA for biochemical analyses of antioxidant enzymes and cell membrane peroxidation level. Without herbicide application, paraquat-resistant populations had higher dry biomass, leaf area, liquid photosynthetic rate, carboxylation efficiency, and stomatal conductance. The recovery of the photosynthetic apparatus by resistant plants after paraquat application is rapid (16 HAA) and, in general, presents physiological improvements in terms of photosynthetic rate and carboxylation efficiency. After paraquat treatment, the antioxidant system enzymes of resistant plants showed increased activity and decreased membrane peroxidation, indicating that these enzymes play an important role in the resistance mechanism of these plants.

A Target-Site Mutation Confers Cross-Resistance to ALS-Inhibiting Herbicides in Erigeron sumatrensis from Brazil.

Cases of weed resistant to herbicides have changed the dynamics of agricultural areas in Brazil, and in recent years, Erigeron species have caused major problems to farmers in the country, mainly in relation to the ineffectiveness of herbicide treatments used. The objective of this study was to confirm the cross-resistance to ALS inhibitors in populations of Erigeron sumatrensis as well as to investigate the existence of mutations in the site of action of ALS-inhibiting herbicides. To do this, 30 populations collected in the 2016/2017 crop season were grown in a greenhouse. Dose-response (chlorimuron-ethyl and cloransulam-methyl), inhibition of cytochrome P-450 with malathion, and ALS gene sequencing experiments were carried out in the F1 generations of two fleabane populations. The results proved the cross-resistance to chlorimuron-ethyl and cloransulam-methyl herbicides applied in the post-emergence of the resistant population of E. sumatrensis. The higher activity of P450 enzymes is unlikely responsible for the resistance of the population studied. The resistance mechanism found in R was the target site mutation Pro197Ser at the ALS gene. This is the first study in Brazil to identify a target-site change as a survival mechanism in E. sumatrensis for the resistance to ALS-inhibiting herbicides.