To evaluate the hormetic effect of glyphosate on Echinochloa colona, two pot studies were done in the screenhouse at the Gatton Campus, the University of Queensland, Australia. Glyphosate was sprayed at the 3-4 leaf stage using different doses [(0, 5, 10, 20, 40, 80 and 800 g a.e. ha-1) and (0, 2.5, 5, 10, 20 and 800 g a.e. ha-1)] in the first and second study, respectively. In the second study, two soil moistures (adequately-watered and waterstressed), and two E. colona biotypes, glyphosate-resistant and glyphosate-susceptible, were included. In both studies, plants that were treated with glyphosate at 2.5-40 g ha-1 grew taller and produced more leaves, tillers, inflorescences and seeds than the control treatment. In the first study, 5 g ha-1 glyphosate resulted in the maximum aboveground biomass (increase of 34% to 118%) compared with the control treatment. In the second study, the adequately-watered and glyphosate low dose treatments caused an increase in all the measured growth parameters for both biotypes. For example, total dry biomass was increased by 64% and 54% at 5 g ha-1 in the adequately-watered treatments for the resistant and susceptible biotypes, respectively, compared with the control treatment. All measured traits tended to decrease with increasing water stress and the stimulative growth of low doses of glyphosate could not compensate for the water stress effect. The results of both studies showed a hormetic effect of low doses of glyphosate on E. colona biotypes and such growth stimulation was significant in the range of 5 to 10 g ha-1 glyphosate. Water availability was found to be effective in modulating the stimulatory outcomes of glyphosate-induced hormesis. No significant difference was observed between the resistant and susceptible biotypes for hormesis phenomenon. The study showed the importance of precise herbicide application for suppressing weed growth and herbicide resistance evolution.
physiological and growth responses of two Australian Echinochloa colona biotypes (glyphosateresistant and susceptible, produced from a single population) to different concentrations of carbon dioxide (co 2) (ambient ~450 ppm and elevated ~750 ppm) and soil moisture (well-watered and waterstressed) were analyzed. Elevated CO 2 and well-watered conditions resulted in E. colona plants with greater biomass, height and numbers of tillers and leaves in both biotypes; however, no significant response was observed for seed production or the amount of photosynthesis pigments with increasing co 2 at both soil moisture levels. In addition, water availability was more influential for growth than co 2 concentration. The mean shoot biomass of the susceptible biotype under elevated CO 2 and wellwatered conditions was significantly greater than the resistant biotype. Although the susceptible biotype showed more vegetative and reproductive growth than the resistant biotype, no significant difference was observed for seed production between the biotypes in the water-stressed condition. In a second experiment, different doses of glyphosate (0, 180, 360, 720 and 1440 g a.e ha −1) were applied to both biotypes grown at two soil moisture levels (well-watered and water-stressed). In the waterstressed condition, glyphosate efficacy was decreased in both biotypes. The resistant biotype in the well-watered condition had only 19% survival at 1440 g ha −1 glyphosate (double the recommended rate), but this value increased in the water-stressed condition by 62%. Our study suggests that future climate change can affect the physiological and growth processes of weeds and their responses to herbicides. Knowledge of their adapting behaviors will be critical to weed management strategies. Climate components such as radiation, temperature and precipitation have a direct impact on the agriculture industry. Therefore, climate change could affect plant biophysiological processes and productivity 1. An increase in the emission of greenhouse gasses (carbon dioxide-CO 2 , methane-CH 4 and nitrous oxide-N 2 O 4), aerosols, temperature and evaporation, as well as a decrease in precipitation will be important factors of future climate change 2. These factors will influence other variables, such as different stresses (drought, salinity, etc), changes in pests' life cycles and soils quality 3-6. The current atmospheric CO 2 concentration, recorded at Mauna Loa Observatory, Hawaii, is 411 ppm 7. Some studies have quantified a difference of 80 ppm in the CO 2 concentration between urban and suburban areas 8,9. According to emission scenarios on climate change as reported by the Intergovernmental Panel on Climate Change (IPCC), CO 2 concentrations are predicted to be in the range between 600 to 1000 ppm at the end of the 21st century 10. Increased levels of CO 2 in C 4 weeds have less beneficial photosynthetic effects compared with C 3 weeds because they already have a pathway for inhibiting photorespiration 11. Different studies assessed that under current temp...
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