Effect of elevated temperature on growth and glyphosate susceptibility of Chloris truncata R.Br., Sonchus oleraceus L., and Conyza bonariensis (L.) Cronq.

A temperature-controlled glasshouse study was conducted to evaluate the influence of elevated temperature (eT - 34/24 +/- 2 degrees C) on the growth and glyphosate susceptibility of windmill grass (Chloris truncata R.Br.), common sowthistle (Sonchus oleraceus L.), and flaxleaf fleabane [Conyza bonariensis (L.) Cronquist]; and to determine the morpho-physiological factors involved in differential glyphosate tolerance under eT. Results showed that elevation of temperature from ambient temperature (aT - 28/20 +/- 2 degrees C) to 34/24 +/- 2 degrees C increased growth and biomass production of C. truncata. In contrast, eT suppressed growth of S. oleraceus and C. bonariensis, resulting in fewer, thicker, and smaller leaves with reduced stomatal conductivity and less total plant biomass. In terms of herbicide susceptibility, the responses to glyphosate under different temperature regimes were species- and rate-specific. Slight variations in glyphosate susceptibility were observed when sprayed at sub-lethal rates at eT. Under eT, C. truncata, S. oleraceus, and C. bonariensis required 1.5, 2.0-, and 1.6-times higher glyphosate rates, respectively, to suppress biomass by 50% compared with plants grown at aT. Depending upon the species and glyphosate rate, differences in leaf characteristics (i.e. leaf chlorophyll content, leaf area/thickness, and stomatal conductance) could have promoted or delayed glyphosate activity under eT over the period, especially at sub-lethal rates. Overall, the glyphosate efficacy was unaffected since herbicide within the recommended rates completely controlled all tested weed species under both temperatures.