Temperature Effects on Glyphosate Resistance in California Populations of Junglerice

This study determined the effect of glyphosate on a glyphosate-resistant (GR) and glyphosate-susceptible (GS) biotype of junglerice (Echinochloa colona L.). Potted plants at the 4- to 5-leaf stage were acclimatized for 72 h in growth chambers programmed for day/night temperatures of 15/10, 25/20, and 35/30 degrees C. On the fourth day, three plants of each biotype were treated with glyphosate at 840 g ae ha(-1) and immediately placed back in the respective growth chambers and grown for seven additional days. Three non-treated plants of each biotype were used as controls for each temperature regime. On the eighth day after glyphosate treatment, the plants were moved to a greenhouse set at 22 degrees C, grown for 21 d, and aboveground dry weights recorded after evaluating phytotoxicity. None of the treated GS plants survived at any of the temperatures while all the non-treated plants grew well at all temperatures. All treated and non-treated GR plants showed no phytotoxicity in the 25/20 and 35/30 degrees C temperature regimes. However, all the treated GR plants subjected to 15/10 degrees C died. The aboveground biomass of the non-treated GR plants was reduced under cooler temperatures but was unaffected in the GS plants. Therefore, this study showed that GR junglerice was susceptible to glyphosate when grown under 15/10 degrees C suggesting that the prevalence of GR plants in the fields could be reduced if glyphosate applications were made in spring when temperatures were at or below 15 degrees C when the plants were just emerging or had developed less than six leaves.