Effects of Temperature and Chemical Additives on the Response of Transgenic Herbicide-Resistant Soybeans to Glufosinate and Glyphosate Applications

Abstract

The influence of temperature and chemical additives on the response of transgenic crops to herbicide treatments was investigated using soybeans engineered with either a metabolism-based resistance (Liberty-Link) or an altered target site resistance (Roundup-Ready). Transgenic soybeans at the V1 stage were grown in chambers with constant temperature settings of 15, 25, or 35°C. Liberty-Link (LL) soybeans were treated with glufosinate rates ranging from 0.25 to 2.0 kg/ha, whereas Roundup-Ready (RR) soybeans were treated with glyphosate rates ranging from 0.5 to 4.0 kg/ha. Chlorophyll measurements revealed a dose-dependent loss of chlorophyll in the first trifoliolate of glufosinate-treated LL soybeans which was greater at 15°C than at 25 or 35°C. Conversely, chlorophyll loss in the terminal trifoliolate of glyphosate-treated RR soybeans was greater at 35°C than at 15 or 25°C. The absorption, translocation, and metabolism of [14C]glufosinate in LL soybeans and [14C]glyphosate in RR soybeans were examined in order to explain the observed injury from these herbicides at 15 and 35°C, respectively. Absorption of foliar-applied [14C]glufosinate by LL soybeans was significantly greater at 25°C than at 15°C at 3, 12, 24, and 48 h after treatment (HAT). Translocation of [14C]glufosinate was similar in LL soybeans grown at 15 and 25°C at all times except 48 HAT. The metabolic conversion of [14C]glufosinate to the [14C]acetyl-glufosinate metabolite at 3 and 12 HAT was slower in LL soybeans grown at 15°C than those grown at 25°C. After 12 HAT, glufosinate metabolism was similar at the two temperature settings. Foliar absorption of [14C]glyphosate by RR soybeans grown at 15 or 35°C was similar at 1, 3, 5, and 7 days after treatment (DAT). RR soybeans grown at 35°C translocated more radioactivity to shoots and leaves above the treated leaf, while those kept at 15°C translocated more herbicide to shoots and leaves below the treated leaf. Treatment with 5% (w/v) of ammonium sulfate (AMS) increased glufosinate absorption in LL soybeans but did not affect its translocation and metabolism. Treatment with 3% (v/v) of pelargonic acid (PA) did not affect glufosinate absorption, translocation, and metabolism. AMS and PA treatments reduced glyphosate absorption in RR soybeans but had no effect on its translocation. These results suggest that glufosinate injury to LL soybeans at 15°C may be due to a slower rate of glufosinate metabolism under cooler conditions. Glyphosate injury to RR soybeans at 35°C may result from an increased translocation of the herbicide to new meristematic areas and could be due to secondary effects caused by glyphosate.