Effect of Glyphosate on Growth, Chlorophyll, and Nodulation in Glyphosate-Resistant and Susceptible Soybean (<i>Glycine max</i>) Varieties

Abstract

Greenhouse and growth chamber experiments were conducted to examine glyphosate [isopropylamine salt of N-(phosphono-methyl) glycine] effects on growth, chlorophyll content, nodulation, and nodule leghemoglobin content of glyphosate-resistant and susceptible soybean (Glycine max[L.] Merr.) varieties. In susceptible soybean, a single application of 0.28 kg/ha reduced chlorophyll content (49%), and shoot and root dry weight (50 and 57%, respectively) at 2 wk after treatment. In glyphosate-resistant soybean, there were no significant effects on these parameters by single application up to 1.12 kg/ha, but 2.24 kg/ha reduced shoot and root dry weight by 25 to 30%. Application of glyphosate 1.12 kg/ha, followed by sequential applications at 0.56 or 1.12 kg/ha, did not affect plant growth and chlorophyll content, but application of 2.24 kg/ha followed by sequential application of 2.24 kg/ha reduced root growth. In glyphosate-resistant soybean, an application of 1.12 kg/ha 3 wk after planting did not affect nodule number or mass, but 2.24 kg/ha reduced these parameters by 30 and 39%, respectively, compared to untreated. Leghemoglobin content of nodules was reduced (6 to 18%) by both glyphosate rates, but effects were inconsistent with rate. At post-treatment temperatures of 18/13_C (day/night), glyphosate at 1.12 kg/ha or 2.24 kg/ha did not affect chlorophyll and growth of glyphosate-resistant soybean. However, at 25/20 and 32/27_C (day/night), glyphosate at 2.24 kg/ha reduced both chlorophyll content and growth of glyphosate-resistant soybean. Overall, treatment of gly-phosate-resistant soybean with glyphosate at 1.12 had little or no effect on chlorophyll content and dry weight of shoots and roots in five of five trials. But treatment of glyphosate at 2.24 kg/ha reduced these parameters in three of five trials, suggesting potential for soybean injury at higher rates. Results showed subtle reductions of nodulation in glypho-sate-resistant soybean using label rates of glyphosate, but these effects may be of minimal consequence due to the potential of soybean to compensate after short durations of stress.