Release of lateral buds from apical dominance by glyphosate in soybean and pea seedlings

Application of a sublethal dose of glyphosate (N-phosphonomethyl]glycine) to the seedlings of soybean (Glycine max L. Merr. cv. Evans) and pea (Pisum sativum L. cv. Alaska) promoted growth of the cotyledonary and other lateral buds. The pattern of the glyphosate-induced lateral bud growth was different from that induced by decapitation. Under the experimental condition, glyphosate did not kill the apical buds. Feeding stem sections of the seedlings with radiolabeled indole-3-acetic acid (2¹⁴C]IAA) and subsequent analysis of free 2-¹⁴C]IAA and metabolite fractions revealed that the glyphosate-treated plants had higher rates of IAA metabolism than the control plants. The treated pea plants metabolized 75% of 2-¹⁴C]IAA taken up in the 4-h incubation period compared to 46.5% for the control, an increase of 61%. The increase was small but consistent in soybean seedlings. As a result, the glyphosate-treated plants had less free IAA and ethylene than the control plants. The increase of IAA metabolism induced by glyphosate is likely to change the auxin-cytokinin balance and contribute to the release of lateral buds from apical dominance in these plants.