INTERACTION OF INDOLEACETIC-ACID AND GLYPHOSATE ON PHENOLIC METABOLISM IN SOYBEANS

This study examined indoleacetic acid (IAA) and 2,4-dichlorophenoxyacetic acid (2,4-D) for effects and possible interactions with glyphosate on growth, extractable phenylalanine ammonialyase (PAL, EC 4.3.1.5) activity, soluble hydroxyphenolic compound levels, and anthocyanin accumulation in intact plants. Root feeding of soybean [Glycine max (L.) Merr.] seedlings in liquid culture with IAA (10 μM) and 2,4-D (2 μM) in combination with glyphosate (0.5 mM) in the light (200 μE · m-2 · sec-1, PAR) did not reverse glyphosate's reduction of elongation or fresh weight. IAA plus glyphosate caused higher levels of hydroxyphenolics (per axis) than did glyphosate alone under light or dark growth. Glyphosate plus 2,4-D in the dark resulted in a phenolic content lower than that of glyphosate alone. Glyphosate's reduction of anthocyanin accumulation in hypocotyls was only marginally reversed by IAA (ca. 15%) and by 2,4-D (ca. 8%). IAA had little effect on extractable PAL in the light or dark, but 2,4-D was a potent inhibitor. Glyphosate increased extractable PAL activity in light- and dark-grown plants and IAA (in light) and 2,4-D (in light and dark) were antagonistic. Alterations of PAL activity, anthocyanin, and phenolic content caused by IAA and 2,4-D were not commensurate with growth reductions, suggesting that IAA (and the parameters altered by IAA) is not primarily involved in glyphosate's mode of action. © 1990.