Effects of Glyphosate on Metabolism of Phenolic Compounds

Abstract

AbstractAmounts of extractable phenylalanine ammonia‐lyase (PAL; E.C. 4.3.1.5) activity increased in the axes of 3–day‐old, dark‐grown soybean seedlings [Glycine max (L.) Merr.] shortly after the seedlings were transferred to glyphosate [N‐(phosphonomethyl)‐glycine] solutions. The stimulation of PAL activity in herbicide‐treated tissue (as compared to control tissue) was detectable as early as 12 h after treatment, whereas growth inhibition (length, fresh weight and dry weight) was not significantly affected until 24 h on a fresh‐weight basis and at 48 h on a dry‐weight basis. PAL activity increased with time (12–72 h) in herbicide‐treated axes when expressed as activity per gram fresh weight, specific activity, and on a per axis basis. PAL activity stimulation correlated positively with glyphosate concentration from 10–4 to 10–3M. PAL activity in control tissues remained nearly constant over the sampling period (12–72 h). Total alcohol‐soluble hydroxyphenolic compound levels in treated axes were not significantly different from the control at any sampling period. The total soluble amino acid pool showed a general decrease with time in glyphosate‐treated tissues. The phenylalanine pool was lowered with treatment time and the ammonia concentration (per g fr. wt. basis) was increased after treatment. No significant differences were noted in the concentration of soluble protein of glyphosate‐treated tissue when compared to controls. Visual effects (stunting, lack of secondary root formation, and necrotic areas) of glyphosate were more obvious in the root than in the hypocotyl. Analysis of various chemical constituents substantiated that other glyphosate effects were more clearly demonstrable in the root than in the hypocotyl or in the intact axis. On a per root basis glyphosate markedly increased PAL activity while reducing free phenylalanine, free tyrosine, soluble hydroxyphenolics, total free amino acids and ammonia content. The effect of glyphosate in the root was greatest on phenylalanine content, reducing it five‐fold. The results indicate that glyphosate could exert its effect through either induction of PAL activity and/or inhibition of aromatic amino acid synthesis.