Accumulation of shikimic acid: A technique for screening glyphosate efficacy

Biochemical changes in leaves of 3-week-old oil seed rape (Brassica napus L. cv. Iris) plants were determined shortly after treatment with different doses of glyphosate. Secondary effects of N-(phosphonomethyl)glycine (glyphosate) on activity of the enzyme phenylalanine ammonia-lyase was not related to dose of glyphosate by any known model within the first 48 h after spraying. Accumulation of shikimic acid was related to the dose of glyphosate by a nonlinear logistic dose-response model-as early as 5 h after spraying. Within the same period shikimic acid accumulation made it possible to distinguish between a formulation of the pure isopropylamine salt of glyphosate and formulations containing different surfactants. ED50 estimates based on accumulation of shikimic acid gave a good indication of the relative strength of the evaluated glyphosate formulations. Ranking of ED50 based on accumulation of shikimic acid was the same as achieved by visual assessment of plant death 14 days after spraying.