Distribution of glyphosate and of its target enzyme inside wheat plants

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Abstract
Enolpyruvylshikimate phosphate (EPSP) synthase activity has been measured in different organs of 1–14‐day‐old wheat seedlings growing in culture chambers. Enzyme activity was first detected after two days, and reached its maximum value at day 4. It remained almost unchanged for 14 days thereafter. During germination, one of the most important sources of enzyme was the seed scutellum. Later, the developed leaves contained 43% of the whole stock of enzyme. Although the enzyme distribution was not identical under light or in the dark, there was no evident light induction of EPSP synthase. High enzyme activity was found in the coleoptile, continuing for the whole life‐time of this organ, and which might be associated with its lignification. Five to 40% of the total enzyme was found in the roots, reaching a maximum at 14 days. At this stage, glyphosate applied to the first grown leaf was phloem‐transported to the sink organs, with no clear relationship being shown between EPSP synthase activity and herbicide quantity in each organ. At day 4, glyphosate treatment, by soaking the seedlings in the herbicide solution, was followed by an active phloem transport from the scutellum to the growing parts. The average internal concentration of radiolabelled compounds (glyphosate plus its possible derivatives) which induced wheat seedling growth decrease, was shown to reach 50 to 100 μM under our conditions.