6533b7dbfe1ef96bd12714b5

RESEARCH PRODUCT

Biological evidence for a 1:1 Ca2+:glyphosate association in deposit residuals on the leaf surface of barley

subject

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It has long been known that calcium ion antagonizes glyphosate, but it was not clear whether the stoichiometry of their interaction is 1:1 or 1:2. Two independent methods were used to determine which stoichiometry was the most probable. First, dose-response curves of barley (Hordeum vulgare L.) plants treated with glyphosate were determined in the presence of 0, 1.25, 2.5, 5 and 10 mM CaCl 2 . The doses of 'free' glyphosate (=not inactivated by calcium ion) were computed using the assumptions of 1:1 and 1:2 stoechiometries. The response curves were redrawn as a function of 'free' glyphosate. Analysis showed that the 1:2 hypothesis could be rejected, whereas the 1:1 hypothesis was highly probable. Second, kinetics of glyphosate penetration into barley leaves were determined in the presence of 0, 2.5 and 10 mM CaCl 2 . The concentrations of free' glyphosate were computed as above. The kinetics of glyphosate penetration at these concentrations were established and were compared to the kinetics of glyphosate penetration in the presence of CaCl 2 . Again, the 1:2 hypothesis was rejected, whereas the 1:1 hypothesis was more probable. These results strongly suggest that the stoichiometry of the Ca 2+ :glyphosate association is 1:1 in deposit residuals on the leaf surface.