Glyphosate Effects on Carbon Assimilation, Ribulose Bisphosphate Carboxylase Activity, and Metabolite Levels in Sugar Beet Leaves

Application of a 17-millimolar solution of glyphosate (GLP) to sugarbeet (Beta vulgaris L.) leaves resulted in an immediate and rapid decline in the level of ribulose bisphosphate (RuBP). Phosphoglyceric acid level began to decrease about 2 hours following the decline in RuBP level. Photosynthesis rate declined linearly with RuBP level, but only when the RuBP level had decreased to about twice the RuBP carboxylase active site concentration. This occurred about 4 hours following GLP-application. At this time starch synthesis also declined abruptly. The activation state of RuBP carboxylase did not change for 8 hours following GLP application and then decreased slightly from 70 to 50% when the RuBP level fell below the RuBP carboxylase active-site concentration. Triose-phosphate, hexose-phosphate, and adenylate energy charge did not change for 8 hours following GLP-application. These data indicate that GLP induced a depletion of carbon or phosphate or both from the photosynthetic carbon reduction cycle, reducing the rate of regeneration of RuBP, photosynthesis, and starch synthesis, while having little effect upon the rate of sucrose synthesis and transport.