The effects of N-(phosphonomethyl)glycine (glyphosate) on the regulation of carbon assimilation, metabolism, and translocation were studied in leaves of sugar beet (Beta vulgaris L., Klein E-type multigerm) under a light regimen that began with gradually increasing irradiance as generally occurs on a natural day. Soon after application, glyphosate caused a marked increase in ribulose bisphosphate and a decrease in phosphoglyceric acid. The response is most simply explained by direct inhibition of ribulose bisphosphate carboxylase activity. The extent of inhibition was small, and the carbon assimilation rate did not decrease. As predicted, photosynthesis declined within an hour after glyphosate was applied to leaves under gradually increasing light. Inhibition resulted from a decrease in ribulose bisphosphate due to depletion of carbon from the photosynthetic carbon reduction cycle. Photoinhibition, a light-dependent limitation of photosynthetic capacity, appeared to be necessary for marked glyphosate-induced inhibition of photosynthesis. As a result, photosynthesis rate increased with irradiance until it exceeded 400 micromoles per square meter per second but then declined as the light level increased beyond 500 micromoles per square meter per second. Glyphosate changed the allocation of newly fixed carbon between starch and sucrose for export. Changes in the levels of ribulose bisphosphate and phosphoglyceric acid produced important effects on the regulation of carbon assimilation and metabolism.
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