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

Abstract

I Application of a 17-millimolar solution of glyphosate (GLP) to sugar-beet (Beta vulgans 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-applica-tion. 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. Glyphosate, N-[phosphonomethyl] glycine, is widely used commercially as a nonselective, post-emergence herbicide. Ap-plication ofGLP2 to green tissue resulted in the accumulation of shikimic acid (8) and a decrease in aromatic amino acids (15). These effects can be attributed to the direct inhibition of 5-enolpyruvylshikimate 3-phosphate synthase by GLP (1, 19) and the indirect effect of GLP on feedback inhibition of 3-deoxy-D-arabino-heptonate phosphate synthase by GLP (16). Glyphosate application, in some cases, has been shown to decrease photo-synthesis rate, but generally only a number of days following application [see review by Cole (3)]. However, in bean (23) and sugarbeet (8, 9), simultaneous decreases in photosynthesis rate and leaf conductance were observed as early as 4 h after GLP application. The mechanism of the GLP-induced inhibition of leaf NCE rate in sugar beet has been shown to be mediated by an inhibition of photosynthetic carbon metabolism (9). Previous reports (8, 11) from this laboratory have also shown that, in addition to a decreased NCE rate, GLP also induced a nearly complete inhibition of starch synthesis but had no immediate ' Supported by grants from Monsanto Agricultural Products Co. and National Science Foundation, grant DMB-8303957(DRG) I Abbreviations: GLP, glyphosate; RuBP, ribulose bisphosphate car-boxylase; PGA, 3-phosphoglyceric acid; rubisco, ribulose bisphosphate carboxylase/oxygenase; Ci, internal leaf CO2 concentration; NCE, net