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Metabolism and Enzymology

Glyphosate Effects on Carbon Assimilation and Gas Exchange in Sugar Beet Leaves ¹

Department of Biology, University of Dayton, Dayton, Ohio 45469-0001

The mechanism responsible for the inhibition of net carbon exchange (NCE) which was reported previously (DR Geiger et al. 1986 Plant Physiol 82: 468-472) was investigated by applying glyphosate [N-(phosphonomethyl)glycine] to exporting leaves of sugar beet (Beta vulgaris L.). Leaf internal CO₂ concentration (C_i) remained constant despite decreases in stomatal conductance and NCE following glyphosate treatment, indicating that the cause of the inhibition was a slowing of carbon assimilation rather than decreased conductance of CO₂. Throughout a range of CO₂ concentrations, NCE rate at a given C_i declined gradually, with the time-series of response curves remaining parallel. Gas exchange measurements revealed that disruption of chloroplast carbon metabolism was an early and important factor in mediating these glyphosate effects, perhaps by slowing the rate of ribulose bisphosphate regeneration. An increase in the CO₂ compensation point accompanied the decrease in NCE and this increase was hastened by stepwise lowering of the ambient CO₂ concentration. Eventually the CO₂ compensation point approached the CO₂ level of air and the difference between internal and external CO₂ concentrations decreased. In control and in glyphosate-treated plants, both carbon assimilation and photorespiration at atmospheric CO₂ level were inhibited to a similar extent of air level of O₂. Maintaining leaves in low O₂ concentration did not prevent the decline in NCE rate.

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