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Thermal dependence of enzyme function and inhibition; implications for, herbicide efficacy and tolerance

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  • The 2002 Annual Symposium of the Southern Section of the American Society of Plant Biologists took place at the University of Georgia on March 4, 2002. The theme of the colloquium was ‘Mechanisms for Regulation of Plant Metabolism’. This issue of Physiologia Plantarum contains three papers based on the invited lectures. Generous support for these lectures was provided by Pioneer Hi Bred International Inc., Syngenta Crop Protection, Inc., and the American Society of Plant Biologists.

  • Names are necessary to report factually on available data; however, the USDA neither guarantees nor warrants the standard of the product, and the use of the name by the USDA implies no approval of the product to the exclusion of others that may also be suitable.

  • Edited by T. Sherman and P. Gardeström

* e-mail: jmahan@lbk.ars.usda.gov

Abstract

Environmental temperature is a critical factor in the lives of almost all organisms. Plants experience periods of thermal stress related to seasonal patterns of temperature and periodic water deficits. Within the range of non-lethal temperatures, there are a number of thermal effects on metabolism that are a result of the thermal dependence of enzymes. The thermal dependence of enzyme kinetic parameters was used to predict that the efficacy of the herbicide pyrithiobac on Palmer amaranth would be reduced at temperatures outside a 20–34°C thermal application range. This prediction is validated in a controlled environment study described in this paper. Palmer amaranth was grown for 16 days in growth chambers with 34/18°C day/night temperature regime. Pyrithiobac was applied to plants at 18, 27 or 40°C. After 1 h at the application temperatures the plants were returned to the 34/18°C regime for 14 days and post-application biomass accumulation (efficacy) was determined. Dry weight accumulation, as a percentage of untreated controls, was 25, 2.5 and 70% for 18, 27 and 40°C application temperatures. Pyrithiobac efficacy was highest for the application within the thermal application range and significantly reduced at temperatures above and below. The validation of the earlier prediction suggests that temperature-related kinetic limitations on herbicide efficacy may also occur in plants with bioengineered herbicide resistance based on herbicide metabolism. The theoretical aspects of such thermal limitations on herbicide resistance mechanisms are discussed.

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