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|Molar mass||181.13 g·mol−1|
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Glufosinate (also known as phosphinothricin and often sold as an ammonium salt) is a broad-spectrum systemic herbicide with the chemical formula C5H15N2O4P. It irreversibly inhibits glutamine synthetase, an enzyme necessary for the production of glutamine and for ammonia detoxification. Application of glufosinate to plants leads to reduced glutamine and elevated ammonia levels in tissues, halting photosynthesis, resulting in plant death.
In the 1960s and early 1970s, scientists at University of Tübingen and at the Meiji Seika Kaisha Company independently discovered that species of Streptomyces produce a tripeptide they called bialaphos that inhibits bacteria; it consists of two alanine residues and a unique amino acid that is an analog of glutamate that they named "phosphinothricin.":90 They determined that phosphinothricin irreversibly inhibits glutamine synthetase.:90 Phosphinothricin was first synthesized by scientists at Hoechst in the 1970s as a racemic mixture; this racemic mixture is called glufosinate and is the commercially relevant version of the chemical.:91-92
In the late 1980s scientists discovered enzymes in these Streptomyces species that selectively inactivate free phosphinothricin; the gene encoding the enzyme that was isolated from Streptomyces hygroscopicus was called the "bialaphos resistance" or "bar" gene, and the gene encoding the enzyme in Streptomyces viridochromeogenes was called "phosphinothricin acetyltransferase" or "pat".:98 The two genes and their proteins have 80% homology on the DNA level and 86% amino acid homology, and are each 158 amino acids long.:98
Glufosinate is a broad-spectrum herbicide that is used to control important weeds such as morning glories, hemp sesbania (Sesbania bispinosa), Pennsylvania smartweed (Polygonum pensylvanicum) and yellow nutsedge similar to glyphosate. It is applied to young plants during early development for full effectiveness. It is sold in formulations under brands including Basta, Rely, Finale, Challenge and Liberty.
Glufosinate is typically used in three situations as an herbicide:
- directed sprays for weed control, including in genetically modified crops
- use as a crop desiccation to facilitate harvesting
Glufosinate also has shown to provide some protection against various plant diseases, as it also acts to kill fungi and bacteria on contact.
Genetically modified crops
Genetically modified crops resistant to glufosinate were created by genetically engineering the bar or pat genes from streptomyces into the relevant crop seeds.:98 In 1995 the first glufosinate-resistant crop, canola, was brought to market, and it was followed by corn in 1997, cotton in 2004, and soybeans in 2011.
Mode of action
Phosphinothricin is a glutamine synthetase inhibitor that binds to the glutamate site. Glufosinate-treated plants die due to a buildup of ammonia and corresponding decrease in pH in the thylakoid lumen, leading to the uncoupling of photophosphorylation. The uncoupling of photophosphorylation causes the production of reactive oxygen species, lipid peroxidation, and membrane destruction.
Elevated levels of ammonia are detectable within one hour after application of Phosphinothricin.
Exposure to humans in foods
As glufosinate is often used as a pre-harvest desiccant, residues can also be found in foods that humans ingest. Such foods include potatoes, peas, beans, corn, wheat, and barley. In addition, the chemical can be passed to humans through animals who are fed contaminated straw. Flour processed from wheat grain that contained traces of glufosinate was found to retain 10-100% of the chemicals' residues.
The herbicide is also persistent; it has been found to be prevalent in spinach, radishes, wheat and carrots that were planted 120 days after the treatment of the herbicide. Its persistent nature can also be observed by its half-life which varies from 3 to 70 days depending on the soil type and organic matter content. Residues can remain in frozen food for up to two years and the chemical is not easily destroyed by cooking the food item in boiling water. The EPA classifies the chemical as 'persistent' and 'mobile' based on its lack of degradation and ease of transport through soil.
There are no exposure limits established by the Occupational Safety & Health Administration or the American Conference of Governmental Industrial Hygienists. The WHO/FAO recommended acceptable daily intake (ADI) for glufosinate is 0.02 mg/kg. The European Food Safety Authority has set an ADI of 0.021 mg/kg. The Acute reference dose (ARfD) for child-bearing women is 0.021 mg/kg.
Glufosinate is a United States Environmental Protection Agency EPA registered chemical. It is also a California registered chemical. It is not banned in any country and it is not a PIC pesticide. There are no exposure limits established by OSHA or the American Conference of Governmental Industrial Hygienists.
Glufosinate is registered for use as an herbicide in Europe; it was last reviewed in 2007 and that registration was set to expire in 2018.
- Topsy Jewell for Friends of the Earth (December 1998). "Glufosinate ammonium fact sheet". Pesticides News No.42. Retrieved March 2015.
- Donn, G and Köcher, H. Inhibitors of Glutamine Synthetase. Chapter 4 in Herbicide Classes in Development: Mode of Action, Targets, Genetic Engineering, Chemistry. Eds Peter Böger, Ko Wakabayashi, Kenji Hirai. Springer Science & Business Media, 2012 ISBN 9783642594168
- "The agronomic benefits of glyphosate in Europe" (PDF). Monsanto Europe SA. February 2010.
- Duke, SO. Biotechnology: Herbicide Resistant Crops. In Encyclopedia of Agriculture and Food Systems, 2nd edition. Ed. Neal K. Van Alfen. Elsevier, 2014. ISBN 9780080931395. Page 97
- Green JM and Castle LA. Transitioning from Single to Multiple Herbicide-resistant Crops. Chapter 4 in Glyphosate Resistance in Crops and Weeds: History, Development, and Management. Editor, Vijay K. Nandula. John Wiley & Sons, 2010 ISBN 9781118043547 Page 112
- Summary of Herbicide Mechanism of Action, HRAC and WSSA
- Watts, Meriel. "Glufosinate Ammonium Monograph" (PDF). Pesticide Action Network Asia and the Pacific. Retrieved 20 April 2015.
- "Chemical Identification and Company Information : DL-Phosphinothricin, Monoammonium Salt" (PDF). Phytotechlab.com. Retrieved 2015-04-25.
- The Rotterdam Convention on Prior Informed Consent (PIC)
- European Commission. Glufosinate in EU Pesticides Database Page accessed August 7, 2015
- Bayer's site of LibertyLink crops
- Basta technical guide
- Glufosinate in the Pesticide Properties DataBase (PPDB)
- Glufosinate-ammonium in the Pesticide Properties DataBase (PPDB)
- Glufosinate-P in the Pesticide Properties DataBase (PPDB)