Glyphosate

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Glyphosate, N-phosphonomethyl glycine, is broad-spectrum herbicide, sold under the brand name Roundup. It is "the most widely used broad-spectrum herbicide on [a] global scale."[1] Some genetically engineered crops, sold as Roundup Ready Crops have their DNA altered to allow them to withstand glyphosate. Commercial formulations of glyphosate were first sold in 1974.[2]

How It Works

Glyphosate is absorbed through a plant's foliage and then transported throughout the stems, leaves, and roots of the entire plant.

"Glyphosate inhibits plant growth by inhibiting the production of essential aromatic amino acids through competitive inhibition of the enzyme enolpyruvylshikimate phosphate (EPSP) synthase. This is a key enzyme in the shikimic acid pathway for the synthesis of chorismate..., which is a precursor for the essential amino acids phenylalanine, tyrosine, and tryptophan."[3]

In other words, glyphosate prevents plants from making amino acids they need to survive. It does this by inhibiting an enzyme needed to make chorismate, a precursor to those amino acids.

A 1984 study found plants that died following treatment with glyphosate were infected with pathogenic fungi, compared to control plants not treated with glyphosate but planted in the same media that did not yield pathogenic fungi.[4] The study concluded that more research was needed but postulated that glyphosate inhibits the plant's defense mechanisms and/or increases nutrient leakage from treated plants.

Impact on Non-Target Plants

Once glyphosate travels to a plant's roots, it is "released into the rhizosphere," (the area immediately around the roots), "where it is immobilized at the soil matrix or microbially degraded.[5]

Impacts of Glyphosate Drift

At sub-lethal doses of glyphosate, such as the amounts a plant might be exposed to from spray drift, plants are still impacted. A study examined sunflowers treated with small amounts of glyphosate (to simulate spray drift) found:[6]

"In conclusion, the results presented in this study showing that glyphosate is especially inhibitory to ferric reductase complement the recently published report (Eker etal., 2006) that glyphosate exerts a strong inhibitory influence on ferric reductase activity of Fe-deficient roots and impairs the uptake and translocation of Fe in plants. These impairments could be a major reason for the increasingly observed Fe deficiency chlorosis in cropping systems associated with widespread glyphosate usage as reported for different parts of the USA (Franzen etal., 2003; Jolley etal., 2004). Such strong interference of glyphosate with root uptake and root-to-shoot transport of Fe in crop plants may represent a potential threat to human and animal nutrition because of possible reduction of Fe in edible plants parts (e.g. seed/grain)."

In other words, low doses of glyphosate equal to the amount plants are exposed to in spray drift, can result in iron deficiencies in the plants. For crops destined as animal feed or as human food, this could result in decreased dietary iron.

Glyphosate in the Environment

Although glyphosate is highly soluble in water, its tendency to bond to soils makes it unlikely to leach into groundwater or runoff "significantly." (Studies have found about 1%-2% of glyphosate may runoff in rainfall after glyphosate is applied.[7])

Glyphosate in the Soil

Glyphosate can reach the soil by washing off the foliage of plants, via spray drift, by exudation from the roots of treated plants, or by the decomposition of treated plants. However, "risks of glyphosate toxicity to non-target organisms in soils are generally considered as marginal,since glyphosate is almost instantaneously inactivated by adsorption to clay minerals and cationic binding sites of the soil matrix (Piccoloetal.,1992;Dong-Meietal.,2004), while glyphosate in the soil solution is prone to rapid microbial degradation (Giesy et al., 2000)."[8][9] In other words, glyphosate residues in the soil are not considered hazardous as it either breaks down quickly or binds to minerals that make it no longer a threat to plants. Glyphosate that biodegrades usually breaks down into carbon dioxide and ammonium (NH4+).[10] In an analysis of 47 studies, 50% of glyphosate broke down in the soil in time periods ranging from 1.2 days to 197.3 days. The arithmetic mean amount of time was 32 days and the geometric mean was 17 days.[11]

Glyphosate in Water

Although most glyphosate applied to soil does not run off into waterways, sometimes glyphosate is applied to aquatic environments directly. In flowing water, it is dissipated via "tributary dilution, dispersion, and loss through processes such as absorption to suspended particulate matter or sediments and microbial degradation."[12] The half-life of glyphosate in water has been estimated to be from 7 to 14 days.[13]

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References

  1. ↑ Tsehaye Tesfamariam, S. Bott, I. Cakmak, V. Römheld, G. Neumann, "Glyphosate in the rhizosphere – role of waiting times and different glyphosate binding forms in soils for phytoxicity to non-target plants," European Journal of Agronomy (2009), 31:126-132.
  2. ↑ John P. Giesy, Stuard Dobson, and Keith R. Solomon, 2000, "Ecotoxicological Risk Assessment for Roundup Herbicide," Rev Environ Contam Toxicol 167:35-120.
  3. ↑ John P. Giesy, Stuard Dobson, and Keith R. Solomon, 2000, "Ecotoxicological Risk Assessment for Roundup Herbicide," Rev Environ Contam Toxicol 167:35-120.
  4. ↑ Gurmukh S. Johal and James E. Rahe, "Effect of soilborne plant-pathogenic fungi on the herbicidal action of glyphosate on bean seedlings," Phytopathology (1984), 74:950-955.
  5. ↑ Tsehaye Tesfamariam, S. Bott, I. Cakmak, V. Römheld, G. Neumann, "Glyphosate in the rhizosphere – role of waiting times and different glyphosate binding forms in soils for phytoxicity to non-target plants," European Journal of Agronomy (2009), 31:126-132.
  6. ↑ Levent Ozturk, Atilla Yazici, Selim Eker, Ozgur Gokmen, Volker Römheld, and Ismail Cakmak, "Glyphosate inhibition of ferric reductase activity in iron deficient sunflower roots," New Phytologist (2008), 177:899-906.
  7. ↑ John P. Giesy, Stuard Dobson, and Keith R. Solomon, 2000, "Ecotoxicological Risk Assessment for Roundup Herbicide," Rev Environ Contam Toxicol 167:35-120.
  8. ↑ Tsehaye Tesfamariam, S. Bott, I. Cakmak, V. Römheld, G. Neumann, "Glyphosate in the rhizosphere – role of waiting times and different glyphosate binding forms in soils for phytoxicity to non-target plants," European Journal of Agronomy (2009), 31:126-132.
  9. ↑ John P. Giesy, Stuard Dobson, and Keith R. Solomon, 2000, "Ecotoxicological Risk Assessment for Roundup Herbicide," Rev Environ Contam Toxicol 167:35-120.
  10. ↑ John P. Giesy, Stuard Dobson, and Keith R. Solomon, 2000, "Ecotoxicological Risk Assessment for Roundup Herbicide," Rev Environ Contam Toxicol 167:35-120.
  11. ↑ John P. Giesy, Stuard Dobson, and Keith R. Solomon, 2000, "Ecotoxicological Risk Assessment for Roundup Herbicide," Rev Environ Contam Toxicol 167:35-120.
  12. ↑ John P. Giesy, Stuard Dobson, and Keith R. Solomon, 2000, "Ecotoxicological Risk Assessment for Roundup Herbicide," Rev Environ Contam Toxicol 167:35-120.
  13. ↑ John P. Giesy, Stuard Dobson, and Keith R. Solomon, 2000, "Ecotoxicological Risk Assessment for Roundup Herbicide," Rev Environ Contam Toxicol 167:35-120.

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