FAQs - Technical

A. Paraquat acts in the presence of light to desiccate the green parts of all plants with which it comes into contact. After application, penetration through the leaf surface occurs almost immediately. This absorption is increased by high light intensity, humidity, and by the specific adjuvant system usually built into the formulation, which ensures good spray retention and wetting of target foliage. Tank mix adjuvants are required if not built into the formulation. The site of action for paraquat is in the chloroplast. Chloroplasts contain the photosynthetic systems of green plants, which absorb the light energy used to produce sugars. Paraquat is known to act on the photosynthetic membrane system called photosystem I, which produces free electrons to drive photosynthesis. The free electrons from photosystem I react with the paraquat ion to give the free radical form. Oxygen rapidly reconverts this free radical and in that process produces super oxides. Chemically highly reactive, the super oxides attack unsaturated membrane fatty acids, rapidly opening up and disintegrating the cell membranes and tissues. The paraquat ion/free radical process then recycles producing further quantities of super oxide until the supply of free electrons ceases. Visible wilting of treated plants is apparent within hours under warm, bright conditions, but may take longer under dull, cool conditions. This is quickly followed by the appearance of brown, desiccated or chlorotic tissue. Light, oxygen and chlorophyll are therefore all required for the rapid and characteristic herbicidal effects of paraquat. It is the rupturing of the cell membranes allowing water to escape from the plant material that leads to the rapid desiccation of the foliage.
A. Paraquat is a broad spectrum non-selective

Description

A chemical product used for eliminating all types of weeds (annual and perennial grasses and broadleaved weeds).

Authoritative On-line References and Resources

http://www.weeds.iastate.edu/ An invaluable source of contemporary information about herbicides and weeds from Iowa State University.
herbicide. It belongs to the Bipyridylium family of herbicides. Pure paraquat salts are white and the technical products, yellow. They are crystalline, odorless, hygroscopic powders. Paraquat is slightly soluble in alcohol and practically insoluble in organic solvents. Paraquat is non-explosive and non-flammable in aqueous formulations. It is corrosive to metals and incompatible with alkylarylsulfonate wetting agents. It is stable in acid or neutral solutions, but is readily hydrolysed by alkali.
Structural formula Paraquat is a strong cation and stays where applied
Description White crystalline solid like sucrose
Solubility Very soluble in water. Not soluble in fat like sodium chloride
Vapor pressure Negligible, below 1 x 10-9 mm Hg like copper coins
Toxicity (45.6% technical) Inhalation not possible. Oral LD50 (technical material in the rat) = 283mg/kg like gasoline

Molecular Structure Paraquat (1,1'-dimethyl-4,4'-bipyridylium dichloride)
 Molecular structure of paraquat (1,1'-dimethyl-4,4'-bipyridylium dichloride)
A. Yes. Like many other herbicides, some resistance to paraquat has been identified.
However the rapid development of crops resistant to glyphosate (another common herbicide) is stimulating the demand for paraquat as we see more evidence of glyphosate-tolerant or -resistant weeds. Paraquat is an increasingly important part of Integrated Weed Management

Description

A decision support system for crop protection which focuses on long-term prevention or suppression of pest problems with minimum impact on human health, the environment, and non-target organisms. IPM takes into consideration all available pest control techniques and tactics (cultural, mechanical, biological, chemical). IPM emphasizes the growth of healthy crops for better productivity with the least possible disruption to agroecosystems.

Authoritative On-line References and Resources

http://www.ipmcenters.org "The USDA's National Site for the Regional IPM Centers' Information System provides information about US commodities, pests and pest management practices, people and issues."
(IWM) programs associated with glyphosate-tolerant crops and in other situations where glyphosate has been intensively used for long periods of time.
Surveys exist to help farmers and advisors understand the extent of herbicide resistance and plan coping strategies. Examples of these surveys can be found here: http://www.weedscience.org/
http://www.resistancefighter.com/
A. No. There are a number of reasons why herbicide tolerance to paraquat has not been commercialized. One key reason is that the mode of action of paraquat is so fast that it is technically difficult to achieve effective resistance. Also, where glyphosate resistant crops are introduced there is always the risk that herbicide-resistant plants emerge as weeds in successive crops and reduce the effectiveness of weed control. Therefore, the major manufacturer of paraquat, Syngenta, made the decision not to introduce transgenic paraquat crops to ensures that paraquat remains a highly effective tool (pre-plant, burndown herbicide) to manage glyphosate-resistant weeds.
A. Amazingly, in over 40 years of research, no other herbicide has been found which offers the same unique benefits of paraquat. Therefore, there is no effective alternative to paraquat. As a result, demand for paraquat continues to be driven by farmers who recognize the unique benefits of paraquat’s rapid, contact action, rainfastness and excellent efficacy.