• How does paraquat kill plants?
• Resistance to paraquat in Australia
• Resistance to paraquat outside Australia
• Products containing paraquat in Australia
• Projects in Australia working on paraquat resistance
• References

How does paraquat kill plants? 
Paraquat is rapidly absorbed into leaves, especially under conditions of high temperatures and humidity. Within the cells paraquat moves into the chloroplasts (Information on chloroplasts) where it reacts with free electrons from photosynthesis to produce molecules which destroy the plant's membranes. Sunlight is necessary for this reaction to occur. This leads to wilting and desiccation (Movies on the effect on paraquat), a process that occurs quickly under sunny warm conditions and slower under low light conditions. Translocation of paraquat is limited by the speed of damage to plant tissues. For this reason paraquat is considered a contact herbicide, however application during late afternoon will allow translocation through the plant during darkness. Paraquat is most effective when applied to seedlings as they have less capacity to reshoot when compared with larger plants. Better coverage of the plant with the herbicide also gives higher levels of control.

Resistance to paraquat in Australia
Three populations of paraquat-resistant annual ryegrass (Lolium rigidum) have been confirmed in south eastern South Australia in 2010 by glasshouse experiments. One population is also resistant to glyphosate. Glyphosate resistance evolved on an irrigation channel and subsequently moved into the paddock, where it was then selected with paraquat.

Other species have previously developed resistance to Group L herbicides in Australia. The first case being northern barley grass in 1983 (Table 1).  Small square weed was the first case of resistance to paraquat in Australia that developed outside of broadacre agriculture. All cases of resistance to paraquat are in situations with long histories of use (>15 years).

Table 1. Species that have developed resistance to paraquat in Australia

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World scene 
Outside of Australia there are 19 species with confirmed populations resistant to paraquat. These include 3 grass and 16 broadleaf species. Paraquat is widely used in tree crops and plantations around the world. The use of paraquat in no-till farming due has increased markedly in recent years due to the development weeds with resistance to glyphosate in these systems. 

Table 2. Species that have developed resistance to paraquat in other countries

Source: International Survey of Herbicide Resistance 2011

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Products containing paraquat in Australia
Below are some examples of trade names of herbicides containing paraquat on the Australian market at time of writing.
Paraquat alone - 48 products including: Gramoxone® 250 Herbicide and Nufarm Nuquat® 250 Non-Residual Knockdown Herbicide
Paraquat + diquat - 34 products including: Spray.Seed 250 Herbicide and Nufarm Revolver® Herbicide
Paraquat + amitrole - One product: Crop Care Alliance® Herbicide

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Projects in Australia working on paraquat resistance 

Risk assessment and management of paraquat resistance in the pasture seed industry 
Rural Industries Research & Development Corporation Project PRJ-006912 

Project leader: Chris Preston (University of Adelaide) 
Summary: This project is attempting to define the extent of resistance to paraquat in annual ryegrass in pasture seed production fields. A survey will be conducted in collaboration with the agronomy companies (Elders, Landmark, Cox Rural) involved in providing advice to pasture seed producers in the south east of South Australia and western Wimmera of Victoria. Seeds will be collected from fields and plants grown from this seed tested for resistance to paraquat, glyphosate and other herbicides. Seedlings will be treated with herbicides at the field rate of products and if more than 20% survive, the populations will be classed as resistant. The project will examine practices used in the pasture seed industry to determine the risks of resistance to paraquat and other herbicides. A survey of seed producers will be conducted in collaboration with the agronomy companies to look at herbicide and other practices used in the pasture seed industry to identify practices that increase resistance risks to the industry and those that decrease risks. A survey of pasture seed samples will be undertaken to determine the potential risk of resistant seed movement to other sectors. A workshop will be held with agronomists, growers and others in the pasture seed industry to deliver the outcomes of the research and identify strategies for better managing resistance.

Understanding and Management of Resistance to Group M, Group L and Group I Herbicides 
GRDC Project No: UA00124 

Project Leader: Chris Preston, University of Adelaide 
Summary: This project will develop better understanding of resistance to glyphosate, paraquat and Group I herbicides to better inform weed management. The project will develop a range of tools for farm advisors to improve their confidence in decision making with respect to reducing the risk of developing resistance to glyphosate, Group I and paraquat. These will include risk assessments, case studies and scenario exploring tools. The project will investigate the potential for alternatives to these herbicides, concentrating on knockdown uses and other herbicides for Brassica weeds, which may prove useful in Australian agriculture. Outcomes will be discussed with commercial providers to explore the potential for future herbicide registrations. The project will establish farm advisor learning groups to work on the application of the research in local areas where resistance is already a major problem and to improve adoption of research from this and other projects. The outcomes of the project will be primarily aimed at farm advisors, but will also be of benefit to grain growers and others in the industry.

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Internet Links
Paraquat Information Centre (Syngenta) - http://paraquat.com/  
International Survey of Herbicide Resistance - http://www.weedscience.org/In.asp 

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References

Busi, R., Powles, S.B. (2011) Reduced sensitivity to paraquat evolves under selection with low glyphosate doses in Lolium rigidum. Agronomy for Sustainable Development, 31, 525-531.

Yu, Q., Huang, S., Powles, S.B. (2010)  Direct measurement of paraquat in leaf protoplasts indicates vacuolar paraquat sequestration as a resistance mechanism in Lolium rigidum. Pesticide Biochemistry and Physiology, 98, 104-109.

Yu, Q., Han, H., Nguyen L., Forster, J.W., Powles, S.B. (2009) Paraquat resistance in a Lolium rigidum population is governed by one major nuclear gene.  Theoretical & Applied Genetics, 118, 1601-1608.

Preston, C., Soar, C.J., Hidayat, I., Greenfield, K. M., Powles, S.B. (2005) Differential translocation of paraquat in paraquat-resistant populations of Hordeum leporinum. Weed Research, 45, 289-295.

Soar, C.J., Karotam, J., Preston, C., Powles, S.B. (2004) Polyamines can inhibit paraquat toxicity and translocation in the broadleaf weed Arctotheca calendula. Pesticide Biochemistry and Physiology, 80: 94-105.

Yu, Q., Cairns, A., Powles, S.B. (2004) Paraquat resistance in a population of Lolium rigidum. Functional Plant Biology. 31: 247-254.

Soar, C.J., Karotam, J., Preston, C., Powles, S.B. (2003) Reduced paraquat translocation in paraquat resistant Arctotheca calendula (L.) Levyns is a consequence of the primary resistance mechanism not the cause. Pesticide Biochemistry & Physiology, 76, 91-98.

Alizadeh, H.M, Preston, C., Powles, S.B. (1998) Paraquat resistant biotypes of Hordeum glaucum from zero tillage wheat.  Weed Research. 38, 139-142.

Purba, E., Preston, C., Powles, S.B. (1996) Growth and competitiveness of paraquat-resistant and susceptible biotypes of Hordeum leporinum Link. Weed Research. 36: 311-317.

Purba, E., Preston, C., Powles, S. B. (1995)  The mechanism of resistance to paraquat is strongly temperature dependent in resistant Hordeum leporinum and Hordeum glaucum.  Planta. 196: 464-468.

Preston. C., Balachandran, S., Powles, S. B. (1994)  Investigations of mechanisms of resistance to bipyridyl herbicides in Arctotheca calendula (L) Levyns.  Plant Cell & Env. 17: 1113-1123.

Purba, E., Preston, C., Powles, S.B. (1993) Paraquat resistance in a biotype of Vulpia bromoides (L) S.F. Gray.  Weed Research. 33, 409-413.

Purba, E., Preston, C., Powles, S.B. (1993) Inheritance of bipyridyl herbicide resistance in Arctotheca calendula and Hordeum leporinum. Theoretical & Applied Genetics.87,  598-602

Preston, C., Holtum, J.A.M., Powles, S.B. (1992) On the mechanism of resistance to paraquat in Hordeum glaucum and H. leporinum. Delayed inhibition of photosynthetic O2 evolution after paraquat application. Plant Physiol. 100, 630-636.

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Updated: 25/01/2012

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