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Paraquat resistance

How does paraquat kill plants? 
Paraquat is rapidly absorbed into leaves, especially under conditions of high temperatures and humidity. Once inside the plant cells paraquat moves into the chloroplasts (Information on chloroplasts). The paraquat then reacts with free electrons from photosynthesis to produce molecules which destroy the cell 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.

In 2013 a population of ryegrass from a Western Australian vineyard was found to be strongly resistant to both paraquat and glyphosate and can be seen in the image below.

ryegrass resistant to both Paraquat and glyphosate

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 (Mitracarpus hirtus) was the first case of resistance to paraquat in Australia that developed outside of broadacre agriculture. This has been followed by the confirmation of paraquat resistance in crowsfoot grass (Eleusine indica), cudweed (Gamochaeta pensylvanica) and blackberry nightshade (Solanum nigrum) in mixed cropping in the Bundaberg area of Queensland in late 2015.

Flaxleaf fleabane (Conyza bonariensis) is the latest species confirmed resistant to paraquat and developed in a vineyard in southern New South Wales. This fleabane population has been sprayed multiple times per year with top label rates of paraquat. Paraquat resistant fleabane is also a significant problem in South African vineyards.

All cases of resistance to paraquat are in situations with long histories of use (>15 years).

Australia currently has 10 species with populations resistant to paraquat (Table 1).

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

Species Common Name Year confirmed State Crop Resistance to other modes-of-action / herbicides
Hordeum glaucum Northern barley grass 1983 Victoria lucerne diquat (L)
Arctotheca calendula capeweed 1984 Victoria lucerne diquat (L)
Hordeum leporinum Barley grass 1988 Victoria lucerne diquat (L)
Vulpia bromoides Silver grass 1990 Victoria lucerne diquat (L)
Mitracarpus hirtus Small square weed 2007 Queensland mangoes diquat (L)
Lolium rigidum Annual ryegrass 2010 South Australia pasture seed A / M - 2 populations
Gamochaeta pensylvanica Cudweed 2015 Queensland tomatoes, sugar cane
Solanum nigrum Blackberry nightshade 2015 Queensland tomatoes, sugar cane
Eleusine indica Crowsfoot grass 2015 Queensland tomatoes, sugar cane
Conyza bonariensis flaxleaf fleabane 2016 NSW grape vines

The poster of Australian paraquat resistant weeds can be downloaded here.

Paraquat resistant weeds poster

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World scene 

Outside of Australia there are 24 species with confirmed populations resistant to paraquat. These include 6 grass and 18 broadleaf species. Paraquat is widely used in tree crops and plantations around the world. The use of paraquat in no-till farming 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


Common name



Common name


Alopecurus japonicus

Japanese foxtail grass


Hedyotis verticillata

woody borreria


Amaranthus blitum ( syn. A. lividus)

Slender / Livid amaranth


Ischaemum rugosum

Saramolla grass


Bidens pilosa

Cobblers' pegs


Landoltia punctata

Dotted duckweed


Conyza bonariensis

Flaxleaf fleabane

Egypt, Japan, South Africa, California

Lepidium virginicum

Virginia peppercress


Conyza canadensis

Canadian fleabane

Japan, Canada, USA, Belgium

Lolium rigidum

Annual ryegrass

South Africa

Conyza sumatrensis

Tall fleabane

Japan, Taiwan, Malaysia, Sri Lanka

Mazus faurieri

Asian mazus


Convolvulus arvensis

European bindweed


Mazus pumilus

Japanese mazus


Crassocephalum crepidioides



Poa annua

Winter grass

UK, Belgium

Cuphea carthagenensis



Schlerochloa dura

Hard grass


Eleusine indica

Crowsfoot grass

Malaysia, Florida, Indonesia

Solanum americium ( syn. S. ptychanthum)

American or Eastern black nightshade

Florida, Canada, New Zealand

Epilobium ciliatrum

American willowherb

Belgium, United Kingdom

Solanum nigrum

Blackberry nightshade


Erigeron philadelphicus

Philadelphia fleabane


Youngia japonica

Asiatic hawksbeard

Japan, China

Source: International Survey of Herbicide Resistance 2018

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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 - over 50 products including: Gramoxone® 360 PRO Herbicide and Nufarm Shirquat® 250 Non-Residual Knockdown Herbicide
Paraquat + diquat - 44 products including: Spray.Seed 250 Herbicide and Nufarm Revolver® Herbicide
Paraquat + amitrole - 3 products: Crop Care Alliance® Herbicide; Mission Paraglide® Herbicide and Imtrade Para-Trooper®

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Past 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) 
Final summary: http://www.glyphosateresistance.org.au/factsheets/PRJ-006912%20Fact%20Sheet.pdf

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 
Final reports - paraquat:

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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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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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How to reference this site
Preston, C.  The Australian Glyphosate Sustainability Working Group.  Online.  Internet.  .  Available  http://www.glyphosateresistance.org.au/

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