Bipyridyliums: Paraquat
I. General Information
A. Discovered by Dyestuffs Division (1958; methyl viologen dyes), ICI Ltd., UK and first
used as a plant growth regulator in 1959
B. Non-selective herbicide: noncrop, pasture renovation, pasture establishment, orchards,
preemergence (e.g. early preplant; no-till), directed postemergence; crop desiccant and
defoliant (cotton, potatoes), soybean harvest aid
B. Use rates of 0.25-1 lb. ai/acre
C. Paraquat: Gramoxone, Cyclone; ICI Americas; diquat (aquatic weed control)
D. Chemistry
1. Herbicidal activity soley dependent on cation. Associated anion not influential,
largely dissociated in water solutions
2. Paraquat:
II. Physiology in plants
A. Mode of Bipyridilium Action
1. In plant cells paraquat undergoes a unique reaction: facile one- electron reduction to
a stable free radical in aqueous solution
2. It forms this stable free radical by Intercepting and accepting electrons from
photosynthetic electron transport in PSI (at ferredoxin; captures electron and oxidizes
NADPH) and transfers them to O2
3. The site of this electron shunting can be seen in these diagrams of photosynthetic
electron transport:
4. Once paraquat forms this free radical it is readily re-oxidized by atmospheric oxygen,
regenerating the paraquat molecule. At the same time it liberates short-lived but very
active, toxic, free radicals such as superoxide (O2-) and hydroxyl (OH*) and peroxide
(H2O2) within the chloroplasts of plant cells
5. These reactions can be summarized:
6. Paraquat acts fastest in the light (electron transport generating many electrons to
steal), but also acts slowly in the dark
7. A catalytic cycle is created in plant tissue giving the paraquat molecule high activity
at low doses in destroying plant tissue. Paraquat is reduced by PSI followed by rapid
oxidation by O2. This interception of electrons doesn't permanently reduce paraquat,
allowing it to be reoxidized over and over again as long as there is a source of new
electron coming from electron transport
B. Mode of Lethality
1. Loss of photosynthesis in affected tissue
2. Destroys unsaturated fatty acids in thylakoids and other cell membranes near site of
free radical production
3. Free radical damage to cells, leaves leading to chlorosis, necrosis, loss of plant
tissue insufficient to maintain growth
C. Uptake and Movement in plants
1. Very rapid uptake by plant leaves; "rainfast"
2. Limited movement and translocation from site of foliar or shoot contact
D. Basis of Selective Toxic Activity between Plant Species
1. Nonselective; no metabolism in plants
2. Physical placement selectivity: directed postemergence spray application
3. Paraquat resistant weeds:
a. Appeared first in no-till and orchard situations where PQ used as sole weed control
several times a year, over several years
b. Poa annua in UK in 1978; continuous usage selected for R types; localized damage
but recovered, spots not spread
c. Paraquat resistance mechanism: not clear which mechanism is it: sequestration or
metabolism; probably both
1) Conyza spp.
-Not due to decreased absorption of PQ in R biotype
-No altered site of action
-Mechanism 1, metabolism: superoxide and other toxic forms of oxygen are rapidly
detoxified
-Mechanism 2: compartmentalization away from chloroplast 2) Perennial ryegrass, italian
ryegrass, 1968: detoxification of superoxide free radical by superoxide dismutase,
catalase, peroxidase
III. Fate in the Environment
A. Soil
1. Bound very tightly to (cation to negatively charged soil) soil colloids; unavailable
for biological activity
2. Some microbial breakdown
3. Very persistent when bound to soils
B. Water: Highly polar, water soluable compound
C. Air: Photodegrades
D. Animal toxicology
1. High acute user toxicity if swallowed, contact of skin or eyes, inhaled; toxicity
expressed in lung damage primarily (even if taken into body at different site)
2. Some references to effects on substantia nigrum (brain cells producing chemical
dopamine) in human brains. MPTP induces Parkinson's disease-like symptoms by destroying
substantia nigrum . Brain changes MPTP into a compound that is chemically similar to
herbicide cyperquat
3. Used to control columnaris, a myxobacterial disease of fish
IV. Plant Injury Symptomology
Paraquat Injury Slides
1 rye leaf damage
2 rye leaf damage
3 barley leaf damage: showing no t-loc, injury above ground of spray, green on foliage
unexposed at time of spray
4 soybeans necrosis
5 corn necrosis
6 corn fungal leaf spot necrosis (several organisms)
7 rapeseed leaf injury; foxtail emerged after spray uninjured: non-residual