Herbicide Resistance Action Committee (HRAC) Groups - C and D
Weed Science Society of America (WSSA) Groups - 5, 6, 22
The common feature of the herbicides in this category is inhibition of the electron transport system of photosynthesis. Photosynthesis is a series of reactions involving light absorption, energy conversion, electron transfer, and a multistep enzymatic pathway that converts CO2 and water into carbohydrates. The photosynthetic process involves two phases: the light reactions, which produce O2, ATP, and NADPH; and carbon-linked reactions, which reduce CO2 to carbohydrate and consume the ATP and NADPH produced in the light reactions.
The reactions of photosynthesis occur in a specialized cell organelle called the chloroplast (Fig. 1). Chloroplasts from higher plants are surrounded by a double-membrane system consisting of an outer and inner envelope and also contain a complex internal membrane system. The internal membrane structure, known as the thylakoid membrane, contains distinct regions. Some thylakoids are organized into stacks of closely pressed membranes called grana. Others are unstacked and exposed to surrounding fluid, known as stroma. The thylakoid membranes are interconnected and inner space created in the grana stacks are known as thylakoid lumen.The two phases of photosynthesis occur in different regions of the chloroplast. The thylakoid membranes contain Photosystems I and II (PSI and PSII), which include the reaction centers responsible for converting light energy into chemical energy. PSI is found primarily in the unstacked and stroma-exposed membranes, whereas PSII is contained mostly in the stacked granal membranes. The carbon-reduction reactions occur in the stroma.
Figure 1. The plant chloroplast contains internal membranes, called thylakoids, that include stacked and unstaked membrane regions. The thylakoid membranes contain Photosystems I and II, which include the reaction centers for converting light energy into chemical energy.
Pigment molecules, such as chlorophyll, absorb light which excites the pigment. A pigment molecule becomes excited when absorption of light energy causes one of its electrons to shift from a lower-energy molecular orbit, which is closer to the pigments's atomic nuclei, to either of two more-distant, higher-energy orbitals. Energy can be lost as heat or fluorescence or it can be transferred to another molecule in close proximity to the excited molecule. The later process, called energy transfer, is an important vehicle for the movement of absorbed light energy through an array of pigment molecules. The excited molecule may also lose an electron to an electron-acceptor molecule through a charge separation event, in which the excited pigment reduces the acceptor molecule. This last mechanism, called photochemistry, converts light energy into chemical products and is central to the process of photosynthesis.
All photosynthetic organisms, including plants, contain two different reaction center complexes, PSI and PSII (Fig. 2). These photosystems contain a reaction center with a full complement of electron transfer components as well as an array of light-harvesting, or antenna, pigments. These antennae function to absorb light energy, transferring it to the reaction center, where the energy is converted into stable chemical products. About 250 chlorophyll molecules are associated with each reaction center. Carotenoids are also commonly found in the antenna. The reaction centers are part of a photosynthetic electron transfer chain that also contains a transmembrane cytochrome complex (Cyt bf); plastocyanin (PC), a water-soluble copper protein; and plastoquinone (Q), a lipid-soluble quinone. Located primarily in the thylakoids, the photosynthetic electron transport chain moves electrons from water in the thylakoid lumen to soluble redox-active compounds in the stroma (e.g., NAPD+). ADP is phosphorylated on the surface of chloroplast ATP synthase, a large stroma-exposed protein complex located in the thylakoid membranes. Light-induced proton (H+) pumping makes the thylakoid lumen acidic. The flow of protons to the stromal side leads to the synthesis of ATP. NADPH is produced by photosystem I and released into the stromal space. The ATP and NADPH produced by the light reactions are then used by the dark reactions of photosynthesis to reduce CO2 to carbohydrates.
Figure 2. The light reactions of photosynthesis.
The Z-scheme (Fig. 3) is a model representation of photosynthetic electron transport. This energy requiring reaction is made possible by the adsoption of light (hv) by photosystem II (P680) and photosystem I (P700). Reduced plastoquinone (QH2 in fig. 2, PQ in fig. 3)) formed by photosystem II feeds electrons into the cytochrome bf complex. Reduced plastocyanin (PC) carries electrons to photosystem I, which generates reduced ferrodoxin (Fdx). Ferrodoxin transfers its electrons to NADP+ to form NADPH. The protein gradient across the thylakoid membrane is generated by the reduction of NADP+ and splitting of water on opposite sides of the membrane. Electrons flowing through the Cyt bf complex also contribute to the proton gradient.
Figure 3. The Z scheme, a model of the chloroplast electron transfer chain that takes into account the differing pigment and reaction center compositions of photosystems I and II.
Several compounds specifically inhibit the chloroplast electron transport chain and, in doing so, act as herbicides. One class of inhibitors binds at the QB site on the D1 protein in PSII and prevents the reduction of QB. The triazine herbicides act at this site. Electrons become diverted from the transport chain, releasing excess energy into the cell and destroying cell membranes.
A second class of inhibitors acts at the reducing end of PSI, inhibiting the reduction of ferredoxin (Fdx). The bipyridillium herbicides, diquat and paraquat, are members of this class. These herbicides become oxidized by accepting electrons from the transport chain. The superoxide radical created by this process is highly reactive and damaging to the photosynthetic apparatus.
Photosystem II Inhibitors - HRAC Group C1, WSSA Group 5
Photosystem II Inhibitors - HRAC Group C3, WSSA Groups 6
Photosystem I Electron Diversion - HRAC Group D, WSSA Group 22
Site of Action
Block the electron transport system in photosynthesis by binding to adjacent sites on the D-1 quinone protein of the electron transport chain.
Diverted electrons produce free radicals that destroy membranes.
History
Large, important family that revolutionized corn and grain sorghum production. Simazine was discovered by CIBA-Geigy Corp. (now Syngenta AG after mergers of many companies) and released in 1956. Atrazine was released by CIBA-Geigy in 1958 and quickly established itself as the most widely used herbicide for production of field corn.
Representative Herbicides
Common name Trade name ametryn Evik atrazine AAtrex cyanazine Use ends in 2002 hexazinone Velpar metribuzin Sencor, Lexone simazine Princep several others Crop Use
Ametryn is used in corn, banana, pineapple, and noncrop areas.
Atrazine is the most widely used herbicide for corn and grain sorghum production. It is also used in warm-season grass establishment and production. Ametryn, metribuzin, and simazine are also used in corn production.
Cyanazine is used in corn and cotton production. It has less carryover concerns than atrazine. Use of cyanazine will end in 2002 because it has been designated a possible carcinogen and has been found in water supplies.
Hexazinone is used for general vegetation control including shrubs and trees. It is selective in conifers and is used for site preparation and release in conifer forests.
Metribuzin controls annual weeds in alfalfa, established asparagus, carrots, field corn, potato, pulse crops, small grains, soybean, sugarcane, and tomato.
Simazine is used in corn, fruit and nut crops, christmas trees, and nursery crops.
Application Target
Soil and foliage depending on the crop and target weeds. Used preplant, preemerge, and early post-emergence. Most are inhibitors of photosynthesis after root uptake. But, those with higher water solubility also have foliar activity.
Translocation Type
Apoplastically systemic
Translocation occurs only in the xylem (upwards only).
Weed Spectrum
Mostly control annual broadleaf weeds with control and suppression of some annual grasses. Weed spectrum varies among the different triazines.
Selectivity
Tolerant species degrade the herbicide. Corn has excellent tolerance to atrazine because it uses three metabolic pathways to degrade it.
Prone to resistant weed development primarily because of repeated use over prolonged time periods.
Reaction in Soils and the Environment
Moderate to strong affinity for soil particles. More available on sandy soils due to fewer adsorption sites and warmer temperatures. Rates must be adjusted for soil texture and organic matter. High pH increases availability and longevity.
Breakdown is mainly by microbial action, but hydrolysis is a major contributer to breakdown at low pH.
Soil persistence varies considerably among the triazines. Persistence is greater under dry conditions, cold temperatures, low soil organic matter, low clay content, and high soil pH. Carryover can be a problem with some. Crops sensitive to atrazine, such as oat, small-seeded legumes, sugarbeet, and vegetable crops should be planted at least 2 years after atrazine application.
Low water solubility (33 mg/L) and strong affinity for soil particles limit leaching in most soils. However, small amounts of atrazine have been found in groundwater. Because of this, atrazine is a restricted use pesticide and the amount of atrazine that can be applied at some sites is restricted by law. Sites most prone to leaching have coarse-textured soils and shallow water tables.
Atrazine has become problematic in some surface waters used for drinking water in the midwest U.S. At first glance its water solubility and half-life would suggest that it should not be much of a problem. However, there are several characteristics of atrazine that make it problematic. One of the major contributing factors is its widespread use. Atrazine is applied at relatively great doses on large acreages throughout the north central U.S. Another factor contributing to water contamination is atrazines ability to move into surface waters while attached to soil particles. Once it moves into anaerobic areas of the water column, the half-life of atrazine can increase to as many as 2 years. Therefore, atrazine is sometimes found in detectable levels in poorly aerated surface waters, especially those containing sediment from crop fields.
Cyanazine will no longer be used in the United States after 2002 because it is suspected of being a human carcinogen.
Half-life in soil – 30-60 days (hexazinone - 90 days)
Symptomology
Most plants are not affected until they emerge and begin photosynthesis. Injury symptoms generally occur after the cotyledons and first true leaves emerge.
Yellowing of the leaf margins or tips and yellowing between the leaf veins in broadleaf plants. Injured leaf tissue eventually becomes necrotic.
Symptoms occur in older and larger leaves first. Necrosis begins on the leaf margins and tip and progress toward the leaf center and base.
There is greater injury on higher pH soils (greater than pH 7.2).
Injury Pictures
Site of Action
Inhibits photosynthesis.
History
Bentazon is the only chemical in this family. It was released by BASF in 1968 and was the first good chemical to control cocklebur in soybean.
Representative Herbicides
Common name Trade name bentazon Basagran Application Target
Foliage
Translocation Type
A contact herbicide with limited translocation in the plant. Weeds must be thoroughly covered with spray solution for good activity.
Crop Use
A postemergence herbicide used to control broadleaf weeds and sedges in grass and large-seeded legume crops. Labeled crops include beans, corn, peanuts, peas, peppermint, rice, sorghum, soybean, and spearmint.
Weed Spectrum
Used in soybeans to primarily control cocklebur, ragweeds, smartweeds, and sunflower.
Selectivity
What makes bentazon selective is not well understood.
Excellent kill - cocklebur, smartweed, ragweed.
Good kill - velvetleaf, nutsedge, sunflower, jimsonweed
Poor kill - lambsquarters, pigweeds.
Grass plants are generally tolerant.
Reaction in Soils and the Environment
No activity at labeled application rates.
Rapidly broken down by soil microbes.
Half-life in soil - 20 days.
Symptomology
Chlorosis and necrosis. Injury is confined to foliage that has come in contact with the herbicide.
Leaf scorch (coppery necrosis) in older leaves.
Plant stunting.
Symptoms start in 3-7 days after treatment and abnormal weather usually helps activity.
Leaf speckling and bronzing can occur in the crop, but plants generally outgrow injury in 10 days or less.
Injury Pictures
History
Selective, contact, postemergence herbicides used to control broadleaf weeds in grass crops. Bromoxynil and ioxynil were both released in 1963 by Rhone Poulenc (now Aventis after merging with Hoescht AG).
Representative Herbicides
Common name Trade name bromoxynil Buctril, Brominal, Bronate, others ioxynil various Crop Use
Bromoxynil is used in field corn, pop corn, sorghum, small grains, seedling forage alfalfa, flax, garlic, onions, grasses grown for seed and sod production, conservation reserve program (CRP) areas, non-residential turf areas, and noncropland/industrial sites.
Site of Action
Inhibit photosynthesis and respiration. Primary action is through disruption of plant cell membranes. These compounds bind to the D-1 quinone protein of photosystem II, blocking electron transport. The electrons that are diverted destroy cell membranes.
Application Target
Foliage
Translocation Type
Contact herbicides applied post-emergence.
Weed Spectrum
Annual broadleaf weeds. Most effective on nightshade, mustards, and smartweeds.
Selectivity
Susceptible species retain the spray longer (more uptake).
Grass plants are generally tolerant.
Reaction in Soils and the Environment
Because of soil sorption they have no soil activity.
Rapid microbial breakdown in soils.
Half-life in soil – 7 days.
Symptomology
Rapid necrosis of sprayed plant parts.
Injury is confined to foliage that has come in contact with the herbicide.
Injury Pictures
From the University of Missouri
Site of Action
Draws electrons from photosystem I to form free radicals that damage cell membranes. Because the final outcome of these herbicides is the destruction of cell memberanes they are often classified as cell membrane disruptors. Specifically, these herbicides reduce molecular oxygen to toxic superoxide radicals. Their action results from the drawing of an electron from photosystem I by the positive bypridinium ion. This forms a stable free radical that continues to react and produce hydrogen peroxide, a superoxide radical O2-, a hydroxyl radical (OH-), and singlet oxygen 1O2. Each of these molecules can damage cell membranes.History
Released by Imperial Chemical Industries of England in 1958. Paraquat is quite toxic to mammals (LD50 in rats is 150 mg/kg) and is a restricted-use pesticide.Representative Herbicides
Common name Trade name diquat Reward, others paraquat Cyclone, Gramoxone Extra Crop Use
Used as a non-selective control to 'burn down" all the plants in a particular area. Paraquat is often used to control weeds before crop emergence and is particularly useful for burndown of weeds in minimum tillage systems. Both paraquat and diquat can used as a directed sprays in noncropland situations, for suppression of sod during reseeding, and as desiccants that aid harvest. Diquat is also used to control aquatic weeds. Diquat's mammalian toxicity is slightly less than paraquat and when diluted by water it has very low toxicity to mammals and fish.Application Target
FoliageTranslocation Type
Contact herbicide with no translocation. However, they are rapidly absorbed into plants and rainfast within one hour or less of application.
Weed Spectrum
Kills contacted foliage of most plant species. Grasses are slightly more susceptible to paraquat than broadleaves.Selectivity
Non-selective herbicide. A very few plants have tolerance.Reaction in Soils and the Environment
No soil activity. Both diquat and paraquat are strong cations and strongly adsorbed to negatively charged clay particles. They are not available for plant uptake, microbial breakdown, or movement in solution. Half-life in soil of about 3 years.Symptomology
Plant leaves will have a limp, water soaked appearance shortly after application. Necrosis is noticeable within hours after application. Spray drift will cause necrotic spots where drift droplets land.Injury Pictures
From the University of Missouri
Agronomy 317 - Principles of
Weed Science
Authored by Dr. Lance R. Gibson
Copyright © 2001 Iowa State University. All rights reserved.
Revised:
July 23, 2004
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