Triazines & Triazinones

Introduction

• Large, very important family including symmetrical triazines (e.g. atrazine) and asymmetrical triazinones (metribuzin).

• First discovered in 1952, introduced commercially in 1957, by Geigy Ltd. in Switzerland.

• Atrazine introduction revolutionized maize production when it was introduced in the late 1950's to early 1960's: finally a dependable herbicide to control broadleaf weeds (and grasses) in corn. The Big Population Shift started by 2,4-D changed direction, dicots replaced by grasses.

• Cyanazine was introduced by Shell Co.

• Herbicides in this family are used to control many annual broadleaf and grassy weeds. Applied in many ways: early preplant, preemergence, preplow, preplant incorporated and postemergence.

• s-Triazine herbicides are used in maize, sorghum, sugarcane, pineapple, etc.

• Metribuzin is used in soybeans, potatoes, transplanted tomatoes and other crops.

• Atrazine is an important herbicide for quackgrass control and other perennial weeds.

• Cyanazine does not control quackgrass but is used were persistence in the soil is a concern and for the control of fall panicum and crabgrass, weeds atrazine often does not control.

Chemistry Triazine(one) Family

Examples of commercial s-triazines:

• chlorinated s-triazines: atrazine, cyanazine, cyprozine, simazine, procyazine, propazine
• methoxy s-triazines: atraton, prometon, secbumeton, simeton
• methylthio s-triazines: ametryn, prometryn, terbutryn, simetryn, desmetryne.
• Atrazine: 6-chloro-N-ethyl-N'-(1-methethyl)-1,3,5-triazine-2,4-diamine
• Asymmetrical (triazinones): metribuzin

The Metabolism of the Triazines in Plants

Mode of s-Triazine Action

• s-triazine herbicides act by inhibiting primary events in photosynthesis in the chloroplast: binding to the D-1 protein in photosynthetic electron transport. This binding stops photosynthesis.
• s-triazine inhibition requires the presence of light and transpiration to move the chemical to foliage. Transpiration: water flow through the plant from uptake from the soil by roots, through the plant, out the leaves
• Atrazine injury is increased with increasing amounts of sunlight and factors that increase transpiration rates in plants: increasing temperature, soil moisture, wind, etc.; and decreasing air humidity.
• Metribuzin injury can be greater if low light, cloudy, weather precedes treatment. The plants carbohydrate reserves are decreased and subsequent high sunlight and temperatures can increase uptake and injury.
• Metribuzin injury can be synergized by treatments with some insecticides: malathion 3 day pretreatment.

Mode of s-Triazine Lethality

• s-triazines kill plants by destroying photosynthetic tissues.
• When they bind to D-1 proteins they steal electrons from photosynthesis and form highly reactive free radicals; these unstable free radicals oxidize and destroy membranes, pigments, etc. and chlorosis follows.

Uptake and Movement of the s-triazines in plants
Uptake.

• Both tolerant and susceptible species take up similar amounts of atrazine.
• Atrazine is absorbed most readily by plant roots, but foliar uptake does occur.
• Atrazine uptake by leaves is enhanced with the addition of oils or surfactants in the spray solution.

Translocation.

• Atrazine is readily translocated in the xylem and movement depends on plant transpiration.
• Atrazine accumulates on leaf edges and tips.

Basis of Selectivity of the s-triazines between Plant Species

Metabolic Resistance.
Atrazine and other s-triazines are degraded in many resistant plants by metabolism of the herbicide, which never reaches the chloroplast to cause injury or death. Corn, wild proso millet, large crabgrass, fall panicum and giant foxtail are especially good at this degradative metabolism.

Triazine Resistance: Metabolism

• Triazine resistant cucumber was developed as a crop by making selections for enhanced metabolic detoxification amongst variants.
• Biotypes of velvetleaf resistant to atrazine were discovered in a Maryland, as well as one in a Wisconsin cornfield( also some populations of perennial ryegrass in Australia) with high levels of resistance due to enhanced degradative metabolism. Unlike other triazine resistant biotypes discussed below, this variant was resistant due to enhanced ability to metabolism the herbicide.

Triazine Resistance: Binding Site Resistance.

• In 1968 another very different form of s-triazine resistance was discovered in Senecio vulgaris treated with simazine. It had been growing in nursery stock in Washington state.
• The basis of this resistance was not discovered until several years later. Subsequent research indicated that atrazine did not bind to the D-1 protein in resistant variants, thus allowing them to survive.
• Atrazine resistant mutants are less photosynthetically efficient than the susceptible types, hence they tend to yield less (yield penalty).
• s-triazine resistant mutants occur very rarely in nature. Selective pressure exerted by continuous applications of atrazine, and the lack of other herbicide or tillage weed control tactics, results in the enrichment of resistant members and their seed in these populations. Within 6-20 years all the susceptible members can be replaced by resistant biotypes.
• Here in Iowa we have populations of Kochia, common lambsquarters, giant foxtail (only case west of Maryland), and Pennsylvania smartweed (unique to Iowa)
• The gene that confers resistance has been transferred to crops: rapeseed (Beversdorf) and rutabaga by conventional breeding techniques from resistance in bird's-rape mustard (Brassica campestris); and to tobacco using cell culture selection techniques.
• Since its first discovery, over 58 different weed species (40+ dicots, 14+ monocots) have been identified with triazine resistance.

as-Triazinones.

• Metribuzin is often injurious to susceptible species, depending on ambient environmental conditions. The dosage difference between susceptible and resistant species is relatively narrow.
• Detoxification metabolism is similar to that of atrazine in most species, being more rapid in resistant species such as soybeans, tomatoes, potatoes. This process is controlled by two genes and results in significant differences in cultivar, varietal, responses to this herbicide.
• Applications made on cloudy days can injure resistant, slow growing, crops due to lower levels of carbohydrate reserves is the plant. Weather and climate conditions that result in slow growth also slow s-triazine detoxification metabolism.

Fate of the s-Triazines in the Environment

Soil.  Atrazine Persistence in the Soil.

• Often application rates of 2 kg ai/ha (2.2 lb ai/a) atrazine will persist in the soil and injure soybeans planted in the same area the next year (above 2 pictures).
• Oats can be injured with atrazine applications less than 1 kg ai/ha (1.1 lb ai/a) the previous year.
• Atrazine and simazine are fairly persistent in agricultural soils. Atrazine persists in soils from 3-12 months depending on the rate applied, the soil type and environmental factors.
• Atrazine is readily adsorbed to soil particles and resists leaching through the soil profile.
• Persistence is greater under dry conditions, cold temperatures and in sandy soils.
• Persistence in sandy soils is greater than that in heavier clay soils primarily due to the greater availability for plant uptake in those lighter soils.
• Atrazine is more available for plant uptake in sandy soils due to the low amount of adsorption sites and warmer temperatures, and despite the lower pH's, typical of these light soils.
• Persistence is greater in soils with lower organic matter and clay contents, lower cation exchange capacities (CEC) and higher pH.
• Postemergence treatments persist longer than preemergence applications, possibly due to the dryer conditions at the time of treatment.
• Preemergence and postemergence applications persist longer than preplant incorporated treatments, possibly due to more concentrated soil localization. Mixing of the atrazine in the soil can speed its breakdown.

Atrazine Carryover "Rule of Thumb"
A crude rule of thumb often used in the moist Northeast US and Canada is the "10% atrazine carryover rule": 10% of the applied active ingredient will remain in the soil the spring of the year following application. This "rule-of-thumb" is subject to many factors and in highly variable, and may not apply to the drier, higher pH soils of Iowa.

Triazine Combinations & Carryover
s-triazines and as-triazinones can combine unexpectedly to cause injury to a crop: metribuzin applied to soybeans (at rates rarely causing injury alone) can be combined with atrazine residues (at amounts rarely causing injury alone) from the previous years maize crop and cause considerable soybean injury.

Triazines & soil pH.

• s-Triazine and as-triazinone herbicides become more available for plant uptake in the soil water solution as the soil pH increases.
• As the soil pH increases, less herbicide is bound to the soil colloidal fraction (organic matter, clay mineral, etc.) due to the pH changing the ionic nature of the herbicide molecule.
• Liming increases the pH of soils and this agricultural practice can cause the release of atrazine from the colloid complex and making it more available for plant uptake.

Triazines & Soil Nutrients.

• Phosphorus fertilizers compete effectively with atrazine for adsorption sites and can lead to greater atrazine availability in the soil water.
• Atrazine applied to manured fields can be very tightly adsorbed to the organic matter of the manure. As the manure decomposes, it is released and becomes available to plants. This occasionally leads to the "skip-year carryover" phenomenon: atrazine applied to manure in year one injures a crop in year 2 despite the fact low, "safe", rates were initially applied and no injury was observed in year 1.
• Atrazine is more available for plant uptake in sandy soils due to the low amount of adsorption sites and warmer temperatures, and despite the lower pH's, typical of these light soils.

Triazine Soil Decomposition.
Atrazine is decomposed in the soil primarily by chemical hydrolysis, which depends on the presence of water, air, and higher temperatures.

Cyanazine.

• persists in the soil 1-3 months.
• Rainfall within 10 days of application is necessary for sufficient amounts of cyanazine to reach the soil water matrix and provide adequate weed control (herbicide "activation").
• Cyanazine does not adsorb to soil colloids as readily as atrazine and is more available for plant uptake. For this reason both cyanazine and metribuzin injury can be greater on lighter soils.
• Greater cyanazine injury to roots can be observed when applied to heavier clay, and high organic matter, soils due to greater adsorption.

Air.
s-triazine and as-triazinone herbicides are relatively non-volatile. As such, they pose little threat of drift in the air to adjacent susceptible crops.

Water.

• Significant amounts of atrazine residues reach the ground water due to applicator and dealer mishandling, as well as by use within recommended guidelines. They also contaminate ground water due to their relatively long persistence in the soil, movement through channels in the soil allowing percolation, and very low breakdown rates at soil depths below 2- 4 feet.
• Atrazine is one of the primary surface water contaminants in the world.
• Levels of atrazine found in ground and surface waters often exceed human health advisory levels, raising the question if this useful agricultural tool should be continued to be used.

Toxicology.

• some evidence atrazine may be a carcinogen; suggested mechanism: High soil pH plus nitrate plus atrazine = nitrosamines
• some evidence triazines may be estrogenic

Plant Injury Symptomology of the s-Triazines in Plants

Chlorosis and Necrosis.

The first symptoms of atrazine injury on susceptible plants often are seen on leaf tips and then leaf margins [cucumber seedlings (left, above); soybeans (center, above) and rapeseed (right, above)].

• Chlorosis, or yellowing, is also typically seen been the main vascular tissues: intervienal chlorosis.
• Chlorotic patterns increase in yellowing, areas of chlorosis enlarge to encompass more of the leaf. Then, necrotic, brown, coloration follows the chlorosis (below). Affected areas can crinkle and blow away. Leaf, then plant, death can ensue.

These symptoms can be confuse with other maladies such as iron (left), magnesium or manganese deficiencies. Interveinal iron chlorosis symptoms are found most frequently on young, developing, upper leaves. Triazine symptoms occur on older leaves that have higher transpiration rates than the younger leaves.

Metribuzin more frequently causes veinal chlorosis patterns.
Metribuzin can also cause "splash burn" when applied to the soil. These herbicides are adsorbed to soil particles on the surface, which are thrown onto plant leaves by rain drop impact.

Other symptoms.

• Marginal growth of leaves can be inhibited due to the destruction of meristems throughout those leaf regions. Triazines don't inhibit meristems directly and plant growth can continue while other plant parts are dead.
• Sometimes, soybeans inhibited by atrazine persisting in the soil the year after application will continue to grow while a steady rate of leaves are lost. In those cases, the plants may stay the same size for a period of the early season, eventually either dying or recovering.
• Root growth of susceptible plants often is unaffected.
• Greater injury, and slower injury symptom development, often occurs with weather conditions resulting in slow growth: cool, wet, overcast.
• Greater injury due to increased s-triazine availability in plants occurs with increased soil pH, solubility in soil water, temperature, wind, soil moisture, plant transpiration rates; and with lower air humidity.
• Cyanazine applied postemergence will cause more injury to corn as it gets older.

©jdekker-1999