Herbicide mode of action can be defined as the primary mechanism of herbicide interference with plant function or metabolism that leads to plant death. Herbicides are often classified according to their mode of action, because as a general rule, herbicides within the same mode of action class will produce similar symptoms on susceptible plants. There are seven major mode of action categories described in this section: cell membrane disruptors, growth regulators, photosynthesis inhibitors, pigment inhibitors, seedling growth inhibitors, ACCase inhibitors, and amino acid synthesis inhibitors. Chemical families, representative herbicides, and some unique characteristics of each are listed within each category. For a more comprehensive explanation of herbicide mode of action and injury symptoms, ask for North Central Regional Publication 377, Herbicide Mode of Action and Injury Symptoms, at your local OSU Extension Office.
I. Cell Membrane Disruptors
Most of the herbicides listed below are effective only when applied postemergence, where they are absorbed by foliage and disrupt cell membranes in susceptible plants. Membrane disruption in treated plants causes the foliage to initially have a watersoaked appearance, which is followed by rapid wilting and eventually a "burned" or frost damaged appearance of the foliage. Compared with herbicides with other modes of action, symptoms of membrane disruptors develop rapidly after application on susceptible plants, usually within a few hours or up to a couple of days, depending on the specific herbicide. Because these herbicides generally have limited translocation in plant tissues, adequate spray coverage and a proper adjuvant are often required for maximum weed control activity. Activity of these herbicides increases with sunlight, temperature, and humidity. There are four classes of chemistry within this mode of action: bipyridyliums, diphenylethers, and N-phenylphthalimides, and aryltriazolinones. Herbicides in the latter class have primarily soil-applied activity, but have the same mode of action as foliar-applied cell membrane disruptors.
II. Growth Regulators
Plant growth regulators mimic the activity of hormones occurring naturally in the plant system. However, growth regulators are toxic to many plants because they are much more potent than natural hormones and can cause secondary effects that inhibit normal plant growth. Most herbicides in this family are highly systemic, meaning they translocate internally to others parts of the shoot, the roots, and other underground vegetative organs (rhizomes. creeping roots) if applied to plants at the proper stage of growth. They are toxic mainly to broadleaf plants, but can injure some grasses, including small grains and corn if applied during sensitive stages of growth. Injury symptoms on susceptible plants develop first on the newly developing tissues in the meristematic regions (growing points) of the plant. Growth regulator herbicides cause growth abnormalities in susceptible plants, and symptoms on broadleaves may include malformed or strapped leaves, parallel leaf veins, twisted stems, and stem splitting or brittleness. Symptoms on grasses include onionleafing, brittle stalks, fused and malformed brace roots, curved stems, and malformed ears or seedheads. Growth regulator herbicides include the following classes of chemistry: benzoic acids, phenoxy acids, and the pyridine carboxylic acids.
III. Photosynthesis Inhibitors
Photosynthesis inhibitors block the process whereby plants convert sunlight into the chemical energy required for further growth processes. Photosynthesis inhibitors are toxic primarily to broadleaf plants, but some herbicides in this class are toxic to certain grass species at an early stage of growth. Some photosynthetic inhibitors are mobile in the plant (triazines, phenylureas, uracils), moving upward from the site of absorption, with water and mineral nutrients. For this reason, these herbicides are usually soil-applied so that weeds will absorb the herbicide via the roots, although they also have foliar activity if applied with adjuvants and good spray coverage. Other photosynthetic inhibitors are not mobile in plants and are classified as postemergence contact herbicides (nitriles, benzothiadiazoles). These herbicides have no soil activity. The most common symptom of mobile photosynthesis inhibitors in susceptible plants is chlorosis (yellowing) of the leaf tissue. In grasses, symptoms first appear on the older leaves near the base of the plant. Leaf tips and margins first show chlorosis and eventually turn necrotic. In broadleaves, the oldest leaves are affected first and show leaf margin and interveinal chlorosis, followed by necrosis.
Mobile Photosynthetic Inhibitors
Non-mobile Photosynthetic Inhibitors
IV. Pigment Inhibitors
In contrast to the photosynthesis inhibitors, pigment inhibitors do not directly block photosynthesis, but inhibit the production of certain plant pigments necessary for photosynthesis. Symptoms include bleaching and chlorosis of the foliar tissue, which sometimes results in plants that appear totally white. Pigment inhibitors are translocated in the apoplast (upward) and are used primarily as soil-applied treatments.
V. Seedling Growth Inhibitors
Regions of active cell division (meristems) in plants are located in both the shoots and roots. Seedling growth inhibitors affect some fundamental process in meristematic regions that prevents normal growth and development of young plant tissue. Seedling growth inhibitors must be soilñapplied because they are taken up by plants after germination until the seedling emerges from the soil. These herbicides are thus effective only on seedling annual or perennial weeds. Plants that emerge from the soil uninjured are likely to remain unaffected, although occasionally root inhibition that occurs early in the plant's development is not noticed until it affects later growth. This broad category can be further subdivided into the following types of herbicides:
(2) Root meristem inhibitors (dinitroanilines - DNA's)
The exact mode of action of the chloroacetamides and thiocarbamates is not known. The dinitroanilines inhibit a crucial step in the process of cell division. Specific symptoms for each chemical class are listed below.
Shoot Meristem Inhibitors
Root Meristem Inhibitors
VI. Amino Acid Synthesis Inhibitors
The amino acid synthesis inhibitors act on a specific enzyme to prevent the production of amino acids, which are the building blocks for protein synthesis and, thus, plant growth and development. Sulfonylurea, imidazolinone, and sulfonamide herbicides inhibit the same enzyme, acetolactate synthase (ALS), blocking the production of three essential amino acids. Glyphosate and sulfosate block the synthesis of three amino acids by inhibiting another enzyme. This category is therefore further subdivided according to the enzyme that is inhibited: ALS inhibitors and EPSP synthase inhibitors. Injury symptoms are slow to develop and include a stunting or slowing of plant growth and eventual death. Symptoms are likely to show in the new plant growth first because movement of these herbicides is to those areas.
C. Glutamine Synthetase Inhibitors
The enzyme glutamine synthetase allows the plant cell to convert ammonia, a product of various plant processes, into amino acids. Glutamine stynthetase inhibitors block the activity of this enzyme. The result is a buildup of phytotoxic ammonia and a lack of essential amino acids, which inhibits photorespiration and photosynthesis and ultimately results in plant death. Glufosinate, currently the only glutamine synthetase inhibitor, has foliar activity only. Translocation within the plant is limited. Primary symptoms of activity are a rapid necrosis of leaf tissue followed by plant death.
VII. ACCase Inhibitors
ACCase or lipid inhibitors prevent the formation of fatty acids, which are essential components for the production of plant lipids. This occurs through the inhibition of a single enzyme involved in fatty acid synthesis. Lipids are vital to the integrity of cell membranes and to new plant growth. Broadleaf plants are unaffected by these herbicides, but most grasses are susceptible.
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