Herbicide Mode of Action
Herbicide effectiveness depends on 1 ) adequate plant contact; 2) absorption by plants; 3) translocation (i.e. movement) within the plants to the site of action, without deactivation; and 4) toxic levels at the site of action. The application method, whether preplant incorporated, preemergence, or postemergence, largely determines when the herbicide will contact plants and the portion of the plant contacted.
Mode of action refers to the sequence of events from absorption into plants to plant death. The mode of herbicide action may determine the application method needed for best results. For example, herbicides which affect photosynthesis but have little soil residual, such as desmedipham (Betanex) or paraquat (Gramoxone Extra), need to be applied postemergence to leaf tissue. Seedling growth inhibitors, such as trifluralin (Treflan) and EPTC (Eptam), need to be applied to the soil to effectively control newly germinating seedlings.
Soil-Applied Herbicide Phytotoxicity in Plants
Seeds of many weed species are quite small and germinate only 0.5 to 1.0 inch below the soil surface, so soil applied herbicides should be concentrated in the top 1 to 2 inches of soil for best weed control. Herbicide positioning can be accomplished by mechanical incorporation or rainfall. Close contact between herbicide and plant is needed for absorption through the roots or shoots and effective weed control. Herbicide absorption through roots will continue as long as the absorbing region near the root tips remains in contact with the herbicide-treated soil. As the roots grow deeper, herbicide uptake declines. Therefore, plants may survive if the root tips grow out of the herbicide-treated soil before herbicide absorption is sufficient to kill the plants.
Many soil-applied herbicides are absorbed through unemerged plant shoots, and plants may be killed or injured before emergence. Volatile herbicides such as the thiocarbamates (EPTC [Eptam] for example) the dinitroanilines (such as trifluralin [Treflan]) can move in the soil and penetrate plant shoots as gasses or liquids. Less volatile herbicides such as the acetanilides (metolachlor [Dual]) probably are absorbed into the shoot only as liquids. Physical and environmental factors that promote rapid crop emergence reduce the length of time that a plant is in contact with a soil-applied herbicide and, therefore, reduce the possibility of crop injury.
Herbicides differ in translocation within a plant. The soil applied dinitroaniline herbicides (such as trifluralin [Treflan]) are not mobile within the plant. Therefore, their primary injury symptoms are mostly confined to the site of uptake. Other herbicides are mobile within the plant, and injury symptoms will generally be most prominent at the site where mobile herbicides concentrate. For example, soil-applied atrazine is absorbed by plant roots and moves upward within the water transport system of the plant to the leaves, where symptoms occur.
Postemergence Herbicide Phytotoxicity in Plants
Effective weed control from Postemergence herbicides depends on adequate contact with above-ground plant shoots and leaves. Therefore, the combination of spray nozzle pressure and volume should be selected to obtain necessary plant coverage and drift control.
Weed control from readily translocated herbicides is affected little by changes in droplet size within a normal range. Relatively large spray droplets may result in less weed control than small spray droplets because of poorly translocated herbicides. Small spray droplets give more complete leaf coverage and are retained more than large droplets on hard-to-wet leaves such as vertical, waxy or small leaves. Large spray droplets will better penetrate a spray canopy and drift less than small droplets. Droplet size will be increased by reducing spray pressure, increas-ing nozzle orifice size, special drift reduction nozzles, addi-tives that increase spray viscosity, and rearward nozzle orientation on aircraft.
Postemergence herbicide rate of uptake and amount absorbed often is determined by the chemical and physi-cal relationships between the leaf surface and the herbi-cide. Factors such as plant size and age, water stress, air temperature, relative humidity, and adjuvants can influence the rate and amount of herbicide uptake. Adjuvants such as oil concentrates, methylated seed oils, surfactants, or liquid fertilizer solutions can increase herbicide uptake by a plant. Hot and dry conditions, old weeds and weeds under drought stress all can reduce herbicide uptake. The amount and rate of herbicide uptake influence the potential for crop injury and weed control and often explain the year-to-year variation in the effectiveness of herbicides. Also, rapid herbicide absorption by plants will reduce the time for possible removal by rain or degradation by sunlight.
Postemergence herbicides, like soil-applied herbicides, differ in movement within a plant. For adequate weed control, non-mobile Postemergence herbicides must thoroughly cover the plant. Non-mobile herbicides are often called contact herbicides and include the bipyridylium, diphenyl-ether, benzothiadiazole, and nitrile families. Other herbi-cides are mobile within the plant and can move from the site of application to their site of herbicidal activity. For example, growth regulator herbicides such as 2,4-D and dicamba (Banvel) generally move both upward and down-ward within the food transport system to the growing points of the shoots and roots. In general, injury symptoms will be most prominent at the sites at which the mobile herbicides concentrate.
Plants may rapidly degrade or deactivate a herbicide to escape that herbicide's toxic effects. For example, corn quickly deactivates atrazine by binding to naturally occur-ring plant chemicals. Soybean tolerance to metribuzin (Sencor, Lexone) is at least partially due to the deactiva-tion of the herbicide by conjugating (binding) to plant sugar molecules. Sugarbeet avoids injury from desmedipham (Betanex) partially through rapid metabolism.
Situations may occur in which a crop is injured by a herbi-cide that is normally not toxic to the crop. This often occurs because environmental stresses such as hot or cold temp-eratures, high relative humidity, or hail decrease a plant's natural ability to reduce herbicide uptake or deactivate a herbicide. Postemergence desmedipham (Betanex) injury to sugarbeet under hot and wet weather conditions is a good example of environmentally induced herbicide injury. An excessive amount of herbicide due to misapplication also can injure a tolerant crop by overwhelming the crop's herbicide degradation and deactivation systems.
An understanding of how herbicides kill weeds (herbicide mode of action) is useful in selecting and applying the proper herbicide for a given weed control problem and for management of weed resistance to herbicides. Herbicide mode of action information also is useful in diagnoses of injury from herbicides.
Although a large number of herbicides are available in the marketplace, they can be divided into groups with similar chemical and phytotoxic properties. Herbicides with a common chemistry have been conveniently organized into "families." In addition, two or more herbicide families may have the same mode of phytotoxic action and thus express the same injury symptoms.
The following sections describe the characteristics of widely used herbicide families grouped by mode of action. These seven major modes of action are: growth regulation, amino acid synthesis inhibition, lipid synthesis inhibition, seedling growth inhibition, photosynthesis inhibition, cell membrane disruption, and pigment inhibition.
Callus tissue - A mass of plant cells that form at a wounded surface.
Chloroplast - A membrane-enclosed structure that contains the green pigment molecules (chlorophyll) essential for photosynthesis (i.e. food production).
Chlorsis - A yellowing in plant color due to a decline in chlorophyll levels.
Contact herbicide - A general classification for herbicides that are unable to move within a plant. A contact herbicide's effectiveness is highly dependent upon uniform coverage of treated soil or plant tissue.
Epinasty - A bending of plant parts (e.g. stems or leaf petioles) downwards due to increased growth on the upper side of an affected plant part. Often associated with the plant growth regulator herbicides.
Herbicide mode of action - The sequence of events from absorp-tion of the herbicide into the plant through plant death. Refers to all plant-herbicide interactions.
Herbicide site of action - The primary biochemical site that is affected by the herbicide, ultimately resulting in the death of the plant. Also referred to as herbicide mechanism of action.
Necrosis - The death of specific plant tissue while the rest of the plant is still alive. Necrotic areas are generally dark brown in color.
Phloem - Plant tissue that functions as a conduit for the movement (translocation) of sugars and other plant nutrients.
Postemergence application - A time of herbicide application occurring after the crop and weeds emerge from the soil. Also referred to as a foliar application.
Preemergence application - A time of herbicide application occurring after a crop is planted but before the crop or weeds emerge from the soil.
Pre-planting application - A time of herbicide application occurring before the crop is planted. Often followed by an incorporation (mechanical mixing) into the top I to 2 inches of soil. Often referred to as a preplant incorporation treatment.
Systemic herbicide - A general classification for herbicides that are able to move away from the site of absorption to other parts of the plant.
Translocation - The movement of water, plant sugars and nutrients, herbicides and other soluble materials from one plant part to another.
Translucent - An absence of leaf tissue pigments that results in the diffusion of light, giving the plant an off-white color.
Xylem - Plant tissue that functions to serve as a conduit for the upward movement (translocation) of water from the roots to above-ground plant parts.