Herbicides
​The world we live in today does no longer bear resemblence to the world our ancestors once lived in. The gardens of our great-grandmothers and the farmlands of our great-grandfathers are not the places where the hindrances of weeds can simply be plucked from the ground. No, instead our world today requires the complexity of multiple chemicals that sink deep down to the biological mechanics of these very weeds and cease their mechanisms of action.
What Are Herbicides?
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Herbicides can be simplisti​cally defined as chemicals that can be utilized to control and even manage undesirable forms of vegetation, as well as interrupt plant growth. They are broadly placed in two categories: Specific - specific herbicides only kill the plants they are designed to kill; the other surrounding plants remain unharmed and intact. The other categorization is Non-specific, which kills every plant it comes into contact with.
Key Terms
To better further your understanding of the page, a few terms have been listed and defined.
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​Mode of Action: The mode of action refers to any type of plant-herbicide relationship. More in depth, it is specifically how the herbicide works to apply injury or death to the plant.
Site of Action: The site of action refers to the exact location or biochemical pathway inside of the plant where the herbicide releases toxicity at a cellular level.
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​Mechanism of Action: The mechanism of action refers to the specific biochemical processes in which the herbicide has affected.
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​Translocation: Translocation is the movement of the herbicide to the specific location within the cell known as the site of action.
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​Site of Absorption: The site of absorption is the location where the plant absorbs the herbicide. It is sometimes referred to as the site of uptake and it can occur via root, shoot, and foliar.
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​Xylem: The xylem moves water and minerals up from the roots through the center of the plant's vascular bundle.
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​Phloem: The phloem translocates food and nutrients up and down the the side of the plant's vascular bundle.
A diagram of a leaf depicting the mode of action of a systemic herbicide that inhibits photosynthesis. In this illustration, the herbicide applied is absorbed through the leaf and translocated to the chloroplasts, playing as the site of action in this diagram, and later inhibiting the process of photoysnthesis, acting as the mechanism of action. (Image Citation)
Application of Herbicides
Before understanding exactly how herbicides affect the biological processes located within the plant, it is important to clarify the various methods of how herbicides are applied to the plant and the specific timing of these applications.
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Herbicides can be applied to plants at three different stages: preplant, preemergence, and postemergence.
The illustrations above depicts the usual timing of herbicide application on crops and weeds. (Image Citation)
The other segment relating to herbicide applications, is the methods used. Application techniques can be divided into two categories: terrestrial and aquatic.
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​Terrestrial application includes the following methods:
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​Foliar Application: ​
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​Foliar application, defined simplistically, is the direct spraying of the herbicide on the leaf of a plant, tree, or shrub.
A visual interpretation of foliar application.
Basal bark: ​
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​Basal bark application is almost similar to the technique of foliar spraying, however the herbicide is applied directly to the bark of a tree.
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Soil Application:
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In soil application,​ herbicides are sprayed directly on the soil making contact with either the seed of the plant or the roots. In order for this method of application to work efficiently, moisture must be present for it to move through the plant, and the herbicide must reach the site of absorption.
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A visual interpretation of soil application.
Hack and Squirt:
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In the hack and squirt method of herbicide application, the bark of the tree is cut to create a "cup" like shape that would eventually hold the herbicide.
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A photograph of the hack and squirt technique. (​Image Citation)
Stu​​mp Cut:
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​​In the stump cut technique, after a tree has been cut - leaving only the stump behind - , herbicides are applied directly onto the stump to prevent growth.
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​Aquatic application includes the following methods:
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​Foliar Application: ​
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​Foliar application, defined simplistically, is the direct spraying of the herbicide on the leaf of a plant, tree, or shrub.
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Water Column:
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Herbicides are applied directly to the water column, the habitat and medium through which fish habitats are connected. From there, the plants uptake the herbicide through the water.
How Do Herbicides Work?
Herbicides operate by supressing or often killing processes essential to the plant such as photosynthesis and respiration. Their effectivity is ultimately based upon four individual factors: 1) they must create adequate contact with the plant; 2) the plant must absorb the herbicide; 3) the herbicide should move through the plant's site of action without deactivation, and 4) the herbicide is obliged to attain the toxic levels at the site of action.
Click on the video to the left to gain an overview concerning the pathways affected when a herbicide has made contact and is absorbed into the plant.
To better comprehend the journey, it is crucially important to know how these herbicides enter through the leaf and soil. As mentioned before in the previous sub-heading, foliar spraying is the direct application of herbicides on a leaf.
The herbicide can now enter the leaf in two ways: either through the cuticle or the stomata.
An illustration of the anatomy of a leaf. (Image Citation)
​The surface of a leaf is protected with a thin waxy layer called a cuticle. Upon closer examination, it is viable to see cell membrane - which is composed of lipids and protein. Hydrophilic herbicides or "water loving" herbicides will not penetrate the membrane while the hydrophobic herbicides or "water-fearing" herbicides will become entrapped.
Another way for the herbicide to enter the leaf is through the stomata, which can be described as openings in the leaf that allow the passage of some compounds, including carbon dioxide.
Importance of Translocation
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​​As mentioned before, translocation is the movement of the herbicide to the specific location within the cell known as the site of action. The important aspect surrounding translocation is that it allows the herbicide to move the active ingredient that it contains to the site of action where a specific plant system can be inhibited.
Modes of Action
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​​Herbicides are generally categorized by their mode of action which describes how injury is applied to the plant. Knowing the mode of action allows you to understand how an herbicide works in applying injury or death to the plant.
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​Lipid Synthesis Inhibitor
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Herbicides that are under this classification inhibit the production of fatty acids also commonly referred to as lipids. The synthesis of lipids within a plant is essential in maintaining normal plant growth and keeping the structure of cell membrane. So how exactly is the inhibition of lipid synthesis occurring?
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To begin with, the herbicide is absorbed through the use of foliar application and is translocated through the phloem towards the areas of injury. Within the anatomy of the plant, the activity of an enzyme by the name of acetyl-CoA carboxylase is blocked prohibiting the production of fatty acids. This process takes place in the stroma of a plastid.
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An illustration depicting the location of lipid synthesis inhibition. (Image Citation)
An illustration depicting the disruption to lipid synthesis. (Image Citation)
For a more in depth explanation regarding lipid synthesis inhibition, click on the video on the right!
Amino Acid Synthesis Inhibition
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​Herbicides under the classification of amino acid synthesis inhibitors prevent the creation of the essential amino acids. Within these plants, there are 20 amino acids – each with its own pathway. When an herbicide inhibits one of these metabolic pathways the formation of more than one vital plant protein is prevented.
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Sulfonylureas, imidazolinones, and amino acid derivatives are the amino acid inhibitors. Herbicides included beneath the family sulfonylurea and imidazolinone inhibit the enzyme acetolactate synthase (ALS) or acetohydroxy acid synthase (AHAS). They can be absorbed into the plant either through foliar application or through the roots – moving through the xylem and phloem to reach the area of injury. The other amino acid inhibitor, the amino acid derivatives, ceases the creation of three aromatic amino acids by inhibiting a key plant enzyme by the name of SEPSP synthase. Amino acid derivatives are absorbed through foliar application and move through the phloem.
An illustration depicting the disruption to the amino acid synthesis.
Pigment Inhibitor
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An herbicide classified under the category pigment inhibitor, destroy the chlorophyll in the leaf tissue. When the chlorophyll is destroyed the plant is no longer capable of going through the process of photosynthesis, leaving the plant to die.
An image depicting the effect on the plant once the chlorophyll has been destroyed.
​Click on the video to the right to see the effects of pigment inhibition over the course of a couple of weeks!