How Herbicides Work

Front 1

To be effective, foliar herbicides must:

Back 1

Be retained on the leaf;

Penetrate the waxy cuticular layer on the surface of the leaf;

Move through the water-filled space around the cell;

Enter a cell, passing though a lipid-like membrane;

Reach the target site (usually an enzyme); and

Bind to and inhibit the target enzyme.

Front 2

To be selective, a herbicide must:

Back 2

Affect the target site of the weed, but not the crop; or

Be metabolized or broken down by the crop plant; and

Not be metabolized by the weed; or

Reach the target plant, but not the weed.

Front 3

Herbicides must work reliably when:

Back 3

The crop plants and weeds are growing rapidly; and

The crop plants and weeds are under stress.

Front 4

What is an enzyme?

Back 4

It is a protein made up of a chain of amino-acid building blocks folded into a complex three dimensional shape.

Front 5

What do enzymes do?

Back 5

Enzymes are catalysts. They facilitate chemical reactions by reducing the amount of energy required and thus allow thousands of reactions to occur at low temperatures.

Front 6

What is the relationship between herbicides and enzymes?

Back 6

It is thought enzymes have active sites, places where substrates bind and where they are modified to form the products.
Herbicides can also bind to the active sites of enzymes. These herbicides stop the substrate from binding or perhaps the product from leaving.

Herbicides may also bind away from the active sites, possibly changing the shape of the enzyme and stopping it from functioning.

In this way, herbicides kill plants.

Front 7

How do target sites relate to herbicides?

Back 7

Herbicides have a specific target site, a place in the plant that herbicides bind to and inhibit function.

Most target sites are enzymes.

By binding to the target site, herbicides stop or slow down enzyme function.

In most cases, herbicides inhibit only one enzyme or target site, despite there being thousands of different enzymes in every cell.

Front 8

How are herbicides divided in North America?

Back 8

They are divided into groups based on target site.

Herbicides that affect the same target site frequently have the same symptoms, application method, constraints and even toxicological profile.

Herbicide groups are also an important consideration in managing herbicide resistance; however, herbicides in the same group can vary in the weeds they control and the crops in which they can be used.

Front 9

While much is known about herbicides and their target sites it is not always apparent why plants die following a herbicide application.

Explain an example.

Back 9

When plants are treated with an inhibitor of photosynthesis, there is the expectation that the plant will "starve" to death by running out of energy. However, photosynthesis inhibitors work too fast for this to be the case. Photosynthesis inhibitors, by blocking electron movement, produce high energy toxic compounds that destroy membranes and lead to plant death.

Front 10

How do glyphosate herbicides work?

Back 10

Enzyme activity is normally carefully regulated, so toxic compounds remain at low levels. By inhibiting the target enzyme, the herbicide causes the substance to build up and damage the plant.

Front 11

How do Group 4 herbicides (such as 2,4-D) work?

Back 11

A de-regulation of the controlled process of cell growth.

Front 12

Herbicides affect a particular target site because they are the right shape, like a key for a specific lock. What are the two additional properties that influence effectiveness?

Back 12

Solubility - a measure of how much herbicides will dissolve in water or organic solvents.

Whether or not they are weak acids - indicating how readily the chemical, when dissolved in water, will lose a hydrogen ion. Those that give up protons readily are called weak acids. Weak acid herbicides have a pKa of less than 7.0

The manner by which a herbicide's solubility and weak-acid nature affect how it enters and moves within the plant.

Front 13

What is the measure of solubility K[ow]?

Back 13

It is determined by measuring the relative amounts of herbicide that dissolve in octanol and water. Herbicides with low K[ow] are water soluble (such as glyphosate - K[ow] = 0.0006) and herbicides with high K[ow] are soluble in organic solvents (such as diclofop-methyl K[ow] = 37,800)

Front 14

What is the driving force for most herbicide movement?

Back 14

Diffusion

Front 15

Describe the herbicide pathway.

Back 15

The cuticle on leaves and stems is waxy; therefore, water-loving (hydrophilic) compounds have difficulty dissolving through this layer. Water-hating, oil-soluble (hydrophobic) compounds may dissolve into the cuticle and never come out.

Next the herbicide encounters a water-filled area (the apoplast) that bathes the cells, and the situation is reversed. Hydrophilic compounds can go into this solution easily, wile hydrophobic materials enter and move through slowly. Additionally, water moves through this area, making its way to the edge of the leaves where most is lost to transpiration. Therefore, hydrophilic compounds can get swept along to the edge of the leaves.

Surrounding the cell is a cell membrane. This membrane is primarily made of oil-like substances called phospholipids and some proteins. Hydrophobic herbicides enter the membrane readily, while hydrophilic herbicides enter very slowly. Once dissolved in the oily membrane, hydrophobic herbicides exit slowly while hydrophilic herbicides exit rapidly.

Inside the cell is a water solution called cytoplasm. Hydrophilic herbicides dissolve readily in the cytoplasm, while hydrophobic herbicides do not.

Front 16

Why are most herbicides an intermediate between hydrophilic and hydrophobic in tendency?

Back 16

Due to the alternation of water-like layers with oil-like ones presenting problems for either extreme.

Front 17

In addition to keeping the cytoplasm within the cell wall, what does feature does the cell membrane have and what does it do to the acidity of the cell?

Back 17

It contains a protein that pumps protons (H+ or positive charges) from the inside to the outside of the cell. This action makes the outside of the cell more acidic than the inside.

Front 18

Does the solubility of weak-acid herbicides change with pH?

How does this affect the uptake of a weak-acid herbicide?

Back 18

Yes. Outside the cell, weak-acid herbicides tend to accumulate a proton (where they are abundant), making herbicides hydrophobic and able to move through the cell membrane. Inside the cell, weak-acid herbicides lose a proton (they are relatively scarce), making them more hydrophilic or water soluble. Thus, weak-acid herbicides tend to get trapped in cells and accumulate.

Front 19

What are the two transport systems that move nutrients (and sometimes herbicides) throughout a plant?

Back 19

The xylem - a one-way stream that moves water and dissolved minerals from the soil, via roots to the shoots where water exits the plant.

The phloem - which moves solutions both up and down in the plant.

Front 20

Describe the xylem transport system in greater detail.

Back 20

Plants have limited control of xylem movement; the environment, temperature and water availability in soil are major influences on water movement in xylem.

To move in the xylem, a herbicide must be water soluble. To reach the shoots, soil-applied herbicides usually move in the xylem. The xylem stream can also move foliar-applied herbicides to the edge of leaves.

The xylem flow moves through dead cells.

Front 21

Describe the phloem transport system in greater detail.

Back 21

Phloem sap moves in living cells giving the plant more control of phloem transport. Phloem is the primary sugar transport system.

During the growing season, sugars move from the leaves, where they are made, to the balance of the plant where they are used as an energy source.

The phloem also moves amino acids, the building blocks of proteins, within the plant.

Front 22

How do perennial plants use the phloem?

Back 22

Perennial plants move sugars up from the roots during spring growth and down after flowering to store them for next year's growth.

Front 23

What herbicides use the phloem for transport?

Back 23

Herbicides that move well in the phloem are those that readily enter living cells and may have intermediate solubility and/or be weak-acid herbicides.

Any herbicide movement in the phloem is purely passive.

Front 24

Why is translocation of interest?

Back 24

Perennial plant control depends on herbicide killing roots and rhizomes at considerable distances from the leaves where the herbicide is applied. If the herbicide translocates, long-term perennial weed control can be achieve. If not, top growth is removed while roots and rhizomes are unaffected; thus, long-term control will not be achieved.

Front 25

What does herbicide selectivity depend on?

Back 25

It depends on the crop's ability to break down or metabolize the herbicide more rapidly than the weed.

Following metabolism, herbicides are frequently joined (conjugated) to sugars or amino acids. It is believed that these forms of the herbicide are non-toxic and move to the cell vacuole.

Analogous to this action is the ability of humans to metabolize caffeine. Caffeine is quite toxic, but humans can degrade it rapidly to reduce its toxic effects.

Front 26

What is a Proherbicide?

Back 26

A biologically inactive chemical form of the herbicide. Plants have the capacity to change the proherbicide into a toxic molecule. Herbicides are formulated as proherbicides because they are better than the active molecule in penetrating the cuticle, are more stable in formulation or are less volatile.

Front 27

What is the LD[50]?

Back 27

The LD[50] is a feeding study to determine non-target toxicity. Rats (or other mammals and fish) are fed herbicides at various doses, and the percentage killed is measured.

The dose that kills one-half of the subjects is called the LD[50] (Lethal Dose for 50 per cent of the population.