Plant herbicide interactions for field grown crops

After applying a herbicide to a field with weeds, the herbicide needs to follow a certain path in order for it to be effective.

• Firstly it has to be absorbed through the leaf cuticle
• Secondly the herbicide must be trans-located through the plant in order to have any effect on plant growth.  Any chemical that is not trans-located cannot have any effect on plant growth, be it positive or negative.
• The herbicide must react with various processes within the plant which will either neutralize the effect of the herbicide or cause more destruction
• The herbicide must have an effect on plant metabolism.  If it does not have any effect, the herbicide is said to be selective.
• Each of these important aspects will be discussed

Absorption of chemicals

Absorption through roots and seeds

Roots are ideal organs to absorb herbicides, in some cases they are better suited that leaves.  Root absorption of herbicides is either passive or active, some herbicides are absorbed actively and others passively.  The vast majority of herbicides are absorbed passively.

Herbicides are absorbed through the first 50mm of actively growing roots.  The herbicides enter roots via the apoplast, the symplast and the apoplast-symplast complex.  Herbicides that enter through the apoplast move exclusively through the xylem.  Herbicides that move through the symplast have to move through cell walls, them into the protoplasm of the epidermis, cortex or both.  The herbicide remains in the protoplasm and sequentially passes into the endodermis, stele and phloem by means of the plasmodesmata.

Herbicides are rapidly translocated upwards from the roots through the xylem, which is powered by the transpiration stream.

Soil applied herbicides are preferred under conditions where the grower would like to either:

• Control weeds prior to emergence or soon thereafter
• Certain herbicides are more effective when absorbed by the underground organs.
• When it is important to kill the weed at a very early age
• The soil applied herbicide must come in contact with the root system or the seed in order to have any effect.

This requires at least one of three methods:

• Diffusion
• Mass movement or mass flow and
• Interception by the roots

Absorption through leaves

There are two area through which leaves may penetrate:

• Through the leaf surface which is the predominant method
• Through stems and buds
• Through the stomates due to volatile fumes

Foliar absorbed herbicides penetrate the cuticle and cell walls by diffusion.  The plasmalemma is penetrated after which the active ingredient of the herbicide can start its work in the cytoplasm (Figure 6).   The movement with which herbicides move through the epidermis is an active energy consuming process.

A graphical representation of the epidermis of a leaf. Note the difference between the contact area of a water or herbicide droplet with and without a surfactant.

The most common areas through which herbicides move on the leaf surface are:

• The ectodesmata such as guard cells of the stomata
• Hairs and other trichomes
• Anti-clinal epidermis walls
• Modified epidermis walls such as those overlying veins
• Openings created by stretched cells beneath the cuticle

It used to be assumed that herbicide entry into stomata was an essential pathway into the plant cytoplasm, however, the latest research has indicated that this is not the case.  Even if the herbicide enters through the stomata, it still has the reach the cytoplasm which is just as difficult as from the leaf surface.  All herbicides, contact and systemic, must move through the epidermis and be translocated through the plant to have any effect.

It has been found that polar solutions are not as effective as non-polar solutions and they have different pathways through which they enter the plant.  Water and salts are polar and oils are non-polar.  The problem that chemists have is that the cuticular wax or is the most hydrophobic (non-polar), or water “hating” part of the leaf, followed by cutin, pectin and cellulose layers with each layer getting more polar.  Thus non-polar herbicides such as oils enter the outermost layer of the leaf relatively easy while the experience more resistance as the move deeper into the leaf epidermis.  The opposite is true for polar herbicides, they have difficulty in entering the wax layers but find it easier the deeper they penetrate.  The goal of the chemists is to design herbicides which are not at the extreme of polar and non-polar, but a mixture in between.

Surfactants increase the absorption rate considerably since they reduce the surface tension of the droplet which increases the area of contact between leaf and droplet (Figure 6).  Surfactants further help penetration as a result of the following:

• Reduced air between the herbicide and leaf surface
• The surfactants act as solvents to help the herbicide penetrate the wax
• Enhances entry of the herbicide into open stomata
• Enhances the movement of the herbicide in intra-cellular spaces
• Many surfactants reduce the volatility of the herbicide which retards the drying out of the spray solution on the leaf surface.

Factors influencing the absorption of herbicides into the leaf are:

• The inherent wettability of the leaf surface
• The surface tension of the herbicide
• The thickness and waxiness of the cuticle
• The hairiness of the leaf surface

Climatic factors influencing the absorption of herbicides into the leaf are:

• Light: It enhances the absorption process
• Atmospheric temperature:  It can increase or decrease absorption rate.  Plants that grow in areas which have high temperature tend to have thicker wax layers.  High temperatures during spraying causes higher loss of herbicide due to volatility.  Low temperatures tend to reduce plant metabolism and reduces the activity of the herbicides dinoseb and bromoxynil.
• Rain: Rain can wash the leaf surface if it follows directly after an application. The influence of rain will depend on herbicide applied and the type of plant sprayed.  Some herbicides tend to be absorbed relatively quickly into the leaf while others take a while.  Non-polar herbicides will tend to stick stronger on the leaf surface than polar herbicides.
• Wind: Wind increases the rate at which the herbicide evaporates and the amount that is lost due to drift.

Absorption through shoots and stems

Shoot absorption is important during the germination of especially grasses.  The coleoptiles that emerges through the soil surface are highly susceptible to herbicide penetration since it has a very thin layer of wax on its leaves and they are usually actively growing seedlings.  The high metabolic rate increases the rate of herbicide translocation which reduces the time the herbicides takes effect on the plant (seedling).

The only time that stem penetration is important is on woody plants.  Although by spraying the herbicide on a plant some of the droplets will inadvertently come in contact with the stem of the plant.  Covering the stem will only increase the surface area of contact which will increase the effectiveness of the herbicide.

Translocation of herbicides in the plant

Once the herbicides have been sprayed on the plant, the herbicides have to be translocated to various areas where it can cause the damage or effect needed.  There are two pathways that herbicides follow:

• The symplastic pathways and
• The apoplastic pathways.

Some herbicides favour the symplastic pathways while others favour the apoplastic pathways and then other use both the pathways equally.

Symplastic trans-location of herbicides

Herbicides are translocated through the phloem involving mass flow of solutes.  Many herbicides block these pathways so that the photosynthetic products do not reach their destination and the plant dies.  In order to kill the root system, a herbicide must not kill the photosynthetic system of the leaves or block any solutes that are transported to the roots, instead the herbicides active ingredient must be transported to the root system where it must be able to cause its damage.  If too high concentration of herbicide is mixed, the leaf photosynthetic system is damaged and the above ground parts of the plant suffer but not the roots.  After a while the above ground parts while re-generate and the weed problem will persist.  The ideal time to apply herbicides that kill the root system are when the plants are growing rapidly which is:

• just after full leaf development
• after sexual reproduction
• Before autumn when plants tend to store reserves in their root systems.

Herbicides that are applied on leaves of weeds have to move through the epidermis as previously explained, then from cell to cell through the plasmodesmata or apoplast after which it can enter the phloem in order to be transported either upwards or downwards (Figure 7).

An example of the trans location of herbicides if applied on a leaf of a weed. The upwards (Acropetal) and downwards (Basipetal) movement of the herbicide is through the phloem

Apoplastic translocation of herbicides

Water is translocated in the apoplastic area of the plant through transpiration forces.  The direction of the apoplastic transport system is upwards towards the leaves. The apoplastic transport system or the non living transport pathway, consists of

• The xylem
• Intracellular spaces
• Cell walls

Absorption of herbicides that use the apoplastic pathway start at the roots.

Effect of herbicides on plant metabolism

Several critical pathways are influenced in various ways by herbicides.  The critical pathways are

• Photosynthesis
• Respiration
• Nitrogen metabolism
• Enzyme activity

The active ingredient of the herbicides do not actually interfere with the above processes but rather they block, or disrupt some of the reactions which take place in these processes.

Photosynthesis

The photosynthetic pathways that are disrupted are:

• The inhibition of the carotenoid or chlorophyll formation process.  The active ingredients that are responsible are: amitrole, fluridone, norflurazon.
• Interference with the electron transfer Photosystem II.  The active ingredients that are responsible are: bromoxynil, buthidazol, Fluridone, Ioxynil, Metribuzin, propanil, pyrazon.
• Generation of phytotoxic hydrogen peroxide through interference with the electron transfer chain in Photosystem I.  The active ingredients that are responsible are bipyridylium herbicides such as diquat and paraquat

Respiration

Since respiration is the most important energy releasing process, it serves as an ideal site where the active ingredient of herbicides can cause severe destruction.  Herbicides interfere with the respiration processes within the mitochondria in the following way:

• They act as inhibitors of the electron transport system.  The active ingredient of the herbicide either combines with the electron carriers or intercepts the transport of the electrons.
• They act as inhibitors of the energy transfer process.  The active ingredient of the herbicide combines with an intermediate of the energy-coupling chain which blocks the high-energy
• They interfere with the utilization of released energy through uncoupling agents

Nitrogen metabolism

Genetic code is made of amino acids which are derived from inorganic nitrogen such as ammonium (NH4+) and nitrate (NO3-).  The process through which these inorganic compounds are converted into amino acids is called nitrate metabolism.  The whole process produces three basic products:

• Amino acids
• Nucleic acids and
• Proteins

Herbicides that target nitrogen metabolism affect the nucleic acid synthesis process through stimulation of inhibition of the activity of the enzymes DNase and RNase.    Their effect is either an increase in proteins, amino acids or available nitrogen in the leaves, stems or other storage organs.

Enzyme activity

Enzymes are proteins but not all proteins are enzymes.  Enzymes consist of two parts: a protein part that determines its specificity and a non-protein part that determines the activity.  Herbicides affect enzyme activity directly and indirectly through.  Direct influences are:

• By changing the molecular structure of enzymes they interfere with normal processes in the nucleus of the cell.
• Herbicides compete with the non-protein cofactor of enzymes for the protein component.
• They compete with the enzyme for the active sites of the substrate.
• They impede the activity or stimulate enzyme activity through complex forming with either enzyme or substrate.
• The active ingredient of the herbicide changes the active component or the cofactor portion of enzyme, thereby alienating the cofactor portion from the protein portion.

Indirect influences are:

• The active component of the herbicide interferes with the production of ATP which adversely affects the production of energy to the plant which is required for all processes especially growth.
• They influence the production and concentration of co-enzymes which reduces the effectiveness of enzyme productivity.
• They affect the conditions that are required for effective enzyme activity

Factors affecting herbicide efficiency

Calibration

The calibration of equipment involves the following:

• Determine the width of the spray from the nozzle
• Determine the length of the spray area
• Determine the volume that needs to be sprayed
• Determine the volume/ha conversion
• Dissolve the correct amount herbicide in the water tanks of the spray equipment
• Apply the herbicide

Calibrating the equipment correctly is essential for good weed control.  Not only does it determine the effectiveness of the active ingredient of the herbicide but it also indirectly influences the cost of controlling the weeds.  Incorrectly calibrated equipment will either:

• Apply to much herbicide per area, thus wasting expensive chemicals without increasing the amount of weeds controlled.
• Kill plants that you do not want to control
• Control some of the weeds due to too low concentration of the chemical
• Partly control since application does not reach all the weeds it has been intended to control
• Cause large amount of drift resulting in too little active ingredient reaching the target and some of the drift affecting neighbouring commercial crops.

Volume coverage

The volume coverage referrers to the concentration of the active ingredient or the total volume of herbicide used per unit of water it is dissolved in.  The following units are commonly used:

• L.ha^-1
• kg.ha^-1
• mg.ha^-1
• ml.ha^-1
• g.100L^-1
• mg.100L^-1
• kg.100L^-1
• L.100L^-1
• ml.100L^-1
• mg.L^-1
• ml.L^-1
• g.L^-1
• g.100m^-1 row
• kg.100m^-1 row
• L.100m^-1 row

The varying volume concentrations used will depend on the characteristic of the surface which is being sprayed.  The total volume sprayed on a soil surface would be more than the total volume that is required if only the leaf area is sprayed.  The higher the total volume used to spray, the more even will the application of the herbicide be.  The higher the concentration of the herbicide is, thus the higher amount the active ingredient dissolved, the more difficult will it be to apply the herbicide evenly.

The type of equipment used

The type of equipment used plays an important part in the effectiveness of the herbicide applied.  Each piece of equipment has its limitations and shortcomings and the grower should be aware of them.  The following factors play an important part in the effective application of the herbicide:

• The speed at which the nozzles move over the infected area
• The height of the nozzles over the weeds
• The pressure with which the herbicides are sprayed on the weeds
• The amount of nozzles attached to the spray equipment
• The nozzle design
• The position of the nozzle on the equipment

Weed leaf area characteristics

The leaf area that need to be in contact the herbicide will have a significant influence on the effectiveness of the method of application as well as the type of surfactants added.  Leaf and growth characteristics such as the following have an influence:

• The leaf growth angle, thus how vertical does the leaf grow.  The more upright the leaf grows, the more difficult will the herbicide stick to the leaf. If the leaf grows horizontally, the herbicide will lie on top of the leaf and has enough time to penetrate the cuticular layer.
• Some weeds have many leaves at the top of the plant preventing the herbicide penetrating the bottom leaves.
• Other weeds have heavy growth lower to the ground so penetration is effective in the top layers.
• The amount of hairs growing on the leaf will have a significant effect on the ‘clinging’ ability of the herbicide especially if the leaf is growing very upright.

Atmospheric conditions

Atmospheric conditions can play a significant role in the effectiveness of the herbicide application.  Atmospheric conditions determine how much of the active ingredient reaches its target in relation to the amount of herbicide applied.  Conditions such as temperature, humidity and wind are important atmospheric conditions that will influence the amount of herbicide reaching its target in relation to the amount applied.  By mixing the herbicide with oil based solutions drift and volatility can be reduced, thus increasing the amount of herbicide reaching the target.  The pressure can also be reduced in order to increase the droplet size.

Factors influencing the uptake of herbicides

pH

Herbicides with dissociable groups attached are pH dependant during uptake.  The dissolvability of the herbicide in oil can be improved by altering the dissociable groups.

Growth stage

Young fast growing leaves are more susceptible than older mature leaves to herbicide penetration and uptake.  Thus a plant with relatively more younger leaves than older ones will absorb the herbicide faster.  Older more mature leaves have thicker and more develop cuticle layers which make penetration slower and less effective.

Leaf that are injured through either insects or diseases are more susceptible towards herbicide penetration.

History of the plant

It is a well known fact that healthy growing plants absorb herbicides faster than stressed out plants.  Quite often growers will apply some fertilizer and water to the soil in order to enhance the growth of the weeds in order to control them better.  The reason is that fast growing weeds have thin cuticular layers making it easier for the herbicide to penetrate.  The fast growing plant has also higher flow rate of cytoplasmic fluids thus the transport system within the plant is also more effective.

Concentration differences between carrier volumes

If a herbicide droplet reaches a leaf, the water will evaporate over time but the active ingredient of the herbicide will not.  The result is that the concentration of the active ingredient in the droplet is increasing over time which increases the rate of uptake per unit of leaf area as time goes on.

Concentration of chemicals

Although it is mentioned above that the higher concentration increases the absorption rate, there are situations that the herbicide might form a precipitate if the concentration exceeds a certain level.  Once the precipitate has been formed, the herbicide loses its characteristics.

In some cases droplet size is more important than concentration.  In other cases the translocation might be impaired due to damage to cells in the transport pathway by using to high concentrations.

Chemo-physical characteristics of the herbicide

Some properties of herbicides have the ability to injure the surface of the plant physically due electrostatic characteristics of both the herbicide and plant surface.  The result is that the active ingredient can penetrate the leaf surface quickly without any barriers.

It is important to note that surfactants do not always enhance the uptake of herbicides.  Some herbicides should not be used in conjunction with a surfactant since their chemo-physical properties have certain properties that are cancelled out if surfactants are added.

Conditions during and after application

The most important factors are relative humidity and temperature.  During periods of high RH, the droplets stay on the leaf longer leaving more time for the herbicide to penetrate before it is evaporated.  The same applies to lower temperatures.  The water status of the plant has also an effect.  By increasing the water status of the plant, through irrigation or just after good rains, the uptake, translocation of solutes are more effective that under water stress.  During water stress periods the stomata are closed, thus reducing the effectiveness of herbicide uptake.

It is obvious that if it rains directly after applying a herbicide its effectiveness will diminish substantially.

Transport

Any factor that influences the rate of translocation of nutrients inside the symplastic or apoplastic channels, will influence the transport rate of the herbicide which has either been absorbed through the above ground organs or below ground organs.

• Dry or moisture stress conditions
• Periods of high humidity and low radiation
• Damaged leaves due to insect infestations
• Shallow root systems or low fertile soils
• Very high temperatures etc.
• Easily drained soils (low clay sandy soils)

These are some of the conditions that reduces water flow in the plant due to stress, be it either moisture or temperature related.

Susceptibility of plants to herbicides

If a growth process of a plant is negatively influenced in the presence of a herbicide, it is said that the plant is susceptible towards the active ingredient of the herbicide.  The degree of the susceptibility varies from being hardly noticed to total collapse of the plants biological processes with the result that the plant dies.  Susceptibility is influenced by the following:

• Stage of plant growth when herbicide is applied
• The amount of active ingredient that is absorbed
• Physiological and morphological plant characteristics that influence the translocation of the active ingredient of the herbicide.
• The toxicity of the herbicide
• Environmental factors during the application of the herbicide which influences its effectiveness

Some plants may be susceptible during early stages of growth but as soon as the cuticle grows thicker or the plant matures,  the plants susceptibility decreases.  Important factors in the susceptibility of a herbicide is:

• The active ingredient must be absorbed by the plant
• The active ingredient must be transported in large enough concentrations in the plant to the specific site where the damage can be done.

There are many barriers within the plant that reduces the effectiveness of the active ingredient of the herbicide.  Many of these are shown in Figure 8.

The many barriers that the active ingredient of a herbicide faces during the process of reaching its active site.[Source adapted from Hilton, J.L., Moreland, D.e. and Jansen L.L. 1960. Herbicides in Plants. In The Nature and Fate of Chemicals Applied to Soils, Plants and Animals, pp. 119-133. Illus. Pp. 129. Agr. Res. Serv.., USDA, ARS 20-9.]

1.3.   Selectivity of herbicides

The ability of some herbicides to affect only certain species is called selectivity and is one of the useful aspects of modern herbicides.  There are two types of selectivity: true selectivity and placement selectivity.

• True selectivity is the reaction of various plant species to applied herbicides.
• Placement selectivity is when the herbicide is applied without coming in contact with other plant species.

Selectivity is never absolute.  By this it is implied that there are many factors that will influence the active ingredients selectivity and the plants susceptibility towards it.  All the factors as mentioned previously in this manual such as concentration, pH, environmental factors, the presence of surfactants, plant growth and age all play an important role in the herbicide selectivity.  Selectivity can be grouped into five categories:

• Physical
• Biological
• Properties if the active ingredient, adjuvants, surfactants and other chemicals included in the final herbicide mix
• Plant-protection mechanisms
• Pesticide interactions