FATE OF HERBICIDES IN THE ENVIRONMENT

Chapter 5

 

As soon as a herbicide is applied to its target, a number of processes immediately begin to remove the compound from the original site of application.  This removal is the process of environmental fate.  For the herbicide which is intercepted by plants, the chemical may be taken up by the plant itself, may be washed off of the foliage by precipitation or irrigation onto the soil, may undergo photodegradation on plant surfaces, or may volatilize back into the air. 

 

 

The herbicide that falls directly upon the soil or is washed onto the soil can undergo a number of processes which may be broken down into two main groupings: degradation and transport processes.  Degradation processes include biological degradation by soil organisms and abiotic chemical and photochemical transformations.  Transport of herbicides within the soil compartment can occur downward into the soil profile (leaching), across the soil surface (runoff), or into the air (volatilization).  Each can be a combination of more fundamental processes including adsorption, convection, and diffusion.

 

 

Most herbicides are organic compounds and are therefore basically unstable in the environment.  Inherent instability is essential to prevent these materials from accumulating in the environment as compounds are repeatedly used.  Accumulation of pesticides not only poses environmental hazards, but prohibits rotation to sensitive crops.  Unfortunately, not all herbicides exhibit optimal stability properties, and therefore numerous examples of insufficient control (too little stability) or carryover (too much stability) have been reported.  Furthermore, persistence in the environment prolongs exposure of the materials to forces that can cause movement of the herbicide away from the application site.  The extent of transport, with the exception of spray drift, is a function of both the stability and the physicochemical characteristics of the herbicide in the environment to which it is applied. 

 

 

 

I.  Biodegradation

 

The environmental stability of a herbicide is strongly dependent on its tendency to decompose into an inactive or nontoxic form (or in some cases a more active or toxic form) as a result of chemical reactions that may involve living organisms.  Biological reactions generally involve bacteria and/or fungi.  Reactions not involving living organisms are abiotic and may involve light or catalytic surfaces such as soil clays.

 

Generally speaking, most abiotic reactions, with the exception of those induced by light in certain compounds, result in only partial modification of herbicides.  Under natural conditions, complete degradation of pesticides to mineral constituents such as carbon dioxide, chloride, or phosphate nearly always requires biological degradation.  However, inactivation or detoxification of a pesticide may require only a single reaction and often occurs via abiotic mechanisms. 

 

Whether inactivation or detoxification is adequate to remove risk associated with herbicides has been a subject of much debate; however, most agree that persistence of an inactive form of a herbicide poses less risk than persistence of the active form.

 

Terminology

 

There often is heated debate about precise usage of terminology, but the following are some general terms you should learn.

 

 

Degradation -            generally refers to the process of chemical modification of pesticides by abiotic and biotic processes.  The mechanism is often used as a descriptor (biological degradation, photochemical degradation, etc.).

 

 

Decomposition -       is often used interchangeably with degradation.

 

Dissipation - is often used interchangeably with the two aforementioned terms.  However, it more accurately denotes the process of removal of the pesticide from the environment, without regard to whether the material is degraded in the process.

 

Inactivation-    usually refers to the loss of pesticidal activity, through either structural modification of the pesticide or transfer to some unavailable compartment (such as irreversible sorption to soil colloids).

 

 

Detoxification -          may refer to inactivation or more commonly indicates loss of ecological toxicity of a pesticide.

 

Transformation -        designates a change in the chemical structure of a pesticide (often used with a modifier such as biotransformation, etc.).

 

 

                                                    Role of Chemical Properties

 

Biological degradation of herbicides for the most part involves recognition of the herbicide by one or more enzymes.  Complete mineralization of herbicide to carbon dioxide may require the cooperation of more than one type of organism.  It is important to remember that nearly all herbicides are xenobiotic substances; that is, they are foreign to biological systems.  Since they are anthropogenic (man-made) and novel, they were not present during the evolution of present-day organisms.  Recognition of a xenobiotic substance, such as a herbicide by an enzyme, requires either a certain sloppiness of enzymes (non-specificity) or some adaptation of the enzymes due to mutation.  It is apparent that some enzymes, particularly those capable of hydroxylating aromatic rings, may recognize xenobiotic substances without prior exposure to the substance by the organism(s) that produce the enzymes.  There are a number of enzymes from a variety of sources that can transform certain herbicides, demonstrating that mutation is not necessary for recognition of xenobiotic substances by biological substances. 

 

 

In the environment, some substances are more rapidly degraded with each passing season (example the thiocarbamate herbicides), probably as a result of some type of microbial adaptation, suggesting that mutations important to pesticide degradation do in fact occur in nature.

 

 

The fact that huge differences in degradation rates exist among herbicides indicates either that some structures are easier for organisms to degrade or that the genetic information for degradation of some structures is not widely distributed among actively growing organisms.  Only very minor differences often exist between some extremely long-lived compounds in the environment from those that are relatively unstable.

 

 

A.  Aerobic Biodegradation Processes

 

Most microbial processes in the environment are strongly influenced by the availability of molecular oxygen.  Many soil organisms are unable to function or compete under anaerobic conditions, and thus the influence of their presence diminishes when oxygen is limiting.  In terrestrial environments, oxygen depletion usually results from limited oxygen supply (typically due to excess water) which is compounded by significant oxygen consumption (driven by carbon sources being degraded by microorganisms).  Many reactions involved in pesticide degradation are carried out more rapidly by aerobic organisms, and some reactions actually require molecular oxygen as a substrate.

 

One of the more common steps in pesticide degradation by aerobic organisms is the manner in which aromatic rings are decomposed.  Microbes tend to activate aromatic rings prior to ring cleavage  by introducing hydroxyl groups.

 

 

B.  Anaerobic Biodegradation Processes

 

Pesticide degradation does not have to stop in the absence of oxygen.  Contrary to previous theories, anaerobic systems are often metabolically very active and utilize novel mechanisms to degrade pesticides.  Anaerobic metabolism of pesticides in usually different from aerobic metabolism but is not necessarily slower or faster.  Among the most noteworthy anaerobic transformations of pesticides are reductive dechlorination (common in lake sediments) and aromatic ring reduction.

 

 

Effect of chemical structure on biodegradability of aromatic and aliphatic compounds.

 

Structure

Compound

Biodegradable

Aromatic ring substituents

Carboxylic acids, phenolic hydroxyls, methoxyl and methyl groups

Aliphatic chain features

Straight unsubstituted saturated chains

Persistent

  Aromatic ring substituents

Nitro, amino, sulfonate, and halogen groups

Other chain features

Branched chains, halogenated hains

 

Some of the characteristics that are common to easily degradable versus persistent compounds are listed in the above table.  Generally, the addition of functional groups that are difficult to remove by oxidative reactions usually increase the persistence of compounds in aerobic environments.  Halogens such as chlorines and bromines fall into this category.  Halogens withdraw electrons from adjacent carbons, rendering them less susceptible to electrophilic substitution and, in general, less susceptible to oxidation.  As the number of halogen substituents increases, the relative persistence of a compound increases.  The fact that the halogen-carbon bond is not commonly found in nature may explain in part why removal of halogens from pesticides is often slow in the environment. 

 

II.  ABIOTIC DEGRADATION

 

Although soil can exhibit intense biological activity, non-biological or abiotic, transformations may become dominant in some cases.  In most soil systems, biological and abiotic transformations occur simultaneously, and efforts to separate the individual processes is difficult to say the least.  The two main classes of abiotic reactions are hydrolysis and photolysis.

 

A.  Hydrolysis

 

            Hydrolysis is defined as the chemical process of decomposition involving the splitting of a bond and the addition of the elements of water.  Hydrolysis is the most prevalent reaction to occur in the liquid phase of soils, the soil water.  Hydrolysis can also take place near the water-solid interface where the surface properties of the solid phase often dominate.

 

            In water environments, hydrolysis is also a prevalent reaction and is greatly influenced by pH and temperature.

 

B.  Photolysis

 

            Photochemical degradation by sunlight is often the primary degradation route for herbicides present in exposed environments including soil and plant surfaces, water, and the atmosphere.  All photolytic reactions begin with the absorption of light energy by some chemical compounds which can then undergo a variety of chemical or physical processes.  Photolysis is often viewed as a detoxifying mechanism, producing products of lower environmental significance than the parent molecule, although there are some exceptions.  Photolysis reactions can be grouped as direct or indirect.

 

Direct photolysis:  The pesticide itself absorbs light energy and then reacts.  The rate of photolytic reaction is directly correlated to the overlap of the absorption spectrum of the herbicide and the spectral distribution of sunlight.

 

Indirect photolysis:  Occurs when a chemical species other than the compound of interest absorbs light energy and initiates a series of reactions that eventually degrades the pesticide.  These other chemical species are known as photosensitizers and can include naturally occurring organic and inorganic species, including humic materials, clay minerals, ozone, and various free radicals produced by the interaction of sunlight with air and water.

 

 

III.  TRANSPORT PROCESSES

  

Soil is a porous medium that consists of minerals and organic aggregates or peds, surrounded by pore space containing soil water and soil air.  The peds themselves also contain pores, although they are much smaller than the inter-particle pores.  Soluble herbicides are primarily transported through the soil profile as solutes dissolved in the soil water while volatile compounds are carried in the soil air.

 

The description of transport can be broken down into a series of fundamental processes, all interacting closely and often simultaneously.  Because the complexity of the soil system, the interactions of the processes are not completely understood.

 

 Sorption

 

Adsorption is defined as the accumulation of chemical at either the soil-water or the air-water interface, or the processes on how a chemical associates with a surface.  These processes may include true adsorption at the molecular scale onto a soil particle surface, precipitation from solution, or even covalent bonding to soil organic matter.  It is often difficult to distinguish the separate processes, so they are often lumped into the more general heading of sorption.  In general, all these processes contribute to the removal of chemical from the soil water phase, rendering them less available for further transport, uptake by plants, or degradation.

 

All soil-applied herbicides are adsorbed to some extent, and their herbicidal activity is reduced in direct proportion to the amount that is retained on the soil colloids by adsorption.  Adsorbed herbicides are in a passive state and are unavailable to biological, physical, and chemical processes until desorption occurs.

 

Absorption is defined as the uptake, or surface penetration, of ions or molecules by, or into, any substance or organ such as the uptake of herbicides or nutrients by plant roots.

 

Desorption is defined as the release, or displacement, of adsorbed or absorbed ions or molecules from the substance to which they are adsorbed or absorbed.

 

Sorption is defined as either absorption or adsorption or both and does not specifically indicate one or the other.

 

In the environment, sorption is a dynamic, highly complex process in which partitioning between soil solution and surfaces occurs continually.  Pesticides can be associated with the solid phase by a wide range of physicochemical processes including van der Waal's forces, hydrogen bonding, dipole-dipole interactions, ion exchange, covalent bonding, protonation, ligand exchange, cation adsorption, and water bridging.

 

As herbicides exhibit a wide diversity of chemical properties, the relative importance of the various sorption processes will vary for each chemical.

 

Convection and Diffusion

 

In convection (mass transport), dissolved chemicals move with the soil water through the soil profile.  Especially in highly structured or fractured soils, mass flow can transport pesticides over large distances in relatively short time.

 

Diffusion is a slow process, moving chemical molecules only very short distances.  Molecules move in solution driven by concentration gradients from areas of high to low concentrations.

 

A.  Leaching

 

Leaching is movement of a herbicide with water, usually, but not always downward.  It is of environmental concern because of the possibility of groundwater contamination.  It can also reduce the effectiveness of a soil-applied herbicide by moving it out of the zone of safe germination for weed seed.

 

Generally, leaching is inversely related to the percent of organic matter and the percent clay, and therefore to adsorption. the greater the adsorption of a herbicide and the adsorptive capacity of a soil, the less leaching will occur.

 

The principle factors that influence herbicide leaching in soils are:

 

1.  Soil texture

 

2.  Soil permeability

 

3.  Volume of water flow

 

4.  Adsorption of herbicide to soil colloids

 

5.  Water solubility of herbicide and other pertinent chemical features.

 

 

B.  Runoff

 

As many herbicides are applied to, or wash onto, the surface of the soil, runoff can be a significant route of transport of herbicides from their initial site of application.  Often, the ultimate sink for runoff is local surface water such as ponds, lakes, and streams, where it is likely to be of practical as well as public concern.  The Federal Water Pollution Control Act Amendments of 1972 requires that states develop guidelines to help control water pollution from non-point sources, such as agricultural runoff.

 

Runoff includes not only the water leaving a field in surface drainage but also any dissolved or suspended material it may contain.  Herbicides may be dissolved in runoff water, suspended as particles, or sorbed to soil carried by runoff water. 

 

A number of factors affect the amount of chemical transported by runoff.  These may be subdivided into climatic, soil, chemical, and management factors; again, the site-specific combinations of all these factors influences the amount of chemical lost by runoff.

  

Climatic factors.

 

The timing of runoff events governs the concentration of herbicide present in runoff water.  Highest concentrations of active ingredients occur at the first significant runoff event after application.  Concentrations decrease with time as the compound dissipates from soil and plant surfaces as a result of environmental processes.  Increased intensity of the rainfall event causes a greater volume of runoff water and can increase the energy of extraction of the compound from the soil.  On the other hand, an increase in intensity (before infiltration rate is surpassed) can leach the chemical into the soil profile, rendering it unavailable for runoff.

 

Soil factors.

 

Water tends to run off more quickly in finer-textured or compacted soils because of decreased infiltration rates.  Topography is an important influence; increasing slope can increase the actual rate of runoff, increase the amount of sediment washed into the runoff, and increase the depth in the soil at which the runoff has an effect.  High organic matter content affects sorption of the chemical as well as infiltration rates, thus reducing runoff potential.  In addition, water content is important, as runoff occurs when the surface soil becomes saturated.

 

Chemical factors.

 

The chemical properties of the applied herbicide and its formulation have a pronounced effect on the degree of loss through runoff.  Research has found that chemicals with water solubilities > 10 ppm were primarily transported in the water  runoff as dissolved compounds while hydrophobic compounds were carried along with soil sediments that were washed away.  Wettable powder formulations are most prone to runoff.  In addition, more persistent compounds which remain near the surface after application, as well as compounds applied at high rates, are more prone to runoff.

 

Management factors.

 

Erosion control practices on agricultural land can reduce runoff losses of compounds sorbed to suspended particles.  However, these practices will have little effect on the transport of soluble compounds unless the erosion control also reduces the volume of runoff water.  Crop residues may also slow sediment transport, although increased loss of chemical may occur if the compound is washed directly into runoff water.  In addition, the use of vegetative buffer strips around application areas can help to intercept captured or dissolved chemicals through sediment deposition or plant sorption.

 

 

C.  Volatilization

 

A herbicide has the capacity to move into the atmosphere by two distinct processes.  These are spray drift, which occurs during application of a herbicide formulation to a site, and volatilization, which usually occurs after the completion of application.

 

 

Volatilization is considered one of the primary pathways for herbicide dissipation from the site of herbicide application.  Under unfavorable conditions, losses that result from volatilization can reach 80 to 90% within a few days although such rates are dependent upon climatic and microclimatic conditions.

 

The moisture content of soil has been determined to be one of the most significant environmental parameters which influences the rate of herbicide volatilization.  In general, herbicides volatilize more rapidly from moist than dry soils.  The reduced volatilization of a herbicide under dry conditions has been attributed to the exposure of additional adsorption sites on the soil by the evaporation of water from the soil surface.

 

 

        GENERAL CONSIDERATIONS FOR REDUCED ENVIRONMENTAL IMPACT

  

First, make good educated decisions when developing a weed management program. Pesticide users have come under public scrutiny recently because of concerns of pesticide occurrence in natural waters.  Furthermore, since pesticide users have greater exposure to pesticides than other groups, concern over health risks to pesticide users has also increased.  Though the pesticide user may not need to know structure biodegradability relationships and complex photochemistry to use pesticides effectively, knowledge of why pesticides pose environmental risk can assist the applicator in making decisions that may determine the environmental impact from the use of pesticides.

 

 Manufacturers choose new candidates for development on the basis of efficacy and environmental and toxicological properties.  Users should also include potential environmental impact in their choice of product lines and formulations.  Once a product is chosen, the chemistry is fixed.  How the product is used thus becomes the deciding factor in minimizing environmental impact.  The user is limited in flexibility by approved use practices specified on the herbicide label.  However, with some understanding of environmental properties of the herbicide to be applied, the user can have a significant effect on the environmental impact.

 

Some sites are more vulnerable than others, because of the soil type and/or topography.  Deep sands are more permeable than clay soils; thus groundwater is more vulnerable at such sites.  Water wells are perhaps the most important features at any site, since contamination of groundwater through well-heads is instantaneous, typically severe, and, most likely, long-term and difficult to remedy.  The user should determine the location of wells on the site and assess their condition.  Spraying and handling should never be done near an uncapped wellhead.  Proximity to surface water is an extremely important feature, since herbicides are commonly transported to streams in runoff water, in detached sediment, or as spray drift.  Use of buffer zones (untreated areas) near surface water is an obvious choice.  Soils that are plowed annually pose a greater risk for surface water contamination than those under long-term reduced tillage.  There is some concern that reduced tillage may at least slightly increase the risk of groundwater contamination relative to annual plowing.  This is because annual plowing breaks up natural macropores (worm holes) which facilitate rapid downward movement of water in undisturbed sites.  Whether reduced tillage actually increases the risk of groundwater contamination by pesticides is not well understood, although decreased risk of surface water contamination as result of reduced tillage has been clearly demonstrated.

 

Timing of application is probably the most important management factor that can be adjusted by the applicator, and, unfortunately, most often cannot be controlled by the applicator. Most pesticide movement either to the surface or to the groundwater occurs in the first major storm event after application.  Heavy losses are reported when application occurs immediately before a major storm.  Light gentle rain lacking significant runoff may help move the compound into the soil micropores, thus reducing the effects of a subsequent heavy rainfall.  This scenario holds not only for transport of chemicals in surface runoff, but also for movement of chemicals to groundwater via macropores.  Applicators obviously cannot control the weather but can be aware of the relationship between timing of application and weather patterns.

  

Proper disposal of pesticides may be more important than proper use in many cases.  Unused materials and tank mixes are concentrated forms of pesticides, which when applied to small areas or dumped on the soil surface may be much more toxic and likely to move than when disposed of properly.  Disposal should follow recommended procedures, follow the label if specific guidelines are listed.  When choices exist, the user may wish to inquire which disposal technique causes the least impact.  It should be noted that the historical practice of rinsing tanks (especially disposing of rinsate on the soil surface near a well) has often been cited as contributing factors to groundwater contamination.  These observations make it clear that management practices other than pesticide use may influence the environmental impact of pesticides.

 

 The LD50 of Some Herbicides and a Few Other Chemicals.

_

Common name

LD50 (mg/kg)

Technical

176 lb person (g required)

Aspirin

750

60

Caffeine

200

16

DDT

87

6.96

Diazinon

66

5.28

Table salt

3320

265.6

Imazethapyr  

>5000

400

Paraquat

112

8.96

Dinoseb

58

4.64

Oryzalin

>10000

800

Bromoxynil    

400

32

Glyphosate    

5600

448

Picloram

8200

656

Ethyl Alcohol

4500

360

 

FATE OF HERBICIDES APPLIED TO PLANTS

 

 

Once a herbicide has contacted the plant surface, six things can happen to the chemical:

 

 

1.         It may be volatilized and be lost to the atmosphere or washed off by rain or irrigation.

 

2.         Remain on the outer surface in a viscous liquid or crystalline form.

 

3.         Remain on the outer surface of the leaf and be broken down by photodegradation.

 

4.         Penetrate the cuticle, but remain absorbed in the lipoidal components of the cuticle.

 

5.         Penetrate the cuticle, enter the cell walls and then translocate (apoplastic translocation) prior to entering the symplasm. 

 

6.         Penetrate the cuticle, enter the cell walls and then move into the cellular system (through the plasmalemma) for symplastic translocation, which includes phloem movement.

 

 

The physical form of the herbicide on the leaf surface after spray application can have a significant impact on its activity.  The most recognized effect is crystallization of herbicide active ingredient on the leaf surface, which reduces effectiveness. 

  

I.  HERBICIDE SELECTIVITY

 

 

A selective herbicide is one that is much more toxic to some plants than others within the limits of:

  

            1) a specified dosage range

 

            2) the method of herbicide application

 

            3) environmental conditions that follow application

 

           

 Thus selectivity is a relative term, indicating that a given herbicide is safer to some plants than others under a specific set of conditions, often including method of application. 

 

Herbicide selectivity is the basis for successful chemical weed management in crop production and is possible as a result of physical and/or biological factors that produce differential herbicide toxicity for crops and weeds.

  

Physical factors that impart selectivity by application method do so by causing a lethal dose of herbicide to be in physical contact with weeds but not crop plants. 

  

Biological factors that make some plants less susceptible to a herbicide than others include 1) genetic resistance, 2) differential absorption, 3) differential translocation, and/or 4) differential metabolism of the herbicide.   

Although selectivity of some herbicides among plants is largely attributable to a single mechanism, e.g., differential metabolism, often a combination of application method and biological tolerance is required to impart the greatest degree of selectivity over a range of herbicide dosages and environmental conditions.  Herbicide selectivity is not absolute.  Virtually all herbicides lose selectivity if applied improperly or under environmental conditions that adversely affect biological tolerance mechanisms.

 

 

A.  Physical Placement

 

Herbicide selectivity by physical placement is discussed here in general terms and refers to any factor(s) that result in a physical (spatial) separation between sensitive crop tissues/absorption sites and a toxic dose of herbicide.  The desired weed and crop selectivity is achieved when a toxic concentration of herbicide is in direct contact with weeds but concentrations toxic to crop plants are prevented. 

 

 

Preemergence application of herbicides used for selective control of small seeded grass and broadleaf weeds in large-seeded annual crops is one example of selectivity by placement.  In this case, crop seed are planted below the zone of greatest herbicide concentration in which the smaller-seeded weeds are germinating. 

 

By a similar mechanism, some perennial crops avoid damage from soil-applied herbicides by having deep roots that avoid contact with toxic concentrations of herbicide.  Perennial weeds often use this mechanism to survive herbicide treatments.

 

 Directed postemergence sprayers, shielded sprayers, rope-wick applicators, and other specialized herbicide application equipment that directs herbicides onto weeds and away from crop plants are other examples of herbicide-application selectivity by placement.  This management technique is good for precluding the development of herbicide resistant weeds since the differential selectivity is due to herbicide placement and not differential metabolism.

 

 

A number of factors other than application method and application timing relative to crop or weed growth stage can ultimately influence physical placement of herbicides, including mechanical incorporation, irrigation, herbicide formulation, chemical properties of the active ingredient, and physicochemical properties of the soil.  Restrictions on herbicide use regarding these factors are routinely included on product labels. 

  

Environmental variables interact with the physical factors affecting herbicide selectivity, and extremes in rainfall and temperature that can affect herbicide movement or alter plants' physiological responses to a herbicide, may reduce selectivity.

 

 

B.  Plant Factors and Selectivity

 

 

Tolerance mechanisms include differential herbicide absorption, translocation, and metabolism.  Herbicide selectivity between crop plants and weeds is usually greatest when crop plants are resistant to rather than tolerant of a herbicide. 

 

 Tolerance is defined as when plants respond variably to a herbicide or are only partially affected by the normal herbicide dosage.  Tolerance is usually associated with plant anatomical or physiological mechanisms that prevent a lethal dosage of herbicide from reaching its target site.

 

 Resistance is defined as the mechanisms by which some plants withstand dosages of a herbicide that are normally lethal to other plants.  Normally resistance is associated with the inability of a herbicide to bind or interact with its normal site of action.

 

1.  Differential Absorption.

 

 Herbicides on plant foliage or in contact with subterranean plant organs must cross several natural barriers before ultimately reaching their biochemical site of action.  The pathway of absorption and translocation is more rigorous for some herbicides than others and depends on the morphological, anatomical, and physiological characteristics of the individual plant on which the herbicide is deposited.  Perhaps the greatest variability in rates of herbicide absorption among plants occurs after foliar applications.

  

Genetically controlled and environmentally influenced morphological characteristics such as leaf angle, leaf arrangement, leaf area, and presence of hairs (pubescence) on the leaf surface first determine the maximum potential herbicide dosage from a herbicide spray that is intercepted and retained by a plant.

 

 

Differential spray retention among plants may play a role in determining selectivity of some herbicides.  For example, studies have shown that selective postemergence control of wild oat in flax with asulam is possible because flax retains less spray than wild oat.  Addition of an adjuvant to the spray mixture can increase herbicide spray retention and thus herbicidal activity in some weeds, but this may be accompanied by a concomitant decrease in weed/crop selectivity.  The role of differential spray retention as a factor in determining selectivity between weeds and crops is probably minor for most postemergence herbicides used in agriculture.

  

Herbicides deposited on a plant surface must first penetrate nonliving barriers before ultimately reaching their site of action in living plant tissue.  The outermost layer covering all aerial plant parts is the cuticle, which represents the most significant barrier to foliar penetration by herbicides.

 

 The cuticle is a complex layer consisting of waxes embedded in a matrix of polymeric cutins with an outermost coating of soluble waxes collectively called the epicuticular wax.  The cuticle prevents excess water loss from transpiration and provides plants some protection against insects and pathogens.  The impervious nature of the cuticle also makes it quite effective at preventing or impeding the entry of many foliar-applied herbicides.

 

Epicuticular waxes are hydrophobic (non-polar and oil-soluble) and decrease leaf wettability by repelling water and other hydrophilic (polar and oil-soluble) substances.  A simplified explanation of the solubility of one substance in another and its dependence on the relative polarities of each is given by the adage that "like dissolves like".  This simply means that hydrophilic substances tend to be readily soluble in other hydrophilic substances, whereas they are insoluble in hydrophobic substances, and vice versa.  This principle is applied when discussing foliar penetration of the waxy cuticle by herbicides.

 

Some herbicides, such as glyphosate, are highly polar and do not easily penetrate the nonpolar epicuticular wax layer.  In such cases, use of an adjuvant is necessary to facilitate herbicide penetration.  Conversely, herbicides of a more hydrophobic, nonpolar nature are more soluble in epicuticular waxes and thus tend to penetrate plant cuticles more readily than polar herbicides.  Herbicides formulated as salts are relatively polar, whereas those formulated as esters are relatively nonpolar.

 

Cuticle thickness varies from plant to plant and even among leaves of the same plant.  Cuticle composition and thickness depends on the age of the tissue and environmental conditions that exist during deposition of cuticular components, including the epicuticular wax layer.  Leaves that develop under conditions of high light, low soil moisture, and low relative humidity tend to develop a thicker epicuticular wax layer than those which develop under low light, adequate soil moisture, and high relative humidity.  Not surprisingly, plants with leaves like this are more difficult to control with most foliar-applied herbicides.

 

Subsequent environmental conditions can also affect cuticle thickness and permeability. 

 

Adequate soil moisture, maximum sunlight, and high relative humidity tend to keep cuticles hydrated and thus more conducive to penetration by herbicides.  Drought or prolonged periods of hot, dry weather can result in an increase in epicuticular wax, cuticle dehydration, and plant stress, and foliar application of herbicides during such periods may result in a loss of effectiveness due to lack of herbicide absorption. 

 

Use of a proper surfactant or other adjuvant can aid foliar wetting and enhance cuticular penetration of herbicides under adverse conditions.  Addition of one or more adjuvants to foliar herbicides is often recommended even when environmental conditions are favorable; however, improper adjuvant selection or dosage may reduce selectivity between crop plants and weeds.

 

Roots, like leaves, have a cuticle through which herbicides must initially penetrate in the process of absorption.  However, the cuticle on root surfaces generally lacks a waxy covering and is much thinner and more permeable to herbicides than the foliar cuticle.

 

Soon after germination but before emergence, developing shoots of some plants may also serve as sites of herbicide absorption from soil and be a source of herbicide selectivity.  For example, the coleoptile as well as the radicle of some grass seedlings are strong absorption sites for carbamothiate herbicides.  As a result, those grasses are especially susceptible to soil-applied carbamothiates during and immediately after germination.

 

 

2.  Differential Translocation

 

 Translocation is the net internal movement of water and dissolved substances from one plant region to another.  After penetration of the cuticle, most herbicides are translocated in the apoplast and/or symplast some distance before reaching their site of action.  The apoplast is a nonliving, continuous network of cell walls, intercellular spaces, and xylem tissue that functions as the conduit for water and mineral nutrition transport from the roots to the shoot.

 

The symplast is the continuous network of living cells, intercellular protoplasmic connections (plasmodesmata), and phloem tissue that transports sugars and other organic solutes from leaves to storage organs and to other areas in the plant.

 

After absorption and before long-distance translocation in the xylem or phloem can occur, herbicides must first undergo short-distance translocation from the epidermis to the vascular tissue. In the foliar tissue, the herbicide moves to the xylem or phloem via normal apoplastic and/or symplastic routes.

 

Herbicides absorbed by roots, however, may encounter an additional barrier if short-distance translocation occurs in the apoplast.  Herbicides in the root apoplast that are traveling toward the xylem and phloem in the center of the root first encounter the endodermis, a layer of cells surrounding the vascular tissue. 

  

The endodermal cell walls are impregnated with suberin, creating a banded layer called the Casparian Strip that is impermeable to herbicide penetration.  Therefore, to reach the xylem or phloem tissue for long-distance translocation, the herbicide must penetrate the cell membrane and enter the symplast in order to bypass the Casparian strip.  Having bypassed the Casparian strip, the herbicide may enter the xylem or phloem for long-distance translocation.

 

Herbicides in the apoplast are able to penetrate the cell membrane and enter the symplast, and virtually all herbicides must penetrate the cell membrane to reach their biochemical site of action inside the cell.  Therefore, no herbicide is totally confined to either the symplast or the apoplast.  In fact, some herbicides translocate extensively in both the apoplast and the symplast.  However, long-distance translocation of many herbicides occurs predominantly in one system or the other.

  

The primary route of herbicide translocation is dependent on physicochemical properties of the herbicide and conditions within the plant.  Some herbicides, for example, may undergo ionization upon entry into the symplast, resulting in an anionic form of the herbicide that is unable to penetrate back across the cell membrane.  The anionic herbicide is thus "trapped" in the cytoplasm and is largely confined to translocation in the symplast.  Conversely, nonionic herbicides that move freely across cell membranes and into and out of living cells undergo translocation primarily in the apoplast.  Differential rates and extent of translocation sometimes play an important role in determining herbicide selectivity among plants.

  

The driving force behind apoplastic translocation is transpiration, so net movement of apoplastically translocated herbicides in plants is upward from site of entry along the gradient from highest water potential (soil solution or soil water) to lowest water potential (the atmosphere) via the transpiration stream.  As a result, apoplastically translocated herbicides are generally soil-applied, where they are absorbed by roots and translocated upward via the water potential gradient to transpiring leaves.  Some apoplastically translocated herbicides are effective against weeds when foliar-applied, but uniform spray coverage of foliar tissue and use of a surfactant are usually required for maximum effectiveness since little downward movement of the herbicide form the site of absorption can occur.

  

Differential apoplastic translocation after root uptake plays a role in determining selectivity of several herbicides, including linuron, metribuzin, simazine, fluometuron, chloramben, prometryn, and terbacil.  In some instances, herbicides become compartmentalized after absorption and are immobilized or sequestered in roots or veins of tolerant species where damage is minimized.  The mechanisms by which some herbicides are immobilized in tissues of tolerant species is not clearly understood, but others are known to undergo partial metabolism (conjugation) which renders them immobile in the apoplast.

 

 

Translocation of herbicides in the symplast is bidirectional and direction of net movement depends on the location of areas of greatest assimilation supply and demand within the plant.  The supply and demand for assimilate in plants are determined by the relationship between sites of net assimilate production (sources) and sites of assimilate utilization (sinks), respectively.  The direction of symplastic transport in green plants is from source to sink.  Mature green leaves carrying on maximum rates of photosynthesis are primary sources, whereas meristematic regions or other tissues with high metabolic activity and rapid growth are sinks.

 

Source-sink relationships ultimately determine the rate, direction, and extent of phloem-mobile herbicide translocation and vary among weeds according to life cycles.  Maximum herbicidal activity on annual weeds with phloem-mobile herbicides generally is attained if plants are nonstressed and treated during period of rapid growth.  During this period, the root and shoot apical meristems are primary sinks into which phloem-mobile herbicides are readily translocated along with the flow of photosynthate from mature leaves, the primary sources.

  

Perennial weed source-sink relationships are more complex than that of annuals, and response to phloem-mobile, symplastically translocated herbicides is highly dependent on stage of development.  Since a key objective in perennial weed management is to destroy over-wintering vegetative propagules (rhizomes, stolons, tubers, etc.) as well as the foliage, phloem-mobile herbicides should be applied during a period in which photosynthate is being accumulated in the propagules.  Successful perennial weed management with foliar-applied, phloem-mobile herbicides such as glyphosate, dicamba, and 2,4-D is based on application timing relative to source-sink relationships.

 

Differential translocation plays a role in determining the selectivity of some phloem-mobile herbicides, including 2,4-D, dicamba, glyphosate, imazamethabenz, and chlorsulfuron.  As with differential apoplastic herbicide translocation, the mechanisms by which tolerant species compartmentalize phloem-mobile herbicides are poorly understood.  In general, plants that exhibit tolerance to phloem-mobile herbicides by not translocating them must also be able to metabolize the absorbed herbicide into an inactive form in order to prevent delayed toxicity.

 

 

3.  Differential Metabolism

 

The most common mechanism that contributes to herbicide selectivity is differential metabolism.  A plant that is tolerant to a herbicide by this mechanism is capable of altering or degrading the chemical structure of a herbicide through enzymatic (and occasionally nonenzymatic) reactions that render the herbicide nontoxic.

 

 

Most plant enzymes that metabolize herbicides are present in fixed amounts in the plant cell and have a relatively broad range of specificity that may allow a single species to metabolize and detoxify a number of different herbicides.  The rate, in addition to the extent of herbicide metabolism, is often an important factor in determining selectivity.

 

Herbicides may be classified according to their ability to undergo biotransformations in plants as 1) stable, 2) metabolically deactivated, or 3) metabolically activated.

 

 

            1.         Stable herbicides are those which do not undergo metabolic deactivation in plants.

 

            2.         Herbicides that are metabolically deactivated may undergo oxidation, reduction, hydrolysis, and/or conjugation reactions that render them nontoxic in tolerant species.

 

            3.         Metabolically activated herbicides, once they are absorbed by sensitive plants, undergo metabolic transformation that results in an enhancement of herbicide phytotoxicity rather than detoxification.

 

 

Glyphosate and paraquat are examples of stable herbicides that are not metabolized to any extent by most plants; this characteristic is not surprising since they are two of the most nonselective herbicides in existence.

 

Herbicides that are extremely selective by metabolic deactivation are the sulfonylureas and imidazolinones, for which differences in sensitivity between tolerant and sensitive species can be several-thousand-fold.  Tolerant species rapidly metabolize sulfonylureas and imidazolinones, whereas the rate of metabolic detoxification is much slower in susceptible plants.

  

The classic example of a herbicide that undergoes metabolic activation is 2,4-DB.  Plants susceptible to 2,4-DB such as common lambsquarters and redroot pigweed enzymatically convert the relatively nontoxic 2,4-DB to the phytotoxic 2,4-D acid via a normal cellular process called betaoxidation.  Tolerance of several legumes to 2,4-DB is based on their rapid metabolic deactivation of 2,4-DB before lethal quantities of 2,4-D can accumulate.

 

 

HERBICIDE ANTIDOTES

 

Selectivity of some herbicides may be attained by the use of chemicals that protect plants against toxic herbicidal action.  Herbicide antidotes, also referred to as herbicide protectants or safeners, allow the use of certain herbicides for selective weed control in crops that otherwise would be susceptible to those herbicides.  Chemical antidotes may be applied to crops as seed treatments or be included as components of the commercial herbicide formulation. 

 

 

The development and technology of herbicide antidotes are relatively new, and presently there are commercial antidotes which protect corn from carbamothiate herbicides and corn and grain sorghum from chloroacetamide herbicides.  The primary mode of action of the herbicide antidotes studied thus far involves the stimulation of herbicide metabolism in the protected plants. Future antidotes may be developed that allow weed-crop selectivity for a wider range of herbicides than those for which antidotes are currently available.

 

 

BIOTECHNOLOGY AND FUTURE DEVELOPMENTS IN SELECTIVITY

 

 

A greater understanding of herbicide detoxification enzymes in plants, herbicide structure-activity relationships, and gene transfer technology has had and will have a major impact on selective chemical weed management.  Recent registrations include Roundup Ready Cotton, Soybeans, and Corn, Liberty Link Corn and Soybeans, and BXN (Buctril resistant) Cotton.  One benefit of biotechnology may be the development of safer and more effective herbicides. 

 

 

Genes that encode resistant target sites or enzymes that detoxify specific herbicides can now be inserted into some crops and will play a major role in chemical weed management in the future.  So instead of using synthetic chemical safeners we are looking at using biological safeners.  Crops for which research and development of herbicide-resistant varieties is being conducted include canola, tobacco, cotton, sweet potato, tomato, sugar beet, oilseed rape, alfalfa, corn, soybean, and flax. 

 

 

However, the ecological impact of using herbicide-resistant crops is unknown, and development of resistant weed populations should be of great concern.  Ideally, new herbicides and herbicide-resistant crops will not be relied solely upon for weed management; rather they will be utilized to complement existing tools in well-planned, long-term integrated weed management systems.

 

 

HERBICIDES AND ENZYME INHIBITION

 

The most recent major advance in herbicide chemistry has been the development of highly active herbicides that target specific plant enzymes.  Some of the newer herbicide families exhibit a high degree of biological activity at relatively low rates of application and are low in mammalian acute toxicity. 

 

 

Enzymes are special proteins that perform the important biological functions of catalyzing metabolic chemical reactions. The compounds on which biosynthetic enzymes act are called substrates, and enzymes have unique active sites that attract and/or interact with only very specific substrates that "fit" the active site.  Enzymes are required for biosynthesis reactions that assemble complex molecules from simpler, intermediate compounds.

  

An enzyme often catalyzes only a single, yet critical biosynthesis reaction.  Some classes of herbicides act as inhibitors of specific enzymes by blocking the enzyme's active site and preventing catalysis of the normal substrate molecules. This leads to a depletion of the critical product of the reaction, and the resulting deficiency leads to disruption of normal plant metabolism. 

  

For example, the sulfonylurea, imidazolinone, and triazolpyrimidine herbicides are three herbicide families that inhibit the enzyme acetohydroxyacid synthase (AHAS or ALS), a plant enzyme that catalyzes the formation of the branched-chain amino acids: leucine, isoleucine, and valine.  Amino acids are the building blocks of all proteins including enzymes, so a depletion in any of the 20 amino acids required by plants quickly leads to a depletion in critical proteins necessary for normal metabolism, growth, and development.

 

The approach of developing herbicides that inhibit specific plant enzymes has been quite successful, resulting in highly selective compounds that are low in mammalian toxicity.  A benefit of this strategy is that other organisms can be genetically resistant to a specific enzyme-inhibiting herbicide either by lacking the target enzyme (mammals do not have AHAS) or by having a different structural form of the enzyme for which the herbicide has low or no affinity.  However, the latter mechanism can also result in the occurrence and development of herbicide-resistant weed populations.