BIOACTIVITIES & PERSISTENCE OF HERBICIDES

BIOACTIVITIES & PERSISTENCE OF HERBICIDES
BY
AYODELE OLATUNDE PHILIP
CRP\98\0188
A Course Seminar Submitted to the Department of Crop, Soil & Pest
Management, School of Agriculture & Agricultural Technology,
Federal University of Technology Akure
July, 2006

INTRODUCTION

Herbicides perform a vital role in the management of weeds. As the name
indicates, herbicides are chemicals that kill or control weeds 7. Although the ultimate
effect of most herbicides is the same (usually weed death), the way they control weeds
is vastly different 1. Physiologists use the term mode of action to describe the way
herbicides affect weeds. It includes the entire sequence of events that occur from the
time the weed absorbs the herbicide to the final plant response (usually death). Thus,
mode of action includes absorption, translocation to an active site, inhibition of a
specific biochemical reaction, degradation or breakdown of the herbicide in the plant
and the effect of the herbicide on plant growth and physiology.
Large differences exist in the length of time for which specific herbicides
provide acceptable levels of control. Persistence refers to the length of time that an
herbicide remains active in the soil 2. Depending on the herbicide, persistence can vary
from a matter of days to a few years. Long persistence is desirable in terms of weed
control but may be undesirable if new plantings are scheduled for a site or if the
herbicide poses a risk of contamination to ground or surface water.
Herbicide resistance which is the inherited ability of a weed or crop biotype to
survive an herbicide application to which the original population was susceptible is a
problem that could be circumvented by the use of herbicides with different sites or
mode of action in rotation 3. Therefore, the knowledge of mechanism displayed by
herbicides to disadvantage weed is crucial in weed management as it may affect the
efficacy of the herbicides when herbicides of same mode of action is continually used.

BIOACTIVITIES OF HERBICIDE

Herbicides kill plants in different ways. An herbicide must meet several
requirements to be effective. It must come in contact with the target weed, be
absorbed, move to the site of action in the weed, and accumulate sufficient levels at the
site of action to kill or suppress the target plant.
1.1 HERBICIDAL MODE OF ACTION
All herbicide interactions with a plant, from application to final effect, are
considered the mode of action. The mode of action involves the absorption into the
plant, translocation or movement in the plant metabolism of the herbicide, and the
physiological plant response.
1.1.1 Herbicide Translocation
Systemic herbicides are translocated in plants, while contact herbicides are not
translocated. To be effective, contact herbicides must be applied to the site of action.
Most foliar-applied contact herbicides work by disrupting cell membranes. Thorough
spray coverage of a plant is essential with foliar-applied contact herbicides to kill the
entire plant. Systemic herbicides can be translocated to other parts of the plant either in
the xylem or the phloem 1. The xylem is nonliving tissue through which water and
nutrients move from the roots to the shoots and leaves of plants. Translocation in the
xylem is only from the roots to the leaves. Phloem is a living, conducting system in
which materials can move both upward and downward. The phloem transports the food
that is produced in the leaves to the roots and to areas of new growth.
Herbicides can be translocated in the xylem, the phloem, or both. Translocation
depends on the chemical and the plant species. Herbicides translocated only in the
xylem are most effective as soil-applied or early post emergence treatments because
translocation is only upward. Atrazine is a good example of an herbicide that is
translocated only in the xylem. Phloem translocated herbicides that move downward
and suppress root and rhizome growth, as well as top growth, provide the best
perennial weed control. 2, 4-D, and Roundup are examples of systemic herbicides that
will translocate in the phloem and provide good, long-term control of certain perennial
weeds 1.
1.1.2 Herbicide Site of Action
Normal plant growth and development involves complex biochemical
reactions. Herbicides adversely affect some of these reactions in susceptible plants. The
following are biochemical influence of herbicide on plant physiology.
* Photosynthesis.
In the presence of light, green plants produce sugar (C6H12O6) from carbon
dioxide (CO2) and water (H2O) in a process called photosynthesis. Photosynthesis is a
two-phase process that occurs in the leaf chloroplasts. During the light dependent
phase of photosynthesis, the plant transforms light energy from the sun into biological
energy in the form of ATP (adenosine triphosphate) and NADPH2 (nicotinamide
adenine dinucleotide phosphate).
In the light independent phase of photosynthesis, ATP and NADPH2 supply
energy for the conversion of CO2 into sugars. Plants subsequently convert sugars into
longer chain carbohydrates, which represent the major stored portion of biological
energy in the plant. Herbicides that directly inhibit photosynthesis interfere with or
block electron transport and prevent ATP and NADPH2 production. This leads to
decreased sugar or food formation. However, the visual injury symptoms (chlorosis,
desiccation or browning of plant tissue) occur too rapidly to be the result of starvation
of the plant.
Alternatively, chlorosis of leaf tissue may be due to the photo-destruction
(damage from excessive light) of chlorophyll and other plant pigments. When
herbicides block electron transport, chlorophyll continues to absorb light energy but
cannot pass this energy on to make ATP and NADPH2. Thus, chlorophyll either selfdestructs
from the energy it is absorbing or passes this absorbed energy on to
oxygen. This forms radical oxygen, which is highly destructive to cell membranes
and other cell structures. Cell-membrane destruction causes leakage of the cellular
contents and results in the desiccation of plant tissue.
Herbicides that inhibit photosynthesis include simazine and bentazon.
Additionally, the modes of action of oxyfluorfen, diquat, oxadiazon and glufosinate
relate to the photosynthetic process 8.
* Amino-Acid and Protein Synthesis.
Plants use proteins in functional, storage and structural roles. Functional
proteins are called enzymes. Enzymes catalyze thousands of chemical reactions
necessary for plant growth and development. Storage proteins commonly occur in
seeds and supply essential amino acids to young, developing seedlings. Both enzymes
and seed proteins consist of long chains of interconnected amino acids. Commonly,
just l7 to 20 different amino acids occur in plants. However, the amino-acid
composition between different plant proteins varies greatly. In the absence of aminoacid
and protein synthesis, plants cannot complete the chemical reactions necessary for
growth. Imazaquin, halosulfuron and glyphosate inhibit the synthesis of specific
amino acids. Without them, protein synthesis decreases, certain metabolic reactions
cease and the plant gradually dies over a period of one to several weeks.
Protein synthesis is under the direct control of DNA (deoxyribose nucleic
acid) and RNA (ribonucleic acid). DNA is located in the nuclei and chloroplasts of
plant cells and contains the genetic information that determines the sequence of amino
acids in the various plant proteins. RNA (messenger RNA) transports the genetic
information that is contained in DNA and is involved in the assembly (transfer RNA)
of amino acids into proteins. Metolachlor and napropamide interfere with nucleic-acid
synthesis, which in turn decreases protein synthesis 8.

* Cell Division
Plant growth includes the process of cell division, or mitosis, which the
nucleus initiates and regulates. During cell division, a mother cell divides into two
identical daughter cells. Herbicides that interfere with cell division are called mitotic
poisons. When these products block cell division, new cell production decreases and
eventually growth stops. Herbicides that inhibit cell division are the dinitroanilines,
pronamide and dithiopyr 8.
* Cell Membranes
A cell wall and a membrane (the plasma membrane) enclose plant cells.
Contact herbicides cause a breakdown of the cell membrane and leakage of the cellular
contents. The plant then undergoes rapid wilting and desiccation, often within hours
of the herbicide application. Plant tissues appear burned.
However, the mode of action of most contact herbicides is not due to actual
burning or caustic action of the herbicide; contact herbicides affect specific
physiological processes. For example, diquat intercepts electrons during the lightdependent
phase of photosynthesis, creating free radicals. These pass electrons to
other compounds that form superoxide radicals and hydrogen peroxide, which are
toxic and break down the cell membrane. Oxyfluorfen and oxadiazon are other
herbicides that cause massive membrane disruption through the process of free-radical
formation 8.
* Fatty Acid Synthesis
Fatty acids are critical components of cell membranes. If fatty-acid synthesis is
blocked or inhibited, plants are unable to form the cell membranes necessary for
normal growth. The post-emergence grass herbicides sethoxydim, fluazifop,
fenoxaprop and clethodim inhibit fatty-acid synthesis in susceptible grassy weeds.
Tolerant plants (that is, broadleaf) have a different structure of the enzyme that these
herbicides do not affect 8.

* Cell-Wall Biosynthesis
As mentioned, a cell wall (in addition to a membrane) composed of cellulose,
hemicelluloses, pectin and other compounds encloses all plant cells. Cell-wall
biosynthesis begins during the process of cell division and continues during the
growth of the cell. The primary purpose of the cell wall is to impart rigidity and
structure to the plant. The mode of action of isoxaben is inhibition of cell-wall
biosynthesis 8.
* Pigment Synthesis
Carotenoids (yellow in color) and chlorophyll (green in color) are plant
pigments located in the chloroplasts of leaf cells. Both carotenoids and chlorophyll
absorb light during photosynthesis. An additional function of carotenoids is that they
protect chlorophyll molecules from photo-oxidation (damage from excessive light).
Norflurazon inhibits carotenoid synthesis.
In the absence of the protective carotenoids, chlorophyll breaks down in
sunlight and susceptible plants become bleached or white in color due to photooxidation.
Plant death occurs slowly due to the eventual depletion of stored food
reserves and the inability of the plant to manufacture new sugars 8.
* Growth Regulation
Auxins are natural plant hormones that regulate plant growth and are under
direct metabolic control by the plant. At low concentrations, auxins promote normal
growth and development. However, at abnormally high concentrations, auxins inhibit
plant growth. Several herbicides, such as 2,4-D and related compounds, mimic the
activity of heavy doses of auxins and thereby cause abnormal plant growth.
The exact mode of action of auxin-type herbicides is unknown. The first
apparent symptom after application is a downward twisting or curvature of the leaves
and stems of susceptible plants, often within hours of application. Although other
symptoms are slower to develop, plants also undergo rapid, uncontrolled cell division
8
and enlargement. Vascular tissues responsible for the transport of food materials and
water become plugged or broken, and the plant slowly dies over a 2- to 4-week period.
Recent evidence indicates that auxin-type herbicides stimulate the production
of excessive amounts of ribonucleic acids (DNA and RNA). This induces uncontrolled
cell enlargement and division and results in the abnormal growth of susceptible plants.
Herbicides that have auxin-type activity include 2,4-D and related phenoxy herbicides,
and dicamba 8.
* Nitrogen Metabolism
Glufosinate inhibits an essential enzyme involved in nitrogen metabolism.
This enzyme helps convert inorganic nitrogen, in the form of ammonia, into amino
acids. Glufosinate interferes with the activity of this enzyme, which causes toxic levels
of ammonia to accumulate in plant cells. This, in turn, directly inhibits photosynthesis.
The result is rapid tissue necrosis and death of the treated plants 8.
However, it is worth to note that some herbicides are capable of trigging off
their effects in more than one physiological process.
1.1.3 Herbicide Selectivity and Metabolism
The herbicide's ability to kill certain plants without injuring others is called
selectivity. The potency of an herbicide to adversely affect or kill all plants varies.
Base on this, herbicides are classified into selective and non- selective herbicide 7.
Herbicide that kill or suppress the growth of most plant species are relatively non
selective. Herbicide selectivity is relative and depends on several factors, including
environment, herbicide application rate, application timing, and application technique.
Plants tolerant to herbicide often metabolize the chemical into non-toxic
substances. Differential metabolism is often the basis for herbicide selectivity 2.

1.2 THE NON TARGET EFFECT OF HERBICIDE
It is established that the soil enzymatic reactions are influenced by herbicide,
so important ecological functions of the soil- those associated with organic matter
decomposition, mineralization of nutrients, and synthesis of humic substances need to
be reviewed in relation to herbicide application. Ismail, B. S. et al., 1997 discovered that
Metsulfuron-methyl at 5· 0 μg/g caused a reduction in amylase and urease activities in
soil throughout 28 days of study. Also, it was reported that herbicides inhibited the
activities of soil enzymes in the early stage of treatment but increase the activities of
urease, L-glutaminase and protease later 5.
In addition, there is evidence to indicate that herbicides applied in field
cultivations of leguminous plant may influence nodulation, dinitrogen fixation and
development of growing plant 6

2.0 HERBICIDE PERSISTENCE
Herbicide persistence or residual life is the length of time an herbicide remains
active in a soil. The soil persistence of an herbicide is often stated as”half-life,” or
‘T1/2”, which is the amount of time it takes to decompose 50 percent of the applied
chemical to an herbicidal inactive form 2. The ideal soil-applied herbicide is one that
controls weeds for a desired period and then rapidly degrades or breaks down in the
soil to non-phytotoxic levels.
Understanding the residual life, or soil persistence, of an herbicide is
extremely important. It not only determines the length of weed control that is expected
but also influences the plant selection of succeeding plantings. Physical, chemical and
microbial processes affect the persistence of herbicides in the soil. Volatility, leaching
and soil erosion by wind and water are physical processes that also affect herbicide
persistence by determining how their movement from the application site.
* Volatility
This is the process by which an herbicide changes from a liquid or solid state
to the gaseous (vapor) state. Once in the vapor state, the herbicide rapidly leaves the
area of application, and poor weed control or injury to non-target plants can occur if
enough of the herbicide volatilizes. Chemical characteristics, soil moisture,
temperature and adsorption of the herbicide to soil colloids all affect herbicide
volatility. For example, under hot, dry conditions, pronamide volatility is high. The
dinitroaniline herbicides (oryzalin, trifluralin, prodiamine, benefin and pendimethalin)
vary in their volatility characteristics. Oryzalin and prodiamine are perhaps
the least volatile, followed by pendimethalin, benefin and trifluralin 8.
Mechanical incorporation, rainfall or irrigation within 1 to 2 days of
application will prevent or dramatically reduce the volatility losses of dinitroaniline
(and other) herbicides, resulting in better control as well as reduced risk of non-target
effects.

* Leaching.
The movement of herbicides in soil by water is called leaching. Leaching of
herbicides can occur in any direction in the soil, but the most common direction is
downward. Soil texture, the adsorption of the herbicide to soil colloids, the water
solubility of the herbicide and the amount of water movement through the soil all
affect the amount of herbicide lost to leaching.
The movement of an herbicide by leaching is important to weed control
effectiveness, herbicide carryover, and the potential for environmental problems.
When an herbicide is leached downward, the concentration of herbicide near the soil
surface is reduced, lessening the chances for herbicide carryover problems 2.
Herbicides, such as the salt forms of 2, 4-D, have a low tendency to adsorb to soil
colloids and readily leach in fine-sand or silt-loam soils. In contrast, the dinitroaniline
herbicides and most other pre-emergence herbicides readily adsorb to soil colloids and
resist leaching 8.
* Adsorption.
Adsorption is the attraction of ions or molecules to the surface of a solid. After
application, many herbicides adsorb (bind) to the clay and organic-matter fractions of
soils. However, herbicides adsorb poorly to the sand and silt fractions of soil.
Therefore, the extent of herbicide adsorption increases as the percentage of organic
matter and clay increases. The dinitroaniline herbicides, dithiopyr, oxadiazon and
most other pre-emergence herbicides readily bind to soils 8. Weed control is inversely
proportional to how much herbicide is adsorbed to the soil.
In general, small increases in the organic matter content of a soil greatly
increase its ability to adsorb herbicides. A soil high in organic matter content will
generally require a higher herbicide rate than a soil with less organic matter. Adsorbed
herbicide molecules are unavailable for biological, physical, and chemical processes
until released from the soil into the soil solution or vapor phase. Herbicides generally
are more tightly adsorbed in dry soils than in wet soils. Water molecules compete and
displace herbicide molecules from adsorption sites, making the herbicides available for
plant uptake 2.
* Photo-decomposition.
Herbicides break down or degrade in sunlight. Specifically, the ultraviolet
(UV) portion of sunlight is responsible for photo-decomposition. Several herbicides,
such as most dinitroanilines herbicides, are photo-degradable. Therefore, they require
incorporation into soil with tillage, rainfall or irrigation to retain their herbicidal
activity 2.
* Microbial processes.
Microbial decomposition is one of the most important processes by which
herbicides break down in the soil. Microorganisms use many organic herbicides as a
food source. Thus, soil temperature, aeration, pH, organic matter and moisture levels
that favor microbial growth also promote rapid herbicide breakdown. Herbicides that
microbes can affect include the dinitroanilines, metolachlor, napropamide, pronamide,
bentazon, dithiopyr, glufosinate, glyphosate and isoxaben 2.

REFERENCES
1. Anonymous
Herbicide Mode of Action. Kansas State University Extension Publication
C-715.
2. Daniel L. Devlin, Dallas E. Peterson and David L. Regehr, 1992
Residual Herbicides, Degradation, and Recropping Intervals.
Kansas State University Agricultural Experiment Station and
Cooperative Extension Service C-707 April 1992
3. Gunsolus, J.L, 1999
Herbicide Resistant Weed. North Central Regional Extension
Publication 468. University of Minnesota Extension Service
4. Ismail, B. S., Yapp, K. F. and Omar, O., 1997
Effects of Metsulfuron-methyl on Amylase, Urease, and
Protease Activities in two Soils. Australian Journal of Soil Research
36(3) 449 - 456
5. Kim, J.E and Hong, J.U., 1988
Effects of Herbicides on enzyme activities in soil environment.
J. Korean Agric. Chem. Soc., 31:79-85
6. Niewiadomska, A. and Sawicka, A., 2002
Effect of Carbendazim, Imazetapir and Thiram on Nitrogenase
Activity, Number of Microorganisms in Soil and Yield of Hybrid
Lucerne (Medicago media). Polish Journal of Environmental Studies
Vol. 11, No. 6 (2002), 737-744
7. Phillips, T.A., 1977
An Agricultural Notebook. Lowe & Brydone Printers Limited,
Thetford, Britain. pp 312
8. Tim Murphy, 1998
Understand the Mode of Action and Persistence of Ornamental
Herbicides. www.grounds-mag.com