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Emad-el-Din Najafi
‫بازدارنده های ساخت اسید آمینه‬
‫چندین نوع علف کش بیوسنتز کننده اسیدهای آمینه را تحت تاثیر قرار‬
‫میدهند که هر یک از آنها مانع فعالیت آنزیم خاصی می شوند‪ .‬اسیدهای‬
‫آمینه واحد های سازنده پروتئین هستند‪.‬پروتئین ها در کار کرد موجودات‬
‫زنده نقش حیاتی دارند‪ .‬پروتئین ها در کارکرد آنزیمی خود به صورت‬
‫کاتالیزور عمل میکنند و باعث تنظیم و سرعت بخشیدن به واکنش های‬
‫بیوشیمیایی می شوند‪.‬برخی پروتئین ها به انتقال ترکیبات در گیاهان کمک‬
‫میکنند در حالی که برخی دیگر به صورت نیتروژن و ترکیبات ذخیره کننده‬
‫انرژی عمل می نمایند‪.‬‬
‫گیاهان همه اسید های آمینه ضروری را می سازند‪،‬در حالی که پستانداران‬
‫از جمله انسان توانایی محدودی در ساخت اسیدهای آمینه دارند‪.‬بنا براین‬
‫ماده شمیایی که در گیاهان مانع ساخت اسید آمینه سود برای گیاه سمی‬
‫است‪،‬البته احتمال وجود محل هدف متناسب برای این مواد شیمیایی در‬
‫حیوانات ضعیف است‪.‬‬
‫اکثر بازدارنده های بیوسنتز اسید های آمینه سمیت بسیار کمی برای پستانداران دارند‪.‬‬
‫هر چند این علفکش ها از نظر ساختمان شمیایی متفاوت هستند‪،‬اما همگی از سنتز اسید های آمینه گیاه‬
‫جلوگیری میکنند‬
A. INHIBITORS OF ACETOLACTATE SYNTHASE
(ALS)—ACETOHYDROXYACID SYNTHASE (AHAS)
Sulfonylureas
In the 1980s the sulfonylureas were introduced by the DuPont Co. Thecore
structure for the sulfonylureas combines the photosynthetic inhibitors
ureas and triazines, but the primary mechanism of action is inhibition
of amino acid synthesis not photosynthesis. Secondarily, they inhibit
photosynthesis,
respiration, and protein synthesis. Plant symptoms include chlorosis,
necrosis, terminal bud death, and vein discoloration. The site of action
for the sulfonylureas catalyzes the fi rst step in the biosynthesis of the three
branched chain aliphatic amino acids valine, leucine, and isoleucine. A
secondary
effect is cessation of plant growth (stunting) due to cessation of cell
division and slow plant death. Tolerance is related to a plant’s ability to
detoxify the herbicide.
Table 13.8 shows the range of selectivity of sulfonylureas and the primary
crop of use for 17 sulfonylurea herbicides sold in the United States. Several
others are available in other countries. A notable attribute of these
herbicides
is that they are active at rates in the range of 8 to 80 g (grams) ha−1.
TABLE 13.8. A Summary of Information About Some Sulfonylurea Herbicides.
Wheat is not affected by chlorsulfuron until soil concentrations
approach 100 ppb. Lentil and sugarbeet, on the other hand, are affected
by soil concentrations of 0.1 ppb. This thousandfold range in activity is
un precedented in herbicide chemistry. Great care is required to use
these
herbicides so that their activity and weed control potential are
exploited,but
untoward environmental problems are avoided. Several weed species
have
developed resistance to these herbicides, some in as little as three years,
afterannual use.
Imidazolinones
The imidazolinones, also developed in the 1980s, are active at low rates.
Their site of action is the same as the sulfonylureas, but their activity is lower.
At this writing there are six imidazolinones available in the United States.
Imazamethabenz is a selective, postemergence herbicide for control of
some annual grasses and broadleaf weeds in wheat, barley, and sunfl owers.
Imazamox is used postemergence to control annual broadleaf and annual
and some perennial grass weeds in alfalfa, edible legumes (e.g., dry beans),
soybean, and crops tolerant of imidazolinones (e.g., canola). Imazapic controls
a wide range of annual broadleaf and annual and perennial grass weeds
in peanuts. Imazapyr is not selective in crops, does not leach vertically or
laterally, and is used for weed control on non-cropland and in imidazolinone
tolerant corn. Imazaquin is not limited to postemergence application as most
imidazolinones are. It is used to control annual grass and broadleaf weeds
in soybean. Imazethapyr is used for pre- or postemergence control of annual
grass and broadleaved weeds in soybean, edible legumes, alfalfa, and peanut.
It has a relatively long soil persistence, and while small grains and rice can
be planted within four months of its use, corn, dry beans, and sorghum
should not be. There can be problems with soil, effects on rotational crops,
and development of weed resistance.
Pyrimidinylthio-Benzoate
Chemically, pyrithiobac is a benzoate and the only herbicide
in this group
available in the United States. When used pre- or
postemergence, it controls
several annual broadleaved weeds in cotton. Although
chemically distinct from
other ALS inhibitors it acts in the same way.
‫کنترل علفهای هرز‬.‫(موسوی‬:‫این گروه دارای چندین علفکش هست از جمله‬
)123‫ص‬
bispyribac-na . Pyrithiobac-na . Piribenzoxim . Pyriftalid
Sulfonylamino-Carbonyltriazolinone
The two herbicides in this chemical group, fl ucarbazonesodium and propoxycarbazone,
are both active against different annual grass weeds when
applied postemergence in wheat.
Triazolopyrimidines
The second edition of this book reported only one triazolopyrimidine, fl
umetsulam,
that had been approved for use in the United States. It is still available,
and three new herbicides have been added to the group. Cloransulam-methyl
is applied pre- or postemergence to control broadleaf weeds in soybean.
Diclosulam is soil-applied to control broadleaf weeds and perennial nutsedge
in peanuts. Florasulam is used in spring or winter cereal crops. Flumetsulam
is used pre- or postemergence in combination with at least one other herbicide
in soybean and corn to control a range of broadleaved weeds. It has
little activity against grasses. Soil life is short so rotation for each of these
herbicides and injury to rotational crops is not a problem. Low use rates
minimize leaching in soil. The four herbicides in this chemical group possess
a diversity of activity against weeds and selectivity in crops. They illustrate
why site of action is a better system of classifi cation than chemical group or
crop of use.
B. INHIBITORS OF 5-ENOLPYRUVYL-SHIKIMATE3-PHOSPHATE SYNTHASE (EPSP)
Glyphosate was released by Monsanto Chemical Co. in 1971. It is now sold
under several trade names by Monsanto and other companies. Its discovery
and release were as revolutionary in weed science as the discovery of 2,4-D.
The structure of the amino acid glycine is underlined in Figure 13.4; glyphosate,
the N-phosphonomethyl derivative of glycine, is a nonselective, foliar
herbicide with limited to no soil activity because of rapid and nearly complete
adsorption. It controls perennial grasses and has an advantage over paraquat,
because glyphosate translocates. It is the only available herbicide that inhibits
EPSP synthase. The enzyme is common in the synthetic pathways leading to
the aromatic amino acids phenylalanine, tyrosine, and tryptophan. These
amino acids are essential in plants as precursors for cell wall formation, defense
against pathogens and insects, and production of hormones (Duke, 1990). The
enzyme is not found in animals. Glyphosate has very low mammalian toxicity.
Secondarily glyphosate affects respiration, photosynthesis, and protein synthesis.
It is active only postemergence because it is completely and rapidly
adsorbed on soil colloids. Its nonselectivity means that it will affect, if not
kill, almost any green plant it contacts. Low application volume is more effective
than high volume, and small plants are more readily controlled than large
ones. Paraquat, a photosynthetic inhibitor, acts quickly (one or two days) on
most plants. Glyphosate activity usually cannot be detected as quickly and may
take several days to appear after application. One glyphosate formulation
is used as an aquatic herbicide. Transgenic crops resistant to glyphosate
have been created and marketed. At least eight species have been reported
to be resistant to glyphosate (www.weedscience.org/Summary/UspeciesMOA
.asp?1stMOAID = 12; accessed March 2006). Resistant species include Palmer
amaranth, common ragweed, hairy fl eabane, goosegrass, Italian ryegrass, rigid
ryegrass, and buckhorn plantain. Resistance has been found in Australia,
Chile, South Africa, Spain, and in 15 US states.
C. INHIBITION OF GLUTAMINE SYNTHETASE (GS)
Glutamine synthetase (GS) is essential for assimilation of organic nitrogen as
ammonia (Duke, 1990). Its lack leads to very high ammonia levels. Glufosinate
(phosphinothricin) is the only available herbicide that inhibits GS. It is available
in the United States for complete weed control in noncrop areas and as a
directed spray in fi eld- and container-grown nursery stock. It is rapidly
degraded in soil with a half-life of seven days. Even though it is not adsorbed
tightly, it does not leach because it is degraded quickly. Glufosinate is nearly
nonselective. It has been made selective in corn because a gene coding for
phosphinothricin acetyl transferase activity was isolated from the soil bacteria,
Streptomyces hygroscopicus, and cloned into corn. The acetyl transferase
enzyme converts glufosinate to its nonphytotoxic acetylated metabolite,
en abling crops to achieve resistance by rapidly metabolizing glufosinate
‫گلوفوزینیت آمونیوم‬
‫این علفکش اثر تماسی و عمومی داشته‪،‬اثر سیستماتیک آن ضعیف است و اثر زیادی بر‬
‫روی ریشه های دائمی و ریزم ها ندارد‪.‬‬
‫علفکشی هست که منشا بیولوژیکی دارد و از روی توکسین باکتری ها ساخته شده است‪.‬این‬
‫علف کش از طریق برگ جذب شده و حرکت روبه باال دارد ‪.‬در گیاهان مانع سنتز گلوتامین‬
‫می شود عدم سنتز گلوتامین موجب جمع شدن آمونیاک تولید شده در چرخه بیوسنتز گلوتامین‬
‫می شود که برای گیاه کشنده است‪.‬‬
‫از آنجا که جمع شدن آمونیاک در حد کشندگی زمان الزم دارد این علفکش نسبتا کند بوده و‬
‫حدود ‪10-7‬روز پس از سمپاشی گیاهان را از بین می برد‪.‬‬
‫این علف کش در ایران با نام تجاری باستا به ثبت رسیده است‪ .‬از طریق رشه اثر زیادی‬
‫ندارد و جذب کلوئید خاک می شود‬
How does herbicide resistance develop?
Herbicide resistance involves a shift in weed biotypes within a particular species.
A biotype is an individual within a species that has characteristics not common
to the population as a whole. In this case, the selection was for giant foxtail
plants with the ability to survive an application of ALS herbicide. Over a period
of five or six years, repeated use of a chemical with the same mode of action
effectively turned the minority population into a majority population.
The development of herbicide resistant weeds is not new. The first case of
herbicide resistance was documented in 1957. As of 1998, the International
Survey of Herbicide Resistant Weeds Internet site reports 218 resistant biotypes
worldwide. There are likely two more known cases of ALS resistant foxtail in
Fond du Lac County but confirmation tests have not been performed
Should I quit using ALS herbicides?
ABSOLUTELY NOT! ALS inhibitor herbicides represent a high
percentage of products currently being marketed. Taking steps to
avoid herbicide resistance insures that these products will be
effective weed control tools for many years to come. However, if
you’ve been using ALS herbicides on the same field for several
years, it’s time to rotate your herbicide program to a product with a
different mode of action. This doesn’t mean you can’t return to an
ALS inhibitor product in future years.
If you have a field(s) in a situation where ALS herbicides have been
applied for four or five years, take time this winter to sit down with
your ag chemical dealer and explore some different herbicide
options for 2000. A list of common ALS herbicide products currently
being marketed is presented in Table 1.
Table 1. Amino Acid Synthesis (ALS)
Inhibitors
Accent
Basis
Beacon
Broadstrike/
Python
Classic
Concert
Exceed
FirstRate
Harmony Extra
Lightning
Permit
Pinnacle
Pursuit
Raptor
Reliance
How do I avoid herbicide resistance?
Aside from rotating herbicide modes of action, there are several things producers
can do to help prevent herbicide resistant weeds from taking hold. They are as follows:
Use herbicides only when necessary.
Use herbicides with multiple modes of action within the same year and on the same field.
Whenever possible, use broad crop rotations that include 3 or 4 different crop species.
Integrate mechanical control practices like rotary hoeing and cultivating
Scout fields regularly and control escaping weeds as needed
Clean tillage and harvest equipment before moving from fields or farms with known resistant weed problems
The questionnaire sent to weed scientists and other
experts in the U.S. resulted in no reports of Agrostis or Polypogon species
that are resistant toglyphosate or glufosinate.
To date there have been no reports of glufosinate resistant weeds
To date only three grass species (goosegrass, Italian ryegrass, and rigid ryegrass) have
developed resistance to glyphosateThis is especially true since the effect of
increased glyphosate use in glyphosate-resistant crops may have impacts on the development of
resistant weeds in the future.
There are several weed species that are very adaptable and are likely candidates for the
evolution of glyphosate or glufosinate resistance. Preliminary studies (Goss et al. 2001; Goss et
al. 2002; Goss and Gaussoin 2003) suggest that applications of glyphosate to successive
generations of annual bluegrass, large crabgrass [Digitaria sanguinalis (L.) Scop.], and
dandelion (Taraxacum officinale Weber in Wiggers) will select for more tolerant types of each
species.By comparison, some other modes of herbicide action such as ALS
(acetolactate synthase) inhibitors, triazines (photosystem II inhibitors), and ACCase (acetyl CoA
carboxylase) inhibitors have 83, 65, and 33 weed species, respectively, that have developed
resistance to them worldwide. Figure 1 presents data for the U.S. indicating the relative risk of
developing resistant weeds when using various herbicide modes-of-action
The top of the triangle indicates groups that may develop resistance
quickly and those at the bottom of the triangle have demonstrated a low risk of
resistance developing over long-term use.
Beaware that low risk does not mean no risk since weeds have developed
resistance to herbicides in these groups as well
Rotation of herbicides and
other pesticides is an important measure to delay the
onset of resistance to any one pesticide group or mode-of-action