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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 notlimited 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. این گروه دارای چندین علفکش هست از جمله:(موسوی.کنترل علفهای هرز ص123) bispyribac-na . Pyrithiobac-na . Piribenzoxim . Pyriftalid Sulfonylamino-Carbonyltriazolinone The two herbicides in this chemical group, fl ucarbazone-sodium 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-SHIKIMATE- 3-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 بالا 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 لازم دارد این علفکش نسبتا کند بوده و حدود 7-10روز 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.
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
