II. Amino acid synthesis inhibitors (SOA 2, SOA 9)
The amino acid synthesis inhibitors include the following herbicide families:
- Imidazolinone
- Sulfonylurea
- Sulfonylamino carbonyltriazolinone
- Triazolopyrimidine
- Amino acid-derivatives
Amino acid synthesis inhibitors act on a specific enzyme to prevent the production of specific amino acids, key building blocks for normal plant growth and development.
Enzymes function as steps in biological processes. They are also extremely specialized in their function. As a result, many different enzymes are involved with the many different biological processes that occur within a plant.
Some herbicides can stop specific enzymes from functioning, as shown in the plants on the right in Figure 1. This results in disruption of specific plant processes that often leads to death of the plant.
This herbicide-enzyme relationship is very specific and any chemical modification of the herbicide or enzyme can eliminate herbicidal activity.
Acetolactate synthase (ALS ) enzyme inhibitors
Imidazolinone, sulfonylurea, sulfonylamino carbonyltriazolinone and triazolopyrimidine herbicides prevent the production of three essential branched-chain amino acids by inhibiting one key plant enzyme, acetolactate synthase (ALS enzyme).
These herbicides can move in the xylem and phloem to areas of new growth and taken up through plant foliage and roots. Herbicides in these families vary greatly in selectivity and may control annual and perennial broadleaf or grass weeds and may be soil or foliar applied.
EPSP synthase inhibitor
The amino acid derivative herbicide inhibits the production of three essential aromatic amino acids by inhibiting another key plant enzyme, 5-enolpyruvylshikimate-3-phosphate (EPSP) synthase. In general, injury symptoms are slow to develop, especially in cold weather (seven to 14 days) and include stunting or slowing of plant growth and slow plant death.
Glyphosate is an example of an amino acid derivate herbicide. It is nonselective and the site of uptake is the plant foliage. Glyphosate moves via the phloem to all parts of the plant and is an excellent perennial weed control herbicide that is active on annual weeds as well. Glyphosate is inactive in soil because of strong adsorption.
Glyphosate-resistant crops with an alternative EPSP synthase enzyme have been developed through genetic engineering.
Herbicide use
- Imazamox (Raptor) for alfalfa, Clearfield canola, dry bean, field pea, soybean and Clearfield lentil and sunflower.
- Imazapyr (Arsenal) for rangeland.
- Imazethapyr (Pursuit) for alfalfa, chickpea, dry bean, lentil, field pea and soybean.
Injury symptoms
The injury symptoms for this herbicide family is the same as the sulfonylurea herbicides. See the next section for injury symptoms.
Site of action
Acetolactate synthase (ALS) enzyme; also referred to as acetohydroxy acid synthase (AHAS).
Herbicide use
- Chlorimuron (Classic) for soybean.
- Halosulfuron (Permit) for corn, dry bean, pastures and rangeland.
- Mesosulfuron (Osprey) for wheat.
- Metsulfuron (Ally) for barley and wheat.
- Rimsulfuron (Matrix) for corn and potato.
- Sulfosulfuron (Maverick) for pastures and wheat.
- Thifensulfuron (Harmony) for barley, SU canola, corn, oat, STS soybean, and wheat.
- Tribenuron (Express) for barley, SU canola, corn, oat, Express Sun sunflower and wheat.
- Triflusulfuron (UpBeet) for sugarbeet.
Injury symptoms
Soil residual injury symptoms
Symptoms from sulfonylurea herbicides are identical to imidazolinone, triazolopyrimidine and sulfonylamino carbonyltriazolinone herbicides. Not all injured plants will exhibit all symptoms, and symptoms may vary from field to field.
Sugarbeet plants may be stunted (Photo 10) and the leaves usually become a bright yellow with first yellowing on young leaves (Photos 11, 12). Phenotype mimics rhizomania-susceptible sugarbeet (Photo 13).
Relatively high levels of herbicide residual in soil may cause the plants to form a rosette rather than a normal sugarbeet plant (Photos 14, 15, 16). The total root and hypocotyl of sugarbeet seedlings may turn brown and shrivel (Photo 17, upper plant) or the root may turn brown and die, starting at the point where the root joins the hypocotyl, about 1 to 1.5 inches below the soil surface (Photo 17, lower plant).
Plants with injury similar to the upper plant in Photo 17 will often die due to a nonfunctional root system, but plants with injury similar to the lower plant often will survive by producing secondary roots from the hypocotyl. However, low moisture in the surface two inches of soil can prevent the successful production of secondary roots and the damaged plant would then die.
Look-alike symptoms
Nearly identical symptoms on roots of seedling sugarbeet also can be caused by dinitroaniline herbicides and Aphanomyces cochlioides, a fungal disease (Photo 18). Plants that survive and grow may produce new leaves that are more strap-shaped than normal (Photo 19).
Postemergence injury symptoms
Symptoms imidazolinone and sulfonylurea herbicides are identical. Not all injured plants will exhibit all symptoms, and symptoms may vary from field to field (Photos 20, 21, 22).
Plant leaves will become prostrate a few hours after exposure, similar to the effect from phenoxy acetic acids, dicamba or pyridines. Older leaves may remain prostrate for several weeks.
However, the petiole epinasty from imidazolinone or sulfonylurea herbicides is less than from phenoxy acetic acid, dicamba or pyridine herbicides.
Yellowing of the youngest leaves begins about four to five days after exposure, and the yellowing intensifies and spreads to the older leaves with time. Severely affected leaves or whole plants may die and turn brown.
Petioles may turn black or have black streaks as symptoms worsen (Photo 23). The color contrast between affected and normal plants can become quite evident (Photo 24). The yellow may disappear later in the season as affected plants recover and begin to produce new leaves.
Some affected plants may develop brown rings in the roots within five to seven days after exposure (Photo 25). These rings still may be present at harvest (Photo 26).
Plants injured by imidazolinone or sulfonylurea herbicides often produce new leaves in clusters rather than in pairs. This can result in more than one crown per root (Photo 27). These plants may be more difficult to defoliate than normal plants (Photo 28). Young seedling exposure to imidazolinone or sulfonylurea herbicides can cause root symptoms similar to those from soil residual (Photo 17).
Site of action
Acetolactate synthase (ALS) enzyme; also referred to as acetohydroxy acid synthase (AHAS).
Herbicide use
- Flucarbazone (Everest) for wheat.
- Propoxycarbazone (Olympus) for wheat.
- Thiencarbazone (Varro) for wheat.
Injury symptoms
Same and sulfonylurea; see previous section.
Site of action
Acetolactate synthase (ALS) enzyme; also referred to as acetohydroxy acid synthase (AHAS).
Herbicide use
- Cloransulam (FirstRate and Surveil) for soybean.
- Flumetsulam (Python, Surestart II and Tripleflex II) for corn and soybean.
- Pyroxsulam (Goldsky and PerfectMatch) for wheat.
Injury symptoms
Same as sulfonylurea; see previous section.
Site of action
Acetolactate synthase (ALS) enzyme; also referred to as acetohydroxy acid synthase (AHAS).
Herbicide use
Glyphosate (several trade names) nonselective weed control before crop emergence, for spot treatments in some crops, pasture and noncropland or postemergence grass, broadleaf and perennial control in glyphosate-resistant (Roundup Ready) crops.
Injury symptoms
Sugar beet injury in susceptible varieties from glyphosate is quite similar to injury from imidazolinones or sulfonylureas. However, the yellowing from exposure to glyphosate starts with the older leaves and moves toward the younger leaves, while injury from imadazolinone or sulfonylurea herbicides starts with the younger leaves and moves toward the older leaves.
Glyphosate can cause browning in the roots similar to the imidazolinone or sulfonylurea herbicides.
Site of action
5-enolpyruvylshikimate-3-phosphate synthase (EPSP synthase) enzyme.
CAUTION: Mention of a pesticide or use of a pesticide label is for educational purposes only. Always follow the pesticide label directions attached to the pesticide container you are using. Be sure that the area you wish to treat is listed on the label of the pesticide you intend to use. Remember, the label is the law.
Reviewed in 2018