Amino Acid Synthesis Inhibitors

The amino acid synthesis inhibitors include the herbicide families: sulfonylurea, imidazolinone, sulfonamide and 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 (See Figure 1).

Sulfonylurea, imidazolinone and sulfonamide herbicides prevent the production of three essential branch-chain amino acids by inhibiting one key plant enzyme. The amino acid derivative herbicides inhibit the production of three essential aromatic amino acids by inhibiting another key plant enzyme. In general, injury symptoms are slow to develop (one to two weeks) and include stunting or slowing of plant growth and a slow plant death.

Herbicides in the sulfonylurea, imidazolinone and sulfonamide families can move in both the xylem and phloem to areas of new growth and can be 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. Glyphosate, an amino acid derivative herbicide, is non-selective 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 and is active on annual weeds as well.

1. Imidazolinones

a. Use: Imazamethabenz (Assert) for wheat, barley and sunflower.
Imazaquin (Scepter) for soybean and non-cropland.
Imazethapyr (Pursuit) for soybean, dry bean, alfalfa, noncropland and pea.

b. Injury Symptoms: Same as for sulfonylureas, see next section.

c. Site of Action: Acetolactate synthase (ALS) enzyme. Also referred to as acetohydroxy acid synthase (AHAS).

2. Triazolopyrimidine sulfonanilide or sulfonamide

a. Use: Flumetsulam (Broadstrike) for corn and soybean.

b. Injury Symptoms: Same as for sulfonylureas, (see next section).

c. Site of Action: Acetolactate synthase (ALS) enzyme. Also referred to as acetohydroxy acid synthase (AHAS).

3. Sulfonylureas

a. Use: Chlorimuron (Classic) for soybean.
Primisulfuron (Beacon) for corn.
Thifensulfuron (Harmony Extra) for small grains.
(Pinnacle) for soybean.
Triasulfuron (Amber) for small grains.
Nicosulfuron (Accent) for corn.
Metsulfuron (Ally) for small grains, grass pastures, and CRP.
Tribenuron (Express, Harmony Extra) for small grains.
Rimsulfuron (Matrix) for potato.
Triflusulfuron (Upbeet) for sugarbeet.

b. Injury Symptoms from Soil Residual: Symptoms from imida-zolinone, sulfonylurea and sulfonamide herbicides are iden-tical. Not all injured plants will exhibit all symptoms, and symptoms may vary from field to field. Sugarbeet plants may be stunted and the leaves usually become a bright yellow with first yellowing on young leaves (Photo 9. An injured plant is on the left and an undamaged plant on the right.) Relatively high levels of herbicide residual in soil may cause the plants to form a rosette rather than a normal sugarbeet plant (Photo 10). The total root and hypo-cotyl of sugarbeet seedlings may turn brown and shrivel (upper plant, Photo 11 ) 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 (lower plant, Photo 11). Plants with injury similar to the upper plant in Photo 11 would probably die due to a non-functional root system, but plants with injury similar to the lower plant often will survive by pro-ducing secondary roots from the hypocotyl. However, low moisture in the surface 2 inches of soil can prevent the suc-cessful production of secondary roots and the damaged plant would then die. Nearly identical symptoms on roots of seedling sugarbeet also can be caused by dinitroaniline herbicides and Aphanomyces cochlioides, a fungal disease. Plants that survive and grow may produce new leaves that are more strap-shaped than normal (Photo 12).

c. Injury Symptoms from Postemergence Exposure: Symptoms from imidazolinone and sulfonylurea herbicides are identical. Not all injured plants will exhibit all symptoms and symptoms may vary from field to field.

Plant leaves will become prostrate a few hours after exposure (Photo 13) similar to the effect from phenoxy acetic acids, Banvel or pyridines. Old leaves may remain prostrate for several weeks. However, the petiole epinasty from imidazo-linones or sulfonylureas is less than from phenoxy acetic acids, Banvel or pyridines. Yellowing of the youngest leaves begins about four to five days after exposure (Photo 14) and the yellowing intensifies and spreads to the older leaves with time (Photo 15). Severely affected leaves or whole plants may die and turn brown. Petioles may turn black or have black streaks as symptoms worsen. The color contrast between affected and normal plants can become quite evident (Photo 16). 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 17). These rings may still be present at harvest (Photo 18).

Plants injured by imidazolinones or sulfonylureas often pro-duce new leaves in clusters rather than in pairs. This can result in more than one crown per root (Photo 19). These plants may be more difficult to defoliate than normal plants. Young seedling exposure to imidazolinone or sulfonylurea herbicides can cause root symptoms similar to those from soil residual (Photo 11).

d. Site of Action: Acetolactate synthase (ALS) enzyme. Also referred to as acetohydroxy acid synthase (AHAS).

4. Amino Acid Derivatives

a. Use: Glyphosate (Several trade names) non-selective weed control before crop emergence, for spot treatments in some crops, in pasture and in non-cropland.

b. Injury Symptoms: Sugarbeet injury 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 (Photo 20) while injury from imidazolinones or sulfonylureas starts with the younger leaves and moves toward the older leaves (Photo 15). Glyphosate can cause brown coloring in the roots (Photo 21 ) similar to the imidazolinones or sulfon-ylureas.

c. Site of Action: 5-enolpyruvyl-shikimate-3 phosphate synthase (EPSP synthase) enzyme.