Photosynthesis Inhibitors

The photosynthesis inhibitors include triazines, phenylureas, uracils, benzothiadiazoles, nitriles, carbamate and dicarboxylic acid. Photosynthesis inhibitors shut down the photosynthetic (food producing) process in susceptible plants by binding to specific sites within the plant chloroplast. Inhibition of photosynthesis could result in a slow starvation of the plant; however, in many situations rapid death occurs perhaps from the production of secondary toxic substances.

Injury symptoms include yellowing (chlorosis) of leaf tissue followed by death (necrosis) of the tissue. Three of the herbicide families (triazines, phenylureas and uracils) are taken up into the plant via the roots or foliage and move in the xylem to plant leaves. As a result, injury symptoms will first appear on the older leaves, along the leaf margin. Foliar applied photosynthetic inhibitors generally remain in the foliar portions of the treated plant and movement from foliage to roots is negligible.

1. Triazines

a: Use: Ametryn (Evik) for corn.
Atrazine for corn and sorghum.
Cyanazine (Bladex) for corn.
Simazine (Princep) for corn.
Metribuzin (Lexone, Sencor) for alfalfa, soybean, potato, pea and lentil.
Hexazinone (Velpar) for alfalfa.

b. Injury Symptoms: Residual of photosynthesis inhibitors in soil does not prevent seedlings from germinating or emerging. Injury symptoms occur after emergence and the speed of appearance of symptoms will be more rapid with sunny days than with cloudy weather. Also, symptoms will be more severe and more rapid as the level of herbicide in the soil increases. Sugarbeet plants may be in the two- to four-leaf stage before symptoms become noticeable but plants can die in the early two-leaf stage. Initial symptoms include browning of the cotyl-edonary leaves and yellowing of the true leaf margins (Photo 27). Browning of leaves will increase with time (Photos 28, and Photo 29) and total desiccation may result (Photo 30). Older and larger leaves are affected before younger leaves. Postemergence triazines cause an initial yellowing followed by desiccation and leaf browning.

c. Site of Action: D-1 -quinone-binding protein of photosynthetic electron transport.

2. Phenylureas

a: Use: Linuron (Lorox) for soybean and corn.
Tebuthiuron (Spike) for grass pasture and noncropland.

b. Injury Symptoms: Same as for the triazine herbicides.

c. Site of Action: D-1-quinone-binding protein of photosynthetic electron transport.

3. Uracils

a. Use: Terbacil (Sinbar) for alfalfa.

b. Injury Symptoms: Same as for triazine herbicides.

c. Site of Action: D-1-quinone-binding protein of photosynthetic electron transport.

4. Benzothiadiazoles

a. Use: Bentazon (Basagran) for soybean, corn, dry bean and grain sorghum.\b. Injury Symptoms: Leaves become chlorotic and later turn brown and die (Photo 31). The older leaves die first. All the older leaves can turn brown while the growing point remains green. Sugarbeet can recover, produce new leaves and produce a nearly normal-size root at harvest if the growing point survives.

c. Site of Action: D-1 -quinone-binding protein of photosynthetic electron transport.

5. Nitriles

a. Use: Bromoxynil (Buctril) for wheat, barley, oats, rye, flax, corn and alfalfa.

b. Injury Symptoms: Leaves become chlorotic and later turn brown and die (Photo 32). Contact with isolated spray droplets may cause a spotting or speckling of the leaves. The older sugarbeet leaves will be affected more than the young leaves. Sugarbeet can produce new leaves and a harvestable root if the growing point survives.

c. Site of Action: D-1-quinone-binding protein of photosynthetic electron transport.

6. Carbamate

a: Use: Desmedipham (Betanex) for sugarbeet. Desmedipham + Phenmedipham (Betamix) for sugarbeet.

b. Injury Symptoms: Desmedipham and phenmedipham are registered for sugarbeet but injury sometimes occurs, most often in a hot and moist environment. Symptoms from des-medipham and phenmediham are very similar to symptoms from bentazon and bromoxynil. Injured leaves may turn brown and die. The older leaves die first and the growing point may remain green and alive even when most leaves are dead (Photo 33, and Photo34). Sugarbeet plants with a surviving growing point will produce new leaves and a nearly normal size root at harvest.

c. Site of Action: D-1-quinone-binding protein of photosynthetic electron transport.

7. Dicarboxylic Acid

a. Use: Endothall (H-273) for sugarbeet.

b. Injury Symptoms: Endothall is registered for sugarbeet but injury sometimes occurs, most often in a hot and moist environment. Symptoms from endothall are very similar to symptoms from bentazon, bromoxynil, desmedipham and phenmedipham. Injured leaves turn brown and die. The older leaves die first and the growing point may remain green and alive even when most leaves are dead (Photo 35). Sugarbeet plants with a surviving growing point will produce new leaves and a nearly normal size root at harvest.

c. Site of Action: D-1-quinone-binding protein of photosynthetic electron transport.


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