V. Photosynthesis Inhibitors (SOA 5, SOA 6, SOA 7)
The photosynthesis inhibitors include these herbicide families:
- Triazine
- Triazinone
- Phenylurea
- Benzothiadiazole
- Nitrile
- Phenyl-carbamate
Photosynthesis inhibitors disrupt the photosynthetic (food producing) process in susceptible plants by binding to specific sites within the photosystem II complex in plant chloroplasts. Inhibition of photosynthesis could result in slow starvation of the plant; however, in many situations, rapid death occurs perhaps from the production of secondary toxic substances.
Injury symptoms include interveinal yellowing (chlorosis) and death (necrosis) of leaf tissue beginning at the leaf margins and progressing toward the center of leaves.
The triazines, triazinones, 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 first will appear on the older leaves, along the leaf margin.
They also have relatively long persistence in soil.
The benzothiadiazoles and phenyl-carbamates are foliar-applied photosynthetic inhibitors and generally remain in the foliar portions of the treated plant. They are contact herbicides requiring thorough spray coverage of the foliage for good weed control. Movement from the foliage to roots is negligible.
Herbicide use
Atrazine for corn.
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.
Symptoms will also 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 cotyledonary leaves and yellowing of the true leaf margins. Browning of leaves will increase with time and total desiccation may result. Older and larger leaves are affected before younger leaves (Photo 39).
Triazines residues are most likely to occur following years with low rainfall because chemical and microbial activity needed to degrade herbicides are limited in dry soil. There are other herbicide tank mixtures that can cause similar visual damage (Photo 40).
Postemergence triazine herbicides cause an initial yellowing followed by desiccation and leaf browning.
Site of action
D1 quinone-binding protein (photosystem II) of the photosynthetic electron transport chain, binding site A.
Herbicide use
Metribuzin (Sencor, Dimetric) for alfalfa, corn, field pea, lentil, soybean and potato.
Injury symptoms
Same as for the triazines; see previous section.
Site of action
D1 quinone-binding protein (photosystem II) of the photosynthetic electron transport chain, binding site A.
Herbicide use
- Linuron (Lorox) for corn, potato and soybean.
- Diuron (Diuron) for alfalfa.
- Tebuthiuron (Spike) for pastures and rangeland.
Injury symptoms
Same as for the triazines.
Site of action
D1 quinone-binding protein (photosystem II) of the photosynthetic electron transport chain, binding site A.
Herbicide use
Terbacil (Sinbar) for alfalfa.
Injury symptoms
Same as for the triazines.
Site of action
D1 quinone-binding protein (photosystem II) of the photosynthetic electron transport chain, binding site A.
Herbicide use
Bentazon (Basagran) for corn, dry bean, field pea and soybean.
Injury symptoms
Leaves become chlorotic and later turn brown and die. The older leaves die first. All older leaves can turn brown while the growing point remains green (Photo 41).
Sugarbeet can recover, produce new leaves and produce a nearly normal-size root at harvest if the growing point survives (Photo 42).
Site of action
D1 quinone-binding protein (photosystem II) of the photosynthetic electron transport chain, binding site B.
Herbicide use
Bromoxynil (Buctril) for alfalfa, barley, corn, flax, oat and wheat.
Injury symptoms
Leaves become chlorotic and later turn brown and die. 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 (Photo 43). Sugarbeet can produce new leaves and a harvestable root if the growing point survives.
Site of action
D1 quinone-binding protein (photosystem II) of the photosynthetic electron transport chain, binding site B.
Herbicide use
Desmediphan+phenmedipham (Betamix) for sugarbeet.
Injury symptoms
Betamix is registered for sugarbeet but injury sometimes occurs, most often in a hot and moist environment. Symptoms from Betamix are very similar to symptoms from bentazon and bromoxynil.
Injured leaves may turn brown and die (Photo 44). The older leaves die first, and the growing point may remain green and alive, even when most leaves are dead (Photo 45).
Sugarbeet plants with a surviving growing point will produce new leaves and a nearly normal-size root at harvest.
Site of action
D1 quinone-binding protein (photosystem II) of the photosynthetic electron transport chain, binding site B.
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