Mode Of Action of Herbicides

Plants are complex organisms with well-defined structures in which multitudes of vital processes viz., photosynthesis, protein, fat, pigment and nucleic acid synthesis, respiration and energy transfer take place in well ordered and integrated sequences.

One or more of the vital processes must be disrupted in order for a herbicide to kill a weed. Knowing the site of action of an herbicide is necessary to plan an effective herbicide rotation scheme to delay the development of herbicide-resistant weeds. Herbicides with the same mode-of-action will have the same translocation pattern and produce similar injury symptoms.

Foliage applied herbicides

Symplastically translocated herbicides

These herbicides are capable of moving from leaves to sites of metabolic activity such as underground meristems, shoot meristems, storage organs and other live tissues. These herbicides have the potential to kill simple perennial and creeping perennial weeds with only one or two foliar applications. The typical symptoms include pigment loss, distortion and stunted growth and the plants die slowly.

1. Auxin growth regulators

These herbicides are used for the control of annual, simple perennial, and creeping perennial and broadleaved weeds. On application, they evince immediate bending and twisting of leaves and stems and form abnormal flowers and roots.

2. Aromatic amino acid inhibitors

The usage of glyphosate and sulfosate is limited to foliar applications only, since these chemicals are rapidly inactivated in the soil. They are nonselective herbicides control grasses, sedges and broadleaved weeds by showing the symptoms like yellowing of new growth and death of treated plants in days to weeks.

Eg. Glyphosate and sulfosate

3. Branched chain amino acid inhibitors

Application of this group of herbicides results in very slow symptom development viz., appearance of yellow, pink and purple symptoms, development of poor root system and shortening of secondary roots. It requires two to three weeks for the development of aforementioned symptoms.

Chlorophyll / Carotenoid pigment inhibitors

The pigment inhibitors evince white new growth, sometimes tinged with pink or purple. Amitrole is the only compound of this group which moves well in the symplast, nevertheless, other compounds show initial movement into shoot tips causing new growth to be devoid of green and yellow pigments. Eg. Amitrole, fluridone, clomazone

5. Grass meristem destroyers

These compounds are used for selective removal of most grass species and are more active when applied as post emergence before the boot stage than soil applied. This group shows symptoms viz., discoloration and disintegration of meristematic tissue at and above the nodes, including nodes of rhizomes, yellowing and reddening of leaves.

Contact herbicides

Cell membrane destroyers: This group of herbicides penetrates into the cytoplasm; cause the formation of peroxides and free electrons which destroy the cell membranes immediately. This prevents translocation to other regions of the plant resulting in water-soaked areas which later turn yellow or brown. Bipyridiliums: Paraquat and Diquat

Diphenyl ethers: These herbicides have both foliar and soil activity and is relatively unaffected by soil texture and organic matter. They show bronzing or burning of leaf tissue after application. Oxyfluorfen is used as pre emergence for cole crops and post emergence for mint and onions. Eg.Oxyfluorfen, Acifluorfen, Nitrofen and Lactofen

Apoplastically translocated herbicides

Photosynthetic inhibitors: Herbicides in these groups have excellent soil and   foliar activity and are used  as pre plant incorporated, pre emergence, and to a limited extent  as early post emergence, for selective control of weeds in annual and established perennial crops. These herbicides translocate only apoplastically. Symptoms develop from bottom to top on plant shoots. Chlorosis first appears between leaf veins and along the margins which is later followed by necrosis of the tissue.

Soil applied herbicides

Cell division inhibitors

1. Root inhibitors: These herbicide groups are applied mostly as  pre plant incorporated and pre emergence for control of grasses and some annual broadleaved weeds in beans, cole crops, herbaceous ornamentals, established turf, and in nurseries, orchards, grapes, Christmas trees, etc. These root inhibitors do not translocate and inhibit cell division and cell wall formation. Incorporation into the soil after application is necessary to prevent volatilization and photo degradation.

Eg. Dinitroanilines: Trifluralin, Benefin, Pendimethalin, Fluchloralin, Nitralin, Ethalfluralin, Prodiamine

2. Shoot inhibitors: The shoot inhibitors are applied in soil as pre plant incorporated for the control of grasses, some broadleaved weeds and suppression of some perennials from tubers and rhizomes. These are used in beets, spinach, tomatoes, potatoes, and ornamentals. On application, the leaves become twisted.

3. Shoot and root inhibitors: These herbicides are applied as pre plant incorporated, pre emergence and sometimes early post emergence for the control of annual grasses, and some annual broadleaved weeds in strawberries, herbaceous perennials, tomato, cole crops, cucurbits and pepper.Eg. Bensulide, Bensulide, Napropamide, Pronamide, Dichlobenil and Dithiopyr

Authors

A.P.Sivamurugan, V.Manivannan and  A.Suganthi

Tamil Nadu Agricultural University, Coimbatore