Plants are complex organisms with well-defined structures in which 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 these vital processes must be disrupted in order for a herbicide to kill a weed. Knowing the site of herbicide action is essential to plan an effective herbicide rotation plan 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 metabolic activity of sites 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.
| Phenoxy aliphatic acid herbicides |
Benzoic acids |
Picolinic Acids (Pyridines) |
| 2,4-D2,4,5-T2,4-DB2,4-DPMCPBMCPAMCPP | DicambaTricambaChloramben2,3,6-TBA |
PicloramClopyralidTriclopyrFluroxypyr |
2. Aromatic amino acid inhibitors
The use of glyphosate and sulfosate is limited to foliar applications only, since these chemicals are rapidly inactivated in the soil. They are nonselective herbicides controlling grasses, sedges and broadleaved weeds showing symptoms like yellowing of new growth and death in treated plants for 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, poor root system development and shortening of secondary roots. It requires two to three weeks for the development of the aforementioned symptoms.
Imidazolinones |
Sulfonylureas |
ImazquinImazethapyrImazapyr |
Metsulfuron, ChlorsulfuronSulfometuron, NicosulfuronPrimisulfuron, Tribenuron |
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 that moves well into the symplast, nevertheless, and other compounds show early movement in shoot movements 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 soil than the boot stage before the post emergence. This group shows symptoms of viz., Discoloration and disintegration of meristematic tissue at and above the nodes, including nodes of rhizomes, yellowing and reddening of leaves.
Aryloxyphenoxypropionates |
Cyclohexanediones |
FenoxapropFluazifop-PQuizalofop |
ClethodimSethoxydim |
Contact herbicides
Cell membrane destroyers: This group of herbicides penetrates into the cytoplasm; Causes 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 are 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: These groups in herbicides 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 grow from bottom to top on plant shoots. Chlorosis first appears between the leaf veins and along the margins which is followed by necrosis of the tissue.
Triazines |
Uracils |
Phenylureas |
Atrazine, Simazine, Metribuzin, Cyanazine, Ametryn, Terbutryn |
Terbacil, Bromacil, Lenacil, Isocil |
Linuron, Monuron, Diuron, Isoproturon, Tebuthiuron |
Soil applied herbicides
Cell division inhibitors
1. Root inhibitors: These herbicide groups are applied 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 some perennials suppression from tubers and rhizomes. These are used in beets, spinach, tomatoes, potatoes, and ornamentals. On application, the leaves become twisted.
Thiocarbamates |
Amides |
EPTC, Butylate, Pebulate, Cycloate, Diallate, Triallate, Monilate, Thiobencarb or Benthiocarb |
Acetochlor, Alachlor, Butachlor, Metolachlor, Propachlor, Diphenamid, Propanil |
3. Shoot and root inhibitors: These herbicides are applied as pre-plant incorporated, pre-emergence and sometimes early post-emergence for control of annual grasses, and some annual broadleaved weeds in strawberries, herbaceous perennials, tomatoes, cole crops, cucurbits and pepper. Eg. Bensulide, Bensulide, Napropamide, Pronamide, Dichlobenil and Dithiopyr
Authors
APSivamurugan, V.Manivannan and A.Suganthi
Tamil Nadu Agricultural University, Coimbatore