Selective Herbicide and Their Movement In Plant

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Selective Herbicide

HERBICIDE SELECTIVITY

• Selectivity refers to the phenomenon wherein the herbicide kills the target plant species in a mixed plant population without harming or slightly affecting the other plants.

• Selectivity is considered as the greatest single factor that helped the chemical weed control to succeed since the advent of 2,4-D. Factors achieving selectivity

1) Differential absorption of herbicide by crop plants and weeds. Also called as physical selectivity or depth protection selectivity

2) Differential translocation of herbicides: After the absorption of herbicide by weed and plant, translocation is more in sensitive one (weeds) and less in tolerant one.

• For 2,4-D the rate of translocation is very slow in sugarcane that’s why 2,4-D got selectivity in sugarcane. The rate of translocation is rapid in beans.

3) Differential rate of deactivation of herbicide by the plants:

• Rate of deactivation differs from plant to plant. In tolerant species rate of deactivation is fast, viceversa e.g., atrazine, simazine – deactivation is very fast in maize as well as sorghum

• Similarly isoproturon is selective to wheat mainly due to its increased detoxification inside the wheat plant , whereas phalaris minor is killed because detoxification is slow

4) Difference in specific physiology of crop:

• The physiological tolerance of plants to herbicides may result from its failure to translocate the absorbed herbicide from the site of absorption to site of action. They also differ in rate of metabolism of herbicides.

• Conjugation is the mechanism by which herbicide molecules are transformed to non-toxic form. 2,4-D is transformed to β-D Glucose ester.

5) Chronolgical selectivity : Achieved by difference in the time of application of herbicides

• Chronological selectivity is achieved through time of application of herbicide and dosage

• Selectivity is also dose dependent

HERBICIDE MOVEMENT IN PLANTS

1) Symplastic movement

• It is source to sink translocation.

• The herbicides move through phloem with sugars or photosynthates produced during photosynthesis.

• Sinks are the sites where sugars translocated from the source are used for growth process or stored.

• Source is nothing but fully expanded and growing leaves

• Sink is nothing but buds, growing shoots, roots, flowers, fruits and seeds.

• Sugars while moving from source to sink through symplast carry along some post emergent herbicides.

• In all foliage applied and post-emergent herbicides the herbicide movement is called symplastic movement.

• It requires metabolic energy and the movement is basipetal (apex to base).

• Symplastic movement is always through phloem. e.g., Glyphosate, 2, 4-D

2) Apoplastic movement

• The herbicides are primarily absorbed by the roots and move predominantly through xylem and intercellular spaces in plants.

• They move along with water (passive movement) or mineral nutrient ions (active movement).

• Therefore the herbicide molecules movement may or maynot require metabolic energy.

• In soil applied or pre-emergent herbicides the usual basis of herbicide movement is apoplastic movement.

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