Absorption and Translocation of Herbicides
If herbicides is to be effective on the physiological and biochemical processes of the Plant, it must be absorbed by the plant and translocated ( Except the contact herbicides) in adequate quantity to the sites of action. Differential absorption and translocation, which form the basis for herbicides selectivity, determine the tolerance and susceptibility of a plant species to particular herbicide.
Absorption of Herbicides:
It is the movement herbicide from the surface into the plant body.
Or
It is the process of penetration of herbicides into the plant tissues.
The herbicides are applied either to the soil or plant foliage. Therefore, absorption of herbicide depends upon the method of application and the plant part with which the chemical comes in contact.
Absorption of Soil Applied Herbicides:
Herbicides applied to the soil as pre-planting or pre-emergence treatment are usually taken up by the root or shoot of the emerging seedlings. Water , salts and water soluble herbicides are taken up by root hairs and cortex, the herbicide molecules migrate through xylum to foliage via the transpiration stream.
Herbicides are absorbed by both passive and active mechanism as like inorganic ions. The passive entrance is primarily along the absorbed water and the herbicides move with the water through out the plant in the apoplast (Interconnecting cells walls and intercellular spaces, including water or air filled xylum elements –a non living system) herbicides may enter the plant and move primarily by one or both of these mechanisms depending upon chemical and physical properties of the molecules.
Besides root the soil applied herbicides are also absorbed by the developing shoots, seeds and also by rhizomes, tubers and other vegetative parts of the perennial weeds In general the shoots active herbicides like atrazine and urea herbicides kill the weeds by absorption through shoots.
In case of soil applied herbicides, the placement of herbicides in soil is important factor governing the efficiency and selectivity of herbicides in weed control. The surface or shallow application of herbicides will ensure greater selectivity for controlling shallow rooted weeds from deep rooted crops. Most of the herbicides applied is usually concentrated in the upper 2 to 8 cm of soil. If the herbicide is not concentrated in the zone where weed grow, the treatment may prove ineffective or less effective.
Absorption of Foliage Applied Herbicides:
In general, the foliage applied herbicide has to meet five major barriers before reaching the interior of individual cell for action. These barriers are i) Surface waxes and Hair ii) Cuticle iii) Periderm iv) cell wall and v) Plasma lemma. The thickness and chemical quality of each barriers varies with the plant species and environmental conditions under which a plant is grown. For instance, a plant growing in shade has thin cuticle as compared to when it is grown in open sunshine. Likewise , a plant in dry land condition has thick cuticle to prevent moisture loss, while in wet situation its cell wall and cuticle are thin. The nature of waxes present on leaves can also vary with plant species and environment. Usually thinner barriers permit rapid absorption of herbicides by the plant shoots. The non-polar herbicide molecules enter the cuticle and waxy barriers through their liphilic constituents, while the polar molecules enter these through the hydrophilic channels present in the barriers. The herbicides are also absorbed by the stem. The absorption of herbicides is more through green and succulent stem ( young stem) than the woody or hard stem (older stem). Trichomes or leaf hairs are also involved in foliar absorption of herbicides. The stomatal absorption is greater through the lower surface than the upper leaf surface as the penetration through the bark or woody plants is very difficult.
Translocation of Herbicides:
It is transfer of herbicides from one part to another in plants. The translocation of shoot active herbicides is conducted through the phloem tissues in plants along with the food material. Therefore, for active translocation of such herbicides to the underground parts of the treated plant, sunlight and other conditions favourable for process of photosynthesis by plant are essential.
The herbicide applied to the soil are absorbed by the plant roots and translocated to the shoots through xylum vessels along the translocation stream. In dry soil the translocation of such herbicides in plants will be much slower as compared to the moist soils. Some herbicides exhibit xylum-Phloem interchange and consequent, simultaneous bi-directional movement in plants. These herbicides are also called circulatory herbicides.