Herbicide absorption through roots will continue as long as the absorbing region near the root tips remains in contact with the herbicide-treated soil. As the roots grow deeper, herbicide uptake declines. Therefore, plants may survive if the root tips grow out of the herbicide-treated soil before herbicide absorption is sufficient to kill the plants. Many soil-applied herbicides are absorbed through unemerged plant shoots, and plants may be killed or injured before emergence. Physical and environmental factors that promote rapid crop emergence reduce the length of time that a plant is in contact with a soil-applied herbicide and, therefore, reduce the possibility of crop injury.
Herbicides differ in translocation within a plant.
Therefore, their primary injury symptoms are mostly confined to the site of uptake. Other herbicides are mobile within the plant, and injury symptoms will generally be most prominent at the site where mobile herbicides concentrate. For example, soil-applied atrazine is absorbed by plant roots and moves upward within the water transport system of the plant to the leaves, where symptoms occur. Postemergence Herbicide Phytotoxicity in Plants Effective weed control from Postemergence herbicides depends on adequate contact with above-ground plant shoots and leaves. Therefore, the combination of spray nozzle pressure and volume should be selected to obtain necessary plant coverage and drift control.