Root uptake distribution through plant

Insecticides with systemic action are taken up relatively quickly by the plants and transported into the vascular system. According to the type of application, uptake occurs through the roots or the parts of the plant above ground. Distribution is chiefly by the xylem, but is also possible by the phloem and by diffusion from cell to cell. The persistence of activity is dependent on the type of substance, the intensity of breakdown in the plant or the soil, and environmental conditions. A much longer period of protection can be maintained if, by application of granulates at drilling or planting out, a depot of the substance is created in the soil from which the active substance is released slowly and taken up by the plants. 

The two main pathways for the uptake of toxic substances by plants are through their root systems and across their leaf cuticles. Stomata, the specialized openings in plant leaves that allow carbon dioxide required for photosynthesis to enter the leaves and oxygen and water vapor to exit, are also routes by which toxic substances may enter plants. The mechanisms by which plants take up systemic pesticides and herbicides, which become distributed within the plant, have been studied very intensvively. 

Lead within soils is distributed between solid and liquid phases, with the latter of major importance to the issue of lead bioavailability, for example, to plant roots where uptake can occur. Studies of lead species in this liquid mobile phase indicate that they exist as both complexed and ionic forms although the latter as simple ions are present in very low concentrations. The extent to which lead can move through soils, in turn, is the extent to which lead binds to insoluble organic and mineralogical inorganic species. The former are typically humic and fulvic acid derivatives, and the latter are surfaces of clays and metal oxides (U.S. EPA, 1986). The factors most important for lead movement within soils are pH, cation exchange capacity of... 

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