3.6 Absorption of water and nutrients by roots
In terrestrial plants, water and solutes must move from the bulk soil through a rhizosphere before entering roots. Within a root, radial transport carries resources to the central stele where they are released into xylem vessels and made available for long-distance (axial) transport. Uptake is achieved via this tortuous route through different matrices with a high degree of control and responsiveness to plant requirements. Inorganic nutrients are delivered to cells along the transpiration pathway and a proportion is subsequently transferred to phloem vessels for use in a wide variety of synthetic events throughout the plant.
For example, inorganic ions (e.g. potassium and orthophosphate) are delivered to growing root tips through the incoming phloem sap and therefore root tips are largely reliant on ion and water uptake elsewhere in roots. Because growing root tips do not absorb most nutrients locally, they are somewhat independent of fluctuations in external nutrient concentrations as roots elongate through soil. Ion uptake and phloem retranslocation therefore both contribute to establishment of new roots by delivering an appropriate nutrient mix to the cytoplasm of immature root apical cells. In soils contaminated with heavy metals, relatively unvacuolated apical cells can be protected from toxic ions because these ions are sequestered in vacuoles of more mature root cells. The entire process of acquisition of inorganic resources is dependent on the initial entry of ions into roots, the subject of Section 3.6.