Roots are the primary way in which plants acquire nutrients. Plant roots can penetrate to significant depths and are branched so that they can explore large volumes of soil. Some nutrients are only “weakly bound” in the soil, and can flow towards the root surface when a plant uses water. Other nutrients can be tightly bound in some soils, thus, continued uptake by plants relies on growth of roots into new areas of the soil. Root hairs, which greatly increase the volume of soil explored adjacent to the main root and also increase the root’s surface area, may be particularly active in nutrient absorption. The degree of phosphate absorption by soil particles depends on the soil type.
Grasses normally have extensive root systems with many fine branches and sub-branches. These features contribute to good soil exploration for nutrients. Other plants have a thick tap-root and relatively fewer fine roots. Legumes form symbiotic relationships with species of soil bacteria and fungi. The bacteria form root nodules and fix nitrogen gas (N2) from the atmosphere and provide it in a form suitable for plants (NH4). Mycorrhizal fungi form a network of hyphae extending into the soil to facilitate phosphorus uptake. In return, the host plant supplies the bacteria and fungi with sugars produced through photosynthesis.
How is selectivity of nutrient uptake achieved?
Mineral nutrients are absorbed by plants from the soil solution as ions. An ion is the charged particle formed by the removal or addition of electrons to any particular atom or molecule. The ions have two possible pathways for roots uptake: through the cell walls and intercellular spaces and movement cell-to-cell in the symplasm. The pathway is blocked at the endodermis by casparian bands in the cell walls. These barriers force the water and ions to move through cellular membranes if they are to be absorbed by the root.
The casparian bands that block the apoplasmic movement of ions are essential to selectivity since this is achieved by specific ion transporter proteins embedded in the membranes of plant cells. Membranes are composed of a lipid bilayer which prevents the free movement of substances in and out of cells. The transporter proteins are highly specific for the transfer of different ions across the membrane. So, for any given nutrient ion to be accumulated by a plant it must be transported across root cell membranes. Regulation of these transporters enables control of internal nutrient composition and concentrations.
Ions absorbed by the roots are then loaded into the xylem and transported with water up to the leaves when the plant transpires.