In this article we will discuss about the Translocation of Mineral Ion’s in the Plant.
Plants obtain their supply of carbon and most of their oxygen from CO2 of atmosphere, hydrogen from water while the rest are minerals which are picked up individually from the soil. Minerals exist in the soil as ions which cannot directly cross the cell membranes. The concentration of ions is some 100 times more in root interior than in the soil. Therefore, all minerals cannot be passively absorbed.
The movement of ions from soil to interior of root is against concentration gradient and requires an active transport. Specific ion pumps occur in the membrane of root hairs. They pump mineral ions from soil to cytoplasm of epidermal cells of root hairs. Energy is provided by ATP. Respiratory inhibitors like cyanide which inhibit ATP synthesis, generally reduce the ion uptake.
The small amount which passes into the root even without ATP must be through a passive technique. For active transport, ATPase’s are present over the plasma membranes of root epidermal cells.
They establish an electrochemical proton gradient for supplying energy for movement of ions. The ions are again checked and transported inwardly by transport proteins present over the endodermal cells. Endodermis allows the passage of ions inwardly but not outwardly.
It also controls the quantity and type of ions to be passed into xylem. Inward flow of ions from epiblema to xylem is along the concentration gradient.
The collection of ions in the xylem is responsible for water potential gradient in the root that helps in osmotic entry of water as well as its passage to xylem. In the xylem, minerals are carried up along with the flow of xylem solution. In leaves the cells absorb the minerals selectively through membrane pumps.
Translocation of Mineral Ions in the Plant:
Though it is generally considered that xylem transports inorganic nutrients while phloem transports organic nutrients, the same is not exactly true. In xylem sap, nitrogen travels as inorganic ions, as well as organic form of amino acids and related compounds. Small amounts of P and S are passed in xylem as organic compounds.
There is also exchange of materials between xylem and phloem. Therefore, mineral elements pass up xylem in both inorganic and organic form. They reach the area of their sink, namely young leaves, developing flowers, fruits and seeds, apical and lateral meristems and individual cells for storage. Minerals are unloaded at fine vein endings through diffusion. They are picked up by cells through active uptake.
There is remobilization of minerals from older senescing parts. Nickel has a prominent role in this activity. The senescing leaves send out many minerals like nitrogen, sulphur, phosphorous and potassium. Elements incorporated in structural components are, however, not remobilized, e.g., calcium. The remobilized minerals become available to young growing leaves and other sinks.