After reading this article you will learn about the 3 Mechanisms of Nutrient Uptake in Plants.
Mechanism # 1. Mass Flow:
Mass flow, the most important of these mechanisms quantity wise, is the movement of plant nutrients in flowing soil solution. Movement of ions in the soil solution to the surfaces of roots is an important factor in satisfying the nutrient requirement of plants. This movement is accomplished largely by mass flow and diffusion.
Mass flow is a convective process in which plant nutrient ions and other dissolved substances are transported in the flow of water to the root due to transpirational water uptake by the plant. Some mass flow can also occur due to evaporation and percolation of soil water.
Mechanism # 2. Diffusion:
Diffusion is the movement by normal dispersion of the nutrient from a higher concentration through soil water to areas of lower concentration of that nutrient.
Diffusion process operates when an ion moves from an area of high concentration to one of low concentration by random thermal motion. As plant roots absorb nutrients from the surrounding soil solution, a diffusion gradient is set up. A high plant requirement or a high root “absorbing power” results in a strong sink or a high diffusion gradient favouring ion transport.
There are mainly three soil factors which can influence the movement of nutrient ions into the root through diffusion mechanism namely diffusion coefficient, concentration of the nutrient in the soil solution and the buffering capacity of the solid phase of the soil for the nutrient in the soil pollution phase.
The effective diffusion coefficient, De, for the diffusion of an ion in soil, is influenced by volumetric water percentage Î¸, tortuosity factor (zigzag path), f, and buffering capacity b. Mathematically it can be written as
De = DwÎ¸. 1/f . 1/b
where, Dw, is the diffusion coefficient for the particular nutrient in water.
Raising Î¸ also reduces tortuosity, which in turn increases diffusion. Buffering capacity usually decreases as nutrient levels in the soil are raised. Reduction in buffering capacity is associated with a rise in the rate of diffusion.
The effective diffusion coefficient varies directly with the square of the absolute temperature.
The importance of both diffusion and mass flow in supplying the root surface with ions for absorption depends on the ability of the solid phase of the soil to supply the liquid phase with these ions. The concentration of ions in solution will be influenced by the nature of the colloidal fraction of the soil and the intensity to which these colloids are saturated with basic cations.
The concentration of certain ions under some conditions may build up at the root surface because the root is unable to absorb ions at a sufficiently faster rate. Under such conditions the phenomenon of “back diffusion” occurs due to concentration gradient where the movement of certain ions will be away from the root surface and back toward the soil solution.
Normally such a condition will not occur, but as root do not absorb all nutrient ions at the same rate, there may on occasion be a buildup of those ions that are less rapidly absorbed, particularly during period of rapid absorption of moisture by the plant.
Mechanism # 3. Root Interception:
Root interception is the extension (growth) of plant roots into new soil areas where there are untapped supplies of nutrients in the soil solution. All these three processes are in constant operation during growth. The importance of each mechanism in supplying nutrients to the root surface for absorption by the root varies with the chemical properties of each nutrient element.
The importance of root interception mechanism for ion absorption is enhanced by the growth of new roots throughout the soil mass and probably also by mycorrhizal infections. As the root system develops and exploits the soil more completely, soil solution and soil surfaces retaining adsorbed ions are exposed to the root mass and absorption of these ions by the contact exchange phenomenon is accomplished.
The exact mechanisms for ion absorption into the root cells are not well understood. The cell walls are porous and the soil solution can move through some or all of the cell walls, causing intimate contact of the soil solution with the outer membranes of the cells.
For a nutrient element to cross a cell membrane into the cell, it is necessary for each nutrient element to be attached to some carrier. The carrier nutrient complex can pass through the membrane into the cell.
The necessary carriers are different for many of the nutrients. Some nutrient elements can be partially but not entirely excluded from absorption, others can be preferentially absorbed, even against a concentration gradient (Fig. 20.4).
As nutrient cations are absorbed, H+ ions are excreted into the soil solution or more organic acid anions are produced inside the cell to balance the absorbed nutrient cations. Similarly, as nutrient anions are absorbed by the plant, more compensating cations are absorbed and/or HCO3 ions are excreted into the soil solution in order to maintain the electron balance in the cell.
Probably the H+ ions and HCO3– ions are excreted into the ‘Soil solution first in order to help solubility of nutrient elements.
Besides these plants absorb nutrients through stomatal opening. Carbon enters almost completely through the stomata as CO2 with the release of O2 produced during photosynthesis in gaseous form. Hydrogen, as a part of water molecules, is absorbed through stomata,-but this absorption is generally very lower as compared to absorption through roots.