Mechanism of Ion Translocation across the Root Tissue

After reading this article you will learn about the mechanism of ion translocation across the root tissue.

Nutrients in the soil solution enter the root either through diffusion or convection, initially through an epidermal layer of cells (outer side). They next pass through a layer of cortical cells with endodermis at the inner boundary. Beyond the endodermal cells, the ions traverse the stele including the xylem vessels and the phloem cells.

An individual cell has a cell-wall, cytoplasm and a vacuole. The cytoplasm (inner side) is bounded by two membranes, the plasma lemma at the outer surface and the tonoplast at the inner surface (boundary for the vacuole). Cytoplasmic connections occur between cells by plasmodesmata assuming as a continuous pathway the symplasm, from the epidermis to the xylem vessels (Figs. 20.5 and 20.6).

The cells of the root cap also rapidly absorb ions, but they cannot effectively transmit ions to the shoot. These three mechanisms (root interception, diffusion and mass flow) are associated with two components of the root system. One is termed outer space or apparent free space and the other is termed inner space.

Before describing the various processes of nutrient uptake and assimilation it is prerequisite to know about the plant cell. The cell wall structure is made up of pectic substances and cellulose. Cellulose tends to aggregate to form chain like structures known as “micro-fibrils”. Inter micro-fibrillar spaces allow the entry of water, air and solute particles into the cell wall.

The plasma membrane or plasma lemma is the membrane boundary between the cytoplasm and the cell wall; the tonoplast is the membrane which separates the cytoplasm from the vacuole. Located within the cytoplasm are the most important organelles within the cell like nucleus, chloroplasts and mitochondria.

Chloroplasts are the organelles in which light energy conversion and CO₂ assimilation take place. In the mitochondria, enzymes present which control the various steps of the Tri Carboxylic Acid (TCA) cycle, respiration and fatty acid metabolism. Various other cell functionaries are present and also engaged in various functions of plant metabolisms.

Biological membranes also play an important role for carrying out the ion uptake process. Biological membranes consist of amphiphilic molecules.

The term amphiphilic indicates the presence of both hydrophilic (OH groups, NH₂ groups, phosphate and carboxylic groups) and hydrophobic regions (hydrocarbon chains) in the membrane. Upids and proteins may thus be bound by electrostatic bonds, H-bonds and by hydrophobic bonds.

Lipids in the biological membranes have a barrier function preventing the diffusion of hydrophilic solutes, e.g. inorganic ions, amino acids and sugars across, the membrane. Proteins extend through the membrane form “protein channels” from one side to the other. Such channels may be of importance for the passage of small hydrophilic particles like water molecules and inorganic ions.

It is now generally accepted that biological membranes contain molecules called “ionophores” which play a crucial role in the transport of ions across the membrane. The tendency of a cation to form a complex with the ionophore increases as its hydration energy falls because the adsorbed water, molecules are replaced by the oxygen atoms of the ionophore during the formation of the complex.

As a rule there is a great discrepancy between the mineral nutrient concentrations in the soil or nutrient solution on the one side, and the mineral nutrient requirement, of plants on the other site. The mechanism by which plants take up nutrients must therefore be selective.

Ion uptake is characterised by the following:

(i) Selectivity:

Certain mineral elements are taken up preferentially, while others are discriminated against or almost excluded.

(ii) Accumulation:

The concentration of mineral nutrients can be much higher in the plant cell sap than in the external solution.

(iii) Genotype:

There are distinct differences among plant species in ion uptake characteristics.