Translocation of Solutes: Have you ever wondered how plants transport the solutes to various cells of the plants without any circulatory system? The vascular tissue (xylem and phloem) facilitates the movement of water and minerals throughout plants. In plants, the solute is transported through the xylem in a unidirectional flow from the source to the sink. The plants absorb the minerals through root pressure developed into the root cells. Let us try to understand how plants absorb mineral ions from the soil.

Uptake and Transport of Mineral Nutrients

In addition to the carbohydrates produced or generated in photosynthesis, plants also require certain minerals. Plants obtain these mineral elements from the atmosphere, soil and water. They get carbon from carbon dioxide and receive oxygen from the atmosphere. Hydrogen for carbon fixation is made available by water, and the remaining minerals are obtained from the soil.

Uptake of Mineral Ions 

1. Most of the minerals are absorbed actively by the roots, unlike water and some minerals. It is because of the following reasons:

(a) Minerals are present in the soil as charged ions that cannot move across the cell membranes.
(b) Mineral concentrations in soil are usually lower than those in roots. So, all minerals cannot be absorbed passively by roots.

2. Due to the above reasons, most minerals enter the root by active absorption into the cytoplasm of epidermal cells. This movement of ions from soil to the root’s interior is against a concentration gradient and requires energy.

3. The active uptake of ions is responsible for water potential gradient in roots and, therefore, helps in water uptake by osmosis. Some ions can also be absorbed passively into the epidermal cells of roots by mass flow and diffusion. Hence, ions are absorbed from the soil by both passive and active transport.

4.  Passive transport occurs through mass flow or bulk flow system and diffusion, while active transport occurs due to specific proteins in the membrane of root hair cells. These specific proteins actively pump ions from the soil into the cytoplasms of the epidermal cells of root hairs. ATP provides energy.

5.  Every cell has many transport proteins in its plasma membrane, including endodermal cells; some solutes cross the membrane but restrict others. The upward movement usually occurs through the xylem, while bidirectional movement occurs through the phloem.

6. Endodermal cell transport proteins are control points where plants can adjust the quantity and type of solutes that reach the xylem according to their variable requirement. Root endodermis is covered with the layer of suberin, and they allow the passage of ions in one direction only.

Translocation of Mineral Ions

1. The inorganic solute substances are carried in the xylem vessels with the ascending sap of water in the transpiration stream. In contrast, phloem is the pathway of downward translocation of organic solutes. Translocation always takes place from the supply end (source),i.e., the region of higher concentration of soluble form, to the region of lower concentration of its soluble form, the consumption end (sink).
2. After the ions have reached xylem vessels through active or passive absorption or a combination of the two, they are further transported upwards to the stem and other parts of the plant through the transpiration stream, and this allows the movement of minerals from their conducting tissue towards the area of their sink. The pH increase will lead to absorption of cations, while pH decrease will lead to absorption of anions.
3. Diffusion occurs at the end of fine veins and lets minerals out. Cells take them up through active uptake. The mineral ions are frequently remobilised inside the plant, particularly from their older senescing parts to younger growing parts.
4. The older falling or dying leaves export much of their minerals to younger leaves and other parts. Elements most readily mobilised are phosphorus, nitrogen and potassium. Only those elements incorporated in structural components are not remobilised, e.g., calcium. Young growing leaves and other sinkers have access to remobilised minerals.

Fig: Translocation of Solutes

5. Translocation of organic solutes is essential in higher plants as during the germination of the seeds, the insoluble reserve food material of the seed is converted into a soluble form that is supplied to the growing regions of the plants, such as the apical and lateral meristems, young leaves, developing flowers, fruits and seeds and the storage organs.
6. In higher plants, only the green parts can manufacture food. So, the location of photosynthesis is not the same as the site at which nutrients are stored. As a result, the nutrients must be moved throughout the plant via translocation, and they must be supplied to other non-green parts for consumption and storage.

Translocation and Storage of Food in Plants

Food, primarily sucrose, is transported from the source to a sink by the vascular tissue, phloem. And this movement of organic food materials or the solutes in soluble form from one place to another in higher plants is called translocation of organic solutes. Though it is generally believed that xylem transports only inorganic nutrients while phloem transports only organic material, it is not valid. Recent research has revealed that some nitrogen in xylem sap travels as inorganic ions; the majority is carried as organic compounds such as amino acids and related compounds. As well, organic compounds such as phosphorus and sulfur are carried in small quantities.

Routes of Translocation

Dissolved minerals can be translocated in various directions after absorption:

1. Downward Translocation of Organic Produces: From leaves to root and other parts of the plant.
2. Upward Translocation of Organic Produces: Roots to leaves or other apical regions.
3. Upward Translocation of Mineral Ions: By active transport through the xylem.
4. Upward Movement of Minerals: Movement of ions from sap to the leaves.
5. Lateral Translocation of Minerals: It occurs in woody stems in the tangential direction.
6. Radial Translocation: The translocation of organic solutes from pith to cortex takes place through medullary rays.

Mechanism of Translocation through Phloem

Several theories have been put forward to explain the mechanism of organic solute movement. Still, they are not entirely satisfactory, and the most accepted theory which explains the mechanism of translocation is the Mass Flow Theory. 

Munch’s Mass Flow or Pressure Flow Hypothesis

According to this hypothesis put forward by Munch (1930) and elaborated by Craft (1938) and others, organic substances or solutes move from mesophyll (the region of high osmotic pressure) to the region of low osmotic pressure along the turgor pressure gradient.

Fig: Munch’s Mass Flow Hypothesis

The principle involved in this hypothesis can be explained by a simple physical system, as shown in the diagram given above. Two chambers, “X” and “Y”, made up of semipermeable membranes, are connected by tube “T” immersed in a reservoir of water. Chamber “X” contains a highly concentrated sugar solution compared to chamber” Y” that has a dilute sugar solution. The following changes were observed in the system:

1. The high concentration sugar solution of chamber “X” is in a hypertonic state, which draws water from the reservoir by endosmosis, and due to this, turgor pressure is increased.
2. An increase in turgor pressure in chamber “X” forces the mass flow of sugar solution to chamber “Y” through the tube “T” along the turgor pressure gradient. The sugar(solute) movement will continue until the solution in both the chambers attains the state of isotonic condition. The system becomes inactive, and then the movement will stop.
3. However, if a new sugar solution is added in chamber “X”, the physical system will repeat the same steps to attain the state of isotonic condition.
According to Munch’s hypothesis, a similar analogous system for the translocation of organic solutes as given in the experiment exists in plants. Chamber “X” is analogous to mesophyll cells of the leaves, which contain a higher concentration of food material insoluble form formed from photosynthesis. In short, “X” is the production point called “source”. Chamber “Y” is analogous to cells of stem and roots where the food material is utilised and converted to insoluble forms. In short, “Y” is a consumption end called “sink”, and Tube “T” is analogous to the sieve tube of phloem, which is placed end to end.
4. The translocation of solutes occurs in a mass from mesophyll through the phloem to the cells of stem and roots, and cell sap through the sieve tubes form a region of higher turgor pressure to low turgor pressure (i.e., along a turgor pressure gradient).
5. The organic solutes are either consumed or converted into insoluble forms. In the stem and root cells, the excess water is released into the xylem (by turgor pressure gradient ) through the cambium.

Demerits of Pressure Flow Hypothesis

1. This hypothesis explains the unidirectional movement of solute only and doesn’t explain the bilateral movement phenomena of materials moving at the same time in opposite directions.  
2. Osmotic pressure of mesophyll cells and that of root hair do not confirm the requirements.
3. This theory gives a passive role to sieve tube and protoplasm, while some scientists demonstrated the involvement of ATP.

Summary

Plants require proper mineral elements for their proper functioning. These mineral elements present in the soil are in a charged form that cannot be absorbed passively by the plants. The roots absorb minerals from the soil through active transport by changing the concentration of the mineral ions within the root cortex, which creates pressure in the root and allows the mineral ions to be absorbed with the water molecules. The mineral ions absorbed by the plants can be transported from source to sink through the xylem. According to a hypothesis put forward by Munch (1930) and elaborated by Craft (1938) and others, organic substances or solutes move from mesophyll (the region of high osmotic pressure) to the region of low osmotic pressure along the turgor pressure gradient.

Frequently Asked Questions (FAQs) on Translocation of Solutes

Q.1. What is the direction of flow in the xylem?
Ans: The movement in the xylem is unidirectional from source to sink.

Q.2. Why do roots not passively absorb all minerals?
Ans: Roots do not absorb all minerals passively as minerals are present as charged particles in the soil.

Q.3. What are the control points in plants where the number of solutes is regulated?
Ans: Transport protein of the endodermal cells acts as control points in plants.

Q.4. How does the transport of water and minerals take place in plants?
Ans: Plants transport water and minerals through the xylem.

Q.5. What is the translocation of solutes?
Ans: Food, primarily sucrose, is transported from the source to a sink by the vascular tissue, phloem. And this movement of organic food materials or the solutes in soluble form from one place to another in higher plants is called translocation of organic solutes.

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