Translocation of Minerals and Its Circulation | Plant Physiology

The movement of solutes through the plant tissues can take place in two pathways, the one is cytoplasmic pathway and the other is cell wall pathway. Solute movement may occur through channels in the cell walls and is, therefore, extracellular. In this way it bypasses the protoplasts and overcomes the barriers in the cross membranes.

In this process the solutes can be freely exchanged with an external solution and this pathway is also usually called free space. The whole of the cell wall continuity is referred to as apoplast or apoplasm (Fig. 10-8). Solutes may also follow the route through the cytoplasm of the protoplasts and are transported to the adjacent cells through plasmodesmata.

The latter channels provide continuum with the adjacent cells in a three dimensional way and is called symplast or symplasm. It may be noted that solutes before their entry into symplasm must pass through plasma membrane and the latter provides high degree of selectivity on this pathway.

The significance of the two pathways varies in different tissues and organs of plant. For instance, in the young root, free space provides the continuum between the external solution and the root cortical cells during ions uptake.

Through free space the solution from outside can move across the cortex up to endodermis and. the latter provides a barrier to any further radial movement through the apoplast pathway. The plasma membrane of the cortical cells is immersed in the external solution and a large surface area is available for the uptake of ions into the cortical cytoplasm.

From the outside medium the ions are selectively accumulated in the root cortex and then to the cytoplasm continuity, the symplast. The selective properties of the uptake mechanisms and selective accumulation of ions across the tonoplast into the vacuole regulate the ionic content within the symplast.

It may be added that symplast is rich in K⁺ whereas Na⁺ is usually preferentially accumulated in the cortical vacuoles and is partially removed from the symplast. Most of the ions are moved radially across the root and into the stele whereas some are used within the symplast in a metabolic capacity.

It is generally suggested that radial movement is brought about through diffusion and the driving force is the decreasing gradient of concentration across the root. The ion movement is further helped by cytoplasmic streaming. If the cytoplasmic movement within the cortical cells is inhibited, radial ion transport is also inhibited.

In the cortical symplast very little longitudinal movement is noticed and the direction of the driving force is centripetal. Using-electron probe microanalyser some evidence of a gradient of K⁺ across the root has been provided and points towards high K⁺ level in the cytoplasm.

A pH gradient in the root cortex is also reported and varies from 6.0 (epidermis) to 7.0 (protoxylem). Ions which are transported radially across the endodermis through the symplast and are set free in the stele for the long-distance transport through the xylem.

Several viewpoints exist on the mechanism of ion release within the stele and some suggest passive while others plead active efflux across the plasma membrane of the stellar parenchyma. Apparently stellar cells have the capacity to accumulate ions when applied through the symplast but cannot be accumulated when isolated from the cortical tissue.

Available data suggest that both anions and cations move into the xylem down the gradient of electrochemical potential. This is based on the passive leakage hypothesis. However, many of the workers emphasize active efflux mechanism within the stele. The conflict still remains unresolved.

Ions which are released in the apoplast of the stele are transported above with the transpiration stream through the xylem elements. If transpiration is absent ions accumulate in the stellar apoplast and lower the water potential. Endodermis acts as barrier for the radial transport of ions. In the endodermal cells legnin and suberin thickenings form the inner limit of the cortical free space.

Autoradiographic, EM localization and ion exchange studies lend support to this contention. Further at the tip of the root where endodermis is not fully differentiated and Casparian bands not well developed, free space continuity exists through the xylem. Solutes and water which have crossed the cortical cells apoplast, cross the plasma membrane at points where plasma membrane and endodermal cells are strongly attached.

In monocot roots some cells of endodermis constitute passage cells and are thin walled. These cells usually lie opposite to the xylem archs of the stele. The presences of large pits in the inner tangential wall ensuresymplastic continuity across the thickened endodermis. In the inner tangential walls plasmodesmata are also reported.

K⁺is translocated easily into the stele all along the root whereas Ca²⁺ is taken rarely in the mature region of the root. Ca²⁺ cannot move in the symplast readily. In summary we may remember that the endodermis is essential for root pressure exudation.

The Xylem Pathway:

Long-distance transport of solutes and water is brought about through the specialized elements of the xylem and of the phloem. In xylem two types of treachery elements are there and these are tracheids and vessel elements. These are thick walled elements with lignified secondary walls and devoid of cytoplasm.

They are like pipes or open tubes which allow movement of large quantities of solution. In the vessels there is no resistance for their movement. For detailed structure and distinction between the tracheids and vessels any book on plant anatomy may be consulted. Xylem elements also have wood fibres and xylem parenchyma cells.

The latter are the only living component and permits lateral movement of water and solutes into and out of the conducting cells. These cells also store sugars and starch. Xylem transfer cells are also reported to be present.

Plasmodesmatal connections are lacking within the treachery elements and xylem at maturity is fully apoplastic with no direct continuity with the symplast. We have already discussed the movement of ions across the root and their entry in the xylem vessels where they are carried upward to the aerial parts through transpiration stream.

The composition of the xylem sap depends upon the solution in the external medium. It is rich in inorganic ions, very little amino acids, some growth substances and carboxylic acids. Cations are more than the anions. Organic ions probably provide the balance.

In the xylem of Ricinns, NO₃^– is the major form of nitrogen but in the roots of some species amino acids are present in high level in the xylem sap due to high NR activity.

Xylem sap has been shown to have ABA, IAA, gibberellins and cytokinins. There is also active removal of some ions like Na⁺. It is also shown that lateral movement of ions out of the xylem presumably occurs throughout the passage from the absorbing roots to the leaves.

These ions usually appear in the phloem exudate. In tall trees the composition of xylem sap may vary at different places before it reaches the leaves.

The role of transfer cells in the selective extraction of ions has also been reported. Divalent ion movement through the xylem is retarded. If Ca²⁺ is given to the plant in a chelated form initially it is taken up slowly but it is highly mobile since it is not absorbed on the exchange sites in the xylem.

When the composition of xylem sap in the initial uptake is compared with the mesophyll cells marked differences are noticed. For instance, the level of K⁺ may be much higher than that of the external solution whereas Na⁺ may be considerably reduced. Shoots of many species have high K⁺/Na⁺ ratio.

Circulation of Mineral Salts:

Salts accumulated in the xylem of the roots are elements translocate to the shoot (Fig. 10-9). For example, mineral salts that are deposited in the leaves may be withdrawn prior to abscission and translocated to other parts of the plant such as younger leaves and reproductive parts.

The circulation of elements takes place in the vascular tissues. In order to determine whether vascular tissues provide passage for salts from one region of the plant to another, we will discuss the movement of salts in xylem and in phloem, laterally between these two tissues and outward from the leaf.

Translocation of Salts in the Xylem:

Upward translocation of mineral salts from the root to the aerial organs occurs in the xylem elements along with the transpiration stream (Fig. 10-9).

The evidence in favour of this hypothesis is as follows:

1. Studies of sap from xylem vessels show that they contain traces of both organic and inorganic solutes to the total quantities utilized. The concentration of the inorganic solutes is high.

2. Ringing experiments show that upward translocation of salts is not prevented by removal of the phloem tissue.

3. The increase in the rate of transpiration increases the salts uptake.

Radioactive Tracer Technique:

Radioactive tracer elements are used to ascertain the path of upward movement of mineral salts in plants.

The bark and xylem were carefully separated. A strip of paraffin paper was then inserted between the xylem and the bark. Radioactive ions of potassium were introduced into the rooting medium.

After a period of 5 hours under conditions following transpiration, distribution of the tracer ion in the stem was ascertained by measuring the magnitude of radioactivity in the xylem and phloem from the stem above, below, and in the region where xylem and phloem were separated with paraffin paper.

The results of these experiments clearly showed that radioactive potassium (K¹²) was relatively abundant in both the bark and the xylem above and below the section of the stem in which the xylem and phloem were separated^–by paraffin paper.

These experiments show that mineral salts absorbed by the roots are translocate upward normally through the xylem. During their upward passage some of the mineral salts have moved laterally from the xylem into phloem. Once the mineral salts have moved laterally from xylem to the phloem they also show upward movement.

It is also demonstrated that upward movement of mineral salts may occur in the phloem. The stem tip growth is impeded by removing ring of bark high up the stem, thus supporting the upward translocation of mineral salts through the phloem.

Because of the high position of the ring on the stem in Curtis’s experiment, we assume that the primary effect on stem tip growth was due to the blockage of salts moving out of the lower leaves and so transported upward in the phloem and not because of the root absorbed salts.

This assumption is based on the observation that ringing a stem near the root has no effect on salt nutrition.

Outward Movements of Salts from Leaves and other Lateral Organs:

Mineral salts enter the leaves briefly through the xylem but prior to abscission they move out of the leaf. These mineral salts are nitrogen, potassium, phosphorus, sulphur, chlorine and under certain conditions iron and magnesium. Those remaining there include calcium, boron, manganese and silicon.

The mineral salts move out of leaves in the phloem. Radioactive phosphorus (P³²) introduced as phosphate into intact leaves of squash plants moved out of such leaves into the stem but did not move out of similar leaves if the petioles were scalded. This shows that, outward movement of the phosphorus containing compounds occurred in the phloem.

When radioactive phosphorous was given to leaves of cotton plant, where the bark of a branch immediately below a leaf petiole, was separated from the wood by waxed paper; after about 3 hours, phosphate was located in the bark both up and down the point of leaf attachment.

Thus outward movement of salts from the leaf into the stem takes place in the phloem. On entering the main vascular stream salts move primarily in a downward direction in the phloem tissue. Some upward translocation of salts also occurs in the phloem. The phloem to xylem lateral ‘ movement implies that both tissues may be involved with the upward translocation of mineral salts moving out from the leaves.

The movement of salts in the phloem tissue appears to occur in direction simultaneously in the same sieve tube or in two different sieve tubes, one towards the tip, and the other towards the base of the plant.

The general picture of the circulation of mineral salts in the plant is as follows (Fig. 10-9):

1. The upward translocation of salts takes place primarily in the xylem tissue, although some upward movement does occur in the phloem.

2. Downward movement of the mineral element takes place in the phloem tissue where, also, upward movement occurs. Movement of salts in the phloem tissue is bidirectional.

3. Lateral movement occurs between the xylem and the phloem and this movement is mediated by the cambium.

4. Movement of salts out of leaves occurs prior to abscission in the phloem tissue.