Translocation of Solutes in Plants (With Diagram)

In this article we will discuss about the translocation of solutes in plants.

Upward Translocation of Solutes:

Here the solutes or term mineral salt is used for those substances which contain nitrogen, sulphur and phosphorus in inorganic and organic combinations. The upward translocation of nitrogen, sulphur and phosphorus occurs in both organic and inorganic combination.

Earlier this was universally agreed that upward translocation of mineral salts takes place through the xylem. But most studies of the sap present in the xylem indicated that it contains more inorganic solutes than organic ones. During the seasons appreciable concentrations of mineral salts are commonly present in the sap of vessels when upward flow of water occurs at its most rapid rate.

The experiments of Clements and Engard (1938), Phillis and Mason (1940) and others have shown that ringing stems of various species do not prevent upward transport of mineral salts through the plant.

This shows that upward translocation of mineral salts can take place in the xylem. Experiments on ringing often show that mineral salts can be conducted in the xylem and experiments on intercepting wood often show that they can be conducted in the phloem.

Stout and Hoagland (1939) employed the radioactive tracer technique in experiments designed to ascertain the path of upward movement of mineral salts in plants.

The results of the experiments conducted by these workers showed that upward movement of mineral salts occurs in the xylem, but the possibility of some such movement in the phloem also is not excluded even though it is clear that lateral movement from xylem to phloem takes place readily.

The similar experiments of Gustafson (1939) also indicate that limited amount of upward translocation of radioactive phosphate may occur through the phloem.

Conclusively it can be said that upward translocation of mineral salts occurs in the xylem, and it appears quite certain that this is the main pathway along which their general upward translocation of mineral salts from roots to leaves occurs. Some upward translocation of mineral salts also occurs in the pholem under certain circumstances.

However, analysis of xylem sap shows the presence of inorganic salts, and by feeding plants with isotopes, it has exhibited that inorganic substances move up the plant through xylem.

General Aspects of Translocation of Solutes in Plants:

From the time when a young plant starts to grow until its death, there is continuous movement of solutes through the conducting elements of every organ of the plants. In the early stages of the development of the seedling, the food is generally translocated upward in the growing stems and downward in the developing roots from the storage tissues of that seed.

Once the leaves are formed and plant begins to photosynthesize, the food starts flowing in downward direction, from the leaves towards the stem and the root. Likewise, as soon as the developing roots make effective contact with the soil particles, absorption of mineral salts begins, and the mineral solutes absorbed by the roots move in an upward direction, towards the shoot.

Some of the solutes absorbed from the soil, especially those containing nitrogen, phosphorus or sulphur might react with the organic molecules in the root cells which have descended into the roots from the leaves. The resulting complex compounds, such as amino acids and amides may then be translocated in the reverse direction from the roots into the aerial parts of the plant.

The mineral salts absorbed by the roots are mostly translocated to young leaves and other do not remain in the organ into which they were first translocated. Most of the mineral salts that reach the young leaves or flowers from the root may be translocated back into the stem and become redistributed to other younger parts of the plant; the translocation patterns within a plant have significant effects upon its behaviour.

For example, when the young fruits and seeds develop, there is a general migration of organic materials from all parts of the plant towards the developing fruits and seeds with the result the growth of vegetative organs are checked. In cotton plants, for example, the vegetative growth decreases sufficiently, when the plant is fruiting heavily.

Here the large part of synthesized carbohydrates and of the nitrogenous compounds in the above ground organs of the plant moves into the enlarging fruits.

The carbohydrate supply reaching the root system becomes insufficient to maintain rapid respiration and root elongation. With the result the rate of absorption of mineral salts decreases, which in turn checks or even stops the vegetative growth of the plant.

Evidence from Ringing Experiments:

Much of the evidence indicating that organic solutes move towards the basal portions of the plant in the phloem has been obtained by ringing experiments. ‘Ringing’ means the removal of a narrow continuous band of tissues external to the xylem.

Since ringing entirely encircles the stem, all tissues external to the xylem are completely intercepted. This operation is also called ‘girdling’. This operation which is performed beneath the foliage region of the stem, produces two effects.

They are as follows:

1. The organic food accumulates in the region above the ring which swells up as a result of this accumulation whereas the food content of the tissue below the ring is greatly diminished on account of their being used up locally.

2. Secondly vigorous growth takes place in the region above the ring and sometimes the adventitious roots are developed. This happens, because of the accumulation of food materials in this region.

The tissues below the ring are soon dried up because of starvation. Here the growth of tissues is inhibited, root development is checked and the tissues ultimately die. If the ringing is done in such a way that the phloem is not interrupted, translocation of food is not affected and the normal development of the roots beneath the ring continues.

The ringing is of much practical value in the case of fruit trees. At the time of the development of fruits the rings are being formed at different places on the trunks of the fruit tree.

The ringing of the stem in such a manner prevents the downward flow of the food across the ring. With the result the food synthesized in the leaves above the ring is diverted to the fruits which become much larger in size. Normally the narrow rings are being made, so that the cambium may be affected and after some time the wound heals up.