Translocation of Solutes in Plants (With Diagram)

The below mentioned article provides an overview on the translocation of solutes in plants.

According to the classical concept inorganic solute substances are carried in the xylem vessels with the ascending sap of water in the transpiration stream whereas phloem is the pathway of downward translocation of organic solutes (synthesised foods like sugars, amino acids, etc.).

Translocation of Inorganic Solutes:

The classical concept of movement-of inorganic solutes upwards through the xylem is unquestionably true but it is difficult to estimate with certainty the proportion of mineral salts thus translocated. Any solute conducted through the xylem is carried along with the ascending streams of water which are pulled up through the plant by transpirational suction.

The rates at which inorganic solutes are translocated through the xylem vessels correspond closely with the rates of translocation of water. There is much indirect evidence for the view—analysis of xylem sap reveals the presence of inorganic salts. By feeding plants with radio-isotopes, it was shown conclusively that inorganic substances move up the plant in the xylem.

Most of the evidence, indicating that organic solutes move downwards from the aerial parts towards the basal portions of stem and that xylem is the main pathway of movement of inorganic solutes upwards, have been obtained by the so-called ringing experiments. Ringing really refers to the removal of a narrow continuous band of the tissue external to the xylem.

In a tree or herb, such a ringing always produces accumulation of organic solutes, chiefly sugars, in the tissues above the ring, while sugars in the stem and root tissues below the ring, become depleted as a result of utilisation of sugars in respiration. Below the ring the development of buds and new roots takes place very slowly or none at all (scarcity of organic food) whereas above the ring new roots are rapidly and profusely formed (supply of food abundant). Conduction of inorganic solutes from the roots upwards is scarcely affected.

The classical concept that mineral substances are carried upwards in the xylem vessels has been shaken, during recent years by some apparently contradictory obser­vations and a re-examination of the classical concept was certainly called for. Although analysis of xylem sap has shown the presence in significant amounts of Ca, Mg and K, 50-90% of the total solutes was sucrose, most of the rest being organic nitrogenous compounds. This even promoted drastic suggestions such as that all substances, both inorganic and organic, move in the xylem.

On the other hand, other investigators have championed for phloem as the main path of movement, upward or downward, of all solutes, inorganic as well as organic! It has also been claimed that the path of movement of solutes depends upon the quantity of solutes available. When the quantity of solute to be translocated is high, it may move in the xylem; when the quantity is low, it moves in the phloem!

Later investigators, have, however, confirmed that xylem is the main path of move­ment of inorganic minerals while others even now stick to phloem.

More recent evidence shows that N, P, S, K, Mg and CI can be translocated in phloem in the downward direction but certainly not Ca. Thus, we have no other alter­native but to admit that inorganic substances may move both in the xylem and phloem but the available evidence as yet favours the view that upward movement of these mineral substances is primarily, if not solely, in the xylem; downward movement pro­bably solely in the phloem.

Since by far the greater part of the movement of minerals is in the upward direction from roots to the leaves, this would assign to the xylem, the main path of movement of inorganic solutes in plants whereas the phloem unquestionably is almost the sole path of movement of organic substances synthesised in plants and the path of downward movement of all solutes, inorganic or organic.

By feeding single leaves with ¹³CO₂, it has been shown that photosynthate, carrying ¹³C label was transported both up and down and found to accumulate at the points of most active growth. On killing a section of stem with hot wax, ¹³C-labelled photosynthate did not move in either direction in such a block. This again, certainly suggests that the phloem is the exclusive path of transport for photosynthate. The classical concept of xylem for inorganic sub­stances and phloem for the organic is therefore reasonably near the truth.

Transport Through Phloem:

On the transport mechanism in plants, the phloem is unquestionably the major path of downward movement of all solutes. In some cases, however, particularly in developing fruits which are attached to stem in such a position that some or all of the food translocated into them must move through the stem in the upward direction. Similar examples can be cited in early stages of development of seedlings when upward translocation of foods certainly occurs from the endosperm and cotyledons to the apical shoot meristems in which rapid growth is taking place.

Other examples are provided in upward translocation of organic foods during the earlier stages of shoot growth from underground stems such as bulbs, rhizome, tubers, etc. Extensive recent investigations favour the idea that these and other similar types of up­ward translocation of organic foods take place in the phloem and not in the xylem and this certainly contradicts the classical idea that all upward transport occurs in the xylem.

Of the various stem tissues, only the xylem and phloem possess structures which are suitable for a longitudinal movement of the solutes with the important difference that whereas xylem vessels are dead, sieve tubes are very much alive. Both these tissues are characterised by the presence of elongated cells which are joined end to end in such way as to form a continuous system—a sort of pipe line.

Like the xylem, the phloem is continuous from the top to the bottom of the plant and ultimate termination of the phloem is in stem tips, lateral organs and in the root tips. Cross-walls (sieve plates), separating the sieve tube cells one from another, are often oblique (present in the side walls of the sieve tubes) and provided with pores. Strands of cytoplasm (plasmodesmata) pass from one sieve tube to the next to form a continues living conducting pipe system.

The Mechanism of Translocation of Solutes in Phloem:

It is evident that the mechanism of the transport of solutes through phloem must be entirely different from the mechanism of transport through the xylem. The fact that the xylem vessels are dead cells points to a fundamental difference of xylem vessels from sieve tubes, with its rich protoplasmic content, highly permeable walls and the conse­quent development of abnormally high turgour pressure.

The xylem vessels, in the con­duction of solutes through them, act only passively with its thick in-collapsible cell walls. The water is pulled up by tension developed in the aerial organs and by the cohesive forces of water molecules between each and the adhesive force between the water and thick, rigid walls of the vessels.

It has long been recognised that simple diffusion is totally inadequate to account for the known rates or the translocation though the phloem which can, in many cases, be as high as 100 cm per hour from a 10% solution of sugar, for the total cross-sectional area of the sieve tube in potato tuber and grasses. The late of movement of ¹⁴C-labelled photosynthetic products has recently been shown to be unexpectedly fast. In soybean plants (Glycine soja) amino acids are known to move through phloem at 370—1,370 cm per hour and sometimes even at 3,000 cm per hour.

Later experiments confirmed these results and showed that a small part of the assimilates moved downwards towards the roots, at over 5,000 cm per hour—an incredible feat of food transport through narrow channels like sieve tubes—almost a yard a minute! So diffusion alone is quite in­sufficient to account for the observed rates of solute movement.

There is general agreement that gradients in concentration are intimately involved in organic food movement in the phloem.

Concerning the mechanism of movement, there are many theories; these may be grouped into three categories;

(A) The Druckstrom or mass-flow (literally pressure-flow) or convectional flow hypothesis visualising a flow of solution along the positive gradient—from higher pressure to lower—of hydrostatic pressure developed osmotically, proposed by Ernst Munch,

(B) Those postulating move­ment of the solute molecules in, through or upon the surface of the sieve tube proto­plasm. By this mechanism the solutes are pictured as moving along concentration gra­dient (i.e., in difference in concentration) but independently of each other and of the solvent water and

(C) The export of sugar from photosynthesising leaves (e.g., sugar cane, sugar beet, etc.), is nowadays thought to be primarily dependent on light and sugar accumulation.

All this diffusion is supposed to be accelerated by protoplasmic stream­ing, activated diffusion, a term implying an accelerated diffusion carried on by the proto­plasm and which certainly involves an expenditure of energy.

A. The Druckstrom or Mass-flow Theory:

The principles involved in this theory are clarified by a reference to a simple physical system:

Two cells with semipermeable walls are connected by a tube to form a closed system and dipped in water. Cell A is assumed to enclose a stronger solution of sucrose than cell B. Water no doubt will enter both the cells but due to greater osmotic pressure in cell A (due to higher conc.) than in B, greater turgour pressure will no doubt be deve­loped in A than in B.

The higher hydrostatic pressure developed in A will be transmitted through the closed system resulting in greater diffusion pressure in the water in B than in pure water. The diffusion pressure of water in B will actually be augmented by the imposed hydrostatic pressure transmitted from A. As a result, water will pass out of the cell B and coincidentally there will be a convectional flow of solution along the tube from A to B and then into the external medium. This mass-movement of solution under pressure will continue until the sugar concentration in both the cells are equalised.

If, however one could Fig. 718. Diagram of an osmotic system in which arrange for continuous addition of mass-flow of solution is supposed to occur, sugar in the cell A as rapidly it is moved out of it (the supplying cell) and continuous removal of sugars from the cell B (the receiving cell) by utilisation or by converting them into some insoluble form (i.e., osmotic pressure in the cell B is never allowed to increase more than cell A), a mass-flow of solution under pressure should continue indefinitely.

Some recent investigators have questioned the principle underlying the whole set-up of this experiment, maintaining that this is based on fundamentally questionable concept and the similarity between this experiment and actual transport of organic food in plants, is too far-fetched to be credible.

The Munch hypothesis, introducing the symplast concept, visualises the interconnec­ted protoplasm of plant through plasmodesmata as constituting a continuous living system —the symplast concept—relatively impermeable on the outer surface but highly per­meable throughout its mass.

Fig. 719, shows diagrammatically the downward translocation of solutes. Cells L₁, L₂ and L₃ represent green photosynthetic cells (supplying cells) of the leaf and correspond to the cell A (Fig. 718). Similarly R₁, R₂ and R₃ represent root cells corresponding to (receiving cells) cell B. The continuous system of phloem correspond to the horizonrtal tube connecting A and B in Fig. 718.

The osmotic pressure of the leaf cells is maintained relatively high as a result of photosynthesis. In the root cells, the osmotic pressure is supposed to be lower because most of the sugars, translocated to them, are utilised in metabolic activities or are converted into insoluble storage form,. Water is supplied continuously from the roots to the leaf cells through the xylem vessels.

This hypothesis assumes that in the supplying leaf cells higher turgour pressure developed due to higher osmotic concentrations will cause a convectional flow of solution downward in the phloem. Plasmodesmata connecting adjacent cells are sup­posed to permit mass-movement of solution from L₃ → L₂ → L₁ and from L₁ into the sieve tubes. Once the convectional flow enters the sieve tubes, the function of sieve tubes is completely passive, allowing the solution under pressure to pass through them.

In other words, the sieve tubes are pictured as making up a specialised mechanism of the symplast when water absorbed osmotically brings about high turgour in the regions of synthesis, i.e., the leaves. The turgour promotes a rapid mass-flow of solution through the sieve tubes, through the sieve pores to the region of utilisation, where assimilation, respiration and condensation for storage reduce the osmotic activity of solutes in the roots, thereby reducing its turgour and allowing the water only to return to xylem via cambium or other receiving root cells. Movement in the sieve tubes would then always have to be from regions of high to one of low turgour pressure.

It could, therefore, occur in an upward direction also from storage tissues to the developing aerial shoots for the osmotic pressure, consequently turgour, is evidently much higher in the cells of the storage organs such as underground stems than in the actively growing cells of the developing shoots. The mass-flow mechanism involves a recirculation of water and of the total amount of water used by the plant; only about 5% is invol­ved in this recirculation. Excess of water in the phloem stream is squee­zed (?) laterally through the cam­bium into the xylem as pholem solutes move downward.

An elaboration of Munch hypothe­sis was provided by Crafts from anato­mical studies. According to Crafts, movement from leaf cells to the sieve tubes is by diffusion, accelerated by protoplasmic streaming (rotation and circulation of protoplast) via the plas­modesmata connection at the expense of released cellular metabolic energy. The sieve tubes allow a ready passage of the solutes in solution from one to another.

Removal of the assimilates (carbohydrates, amino acids etc.) from sieve tubes is pictured as active absor­ption (that too requires respiratory energy) of solutes of the sieve tubes by the adjacent living parenchyma Fig. 719. Diagram to illustrate the mechanism of solute cells. This is followed by symplastic movement, i.e., movement along the interconnected continuous protoplast of the plant. Undoubtedly if this modification of Munch hypothesis by Crafts is correct, it would, as has been pointed out before, require the expen­diture of considerable respiratory energy.

The strongest experimental evidence in support of Munch theory is that sap will often exude rapidly from a cut made into the phloem of a stem and this phloem exuda­tion has been seen to continue for a period of more than 24 hours. This phloem exudate is apparently cell sap with high concentration of normal translocation substances and not cytoplasm. This suggests that the phloem exudate is under considerable pressure.

Evidently a turgour push of considerable magnitude developed originally in the leaf cells is operating in the phloem elements of intact stems. It has been shown that after a sieve cell is punctured by an aphid, it continues to exude sucrose solution for a long period, until the exudate exceeds by many thousands of times the volume and sucrose content of single sieve cell. This may be a pathological disturbance of the normal system, but it does certainly suggest that a free flow of sugar solution is theoretically possible through the sequence of interconnected sieve tubes.

On the whole the mass-flow mechanism is supported by evidences, such as per­meable condition of the functioning sieve tube, the presence of a concentration gradient of osmotically active materials in the phloem and the last, but not the least, the pheno­menon of phloem exudation. The elegant idea of establishment of ‘source’ and ‘sink’ by regulating assimilation, with hydrolysis at the ‘source’ and utilisation and storage at the ‘sink’, accompanied by recirculation of the solvent component, water, fascinates to the point of belief.

The most serious objection against Munch hypothesis is that the experimental determination, of osmotic pressure of the supplying cells and receiving cells, has consis­tently shown greater values for receiving cells than supplying cells. For normal operation of Munch theory, exactly the reverse must be true, that is a pressure gradient from the supplying cells to the receiving cells.

Recent investigations further show that the osmotic pressure of the sieve tubes is actually greater than the osmotic pressure of the mesophyll cells. Another difficulty encountered is that cytological observations do not normally reveal interconnecting plasmodesmata channels from one sieve cell to another along which a continuous flow of solution is supposed to take-place. Moreover if such channels do exist, they must be so narrow that resistance to such a flow would be very great indeed.

Other weaknesses of mass-flow theory include a complete lack of our knowledge relating to the resistance offered by cross-sectional walls in the phloem and also by the plasmodesmata connections of the leaf and root cells. The resistance offered by passage through phloem parenchyma or companion cell will also be considerable and it seems incredible that turgour pressure of such magnitude could develop in the leaves and other supplying tissues to move a solution for any great distance and at such a speed in the phloem.

An interesting observation which cannot be explained by Munch hypothesis is that the composition of phloem exudates in Cucurbita shows fundamental discrepancy as contrasted with the composition of its fruit.

The mass-flow theory will account for translocation in the phloem, not to speak of individual sieve tubes, always in one direction at any one time and there is some evidence that at least in some plants (e.g., maize, sugar beet, etc.) the translocation of carbohydrate is polar; i.e., occurs only in one direction.

There are also some metre convincing evidences, however, that translocation in the phloem is simultaneously bi-directional, even in individual sieve tubes. By use of isotopic ¹⁴C and ³²P, strong evidence has been produc­ed of the simultaneous movement of carbohydrates and phosphates in opposite direc­tions, even in one individual sieve tube. If this is generally true, this speaks strongly against the pressure-flow hypothesis of Munch.

B. Protoplasmic Streaming: Accelerated Diffusion Theory

The fact that the protoplasm of most living plant cells is frequently in active motion has been widely observed in the cells of many aquatics like Chara, Vallisneria, Elodea, etc., as well as in root hair cells, staminal hairs and also in many kinds of leaf hairs. This is known as cyclosis. The mechanism of this movement (shown by the movement of course, protoplasmic granular plastids which are actually taken along the stream) is not known but expenditure of energy seems clearly indicated.

In the mass-flow hypo­thesis the function of sieve tubes was taken as completely passive, whereas according to the protoplasmic streaming hypothesis, oxygen is necessary for the sieve tubes to operate in translocation, i.e., living cells are indispensable for translocation of solutes. The functioning sieve tubes are pictured as active elements, rich in protoplasm and capable of continued expenditure of metabolic energy. It has been shown that the conducting tissues of sugar beet and Heracleum have high respiratory intensities in both xylem and phloem.

According to this concept, solute molecules move in, through or upon the surface of the sieve tube protoplasm and are carried along the protoplasmic streaming move­ment from one end of the sieve tube to the other. The solute molecules are usually assumed to pass from one sieve tube to the next by diffusion through the pores of the sieve plates.

Clearly the movement is pictured as due to a combination of simple diffu­sion and protoplasmic streaming. Some investigators have claimed that streaming protoplasm may be continuous from sieve tube to the next through sieve pores.

Anyway, within a particular sieve tube, the solute molecules which have diffused in from the sieve tube above, would be carried downward by protoplasmic streaming; those that diffused in from the cell below, would be carried upward by the same cyclic streaming. Thus this theory may account for the known movements of substances along concen­tration gradients and also for the apparent fact that two substances can be simulta­neously translocated in opposite directions—bi-directional—in individual sieve tubes. The solutes can thus move along their own concentration gradients but independently of each other and of solvent water.

The most serious objection against the protoplasmic streaming hypothesis is that active streaming, though easily observed in young sieve tube elements, is difficult to observe in mature sieve tubes. Careful technique has, however, succeeded in demons­trating that streaming does occur in mature sieve tubes.

Again, the rate of streaming seemingly required to account for the measured rates of transport would be very high and it seems improbable that protoplasmic cyclosis occurs at such rates in any living cell, not to speak of sieve tubes. The possibility, however, remains that streaming of cytoplasmic films or layers which are below the usual range of microscopic visibility might occur at rates, which may be theoretically sufficient to account for these observed rates of solute translocation.

Perhaps the most difficult point to reconcile with this theory is the fact that phloem exudate in many plants is apparently cell sap and not cytoplasm and yet the cell sap contains high concentrations of organic foods. This would seem to indicate that organic food translocation is not through or upon the protoplasm but rather via the sieve tube vacuole.

To conclude, we may state that the protoplasmic streaming theory is supported by evidence that under certain conditions, solutes appear to move independently of each other in the phloem’, that the sieve tubes have high respiration rates and are rich in proto­plasmic contents and radioactive ³²P absorbed into the phloem move simultaneously in both upward and downward directions in the stem. The greatest weakness of this theory is that no physical mechanism is known or has been pictured by the advocates of this theory that will account for the measured rates and for such rapid transport of large quantities of materials along the protoplasm, independent of the solvent, water.

Since no theory of translocation of solutes is satisfactory, several other theories have been proposed from time to time. We shall discuss only two of them. These are based primarily on electron microscopic observations of the internal structures of sieve tubes and some speculations, which seem to be theoretically sound.

Through the pores of sieve plates micro-fibrils are believed to pass; these micro-fibrils are composed of a protein,, named P-protein, which has contractile properties. The micro-fibrils are 60-280 A in diameter and the interstices are large enough to accommodate sugar molecules (10 A in diameter) or hydrated K⁺ ions (5 A in diameter).

According to Spanner the surface of the pore walls is negatively charged and positively charged ions (e.g. K⁺) are held rather firmly by intermolecular forces; ions in the outer wall are held only weakly and are thus mobile. This results in the movement of a current of K⁺ ions from one tube to another. Each K+ will carry with it the water molecules which hydrate it along with the dissolved uncharged sugar molecules.

ATP from bordering com­panion cells of the sieve elements provide the energy required for driving the K⁺ ions through the sieve plate pores overcoming any resistance; the internal surface of the sieve tube cell wall is believed to be rough. So that the adjacent plasma membrane has a large surface area, which further helps in the transport of the K⁺ ions. Solute move­ment through sieve tube thus, according to Spanner is by electro-osmosis.

However, Fensom thinks that the swishing of micro-fibrils results in the development of a fast wave and particles attached to micro-fibrils were actually found to move with a sort of bouncing motion, as observed in the case of Brownian movement; this results is a slower motion involving mass flow of the fluid in the sieve tube. This explains the bidirectional movement of solute in the same sieve tube. This observation and the theory, as also that of Spanner, await verification by other workers.