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Food materials are manufactured mainly in the leaves and are translocated to the other regions of the plant through the phloem. Some of this is utilized for the growth of the plant, while the excess is stored. Whenever needed, the stored food material is drawn to the regions of necessity again through the phloem. The organs of the plants that store the food materials are called the storage organs.
The storage of food materials occurs in fruits and seeds. In some plants root and stems are modified to perform this function, for example, rhizomes, bulbs, corms and tubers are the modifications of the stem which store the food in the form of carbohydrates, fats and proteins.
The conversion of sucrose into starch at the storage region maintains the sucrose concentration low at the storage region and, thus, the concentration gradient is maintained until a sufficient quantity is accumulated. In the storage cells, starch is usually stored in the form of grains. Starch grains are formed in plastid, known as amyloplasts (leucoplasts).
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Starch is the common storage material in seeds and cereal grains. The growth of a starch grain takes place by apposition of successive shells around a growth centre of hilum. Some of the other polysaccharides of storage are hemicellulose and inulin.
Photosynthates are energy rich carbon compounds formed during process of photosynthesis, are transported from the leaf to non-photosynthetic organs and tissues such as roots, stem tissues and developing seeds. This type of long distance transport of photosynthates occurring through phloem is called translocation.
Sucrose is the principal form of carbohydrates that is translocated from leaf to the non- photosynthetic organs. Sucrose is a non-reducing sugar, and therefore, it is chemically stable. Due to this property, this does not react with other substances during translocation. Photosynthates provide energy to non-photosynthetic tissues through respiration.
Mechanism of Translocation of Solutes in Phloem:
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Several theories have been advanced to explain the mechanism of the translocation of solutes in phloem tissues, none theory has ever received general acceptance. In order to consider the principal theories that have been proposed, a general review of various facts regarding the problem of translocation of solutes in the phloem will be useful.
1. The translocation in the conducting elements of the phloem ceases when the cells are killed. This shows that living cells are absolutely essential for the purpose.
2. The movement of solutes may be bidirectional. There is no doubt that movement of solutes in the phloem can sometimes occur in one direction and sometimes in the other.
Experiments conducted by various workers have shown that both carbohydrates and phosphates move both up and down into the phloem of a stem from the point at which they enter it from the petiole of a leaf. But this phenomenon does not involve a bidirectional movement through the same phloem at the same time.
3. Relatively the large amounts of solutes are transported. The amount of carbohydrate translocated through the phloem is sufficiently large in the case of fleshy roots, tubers or fruits. Crafts and Lorenz (1944) calculated that in 33 day growing period the pumpkin fruit gained the weight to be about 5500 gms.
A large amount of the increase is water but they showed that every hour the fruit gained 0.61 gram of dry weight. All this food material entered the developing fruit through one slender stalk that connects the fruit to the vine.
4. The velocity of translocation of organic solutes through the sieve tubes is quite rapid.
5. The movements of organic and inorganic solutes exhibit a periodicity. The movements of these solutes do not occur at a constant rate both day and night. There is more rapid translocation of radioactive phosphate from leaves of bean plants during the day than during the night.
Mason and Mashell (1928) have reported that most of the carbohydrates that enter the developing bolls of cotton appears to move into add them during day time rather than at night. These diurnal variations occur in plants but do not follow the same pattern in all species.
The Mass Flow Hypothesis:
This theory of translocation of solutes through phloem was advanced by Munch (1927, 1930). This theory has also put forth in modified forms by Crafts (1933, 1938) and others. The principles involved in this hypothesis can be clarified by a simple physical system.
This is as follows:
As shown in the figure, two bulbs with membranes permeable only to water, are dipped in a through containing water and are connected by a tube to form a closed system. Membrane X encloses a stronger solution of sucrose than membrane Y.
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Firstly, the water will enter both membranes, but greater turgor pressure develops in X which is soon to be transmitted throughout the system. With the result greater pressure in the water in Y membrane than is the pure water of the trough in which the membranes are immersed.
As a result the water passes out of the membrane Y, and there is a flow of solution along the horizontal tube from X to Y. This mass flow of solution from X to Y continues unless and until the concentrations of the sugar solutions in both membranes become equal. At this point the flow of solution in the tube stops and a dynamic equilibrium establishes between the solution in the closed system and the surrounding water.
If such an apparatus could be set up that we are able to arrange removal of sugar from the weaker solution (Y) as soon as it reaches it and replace the sugar lost by the stronger solution (X), flow of solution from X to Y would continue indefinitely.
According to Munch hypothesis an analogous system exists in plants. This hypothesis assumes that the translocation of solutes in plants through phloem is similar to the situations found in the experiment described in the preceding paragraph. The adjacent figure shows how it is supposed to operate as applied to the downward translocation of solutes.
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The photosynthetic cells of the leaf correspond to membrane X; the root cells correspond to Y and the continuous system of phloem connecting leaf and cells correspond the horizontal connecting tube. Besides xylem and cambium are also there. The osmotic pressure of the leaf cells is high as a result of photosynthesis.
In the root cells the osmotic pressure is usually lower because most of the sugars translocated into them are used in metabolic activities or are converted into insoluble storage forms. Water supply to the leaf cells remains continuous through the xylem.
