11th Class Biology Means Of Transport In Plants Translocation Of Organic Solutes

Translocation Of Organic Solutes

Category : 11th Class

"The movement of organic food or solute in soluble form, from one organ to another organ is called translocation of organic solutes."

The process of translocation requires expenditure of metabolic energy and the solute moves at the rate of 100 cm/hr.

Directions of translocation

Downward translocation : It is of most important type, i.e., from leaves to stem and roots.

Upward translocation : From leaves to developing flowers, buds, fruits and also during germination of seeds and tubers, etc.

Radial translocation : From pith to cortex and epidermis.

Path of translocation

(1) Downward translocation of organic solutes : Phloem is the path for downward translocation of organic food. Following evidences are in support of it :

(i) Elimination of other tissues : Xylem is responsible for upward movement of water and minerals, so it cannot account for downward translocation of solute at the same time. Thus only phloem is left (where there is end to end arrangement of sieve tubes united by sieve pores). Which is responsible for translocation of solutes in downward direction.

(ii) Chemical analysis of phloem sap and xylem sap : Chemical analysis of sieve tube sap proves that concentrated solution of sucrose is translocated from the place of synthesis to other parts of the plant body. Glucose and fructose are sometimes found in traces only. The amount of sucrose is more in phloem sap during the day and less in night. In xylem the amount of sucrose is in traces and also there is no diurnal fluctuation.

(iii) Blocking of phloem : Blocking of sieve pores by 'callose' during winter blocks translocation of solutes.

(iv) Ringing or Girdling experiment : It was first performed by Hartig (1837). On removing the ring of bark (phloem + cambium) above the root at the base of stem, accumulation of food occurs in the form of swelling just above the ring, which suggests that in absence of phloem, downward translocation of food is stopped.

(v) Structure of phloem : The structure of phloem tissue is well modified for conduction of solutes. Phloem tissue of an angiosperm consists of sieve tubes, companion cells several kinds of parenchyma cells, fibres and scleroids. Of these sieve tubes are involved in sugar translocation.

(2) Upward translocation of organic solutes : According to Curtis upward conduction of foods also takes place through phloem.

Mechanism of translocation

Diffusion hypothesis : Mason and Maskel (1928) working on cotton plant demonstrated that the translocation of foods occurs from the place of high concentration (place of manufacture or storage) to the place of lower concentration (place of consumption) but it is very slow process so Mason and Phillis (1936) modified this concept and proposed activated diffusion hypothesis. According to this concept the food particles are first energy activated then translocated. This hypothesis is not accepted due to lack of experimental evidence.

Protoplasmic streaming hypothesis : This concept was proposed by de Vries (1885). According to him the food is transported across by streaming current of protoplasm. The cell protoplasm shows a special locomotion movement called cyclosis. It is of two types, rotation and circulation. While rotation is circular movement of protoplasm, circulation is radial movement forming eddies around the vacuoles. The hypothesis involves two phenomenon, such as streaming of sieve protoplasm and diffusion of metabolites through sieve pores.

This hypothesis not only explains faster rate of translocation but also the bidirectional movement of metabolites across a single sieve element. This hypothesis was supported by Curtis (1950).

Transcellular streaming : Thaine (1964) suggested modification to cytoplasmic streaming theory. He defined transcellular streaming as "the movement of the particulate and fluid constituents of cytoplasm through linear files of longitudinally oriented plant cells. "He further proposed that transcellular strands are proteinaceous and characteristic microtubules to afford rhythmic contraction. Thus, transcellular streaming is an attractive mechanism as it would explain the phenomenon of bidirectional translocation.

Electro-osmotic hypothesis : A mechanism involving electro-osmosis was proposed independently by Fensom (1957) and Spanner (1958). According to this hypothesis the solute moves in the positive direction of the electrical gradient along with \[{{K}^{+}}\]ions.

Munch's mass flow or pressure flow hypothesis : The mass flow or pressure flow mechanism was first proposed by Hartig (1860). It was later modified by Munch (1930). Crafts elaborated it further in (1938). Munch assumed that the protoplasm of sieve tube is connected through plasmodesmata and forms a continuous system, called as the symplast. The translocation of solutes occurs in a mass alongwith cell sap through the sieve tubes form a region of higher turgor pressure to low turgor pressure (i.e., along a turgor pressure gradient).

 

 

Munch's hypothesis has been supported further by the following :

When a woody or herbaceous plant is girdled, the sap containing high sugar content exudates from the cut end. Positive concentration gradient disappears when the plants are defoliated. Movement of viruses and growth hormones is fast in illuminated leaves as compared to shaded leaves.

Objections :

The hypothesis fails to explain bidirectional movement of metabolites which is common in plants. Osmotic pressure of mesophyll cells and that of root hair do not confirm the requirements. Munch's hypothesis gives a passive role to the seive tube elements and the protoplasm.

Factors affecting translocation

Temperature : Optimum temperature for translocation ranges between 20-30°C. The rate of translocation increases with the increase of temperature upto an upper limit and then starts declining. At low temperature, the rate of translocation decreases.

Light : Hartt and his coworkers (1964) proposed that the movement of assimilates of a leaf can depend upon radient energy. The increase in light intensity more food starts being translocated to roots than to shoots. At lower intensity the growth of root and shoot is inhibited thereby the rate of translocation also decreases.

Hormones : Cytokinins have a pronounced effect on the translocation of water soluble nitrogen compounds.

Oxygen : Oxygen is necessary during transfer of food from mesophyll cells into phloem which is called as phloem loading.

Minerals : Boron is highly essential for translocation of sugar. Phosphorus also helps in translocation of solutes.

Water : Translocation of photosynthates out of the leaves is highly sensitive to the amount of water in the plant cells.

Metabolic inhibitors : The metabolic inhibitors which inhibit the process of respiration (e.g., iodoacetate, HCN, carbon monoxide etc.) adversely affect the process of translocation because phloem loading and unloading require ATP.

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