What are the Different Theories of Translocation of Solution in Plants ?

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Besides up taking of water from soil, plant obtains a considerable amount of mineral salts and nutrients along with water in solution form. These solutes dissolved in water are ascended up through the xylem or across the cell wall and plasma membrane. Mineral absorption involves the entry of solutes in form of ions from the soil solution into the root hair cell in various methods. But their translocation is a distance physiological process.

On the other hand the organic food being prepared in the leave should be distributed through out the plant body in the downward translocation through phloem.

Mechanism of soil translocation is a debated problem physiology. These are several theories has been put forwarded by different workers in physiology which are outlined below.

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I) Protoplasmic streaming theory:

This theory was proposed by de Vries and supported by Curits. This theory explains that, the movement solute solution is caused by combination of diffusion and cytoplasmic streaming. Diffusion occurs from seive tube to seive rube across the porus seive plate and by cytoplasim stream solutes carrier up and down for a little distace. This process can be comparable to cyclosis, but it’s magnitude is very low.

Thaine reported that cyclosis does not occur in matyre seive tube cells.

II) Transcellular streaming theory:

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Thaine further suggested that there are presence of transcellule fibrils in form of strands in the seive tubes, which contain particles. These strands move up and down, so that the fluid also moves along these stands during lungitudinal flow of solutes.

Thus trancellular streaming is an attractive mechanism which explanins bi-directonal translocation.

There are many criticisms of this theory, that how bio-energy could be transced into a physical force within the sieve tube. This theory failed to explain how does liquid solute other than particles transocate or whether these strands are solid or hollow is still confusing.

III) Contractile proteins theory:

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Fensom and Williams observed that a network of interlinked microfibrils in the sieve tybe which oscillated in a manner resembling flagellar lashing in other organisms.

This theory also explain that particles which attached to microfibrils move with a bouncing motion. The microfibrils composed of contractile threads of protein.

IV) Electro osmotis theory:

Spanner and Jones suggested that an electrometric mechanism may account for the translocation of sugars in sieve tubrs.

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This theory states that a current of potasiumion passes through the seive poes by electroomosis and the sugar molecules adhered tightly to K ions are carried along with them.

Them theory is not supported by experimental evidences.

V) Pressure flow theory:

Munch postulated that the translocation is a kind of circulation like that of blood of animals in which leaves acts as hearts and seive tuebs acts as blood vessels. This theory believes that there is an osmotic mechanism involved in the movements of solute in side the phloem.

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During photosynthesis cell sap concentration in the mesophyll cells become high to increase the osmotic pressure, As a result osmotic pressure increases and caused abserption of water. Thus tuger pressure becomes high in mesophyll cells.

This pressure transmitted to the phloem of leaf from mesophyll by plasmodemata. Through these connections solution is forced down to seive tubes due turgor pressure. Loss of solute in mesophyll compensated by the photosynthetic products.

This theory has been supported by crafts and esau, maskell and zimmerman in who explained that in phloem a positive concentration gradient exist.

Criticism for the Pressure Flow Theory:

i. This theory holds that there is unidirectional flow of solute in plant but naturally it is bidirectional.

ii. Pressure flow theory does not accommodate active participation of cytoplasm and munch belies translocation as a purely physical process.

iii. Swanson suggested that ATP is required for translocation and diffusion of sugar takes place in seive tubes against concentration gradient is not taken into account in pressure flow theory.

iv. Other physiologists suggested that sieve pores remain plugged by protoplasm and acts as a barrier against mass flow.

Further this pressure flow theory has been modified in to flowing.

VI) Volume flow theory:

Eschrich, Evart and Young believed that pressure difference which acts as a driving force are more localised even at the level of individual sieve tube. The mathematically analysed the flow of solutions along tubular semipemeable membrane.

VII) Solution flow theory:

Marshall and Wardlaw proposed that materials moves in the form of a solution, both solute and solvent travel concurrently. Different solute move at same speed under the physical influences on solution.

VIII) Active mass flow theory:

Mason and Phills found out that oxygen is required to maintain some special state of cytoplasm which enables translocation to occur. They proposed that translocation takes place by active diffusion of sugar enmasse.

IX) Bimodal theory of translocation:

Fenson proposed that two distinct processed involves in the translocation. One is micro peristaltic movement along hollow fibrils of contractile protein present along sieve tubes. Secondly he considered that as a mass flow of solution around the contractile protein fibril. This mass flow of solution is relatively slow.

He also stated that microfibrilar material could also participate directly in the movememnt of solutes.

There are several factors like light temperature oxygen availability moisture content and rate of metabolism influence solute translocation.

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