Plants largely depend on the mineral nutrients present on soil for their metabolism Along with water minerals are taken up by plants. But absorption and translocation of minerals in from of solutes is independent of water up taking. There involve special physiological process in transportation of minerals. Mineral solutes dissolved in water. are ascended up through xylem or across the cell wall and plasma membrane. Absorption of mineral involves the entry of solutes in form of ions from the soil solution into the root hay cell in various methods. But translocation or transportation of these solutes is a distinct physiological phenomena. Besides minerals organic food prepared in leaves are distributed throughout the plant body in downward translocation through phloem in form of solutes
Thus mechanism of solute transport in plants is a debated matter for physiologists. There are several theories have been put forwarded by different physiologist to explain the method of transport of solutes in plants. These theories are outlined below.
(I) Protoplasmic Streaming Theory:
This theory was propounded by De Vries (1885) and supported by Curtis (1935). According to this theory the movement of solute is caused by combined effect diffusion and cytoplasmic streaming Diffusion occurs from sieve tube to sieve tube across the porus sieve plate. By cytoplasmic streaming movement solutes carnids by up and down movement from one end of sieve to other end. This process continue to transport solute for a very little distance This process can be comparable to cyclosia, but its magnitude is very low. Thaine reported that cyclosis does not occur in mature sieve tube cells.
(II) Thanscellular Streaming Theory:
Thaine also suggested that. there are presence of transcellulat fibrils in form of strands in the sieve tubes, which contain particles. These strands move up and down, so that the cell fluid containing solutes moves along these strands during longitudinal flow.
Thus transcellular steaming is an attractive mechanism which explains bi-directional translocation.
There are many criticism of this theory, that how the bioenergy could be transformed in to a physical force within the sieve tube cells. This theory fails to explain how does fluid solutes other than the solid particles translocate. It also fails to explain whether the strands are solid or hollow.
(III) Contractile Protein Theory :
Fenson and Williams (1974) observed that a network of interlinked microfibrils in the sieve tube oscillated in a manner resembling flagellar lashing in other organisms like protozoa. Microtibrils composed of contractile threads of protein. The contractility is due to interconversion of protein to lecithin and vioeversa.
This theory further explains that particles which are attached to microfibrils move with a bouncing motion.
(IV) Electro Osmotic Theory:
Spenner and jones (1958.1970) suggested that an electro osmofic mechanism may account for the translocation of sugars in the sieve tubes. This theory explains that the current of potassium ion passes through the sieve pores by electroosmosls and the sugar molecules adhered tightly to K ions are carried along the sieve tubes through the pores.
There is no experimental support to this theory.
(V) Pressure Flow Theory:
This theory was postulated by Munch (1926) According to this theory, translocation is a kind of circulation like that taking place in blood of animals. In plants leaves acts as the pressure pump like the heart and sieve tubes acts as blood vessels. This theory believes that there is an osmotic mechanism involved in the movement of solute inside the phloem.
During photosynthesis cell sap concentration in mesophyll becomes high to increase the osmotic pressure, as a result of which water is absorbed so that turgor pressure, become high in mesophyll cells. This pressure transmitted to the phloem of leaf from mesophyll by plasmodesmata. Through these connections solution is forced into sieve tubes due to turgor pressure. Loss of solute from mesophyll compensated by the photosynthetic product.
This theory was supported by Crafts (1961), Esaue (1966), Maskell(1921) and Zimmerman (1958).
But there are lot of criticism of this theory-
It holds the view of unidirectional flow only.
It fails to explain active participation of cytoplasm
It does not explain the necessity of ATP for translocation
(VI) Volume Flow Theory:-
Eschrich, Evart and Young (1972) were in the view that pressure differences which as a driving force for transportation of solute is more may localized, even at the level a individuals seive tubes. They mathematically analyzed the flow of solutions along tubular semi-permeable membrane and came to the conclusion that the factor for flow of solute depends on its volume rather than the pressure only.
(VII) Solution Flow Theory:-
Marshall and Wardlaw (1973) proposed that materials move in the form of a solution. Both solute and solvent travel concurrently obeying the principles of flow of solution. Different solutes move at same speed in the solution under the physical influence of solution in relation to the sieve pores.
(VIII) Active Mass Flow Theory:-
Mason and Phillips (1936) proposed that translocation takes place by active diffusion of sugars enmasse. They also found out that oxygen is required to maintain some special state of cytoplasm which enable translocation to occur.
(IX) Bimodal Theory:-
Fenson (1971) proposed that two district processes are involved in the translocation. One is microperistaltic movement along hollow contratile protein fibrils present along the cytoplasm of sieve cells. Secondly he considered mass flow of solution around the contractile protein fibrils. He stated that microfibrilar material could also participate directly in the movement of solutes.
Factors effecting translocation of solute:-
There are several internal and external factors responsible for translocation of solute. Internal factors are cell sap concentration, volume of water absorbed by leaves, rate at which leaves produce sugar etc.
External factors includes light, Oxygen availability, Moisture content, temperature etc, which influence the rate of translocation.