MASS FLOW HYPOTHESIS:
· Mass flow hypothesis was proposed by German scientist Ernst Munch in 1930, who described the movement of sap via phloem.
· It is also known as Pressure flow hypothesis.
· Munch explained the translocation of the organic food material through the phloem tissue. This flow occurs along the gradient of the turgor pressure from a region of higher solute concentration to a region of lower solute concentration.
· Munch visualizes the pumping action in mesophyll cells. According to munch’s hypothesis, the sieve tubes are connected to one another by means of cytoplasmic connections, forming a continuous system called symplast. This symplast is impermeable on the outer face.
· Mass flow hypothesis can be explained as the mass flow of solute taking place from the source i.e. the mesophyll cells where the solutes are produced, it is then transported to the other parts of plant where photosynthesis is needed which is known as sink.
· This movement of the food prepared in the leaves i.e. is the source to the sink where the photosynthate is used up known as translocation. The solutes are actively loaded into the phloem tissues of leaves which thereby increase the osmotic potential. Since the photosynthate is continuously synthesized in the mesophyll cells. This results in the increase of the osmotic potential and the turgor pressure in the source.
· A pressure gradient is created between the source and the sink which results in mass flow of solutes along the gradient.
Mass flow hypothesis is as follows:
· A very high concentration of nutrients is present in the mesophyll cells due to two reasons:
a) Water is transported continuously from the mesophyll cells.
b) Food is continuously synthesized.
Mechanism of Mass Flow Hypothesis:
· When movement of minerals and water via xylem is driven mostly by negative pressure and movement via phloem is driven by hydrostatic pressure. This process is called translocation and accompanied by process known as phloem loading and unloading.
· Cells in sugar sources load a sieve tube by osmosis developing pressure that pushes the sap low. The cells deliver solutes out of the elements of sieve tubes and produce opposite effects. The sugar gradient from sources creates pressure flow via sieve tubes towards sink.
· Glucose is formed by photosynthesis in the cells of mesophyll cells and some glucose is utilized in the cells during respiration. The left over glucose is transformed into non-reducing sugar.
· Sucrose is delivered to the neighboring cells through minute veins of the leaves.
· Sucrose diffuses from neighbor cells to the elements of sieve tubes via plasmodesmata. Hence, the amount of sucrose rises in the elements of sieve tubes.
· Water travels from the close xylem to the leaf vein by osmosis and raises the hydrostatic pressure of the elements of the sieve tubes.
· Hydrostatic pressure shifts the sucrose along with other substance via cells of sieve tubes towards the sink.
· In storage sinks, sucrose is eliminated into apoplast before entering the sink’s symplast.
· The water trans-membrane out of the cells via osmosis and lowers the hydrostatic pressure in them. Hence, a gradient of pressure is developed as a result of entry of sugar at source and elimination of sucrose at the sink.
· The phloem sugar is eradicated by the cortex of root and stem and utilized by cellular respiration. The starch is insoluble and doesn’t exert any osmotic effect. Ultimately, pure water is left and drawn into xylem vessels by transpiration pull.
The main aspect of mass flow is that it requires a positive hydrostatic pressure in the sieve tubes and a continuous supply of sugars in the leaves to generate enough pressure. Experimental evidences have clearly pointed to the pressure of a positive pressure flow in phloem.
Plants need water to grow and maintaining the turgidity of cell. There are three types of transport occur in plants: Diffusion, facilitated diffusion and active transport.
Water is the most abundant constituent of all physiologically active plant cells. Several physiological processes of plants like osmosis, imbibition, plasmolysis are accomplished by water.
Transpiration explains how water moves up the plant against gravity in tubes via a xylem. Plants pump the water up from the soil to deliver nutrients to their leaves. It is influenced by several factors.
Opening and closing of stomata is controlled by concentration of solutes in guard cells. Some factors like light, temperature, CO2, O2,hormones regulate the opening and closing of stomata.
Diffusion is the movement of particles from the region of higher concentration to the region of lower concentration. It is affected by temperature, pressure, density of medium, distance,number of particles per unit volume.
Minerals are move into the roots by using energy in the form of ATP. After the uptake minerals are transported to all parts by transpiration pull. Later they are absorbed by active or passive transport.
Food is synthesized through the process of photosynthesis. It can be transported to different parts of the plant, this function is performed by phloem.
Water transport is important for the uptake of mineral nutrients from the soil. There are three pathways which facilitate the movement of water in plants like apoplast, symplast, trans-membrane pathways.