Mass flow or bulk flow of solutes in sieve elements was first proposed by the German Scientist Munch (1931). Munch visualises a 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. The symplast is impermeable on the outerface.
Mass flow of solutes as per this hypothesis is as follows. A very high concentration of nutrients is present in the mesophyll cells due to two reasons – a) water is transpired continuously from the mesophyll cells and b) food is continuously synthesised. Solutes are actively loaded into the phloem thus increasing their osmotic potential.
This increased potential draws water from the xylem vessels by means of osmosis. A hydrostatic pressure is built up in the sieve tubes. In the cells of the root, food material is continuously utilized reducing their osmotic potential as well turgor pressure. Thus a turgor Pressure gradient is established resulting in the mass flow of solution from
supply end (leaf) to consumption end (root). At the consumption end water diffuses into the xylem to be transported up to the leaves. Thus a circulatory system is established.
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The principle of mass flow may be illustrated with the help of a simple experiment. Two chambers A and B with selective permeability are connected by a tube T containing water (fig 3.28) Here chamber A is Comparable to mesophyll cells, chamber B to the root cells. Tube T is comparable to phloem. This forms a closed system similar to what is seen in plants.
Chamber A contains a highly concentrated sugar solution, while in chamber B its concentration is low. The system is dipped in a water filled vessel. In this experimental set up A corresponds to supply end, B to consumption end and T to longitudinal sieve tube system. The outer vessel having water corresponds to xylem. The concentration at A causes rapid diffusion of water into it resulting in a high turgor pressure.
The solution from A therefore flows en masse to B until turgor pressure gradient is maintained. The flow stops, when turgor pressure both in A and B are equal. However if a high turgor pressure is maintained at A, there is a continuous flow from A to B This is possible in the plant by the continuous synthesis of food On reaching chamber B the water will flow into the outer vessel (xylem).
The main aspect of mass flow is that it requires a positive hydrostatic pressure in the seive tubes and a continuous supply of sugars in the leaves to aenerate enough pressure. Experimental evidences have clearly pointed to the presence of a positive pressure flow in phloem.
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An evidence in favour of this theory is that growth substances or viral particles are transported rapidly in sieve elements, when the leaves are illumminated. This suggests that sugars produced during photosyntheiss will help generate the pressure gradient.