THE MASS FLOW HYPOTHESIS OF TRANSPORT IN PHLOEM
The mass flow hypothesis was put forward by Munch to explain the translocation of the organic food materials takes place as mass 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.
The mass flow hypothesis can be explained as the mass flow of solute taking place from the source that is the mesophyll cells where the solutes are produced. It is then transported to the all other parts of the plants where the photosynthate is needed which is termed as the sink. This movement of the food prepared in the leaves i.e. is the source to the sink where the photosynthate is used up is known as the 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, it 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 the mass flow of solute along the gradient.