Question

Explain the pressure-flow hypothesis of the translocation of sugars in plants.

Answer 1

Hint: Phloem translocates solutes(sugars) from its place of synthesis to its place of requirement or storage. The pressure- flow hypothesis revolves around the phenomenon of osmotic pressure created by the addition of solutes in a solvent.

Complete step by step answer:
- The mechanism used for the translocation of sugars from source to sink is called pressure flow or mass flow hypothesis. It was proposed by E.Munch in 1926.
- The material in the phloem is translocated from source to sink. The source is any organ or tissue which produces more sugars than it requires for its own metabolism and growth. E.g mature leaves, cotyledons, or endosperms of seeds. The sink is the organ or tissue which requires the import of these sugars. E.g. root, fruits.
- During phloem loading, the sucrose after synthesis in the source tissue moves into the sieve tube cells via companion cells through active transport. More solute in the sieve tube elements leads to the entry of water from nearby xylem elements.
- At the sink, phloem unloading takes place and it is the end of the translocation pathway. The sucrose leaves the sieve tube and thus lowers their osmotic pressure due to low solutes and more water. This creates a pressure gradient between the source and sink. The osmotically generated pressure gradient leads to the bulk or mass flow of materials between source and sink.

Note:
- The source and sink may be reversed depending on the season or the plant’s needs.
- As the relationship of source and sink is variable, the direction of movement in the phloem can be upwards or downwards i.e bidirectional.