PHLOEM TRANSPORT IN PLANT, MASS FLOW OR PRESSURE FLOW HYPOTHESIS

Phloem Transport:-

• Generally the food especially sucrose are transported by the phloem from source (synthesizing part of food i.e. leaf) to sink (Storage of food).
•   Source and sink may be reversed according to season or plant need. E.g. root as a source in early spring.
• The movement of phloem is bi-directional.
•  Phloem sap = Water + sucrose with sugars + hormones + amino acids.

Mass Flow or Pressure flow hypothesis:-

• The mechanism for translocation of sugars from source to sink is called pressure flow hypothesis.
• After synthesis of glucose by photosynthesis, converted into sucrose and that moves through the phloem sieve tube cells by active transport.
• A s the loading of the sucrose, hypertonic condition are developed in the phloem so some amount of water inters into the phloem by osmosis to build a osmotic pressure due to which phloem sap move from the higher osmotic pressure to the lower osmotic pressure and at the sink the osmotic pressure completely reduced.
• As sugar reduced the osmotic pressure decreases the water moves out from the soil.
•  The phenomenon of transport of all these substances over a long distance through the vascular system i.e. xylem and phloem called translocation.
•   In this processes the direction of flow is most important i.e. in xylem the flow of water and minerals is unidirectional from root to stems as well as organic and mineral nutrients are flow though transported in multi-directional.

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