Explain the pressure flow hypothesis of translocation of sugar in plants.

(N/A) $ \Rightarrow $ The accepted mechanism for the translocation of sugars from source to sink is called the pressure flow hypothesis.
$ \Rightarrow $ Glucose is prepared at the source (by photosynthesis) and converted into sucrose (a disaccharide). The sugar is then moved in the form of sucrose into the companion cells and subsequently into the living phloem sieve tube cells by active transport. This process of loading at the source creates a hypertonic condition in the phloem. Water from the adjacent xylem moves into the phloem by osmosis.
$ \Rightarrow $ As osmotic pressure builds up, the phloem sap moves to areas of lower pressure.
$ \Rightarrow $ At the sink, the osmotic pressure must be reduced. Active transport is again necessary to move the sucrose out of the phloem sap. As sugars are removed, water also moves out of the phloem.
$ \Rightarrow $ To summarize, the movement of sugars in the phloem begins at the source, where sugars are loaded into a sieve tube. Loading of the phloem establishes a water potential gradient that facilitates mass movement in the phloem.
$ \Rightarrow $ Structure of Phloem: Phloem tissue is composed of sieve tube cells, which form long columns with perforated end walls called sieve plates. Cytoplasmic strands pass through the holes in the sieve plates, forming continuous filaments. As hydrostatic pressure in the phloem sieve tube increases, pressure flow begins and the sap moves through the phloem. Meanwhile, at the sink, incoming sugars are actively transported out of the phloem and utilized for metabolism or storage. The loss of solute increases the water potential in the phloem, and water eventually passes out, returning to the xylem.
$ \Rightarrow $ Experiment: Girdling
$ \Rightarrow $ $ A $ simple experiment was used to identify the tissues through which food is transported. On the trunk of a tree, a ring of bark up to the depth of the phloem layer can be carefully removed. In the absence of downward movement of food, the portion of the bark above the ring on the stem becomes swollen after a few weeks. This experiment demonstrates that phloem is the tissue responsible for the translocation of food and that transport takes place towards the roots.