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15.     Explain pressure flow hypothesis of translocation of sugars in plants.

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Pressure flow hypothesis is the most accepted theory for the translocation of sugar from source to sink. Glucose is prepared at the source by photosynthesis which is converted into disaccharides sucrose which further moves into companion cells and then into sieve tube cells by active transport. Loading of phloem at source creates a water potential gradient that facilitates the mass movement in the phloem. Sieve tube cells of phloem form a long column with holes in their wall called sieve plates. Cytoplasmic strands pass through the hole in the sieve plates to form a continuous filament. Hydrostatic pressure developed in sieve tube cells moves the sap in the phloem. At the sink, incoming sugar is actively moved out of the phloem as complex carbohydrates. The loss of solute produces a high water potential in the phloem and water passes out and returning into the xylem.

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