Increased osmotic pressure in the phloem tissue, resulting from the active transport of sucrose and other substances, causes water to move into the phloem from surrounding tissues. This increased pressure then drives the movement of material within the phloem to areas with lower pressure, allowing for efficient nutrient distribution throughout the plant.
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Increased osmotic pressure in the phloem facilitates translocation by creating a pressure gradient that propels sap flow. Source tissues, where sugars are produced, actively load the phloem with sucrose, increasing solute concentration. This accumulation of solutes lowers water potential in the phloem, leading to water influx by osmosis. The resulting turgor pressure builds up, creating a positive pressure at the source. This pressure, known as pressure flow, propels the phloem sap towards sink tissues with lower turgor pressure. The osmotically driven movement of water and solutes ensures the efficient and bidirectional translocation of nutrients within the plant.