pressure-flow hypothesis

botany

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July 13, 2026

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The pressure-flow hypothesis is a theory explaining how sugars produced during photosynthesis move through the phloem of plants. It suggests that sugars are loaded into the phloem in "source" regions, like leaves, increasing the osmotic pressure. In "sink" regions, such as growing tissues or storage organs, sugars are removed, lowering the osmotic pressure. This creates a pressure gradient that drives the flow of sugary solution through the phloem from source to sink. This movement is essential for distributing nutrients throughout the plant. The exact mechanism of phloem transport is still a subject of research.1

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angiosperm
Process of phloem transport
Mass-flow hypotheses include the pressure-flow hypothesis, which states that flow into sieve tubes at source regions (places of photosynthesis or mobilization and exportation of storage products) raises the osmotic pressure in the sieve tube; removal of sugars from sieve tubes in sink regions—i.e., those in which sugars are removed…
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Mass-flow hypotheses include the pressure-flow hypothesis, which states that flow into sieve tubes at source regions (places of photosynthesis or mobilization and exportation of storage products) raises the osmotic pressure in the sieve tube; removal of sugars from sieve tubes in sink regions—i.e., those in which sugars are removed…
Read More