The pressure flow hypothesis, also known as the mass flow hypothesis, is the best-supported theory to explain the movement of sap through the phloem of plants.[1][2] It was proposed by Ernst Münch, a Germanplant physiologist in 1930.[3]
Organic molecules such as sugars, amino acids, certain hormones, and messenger RNAs are known to be transported in the phloem through the cells called sieve tube elements. According to the hypothesis, high concentration of organic substances, particularly sugar, inside the phloem at a source such as a leaf, creates a diffusion gradient (osmotic gradient) that draws water into the cells from the adjacent xylem. This creates turgor pressure, also called hydrostatic pressure, in the phloem. The hypothesis states that this is why movement of sap in the plant flows from the sugar producers (sources) to sugar absorbers (sinks).
