Which of the following best explains the pressure flow hypothesis of phloem transport?

Correct answer: (b) Osmotic pressure difference between source and sink

Explanation

The pressure flow (mass flow) hypothesis, proposed by Ernst Münch in 1930, explains how food moves in the phloem:

• at the source (such as a leaf), sugar is loaded into the sieve tubes. Water follows from the xylem by osmosis, and the pressure rises;
• at the sink (such as a root, fruit or growing tip), sugar is removed and water leaves, so the pressure falls;
• the sap flows from high pressure to low pressure, from source to sink.

So it depends on the osmotic pressure difference between source and sink. Food moves from source to sink, up or down, not from sink to source, and gravity is not the cause.