The pressure flow hypothesis is another name for the mass flow model of translocation. It states that phloem sap is pushed from source to sink by a difference in hydrostatic pressure, created by loading sucrose at the source and unloading it at the sink. Scientists test the hypothesis by comparing its predictions with evidence.
Evidence supporting the hypothesis1. If an aphid feeding on phloem is cut off, leaving its stylet (mouthpart) in the sieve tube, sap flows out of the stylet. This shows that the phloem contents are under pressure. The sap flows out faster near the leaves than near the roots, suggesting a pressure gradient.
2. The concentration of sucrose is higher in the leaves (source) than in the roots (sink).
3. Metabolic inhibitors that stop ATP production also stop translocation, showing that active loading is needed.
4. Radioactive tracers show that sugars move from the source towards the sinks.Evidence against the hypothesis1. Sieve plates would be expected to slow the flow of sap considerably, yet translocation is fast.
2. Different solutes, such as sucrose and amino acids, can move at different speeds, which does not fit the idea that everything moves together by mass flow.
3. Sucrose is sent to different sinks at similar rates, rather than faster to the sinks with the lowest sucrose concentration.Most biologists accept the hypothesis as the best current explanation, although it may not account for every observation.
Evidence supporting the hypothesis1. If an aphid feeding on phloem is cut off, leaving its stylet (mouthpart) in the sieve tube, sap flows out of the stylet. This shows that the phloem contents are under pressure. The sap flows out faster near the leaves than near the roots, suggesting a pressure gradient.2. The concentration of sucrose is higher in the leaves (source) than in the roots (sink).
3. Metabolic inhibitors that stop ATP production also stop translocation, showing that active loading is needed.
4. Radioactive tracers show that sugars move from the source towards the sinks.Evidence against the hypothesis1. Sieve plates would be expected to slow the flow of sap considerably, yet translocation is fast.
2. Different solutes, such as sucrose and amino acids, can move at different speeds, which does not fit the idea that everything moves together by mass flow.
3. Sucrose is sent to different sinks at similar rates, rather than faster to the sinks with the lowest sucrose concentration.Most biologists accept the hypothesis as the best current explanation, although it may not account for every observation.