Evidence of mass flow hypothesis

• Support mass flow hypothesis: Sap released when sieve tubes are cut

  • There is a pressure within sieve tubes, as shown by sap being released when they are cut
• Support mass flow hypothesis: Sucrose higher in leaves than roots

  • The concentration of sucrose is higher in leaves (source) than in roots (sink)
  • Supports the presence of sources and sinks
• Support mass flow hypothesis: Downward flow in phloem in day, stops at night

  • Downward flow in the phloem occurs in daylight when photosynthesis is occurring but stops at night
  • Greater respiration at night and lack of photosynthesis would mean there is no longer a concentration/pressure gradient
• Support mass flow hypothesis: Companion cells produce ATP

  • Preventing ATP production stops translocation but not water movement
  • Lack of ATP for active transport in companion cells means that they would die which would prevent mass flow
• Against mass flow hypothesis:

  • Not all solutes move at the same speed - they should do so if movement is by mass flow
• Against mass flow hypothesis:

  • Sucrose is delivered at more or less the same rate to all regions, rather than going more quickly to the ones with the lowest sucrose concentration/ highest water potential, which the mass flow theory would suggest
• Against mass flow hypothesis: Sieve tubes function

  • The function of the sieve plates is unclear, as they would create a barrier to mass flow, or slow it down
  • So why are they present?