The Physiology of Flowering Plants - KHAM PHA MOI

THE MECHANISM OF PHLOEM TRANSLOCATION 149Fig: 5:6 The Münch mass-flowhypothesis. (A) Principledemonstrated by a simplelaboratory experiment. Two dialysissacs permeable to water but not tosucrose are filled respectively with aconcentrated sucrose solution (D 1 )and a dilute sucrose solution (D 2 ),then connected with glass tubing Tand immersed in beakers of distilledwater. The greater C gradientbetween water and D 1 results in agreater rate of water uptake intothis sac; a ‘turgor’ pressure buildsup, solution can be seen to passalong tube T into D 2 and pressure isregistered by manometer M. Theflow stops when the sucroseconcentrations in the sacs haveequalized. (B) Operation in theplant. A continuous loading of thephloem at the source and unloadingat the sink can keep up the turgorgradient and the flow indefinitely.along a turgor pressure gradient, driven by a physiologically maintainedgradient of osmotic potential (Fig. 5.6). At the source end,sugars (or other solutes) are loaded into the sieve tubes; this lowerstheir water potential and induces an inflow of water. The turgorpressure in the sieve tubes rises and the solution is pushed alongthe phloem by the pressure. At the sink end the solutes are unloadedand water moves out with them, according to the water potentialgradient, so that the turgor pressure here is kept low. A lateral movementof solutes along the way into surrounding tissues of stem androot, which all need to be supplied, would also work to maintain asolute concentration gradient (and hence pressure gradient) fromsource to the final sink. Water would be absorbed into the phloemfrom the xylem at the source and released at the sink, returning into