Functional Plant Biology Functional Plant Biology Society
Plant function and evolutionary biology

Phloem hydrostatic pressure relates to solute loading rate: a direct test of the Münch hypothesis

Nick Gould A E , Michael R. Thorpe B , Olga Koroleva C and Peter E. H. Minchin D

A Department of Biology, University of Waikato, Private Bag 3105, Hamilton, New Zealand.

B Chemistry Department, Brookhaven National Laboratory, Upton, NY 11973, USA.

C John Innes Centre, Colney Lane, Norwich, UK.

D ICG-III Phytosphaere, Forschungszentrum Jülich, D-52425, Jülich, Germany.

E Corresponding author. Current address: HortResearch, Ruakura Research Centre, Private Bag 3123, Hamilton, New Zealand. Email:

Functional Plant Biology 32(11) 1019-1026
Submitted: 17 February 2005  Accepted: 3 August 2005   Published: 28 October 2005


According to the Münch hypothesis, a flow of solution through the sieve tubes is driven by a hydrostatic pressure difference between the source (or collection) phloem and the sink (or release) phloem. A high hydrostatic pressure is maintained in the collection phloem by the active uptake of sugar and other solutes, with a concomitant inflow of water. A lower pressure is maintained in the release phloem through solute unloading. In this work we directly test the role of solute uptake in creating the hydrostatic pressure associated with phloem flow. Solute loading into the phloem of mature leaves of barley and sow thistle was reduced by replacing the air supply with nitrogen gas. Hydrostatic pressure in adjacent sieve elements was measured with a sieve-element pressure probe, a cell pressure probe glued to the exuding stylet of aphids that had been feeding from the phloem. Sieve element sap was sampled by aphid stylectomy; sap osmotic pressure was determined by picolitre osmometry and its sugar concentration by enzyme-linked fluorescence assays. Samples were taken with a time resolution of ~2–3 min. In accordance with Münch’s proposal a drop in osmotic and hydrostatic pressure in the source phloem following treatment of the source leaf with N2 was observed. A decrease in sugar concentration was the major contributor to the change in osmotic pressure. By observing these variables at a time resolution of minutes we have direct observation of the predictions of Münch.

Keywords: anoxia, aphid stylectomy, Münch hypothesis, phloem loading, phloem pressure probe.


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