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
+ Author Affiliations
- Author Affiliations

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.


This work was funded by the Marsden Fund (Royal Society, NZ). In addition thanks go to Professors Roy Daniels and Warwick Silvester at the University of Waikato for providing a laboratory space and support in carrying out this work.


Fisher DB, Frame JM (1984) A guide to the use of the exuding-stylet technique in phloem physiology. Planta 161, 385–393.
CrossRef |

Gould N, Minchin PEH, Thorpe MR (2004a) Direct measurements of sieve element hydrostatic pressure reveal strong regulation of sieve element hydrostatic pressure after pathway blockage. Functional Plant Biology 31, 987–993.
CrossRef |

Gould N, Thorpe MR, Minchin PEH, Pritchard J, White PJ (2004b) Solute import to elongating root cells of barley as a pressure driven bulk flow. Functional Plant Biology 31, 391–397.
CrossRef |

Grodzinski B, Jahnke S, Thompson RG, Sybesma C (1984) The effect of leaf anoxia on translocation profiles of 11C and 13N labeled assimilates in petioles of Helianthus annuus L. and Lupinus albus L. Advances in Photosynthesis Research 4, 279–282.

Hammel HT (1968) Measurement of turgor pressure and its gradient in the phloem of oak. Plant Physiology 43, 1042–1048.

Jones MGK, Outlaw WH, Lowry OH (1977) Enzymic assay of 10-7 to 10-14 moles of sucrose in plant tissues. Plant Physiology 60, 379–383.

Lalonde S, Tegeder M, Throne-Holst M, Frommer WB, Patrick JW (2003) Phloem loading and unloading of sugars and amino acids. Plant, Cell & Environment 26, 37–56.
CrossRef |

Malone M, Leigh RA, Tomos AD (1989) Extraction and analysis of sap from individual wheat leaf cells: the effect of sampling speed on the osmotic pressure of extracted sap. Plant, Cell & Environment 12, 919–926.

Minchin PEH, Ryan KG, Thorpe MR (1984) Further evidence of apoplastic unloading into the stem of bean: identification of the phloem buffering pool. Journal of Experimental Botany 35, 1744–1753.

Minchin PEH, Thorpe MR, Farrar JF, Koroleva OA (2002) Source–sink coupling in young barley plants and control of phloem loading. Journal of Experimental Botany 53, 1671–1676.
CrossRef | PubMed |

Münch, E (1930). ‘Die Stoffbewegungen in der Pflanze.’ (Gustav Fisher: Jena)

Patrick JW, Zhang W, Tyerman SD, Offler CE, Walker NA (2001) Role of membrane transport in phloem translocation of assimilates and water. Australian Journal of Plant Physiology 28, 695–707.

Smith JAC, Milburn JA (1980) Osmoregulation and the control of phloem-sap composition in Ricinus communis L. Annals of Botany 58, 577–588.

Thorpe MR, Minchin PEH (1987) Effects of anoxia on phloem loading in C3 and C4 species. Journal of Experimental Botany 38, 221–232.

Thorpe MR, Minchin PEH (1988) Phloem loading and transport of endogenously and exogenously labelled photo-assimilate in bean, beet, maize and cucurbit. Journal of Experimental Botany 39, 1709–1721.

Thorpe MR, Minchin PEH, Dye EA (1979) Oxygen effects on phloem loading. Plant Science Letters 15, 345–350.
CrossRef |

Thorpe, MR , Minchin, PEH , Gould, N ,  and  McQueen, J (2005). The stem apoplast: a potential communication channel in plant growth regulation. In ‘Vascular transport in plants’. pp. 201–220. (Elsevier / AP co-imprint: Oxford)

Wright JP, Fisher DB (1983) Estimation of the volumetric elastic modulus and membrane hydraulic conductivity of willow sieve tubes. Plant Physiology 73, 1042–1047.

Full Text PDF (156 KB) Export Citation Cited By (37)