, Volume 248, Issue 3, pp 661–673 | Cite as

Ratio of sugar concentrations in the phloem sap and the cytosol of mesophyll cells in different tree species as an indicator of the phloem loading mechanism

  • Daniel Fink
  • Elena Dobbelstein
  • Andreas Barbian
  • Gertrud LohausEmail author
Original Article


Main conclusion

Sucrose concentration in phloem sap was several times higher than in the cytosol of mesophyll cells. The results suggest that phloem loading involves active steps in the analyzed tree species.

Phloem loading in source leaves is a key step for carbon partitioning and passive symplastic loading has been proposed for several tree species. However, experimental evidence to prove the potential for sucrose diffusion from mesophyll to phloem is rare. Here, we analyzed three tree species (two angiosperms, Fagus sylvatica, Magnolia kobus, and one gymnosperm, Gnetum gnemon) to investigate the proposed phloem loading mechanism. For this purpose, the minor vein structure and the sugar concentrations in phloem sap as well as in the subcellular compartments of mesophyll cells were investigated. The analyzed tree species belong to the open type minor vein subcategory. The sucrose concentration in the cytosol of mesophyll cells ranged between 75 and 165 mM and was almost equal to the vacuolar concentration. Phloem sap could be collected from F. sylvatica and M. kobus and the concentration of sucrose in phloem sap was about five- and 11-fold higher, respectively, than in the cytosol of mesophyll cells. Sugar exudation of cut leaves was decreased by p-chloromercuribenzenesulfonic acid, an inhibitor of sucrose–proton transporter. The results suggest that phloem loading of sucrose in the analyzed tree species involves active steps, and apoplastic phloem loading seems more likely.


Fagus sylvatica Gnetum gnemon Magnolia kobus Non-aqueous fractionation Phloem loading Phloem sap Subcellular metabolite concentration Sugar 



Bundle sheath cell


Companion cell


Mesophyll cell


Sieve element


p-Chloromercuribenzenesulfonic acid



The authors would like to thank Elisabeth Wesbuer and Tim Kreutzer for technical assistance, as well as Sarah Rau for help with the electron micrographs. We are grateful to Rosi Ritter and Christoph Senges for critical reading of the manuscript.

Compliance with ethical standards

Conflict of interest

The authors declare that they have no conflict of interest.


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Copyright information

© Springer-Verlag GmbH Germany, part of Springer Nature 2018

Authors and Affiliations

  1. 1.Molecular Plant Science/Plant BiochemistryUniversity of WuppertalWuppertalGermany
  2. 2.Core Facility Electron Microscopy, UKDHeinrich Heine University DüsseldorfDüsseldorfGermany

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