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Relationship Between Apoplastic Nutrient Concentrations and the Long-Distance Transport of Nutrients in the Ricinus communis L. Seedling

  • E. Komor
  • G. Orlich
  • H. Bauer-Ruckdeschel

Abstract

The seedling of Ricinus communis was used to study the fluxes of nutrients from the apoplastic space into the symplast of the cotyledons, from the apoplast and the symplast of the cotyledons into the phloem, and the recirculation of nutrients into the cotyledons via the xylem. Cotyledons of the intact seedling were incubated in media of defined nutrient concentrations till a steady-state of fluxes was reached, then a small cut of two to three bundles at the hypocotyl hook was made and the exudates from phloem and xylem were sampled and analysed. Together with the information obtained for nutrient concentrations in the medium (apoplast) and in the leaf (symplast) a quantitative scheme of nutrient fluxes at different apoplastic concentrations was designed. All tested nutrients (sucrose, glutamine, potassium ion) were found in higher concentrations in the symplast and in the phloem sap than in the apoplast, but sucrose was the only substrate which was clearly enriched in the phloem sap compared to the symplast. Recirculation (i.e. export via the phloem and re-import via the xylem) of sucrose was negligible at all tested sucrose concentrations, whereas recirculation of glutamine and potassium ions was up to 80{%} in case of high supply at the cotyledon apoplast. The recirculation of these nutrients enables the apoplastic space of the cotyledons to function as a storage space for homeostatic nutrient supply to the phloem for up to 1h. The concentrations of all tested nutrients in phloem and in xylem were determined by the high accumulation capacity of the cotyledons’ symplast and the high phloem loading activity. Therefore, the concentrations of nitrogenous compounds and of potassium reached in the xylem by providing these nutrients to the roots were not higher than obtained through recirculation of phloem-derived nutrients. This situation may be typical for the seedling stage with low transpirational water flux and high nutrient resources in the cotyledons.

Key words

glutamine nutrient circulation phloem transport potassium sucrose xylem transport 

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Copyright information

© Springer 2007

Authors and Affiliations

  • E. Komor
    • 1
  • G. Orlich
    • 1
  • H. Bauer-Ruckdeschel
    • 1
  1. 1.Pflanzenphysiologie, Universität BayreuthGermany

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