Tracing the opposing assimilate and nutrient flows in live conifer needles

Chen Gao, Sean J. V. Marker, Carsten Gundlach, Henning F. Poulsen, Tomas Bohr, Alexander Schulz*

*Corresponding author for this work

Research output: Contribution to journalJournal articleResearchpeer-review


The vasculature along conifer needles is fundamentally different from that in angiosperm leaves containing a unique transfusion tissue inside the bundle sheath. To identify the pathway of photoassimilates from mesophyll to phloem, and the opposing pathway of nutrients from xylem to mesophyll, we used specific tracers. For symplasmic transport we applied esculin to the tip of attached pine needles and followed tracer movement down the phloem. For apoplasmic transport we let detached needles take up a membrane-impermeable contrast agent and used micro-X-ray computed tomography to map critical water exchange interfaces and domain borders. Microscopy and segmentation of X-ray data enabled us to render and quantify the functional 3D structure of the water-filled apoplasm and the complementary symplasmic domain. The transfusion tracheid system formed a sponge-like apoplasmic domain that was blocked at the bundle sheath. Transfusion parenchyma cell chains bridged this domain as tortuous symplasmic pathways with strong local anisotropy which, as the accumulation of esculin, pointed to the phloem flanks as preferred phloem loading path. Simple estimates support the pivotal role of the bundle sheath, showing that a bidirectional movement of nutrient ions and assimilates is feasible and emphasising the bundle sheath's role in nutrient and assimilate exchange.
Original languageEnglish
JournalJournal of Experimental Botany
Issue number21
Pages (from-to)6677–6691
Publication statusPublished - 2023


  • 3D imaging
  • Assimilate flow
  • Bundle sheath
  • Conifer needle
  • Endodermis
  • Esculin
  • Fluorescence microscopy
  • Micro-X-ray CT
  • Nutrient flow
  • Phloem
  • Pinus
  • Transmission electron microscopy
  • Xylem


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