The unique transfusion tissue in conifer needles: the bridge between production and transport. II. Transport efficiency*

The transfusion tissue occupies most of the central “stele” of conifer needles, bounded on the outside by the bundle sheath and on the inside by the axial xylem and- phloem cells (tracheids and sieve elements). It serves the dual purpose of 1. conducting water from the axial xylem via the transfusion tracheids outward through the bundle sheath to evaporate through the stomata or to enter the mesophyll to assist photosynthesis, and 2. to conduct sugars from the production sites in the mesophyll inward through the bundle sheath via transfusion parenchyma to the axial phloem. The structure resembles a sponge, where the parenchyma cells form the holes in the sea of tracheids - with one important modification: In some regions the parenchyma cells are strongly stretched in the axial direction, giving the tissue a significant, spatially varying anisotropy. This emphasizes the 3-dimensional nature of the osmotically driven sugar flow through the tissue, as opposed to a more intuitive radial inflow.  We argue that this is related to the way sugars are loaded into the phloem for efficient sugar transport. As described by Liesche et al., New Phytologist (2021) the sugar is only loaded at the flanks of the phloem to avoid blocking the sugar export in sieve elements further upstream.