The unique transfusion tissue in conifer needles: the bridge between production and transport. I. Structure and function

Conifer needles have a unique transfusion tissue in which the axial vascular elements of xylem and phloem are imbedded. The existence of this tissue has been known for a long time, since the pioneering work of Strasburger (1891). The transfusion tissue fills most of the stele and consists of two cell types: dead transfusion tracheids and living transfusion parenchyma cells, bounded by the bundle sheath. These cells take care of the difficult task of transporting water for photosynthesis and transpiration outward from the xylem, and sugars, produced in the mesophyll, inward to the phloem. Details of this two-way traffic are not known, and it is complicated by the existence of a Casparian strip between the cells of the bundle sheath, forcing both water and sugar at this point to go symplasmically in opposite directions through the same cells. We have studied the needles of P. pinaster using X-ray tomography on living needles and by TEM. Our results reveal a surprising structure reminiscent of a sponge where the water moves out through the continuum and the sugar moves through the holes. We shall discuss main structural features of the tissue and how they relate to sugar transport and the harsh environmental conditions, in particular draught, which conifers are adapted to.