Sap flow and sugar transport in a pine needle: how to deal with an adverse environement

The survival of green plants depends on efficient use of photosynthesis in the leaves, where sunlight, water, and CO₂ are transformed to sugar – the raw material, which builds up even the largest trees. The dissolved sugars are transported by osmosis through the sieve tubes of the phloem, a vascular system, which runs through the veins of the leaves and on through the stem, all the way down into the roots. This basic mechanism is at work in all trees - somewhat surprisingly since the older class of gymnosperms (conifers) predate the “modern” angiosperms (broad leaf trees) by more than 100 mio years. The sugar production sites (mesophyll cells) are distributed along the entire leaf, and it is important for the functionality of the leaf that they are all able to export their sugars. For conifer leaves (needles) the linear venation architecture makes this challenging, since the osmotic sugar transport in a long sugar filled pipe will tend to stagnate at the tip. To overcome this, the phloem tubes (sieve tubes) of conifer needles are divided in groups starting at different points along the needle. In addition the needles contain an extra “transfusion tissue”, taking up a considerable volume, which bridges between production and transport and directs the sugars to the correct (outermost) sieve tube group. Micro X-ray tomography on intact needles reveals the complex network of interdigitated water -and sugar carrying cells (both much larger than the xylem (tracheid) cells and the phloem (sieve) cells) making up the transfusion tissue and allows us to understand the pathways for water and for sugars (running in opposite directions) with huge pressure differences (say 3 MPa) across tiny length scales (say 5 microns). Presumably this structure is also important for understanding why conifers are among the largest and most drought/cold resistant organisms on earth.