Tomas Bohr: Sap flow and sugar transport in a pine needle

The survival of green plants depends on the efficient use of photosynthesis in the leaves, where sunlight, water, and CO2 are transformed into 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. The sugar production sites (mesophyll) are distributed over the entire leaf, and it is important for the functionality of the leaf that they are all able to export their sugars. For conifer needles the linear venation architecture makes this challenging, and they have an extra “transfusion tissue” that bridges between production and transport. We are currently studying this complex collection of interdigitated water -and sugar-carrying cells by micro X-ray tomography on intact needles and by network modelling, 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).

Thomas Bohr is Professor of Physics at the Physics Department of the Technical University of Denmark.