Sap Flow and Sugar Transport in Plants

Thursday, November 21, 2013

11:00-12:00, Assembly Hall, The Inn

Dr. Tomas Bohr

Technical University of Denmark

 Plants have highly effective vascular systems, which can transport fluid over large distances. The xylem system carries water from the roots up to the leaves and the phloem system carries sugar solutions from sources (leaves) to sinks (roots, fruits etc.) and thus provides the necessary material for growth. There are many important fluid dynamical problems connected with these flows, and I shall discuss some of them. In the 1920’ies, Ernst Münch proposed that sugar transport in the phloem is driven by passive osmotic pressure gradients generated by loading and unloading sugar into the phloem tubes (sieve elements) of the leaves. It has been strongly debated whether this hypothesis can actually account for long distance translocation, e.g., from canopy to root of a large tree. In the lecture, I will argue that optimization of the efficiency of the sugar transport leads to a universal scaling of the width of the phloem tubes with the loading (leaf) length and the translocation (stem) length in plants. These predictions have been tested for plants ranging from 10 cm herbacious plants to 60 m trees – both hardwood and conifers – and provide the first quantitative test of Münch’s ideas. For both the xylem and the phloem, the leaves provide the driving force for the sap flow, and I shall discuss current ideas how this complex feat is accomplished.

The lecture is partly based on recent papers:

K. H. Jensen, J. Lee, T. Bohr, H. Bruus, N. M. Holbrook & M. A. Zwieniecki: Optimality of the Münch mechanism for translocation of sugars in plants, Journal of the Royal Society Interface 8, 1155–1165 (2011)

K. H. Jensen, T. Bohr & H. Bruus:  Osmotically driven flows in microchannels separated by a semipermeable membrane, Lab on a Chip 9, 2093-2099 (2009)

K. H. Jensen, J. Liesche , T. Bohr & A. S. Schulz: Universality of phloem transport in seed plants, Plant, Cell & Environment 35, 1065-1075 (2012)

K. H. Jensen, K. Berg-Sørensen, S. Friis & T. Bohr: Analytic solutions and universal properties of sugar loading models in Münch phloem flow, Journal of Theoretical Biology 304, 286-296 (2012)

K.H. Jensen, D.L. Mullendore, N.M. Holbrook, T. Bohr, M. Knoblauch & H. Bruus: Modeling the hydrodynamics of phloem sieve plates, Frontiers in Plant Science 3, article 151 (2012)

L. S. Haaning, K. H. Jensen, C. Hélix-Nielsen, K. Berg-Sørensen & T. Bohr: Efficiency of osmotic pipe flows, Physical Review E 87, 053019 (2013)

*This is a joint seminar co-sponsored by the Department of Engineering Science and Mechanics, the Department of Mechanical Engineering, and the MultiScale Transport in Environmental and Physiological Systems (MultiSTEPS) program.