title: Scaling of phloem structure and optimality of photoassimilate transport in conifer needles.
abstract: The phloem vascular system facilitates transport of energy-rich sugar and signalling molecules in plants, thus permitting long-range communication within the organism and growth of non-photosynthesizing organs such as roots and fruits. The flow is driven by osmotic pressure, generated by differences in sugar concentration between distal parts of the plant. The phloem is an intricate distribution system, and many questions about its regulation and structural diversity remain unanswered. Here, we investigate the phloem structure in the simplest possible geometry: a linear leaf, found, for example, in the needles of conifer trees. We measure the phloem structure in four tree species representing a diverse set of habitats and needle sizes, from 1 (Picea omorika) to 35 cm (Pinus palustris). We show that the phloem shares common traits across these four species and find that the size of its conductive elements obeys a power law. We present a minimal model that accounts for these common traits and takes into account the transport strategy and natural constraints. This minimal model predicts a power law phloem distribution consistent with transport energy minimization, suggesting that energetics are more important than translocation speed at the leaf level.
authors.affiliation: Max Planck Institute for Dynamics and Self-Organization, Am Faßberg 17, 37077 Göttingen, Germany henrik.ronellenfitsch@ds.mpg.de.
authors.forename: Henrik
authors.initials: H
authors.lastname: Ronellenfitsch
authors.affiliation: Department of Plant and Environmental Sciences, University of Copenhagen, 1871 Frederiksberg C, Denmark.
authors.forename: Johannes
authors.initials: J
authors.lastname: Liesche
authors.affiliation: Department of Organismic and Evolutionary Biology, Harvard University, Cambridge, MA 02138, USA Department of Physics, Technical University of Denmark, 2800 Kgs. Lyngby, Denmark.
authors.forename: Kaare H
authors.initials: KH
authors.lastname: Jensen
authors.affiliation: Department of Organismic and Evolutionary Biology, Harvard University, Cambridge, MA 02138, USA.
authors.forename: N Michele
authors.initials: NM
authors.lastname: Holbrook
authors.affiliation: Department of Plant and Environmental Sciences, University of Copenhagen, 1871 Frederiksberg C, Denmark.
authors.forename: Alexander
authors.initials: A
authors.lastname: Schulz
authors.affiliation: Max Planck Institute for Dynamics and Self-Organization, Am Faßberg 17, 37077 Göttingen, Germany.
authors.forename: Eleni
authors.initials: E
authors.lastname: Katifori
mesh_terms: D050260:Carbohydrate Metabolism; D008954:Models, Biological; D052585:Phloem / Q000033:anatomy & histology / Q000378:metabolism*; D028201:Pinaceae / Q000033:anatomy & histology / Q000378:metabolism*; D018515:Plant Leaves / Q000033:anatomy & histology / Q000378:metabolism
keywords: mathematical modelling; optimization; phloem structure; photoassimilate transport