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Abstract Detail

Developmental and Structural Section

Carvalho, Monica R. [1], Niklas, Karl [1].

Murray or DaVinci? Phloem architecture in leaves of Populus tremuloides x alba.

Fluid flow properties affecting the transport of water and sugars in plants are determined by the spatial arrangement and geometry of conducing conduits. Most biological hierarchical branching systems can be described either as conserving mass and following a squared relationship between parent and daughter branch radii (DaVinci’s rule: rp2 = rd12 + rd22), or as conserving flow energy and following a cubed relationship (Murray’s Law: rp3 = rd13 + rd23). Xylem dimensions in open dichotomous systems such as branches act as energy conserving systems, in consistency with Murray’s Law. Angiosperm leaf vasculature mediates the effective export of photosynthate and constitutes a major portion of the water transpiration pathway in leaves. The major and minor veins forming these hierarchical networks show clear anatomical and functional distinctions, consistent with the division of labor typical of reticulate systems. Minor veins are marked by the presence of a bundle sheath that contrasts the rib tissue of major veins thought to provide structural support to veins. Functionally, most of the radial diffusion of water into mesophyll occurs through minor veins, whereas major veins act as distribution pipelines and structural support for the leaf. The functional distinction between major and minor veins is also reflected in overall leaf venation geometry. Across many angiosperm taxa, major veins have diameters that mimic expectations from a mass-conservation system, following DaVinci’s rule. Contrarily, the diameters of minor veins best match the expectations of Murray’s Law, similar to open dichotomous systems. This distinction shows a profound difference in structure between major and minor veins in angiosperm leaves that has implications for flow velocities of water and the transport of photosynthates. However, measured diameters of leaf vein branching systems are focused on vein widths, and do not distinguish cross-sectional areas of conducing tissues from associated bundle sheath or rib tissues. A direct analysis on conducing tissue cross-sectional areas in leaf veins is needed to better test for mass-conserving or energy-conserving branching systems on leaves, and to address the implications of vein branching geometry on transport within leaves. Here, we test predictions of Murray’s Law on phloem conduit dimensions in leaves of the model tree species Populus tremuloides x alba, and consider the implications on phloem flow dynamics at the leaf level.

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1 - Cornell University, Department of Plant Biology, Ithaca, NY, 14853, USA

leaf veins
Fluid flow

Presentation Type: Oral Paper:Papers for Sections
Session: 45
Location: Salon 11/The Shaw Conference Centre
Date: Tuesday, July 28th, 2015
Time: 2:00 PM
Number: 45003
Abstract ID:1201
Candidate for Awards:Katherine Esau Award

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