Dense but flexible wood – How leaf nodes impact xylem mechanics in Juglans californica

in IAWA Journal
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A node is the point of attachment of the leaf to the stem of a plant; gaps associated with nodes have been viewed as discontinuities of the stem vascular system. We tested the hypothesis that the node/gap is a spring-like joint that impacts stem flexibility even well after the leaves have been shed, with some stems specialized for elongation and others for flexibility. Four-point bending tests were done using an Instron Mechanical Testing Device with the independent variable being the number of nodes in the stem segment and dependent variables being Modulus of Elasticity (MOE), Modulus of Rupture (MOR), and xylem density. Node anatomy was examined microscopically to assess structure and function. The stiffness of the stem was inversely proportional to the frequency of leaf nodes. Surprisingly, xylem density was inversely proportional to the frequency of leaf nodes in stems of adult trees. The tissue around nodes/gaps consisted of twisted and contorted cells that may be effective at absorbing compressive and tensile stresses. Because nodes behave as spring-like joints, the frequency of nodes relates to function, with some stems specialized for vertical expansion and others for light capture and damping of wind stress. The ultimate stems on a tree are the most bendable, which may allow the trees to avoid breakage.

Dense but flexible wood – How leaf nodes impact xylem mechanics in Juglans californica

in IAWA Journal



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    Diagrams showing a node on a 10-mm-wide stem of a Southern California black walnut tree. – A: with intact tissues. – B: the bark has been removed to reveal the wood grain. – C: a radial longitudinal section view; the cross-hatched area in the middle of the stem represents the chambered pith.

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    Various stem mechanical parameters as functions of the frequency of leaf nodes on the stem (leaf nodes per meter), including, from top to bottom, flexural stiffness (exponential curve fit), xylem density (logarithmic curve fit), Specific Modulus of Rupture (MOR – exponential curve fit) and Specific Modulus of Elasticity (MOE – logarithmic curve fit). A total of 294 stems were sampled as specified in Table 1. Data were binned at intervals of 6 leaf nodes m- 1 with frequencies from 125 to 250 nodes m-1 pooled as a single bin. Values are means ± 1 standard error.

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    A comparison of the mechanical properties of 10 mm diameter stems for one-year-old resprouts versus adult lateral branches. The differential node frequency enhances support for rapidly growing vertical shoots in resprouts and flexibility for slow growing lateral shoots in adults. Parameters included Leaf nodes per meter, Flexural stiffness, Specific MOE, and Specific MOR. For each parameter N = 20; means (± 1 SE) are shown. * differences between growth form were significant at P < 0.05.

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    Stem microscopic sections showing node anatomy. A lower magnification tangential longitudinal section (A) showing a bud gap (bg) and leaf gap (lg). The box in A outlines the area shown at higher magnification in B, which shows the contorted xylem (cx) tissue adjacent the bud gap. – C: a transverse section showing a bud gap (bg) that runs from the central pith, with a pith chamber (p), to the exterior of the xylem.


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