Mechanical properties of stem vascular bundle --Cheng, WY, P C Cheng, PC, Walden, DB, Shinozaki, DM Mechanically, the maize plant can be characterized as a "foam stick" with a reinforced outer shell, which resembles a concrete pillar in modern constructions. The peripheral region can be considered as the steel reinforcing bars and cylinder, and the interconnecting nodal networks are the steel bracings ("rebar") found in a concrete pillar. In the early stage of development, the vasculatures act as the tensile element, while the highly turgid parenchyma cells are the compression element in the model. In the later stage while the parenchyma cells become air-filled, the highly lignified para-epidermal bundles become the structural element. The binding between individual para-epidermal vascular bundles by lignified sclerenchyma cells is an important structural development. This binding transforms those loosely parallel arranged vascular bundles into a solid cylindrical structure as depicted in a model built of straws (as vascular bundles) and foam boards (as node (N) and internode (IN)) (Fig. 1a). If the individual straws are not glued together, the "stem" is subject to a "twisting" motion (Fig. 1a-left; arrow), but the stem becomes rigid when the straws are bound together (Fig. 1a-right).

To isolate the vascular bundle, mature stems (Oh43/KYS) were placed in cellulase-pectinase solution to remove parenchyma cells. (Cellulase from T. longibrachiatum; Cat. WA18420, and pectinase from A. niger; Cat. 18579; Fluka, Switzerland.) This digestion process took 1-6 days, depending on the age of the specimen. Heavily lignified vascular bundles are resistant to the enzymatic digestion and remain intact. The longitudinal vascular bundles in the stem can be isolated by enzymatic treatment as long fibers (Fig 1b, 1c), which allow us to perform mechanical testing on individual bundles. A tensile load-displacement curve was obtained from individual bundles using a computer-controlled thin-film tensile tester developed by D M Shinozaki (Faculty of UWO).

Preliminary material testing on the isolated vascular bundle reveals the structure is moderately strong (Fig. 2); the individual vascular bundles fractured around 100g (or about 1 N), with an elongation of about 1200µm. For a bundle diameter approximately 0.5mm, the effective stress is about 5MPa at fracture. This is probably lower than the real value since it fractured at the grip in our preliminary experiment. Since the vascular bundle is not a solid rod, the true stress at fracture is much higher. Therefore, effectively the bundle may be stiffer than polyethylene. This test was done wet (in 30% iso-propanol); therefore, the strength probably increases when it is dried.

This article is part of a report by WYC for Siemens Westinghouse Science and Technology Competition (Semi Finalist) and Intel Science Talent Search (2001).

Figure 1. (a) a stem model demonstrating the effect of torsion force (arrow), (b, c) Isolated bundles. N: node, IN: internode.

Figure 2. 30mm isolated vascular bundle of Oh43/KYS fractures at approx. 100 g. with an elongation about 1200 µm
 
 


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