Mechanical Properties of the Rice Panicle

Abstract

Curvature, bending moment, and second moment of stem cross-sectional area were evaluated from photographic data and used to compute flexural rigidity and Young's modulus in the panicle rachis of rice, Oryza sativa L.'M-101.' Flexural rigidity C, and its components E, Young's modulus, and 1, the moment of inertia of the area about the neutral axis, were evaluated 1.5 cm (tip), 9.5 cm (mid), and 16.5 cm (base) from the tip of the panicle rachis. In dynes per square centimeter, C increases from 1.1 x 103 near the tip to 1.09 x 104 in the middle to 5.35 x 104 in the basal region of the rachis. Of the components of C, the I changes have the larger effect, increasing from 2.12 x 10-7 centimeters4 near the tip to 8.21 x 10-7 centimeters' in mid regions to 6.0 x 10-6 centimeters4 in the basal regions. When the rice panicle emerges from its sheathing leaf, it is vertical. As grains fill, the panicle droops so that it is bent in a plane curve with the tip often deflected past the horizontal plane which contains the insertion of the basal branches. Removal of the ripe grains leads to restoration of the vertical orientation. This suggests the beam bending equation as a model relating grain weights to shape changes during grain ripening. According to the engineering theory of bending (4), shape (local curvature) is proportional to bending momentwhere K equals curvature, M is the bending moment, and C is a coefficient, the flexural rigidity, which describes the resistance to bending. In a homogeneous beam, the flexural rigidity is the product of E, Young's modulus (the ratio of stress to a longitudinal strain produced by the stress) and I, the moment of inertia of the area about the neutral axis MATERIALS AND METHODSThe Rice Panicle as a Loaded Beam. Mechanically, the intact rice panicle is a complex structure. Eight to ten branches, each bearing grains, are borne on the rachis (main axis). The branches are wiry and twist to some extent around the stem. Some appear to support the rachis, increasing its rigidity and decreasing local curvature, while others hang below the stem and may act as concentrated loads. To study the mechanical properties of the rachis, the structure was simplified by removing branches with sharp scissors until only the most apical branch remained. Bending moments and curvatures were determined on the singly branched rachis, as described below. Then the branch was removed, and the measurements were repeated on the rachis with its ten grains.The beam model is sketched in Figure 1. Ripening grains act as loads to bend the panicle rachis. A single rice grain attached to the rachis at d produces a bending moment at s, where both d and s are measured from the tip. The bending moment has a direction, which indicates the direction of rotation, and a magnitude, which measures the tendency of the force to make the rigid grain rotate about a fixed axis through s perpendicular to the plane of the curve. The magnitude of the bending moment is the product of the force exerted by the grain and the moment arm ...

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