Histological Studies on Breaking Resistance of Lower Internodes in Rice Culm : IV. The roles of each tissue of internode and leaf sheath in breaking resistance

Abstract

In order to clarify the roles of EF (epidermis-cortical fiber compound) and P (fundamental parenchyma) in breaking resistance, several dynamic characteristics of the fourth internode from the top of rice culm, var. Koshihikari, were studied at the time of harvesting. Breaking strength and deflection of PEF (whole internode tissues) and P, obtained by shaving off the EF from PEF, were measured with aid of the stalk bending hardness tester (type EO-III). Tensile strength, compressive strength and shearing strength were measured by traditional methods used in the material testing. These data are shown in Table 1. Some calculations with these data verify that EF/P ratio in YOUNG's modulus, tensile strength and compressive strength are 113, 63 and 19, respectively. Besides, the tensile strength of EF in rice internode was ascertained to be almost the same to that of the xylem tissue of boxwood (Buxes microphylla var. suffruticosa Makino). Moreover, about 89% of bending stress (tensile and compressive stress) is supported with EF, and about 78% of shearing stress is supported with P in the bended internode. Therefore, with regard of bending stress, the internode of rice culm can be considered dynamically to be a pipe composed of thin EF as illustrated in Fig. 1. The value of maximum stress M can be obtained by the formula (6), and d2t [(diameter of internode)2×(thickness of EF)] can be considered to be a very important factor in breaking resistance. On the other hand, the main role of P in breaking resistance is to prevent sectional deformation and to maintain cylindrical form in the bended internode. Breaking strength of the fourth internodes with or without leaf sheath was investigated in the ripening period as shown in Fig. 2. The breaking strength had a peak soon after heading and a minimum at 25-30 days after heading. It may be thought that the slight increase in breaking strength at the time of harvesting was caused by reaccumulation of starch in the internode.