BENDING STIFFNESS OF CYLINDRICAL PLANT ORGANS WITH A CORE-RIND CONSTRUCTION - EVIDENCE FROM JUNCUS-EFFUSUS LEAVES

The mechanical behaviour of leaves of Juncus effusus L. in bending was investigated in terms of a closed-form analytical solution derived to predict the bending stiffness of a cylindrical sandwich beam consisting of an outer 'rind' (sclerenchyma and chlorenchyma) and an inner 'core' (aerenchyma). The elastic moduli E(TOTAL)) of intact leaves was measured by means of multiple resonance frequency spectra and compared to that of leaves for which the aerenchymatous core was surgically destroyed. Based on ten leaves, E(TOTAL)) = 22.33 x 10(4) +/- 5.37 x 10(4) kg. cm-2 while the elastic modulus of the 'rind' was 22.29 x 10(4) +/- 5.69 x 10(4) kg. cm-2. The elastic modulus of the 'core' was estimated at 3.12 x 10(4) +/- 1.42 x 10(4) kg. cm-2. Load-deflection curves for three leaf segments indicated leaves were linearly elastic within the range of loading and could be predicted with considerably accuracy based on the closed-form solution. The aerenchymatous core was found to contribute very little to the bending stiffness of leaves, although its contribution appeared to increase as leaf diameter decreased. Leaves mechanically failed by Brazier buckling when excessively loaded and were best considered to mechanically operate as hollow tubes. Nonetheless, the analytical solution for bending stiffness could be applied and, in theory, can be used to predict the behavior of other plant organs with a 'core-rind' construction.