Complementary patterns of stiffness in stem and leaf sheaths of Triticale : Measurements of ultrasound velocity

The variation in the stiffness of stem and leaf sheaths along the shoot axis of Triticale (Triticosecale W., cv. Jago) was examined, using an ultrasonic method, at two stages of development, (i) at the stage of high stem mechanical instability when upper internodes are forming (heading), and (ii) at milk maturity when development of strengthening tissues is completed (three weeks after anthesis). The squared velocity of low-frequency longitudinal pulse waves was used as a measure of the specific modulus of elasticity, averaged over the whole cross section of the structures and related to the unit density of the material. Structural material of varying effective stiffness was found to be utilized along leaf sheaths with a pattern complementary to that in growing stems. The stiffness increased basipetally along leaf sheaths in the direction of increasing flexibility of internodes. Maximum values of the specific modulus of elasticity in particular leaf sheaths were enhanced acropetally, ensuring the stronger mechanical protection of those meristematic zones which were actively elongating and were located at the upper internodes. The stiffest material present in leaf sheaths covered only those stem sections which could be the most critical for plant safety during stem elongation. This characteristic and very regular pattern of stiffness alteration along the shoot axis was structurally determined as it remained similar after air drying the specimens. It is concluded that adaptation of cereals to withstand environmental loads is realized not only on morphological and anatomical levels but is also reflected in a specific heterogeneity in the material properties of the cell walls which support the plant.