| Development analysis of the rachis disarticulation
in Triticum K MATSUMOTO, T. TERAMURA and J. TABUSHI Osaka University of Liberal Arts and Education, Kyoto University and Kyoto Kasei College, Japan Two types of rachis disarticulation are known in Triticum, i. e. wedge type (W - type, rachis breaking at the upper side of the spikelet base) and barrel type (B - type, rachis breaking at the lower side of the spikelet base). W - and B - types have abscission zones or layers at different locations of rachis joint, i. e. W - and B - type layers being located at the upper and the lower parts of the rachis joint, respectively. Moreover, various degrees of fragility of rachis are present among the species belonging to each type. Morphological observations of the rachis disarticulation in several species of Triticum and Aegilops were reported by ZIMMERMANN (1934). The present study was concerned with the developmental analysis of the rachis disarticulation in Triticum. The authors analyzed the development of disarticulation at five stages (I-V) in two Triticum species belonging to the W - and B - types. And seven species of Triticum, four strains of synthesized 6 x wheat (by KIHARA et alii, 1948) and one of their parental species, Aegilops squarrosa, were also studied comparatively at stage V. Longitudinal hand sections of rachis joint were microscopically examined, and also tested for lignification and suberization with hydrochloric acidphloroglucine and sudan III. The results obtained are presented in Table 1 and 2, which indicate that the degree of fragility of each type was determined by the following main factors. (1) Constriction of rachis exterior. In both W - and B - types, a constriction is formed at the exterior part of each abscisson layer, and the deeper the constriction is the more fragile the rachis becomes. In the W - type, the constriction in the rachis side develops more remarkably than in the rachilla side in rachis joint, while it is vice versa in the B - type. (2) Cell growth in rachis cortex. In the W - and B - types, localized cortical cells of rachis joint do not elongate, in the longitudinal direction, remaining as linearly arranged short type cells to form an abscission layer, while the outside cortical cells of each abscission layer become elongated, in the longitudinal direction, or enlarged as irregularly arranged long type cells resulting in tough tissues. It is observed that rachis actually breaks at the crevice formed in the intercellular spaces of linear short cells in the layer. Therefore, the degree of fragility of each rachis is partly determined by the difference in cell growth between the localized and outside cortical cells in the W - and B - types. (3) Cell differentiation in rachis cortex. In the W - type, rachis becomes more fragile as lignification increases in the localized cortial cells but not in the outside cortial cells. In the B - type, however, lignification in the outside cells but neither lignification nor suberization in the localized cells causes the fragility of rachis. This result indicates that the fragility of each rachis is also determined by the degree of cell differentiation between the localized and outside cortical cells in the W - and B - types. |