Abstract
To elucidate the mechanism of flower differentiation, ultrastructural changes, particularly the changes in amount and shape of organelles were investigated in the apical meristem of the buds of Japanese pear (Pyrus pyrifolia Nakai cv. Shinsui) during flower bud differentiation.
1. The most important stages in the flower bud differentiation were from the beginning of activation of corpus cells on June 26 to the differentiation of sepal primordia on July 17. During these periods all the organelles in the cell underwent remarkable changes.
2. During flower differentiation the number of dictiosomes markedly increased with concomitant cytomorphological changes such as many larger vesicles and the reduction in length of dictiosome itself. Distribution of endoplasmic reticulum (ER) also increased with the increase in ribosome. ER evolved into a kind of network from a discontinuous linear shape. There also existed active dictiosomes associated with ER.
3. The number and size of mitochondria increased during flower differentiation. As the size increased the shape became spherical or oval. The number and size of plastids in the cells of active tissues did not differ between flower buds and vegetative buds. The size of plastids in the cells of quiescent zone changed markedly, increasing initially and then decreasing, because the size was closely related to the amount of starch grains the plastids contained.
4. While flower differentiation progressed, chromatin in the nucleus increased and became densely stained in the cells of apical meristem of the flower buds. Nucleoli also increased in size, and occasionally two nucleoli were observed in a nucleus. Actively dividing cells in the apical meristem of flower buds were also characterized by many small vacuoles.
5. These ultrastructural changes of the organelles in the cell of flower buds were discussed in relation to their physiological roles.
