Localization of Calcium in the Cells of Apical Meristem during Flower Differentiation of Japanese Pear, Pyrus pyrifolia Nakai

Abstract

To investigate the possible involvement of calcium in the process of flower differentiation, calcium ion in the cells of apical meristem was detected electronmicroscopically using buds on spurs and vegetative shoots of Japanese pear (Pyrus pyrifolia Nakai) cv. Shinsui as materials.
1. Cells at samples fixed with glutaraldehyde-formaldehyde and post-fixed with osmium tetroxide, both of which contained pyroantimonate, precipitated many black particles whereas they were not observable in photomicrographs of cells fixed with fixatives without antimonate. Most of the black particles disappeared when the preparations were treated with EGTA which indicate that the black particles are calcium-antimonate complexes. The localization, density and the size of the particles within cells of apical meristem changed dramatically as flower differentiation proceeded.
2. Density of calcium particles differed depending on the activity of the cells. Until June 5, only a few particles were observed; they were mainly localized in plastids of cells from buds on spurs. The particles subsequently increased, and then decreased temporarily just before the onset of floral differentiation; another increase to a maximum level on June 26 was noted at which time corpus cells became very active judging from dense staining of cytoplasm and increased dictiosomes and endoplasmic reticulum (ER). The particles decreased thereafter through 3 and 10 July and reached a low value on July 17 when calyx primordia began to differentiate.
3. Ca²⁺ density in nuclei of cells was low before flower differentiation, but it increased, indicative that floral differentiation was occurring. Within nucleus, Ca²⁺ distri- buted mainly in nuclear fluid, some in granular region of nucleolus and its releasing materials, but not in the fibrous area of nucleolus, chromosomes or chromatin.
4. Although Ca²⁺ is mainly concentrated in the cell wall, its level in the cell wall decreased as Ca²⁺ in the vacuoles increased at the floral differentiation stage. The vacuole normally contains little Ca²⁺, but during this period it increased dramatically.
5. Plastids contained almost all Ca²⁺ in the cell before June 5, and after that, continued to be one of the main distribution area of Ca²⁺. But Ca²⁺ in plastids decreased when Ca²⁺ in cells markedly increased on June 26. On the other hand, Ca²⁺ contents in mitochondria, dictiosomes, and ER changed parallel to the changes of Ca²⁺ in cytoplasm, and increased after flower differentiation stage on June 26.
6. The size of the Ca²⁺ particles also changed during flower differentiation. The size became smaller as the flower differentiation proceeded, smallest on July 10, but after that became larger when Ca²⁺ density rapidly decreased.
7. Possible role of Ca2 + in the cell of apical meristem in flower differentiation of Japanese pear was discussed.