Flower differentiation of Japanese pear with reference to ultrastructural observation of cells of apical meristem.

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The morphological, cytological and ultrastructural changes in apical meristem of Japanese pear (Pyrus pyrifolia Nakai cv.Shinsui) during flower differentiation were followed as well as the detection of calcium ion in the cell.First sign of morphological flower differentiation became evident on June 26 ; the apical meristem began to enlarge. During these period starch grains started to accumulate in the cells of quiescent zone, the activity of corpus cells was enhanced, shape of the cells of corpus and quiescent zones changed, and the number of tunica cell layrs decreased while that of corpus cell layrs increased. The numbers.of dictiosomes and mitochondria markedly increased. Distribution of endoplasmic reticulum (ER) also increased with the increase in ribosome. At the same time chromatin in the nucleus increased and became densely stained. Nucleoli also increased in size. Cells at samples fixed with pyroantimonte containing fixatives precipitated many black particles which are Ca^<2+>-antimonate complexes. Density of Ca^<2+> increased to a maximum level on June 26 when corpus cells became very active, decreased thereafter, reaching a low value on july 17, Ca^<2+> density in nuclei of cells increased during floral differentition. Within nucleus, Ca^<2+> distributed 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. Ca^<2+> contents in mitochondria, dictiosomes and ER changed parallel to the changes of Ca^<2+> in cytoplasm.These results suggest that the synthesis of DNA,RNA and protein becomes very active in the cells of apical meristem just before the flower differentiation, and that Ca^<2+> ion plays an important role in flower differentiation.