Abstract
Ultrastructural changes in apical meristem cells of flower buds of apple (Malus domestica Borkh.) were studied in relation to variations in dormancy and cold tolerance. During rest from late October to early December, the cold tolerance of the flower buds increased as the air temperature decreased. At this stage, microvacuolation and an increase in the volume of the cytoplasm progressed. The cells were rich in organelles involved in protein synthesis, such as cisterna-like endoplasmic reticulum (ER), polysomes, dictyosomes, and vesicles. Protein-lipid bodies were also abundant, and organelles resembling ribosomes were attached to their surface. The plastids were located near the nucleus. Simultaneously, in the plastids, a prolamellar body, characteristic of etioplasts developed, and large starch granules disappeared. In mid-December when rest was broken, cisterna-like ER was replaced by vesicular ER. In mid-January, when maximum cold-tolerance was achieved, the microvacuolation was completed. In early February, 7-8 weeks after the breaking of rest, plastid initials formed from the mature plastids (etioplasts) by budding and subsequent division by constriction. During deacclimation from late March to early May, an increase in vacuole volume and reduction in cytoplasmic volume occurred in the cells, while organelles involved in protein synthesis became apparent. Also, the spatial relationship between the nucleus and etioplasts disappeared as the latter were replaced by plastids. Mitochondria were always present in the winter.
