Alterations in ultrastructure and subcellular localization of Ca2+ in poplar apical bud cells during the induction of dormancy

In poplar (Populus deltoides Bartr. ex Marsh), bud dormancy and freezing tolerance were concomitantly induced by short-day (SD) photoperiods. Ultrastructural changes and the alteration in subcellular localization of calcium in apical bud cells associated with dormancy development were investigated. During the development of dormancy, the thickness of cell walls increased significantly, the number of starch granules increased, and there was a significant accumulation of storage proteins in the vacuoles of the apical bud cells. The most striking change was the constriction and blockage of the plasmodesmata. It was demonstrated that antimonate precipitation is a reliable technique for studying subcellular localization of calcium in poplar apical bud cells. Under the long day (LD) photoperiod, electron-dense calcium antimonate precipitates were mainly localized in vacuoles, intercellular spaces and plastids. Some antimonate precipitates were also found in the cell walls and at the entrance of the plasmodesmata. However, there were few Ca2+ deposits found in the cytosol and nucleus. After 20 d of SD exposure, when development of bud dormancy was initiated, calcium deposits in intercellular spaces were decreased, whereas some deposits were found in the cytosol and nuclei. From 28-49 d of SD exposure, while dormancy was developing, a large number of Ca2+ precipitates were found in the cytosol and nuclei. When deep dormancy was reached after 77 d of SD exposure, Ca2+ deposits became fewer in both cytosol and nuclei, whereas numerous deposits were again observed in the cell walls and in the intercellular spaces. These results suggest that under the influence of SD photoperiods, there are alterations in subcellular Ca2+ localization, and changes in ultrastructure of apical bud cells during the development of dormancy. The constriction and blockage of plasmodesmata may cause the cessation of symplastic transport, limit cellular communication and signal transduction between adjacent cells, which in turn may lead to events associated with growth cessation and dormancy development in buds.