We periodically investigated the lateral flower bud morphology of 1-year shoots of 'Kosui' pears (Pyrus pyrifolia Nakai) in terms of dormancy progression, using magnetic resonance imaging. The size of flower buds did not change significantly during endodormancy, but rapid enlargement took place at the end of the ecodormancy stage. To gain insight into the physiological status during this period, we analyzed gene expression related to cell cycle-, cell expansion- and water channel-related genes, namely cyclin (CYC), expansin (EXPA), tonoplast intrinsic proteins (TIP) and plasma membrane intrinsic proteins (PIP). Constant but low expression of pear cyclin genes (PpCYCD3s) was observed in the transition phase from endodormancy to ecodormancy. The expression levels of PpCYCD3s were consistent with few changes in flower bud size, but up-regulated before the sprouting stage. In contrast, the expression of pear expansin and water channel-related genes (PpEXPA2, PpPIP2A, PpPIP2B, PpIδTIP1A and PpIδTIP1B) were low until onset of the rapid enlargement stage of flower buds. However, expression of these genes rapidly increased during sprouting along with a gradual increase of free water content in the floral primordia of buds. Taken together, these results suggest that flower bud size tends to stay constant until the endodormancy phase transition. Rapid enlargement of flower buds observed in March is partly due to the enhancement of the cell cycle. Then, sprouting takes place concomitant with the increase in cell expansion and free water movement.

The effects of global warming have been observed in several crops as changes in temperature-dependent processes, such as dormancy in perennial fruit trees. We exposed potted Japanese pear 'Housui' plants to 600 chilling hours (CH) below 7.2°C, which is below the theoretical requirement. Such low chilling accumulation was initiated at different times (three treatments) and carried out during four consecutive seasons in order to observe the influence of these factors on the incidence and severity of floral primordia necrosis, water dynamics, phenological, and morphological changes during dormancy in lateral mixed buds, which is comprised of vegetative and reproductive primordia, obtained from the current season's shoots. The submitted amount of chilling temperatures brought about the release of dormancy stage, regardless of seasonal variations in the duration to accumulate 600 CH, but caused a remarkable increase of floral primordia necrosis and a great increase in the number of floral primordia as a consequence of the development of new inflorescences; however, these morphological changes did not increased number of opened flowers because of the progression of floral primordia necrosis. Consecutive seasons of mild winter conditions anticipated the incidence of floral primordia necrosis. The results obtained from magnetic resonance imaging in the buds of plants subjected to low chilling accumulation showed low water mobility and water content especially in scales compared to plants grown under natural conditions. In contrast, increased water mobility and relative content were observed in specific portions of the bud base in treated buds, which was more accentuated in the mixed buds of plants grown for several seasons under mild winter conditions. Moreover, abnormal functional water movement inside dormant mixed buds might result in the necrosis of floral primordia, giving way to the development of new inflorescences.