Inhibition of carbohydrate metabolism by thermal fluctuations during endodormancy lead to negative impacts on bud burst and incidence of floral necrosis in 'Housui' Japanese pear flower buds

Challenges related to bud dormancy release are important limiting factors to commercial fruit production of Japanese pear (Pyrus pyrifolia Nakai) in areas under mild winter conditions. This study aimed to elucidate the effects of thermal fluctuations that occur in areas with mild winters on endodormancy release of Japanese pear flower buds. 'Housui' Japanese pear trees exposed to thermal fluctuations were left to accumulate 600 chilling hours (CH) in order to simulate mild winter conditions and subsequently moved into a greenhouse to observe bud burst and number of flowers/cluster. During endodormancy progression, the concentration of sugars, starch and α-amylase activity were analyzed in lateral flower buds and stems collected at 0, 300 and 600 CH. In addition, necrosis of floral primordia was examined in buds collected over the same period. Thermal fluctuations during endodormancy caused floral primordia necrosis at 300 CH and 600 CH and tended to decrease bud burst percentages. Low concentrations of soluble sugars were observed in buds and stems subjected to thermal fluctuations during endodormancy. Lower percentage of bud burst and incidence of floral primordia necrosis may be a result of low sucrose observed in buds. Hexoses and sorbitol that are important for energy and carbon supply as well freeze tolerance during endodormancy were lower in buds and stem exposed to thermal fluctuations. Thermal fluctuations inhibited the activity of α-amylase in buds and retarded the degradation of starch, leading to low accumulation of soluble sugars during endodormancy. These results suggest that carbohydrate metabolism is inhibited by thermal fluctuations during endodormancy under mild winter conditions, and that low availability of sufficient carbohydrate during winter may lead to floral primordia necrosis and abnormal patterns of endodormancy release in Japanese pear.