Prediction of Differences in the Blooming Dates between Japanese Apricot ‘Nanko’ and Pollinizer Cultivars Using Development Rate Models

The selection of appropriate pollinizer cultivars is essential for stable fruit production of Japanese apricot because this species has many self-incompatible cultivars. In this study, the chilling responses of flower buds during endodormancy and the heat responses of flower buds during ecodormancy were quantified as development rates (DVRs) in ‘Kotsubu-nanko’, ‘Hakuo’, and ‘NK14’, three pollinizer cultivars of the leading cultivar ‘Nanko’ in Wakayama Prefecture. Approximating functions of DVRs for both endodormancy release and ecodormancy release were obtained on the basis of chilling and heat exposure tests on young trees with various combinations of temperatures and periods. These analyses demonstrated that temperatures over 15°C were only effective for endodormancy release of ‘Hakuo’ buds. Compared with the buds of ‘Kotsubu-nanko’ and ‘Hakuo’, those of ‘NK14’ were less sensitive to 15–20°C during the ecodormancy stage. Blooming date prediction models for these cultivars were constructed using the DVR values. Then, the applicability of the three cultivars as pollinizers for ‘Nanko’ was evaluated based on whether their predicted blooming times overlapped with that of ‘Nanko’. The models were optimized by adjusting the threshold of blooming percentages and initial points of heat accumulation during ecodormancy release to achieve the smallest differences between predicted and observed blooming dates (root mean squared error = 3.72–5.90). Simulations under different temperatures revealed the most suitable pollinizer cultivar for ‘Nanko’ going forward. Our predictions indicate that, in the simulated warmer conditions, the blooming date of ‘Kotsubu-nanko’ will be delayed 10 or more days compared with that of ‘Nanko’, and larger differences between the blooming dates of ‘Hakuo’ and ‘Nanko’ will occur under lower and higher temperatures. The blooming period of ‘NK14’ was predicted to remain stably synchronized with that of ‘Nanko’ under a range of simulated warmer and colder temperature conditions. These results showed that ‘NK14’ is an appropriate pollinizer for stable fruit production of ‘Nanko’ in the future, and highlight the importance of blooming time assessments based on the temperature responses of flower buds.