Seasonal Fluctuation of Freezing Tolerance and Soluble Sugar Content in Three Sweet Persimmon Cultivars

Sweet persimmon (Diospyros kaki Thunb.) exhibits insufficient freezing tolerance (FT) in comparison with other temperate fruit trees. Information about FT of sweet persimmon is scarce. In this study, FT was analyzed in three sweet persimmons cultivars, ‘Fuyu’, ‘Taishuu’, and ‘Romang’, using an electrolyte leakage method for two successive years (Year 1, November 2017 to April 2018; Year 2, November 2018 to April 2019). Current-year shoot samples from 8-year-old sweet persimmon trees of each cultivar were collected to analyze FT in an experimental orchard (Pear Research Station, Naju, Korea). Average daily air temperatures increasingly declined from November 2017, reaching the minimum in mid-January 2018. They then steadily increased until April 2018 (Year 1). Patterns of average daily air temperatures of Year 2 were similar to those of Year 1 except that they reached the minimum at the end of December 2018. FT of the three cultivars constantly increased during cold acclimation (from November to January), whereas FT decreased during deacclimation (from February to April) for both years. Shoot samples (from Year 2) of the most freezing-tolerant ‘Romang’ and the most freezing-susceptible ‘Taishuu’ were used for soluble sugar analysis with HPLC. In both ‘Taishuu’ and ‘Romang’, total soluble sugar content was the highest in December 2018 and started to decrease from January 2019 until April 2019. Fructose and glucose contents showed the same pattern as total soluble sugar. Meanwhile, sucrose and maltose of ‘Taishuu’ and ‘Romang’ were detected minimally throughout the whole experimental period. These results indicated that fluctuations of total soluble sugar, fructose, and glucose contents paralleled FT of the three cultivars (p < 0.001). Additional key words: carbohydrate, cold acclimation, deacclimation, freezing injury, low temperature