Freezing Injury Indicator of Tasty Kiwifruit During Overwintering Period

China has the largest artificial planting area of kiwifruit, of which 67% is tasty kiwifruit. Tasty kiwifruit is mainly distributed in the north foot of Qinling Mountains in Shaanxi Province (about 40% of the national kiwifruit industry scale), and Funiu Mountain and Tongbai Mountain in Henan Province (about 20% of the national kiwifruit industry scale). These two producing areas locate in the northern edge of China's kiwifruit planting area, where the risk of overwintering freezing injury is high to cause great economic losses to fruit farmers. Therefore, it is especially important to study the freezing injury indicator of tasty kiwifruit for the introduction and expansion of tasty kiwifruit, disaster prevention and mitigation, and stable development of the industry.Through disaster investigation, expert interview, data research and other methods, 7 large-scale freezing injury cases of tasty kiwifruit in 1991, 2002, 2007, 2009, 2011, 2015 and 2020 are selected. The disaster cases of four counties, namely Huyi, Zhouzhi, Meixian and Tongbai are used as samples. The critical occurrence indicator of the overwintering freezing injury of tasty kiwifruit is determined as -8.0 ℃. The low temperature indicator (T_D) of mild, moderate and severe freezing injury of tasty kiwifruit are determined as follows: -12.0 ℃ < T_D ≤ -8.0 ℃, -15.0 ℃ < T_D ≤ -12.0 ℃, and T_D ≤ -15.0 ℃. Four factors are defined as: Winter minimum temperature (X₁), days of low temperature below -8.0 ℃ (X₂), absolute value of negative accumulated temperature below -8 ℃ (X₃), and process harmful accumulated cold below -8 ℃ (X₄). Using principal component analysis and K-means clustering analysis, the comprehensive indicator (I_h) of tasty kiwifruit overwintering freezing injury is constructed, and the ranges of I_h for different levels of freezing injury are as follows: -2.08 < I_h ≤ 0.82 for mild freezing injury, 0.82 < I_h ≤ 2.15 for moderate freezing injury, and I_h>2.15 for severe freezing injury. The indicators mentioned above are verified by the freezing injury cases of tasty kiwifruit in Changan, Wugong and Xixia counties. The results show that low temperature indicators and comprehensive indicators of overwintering freezing injury of tasty kiwifruit are basically consistent with the actual situation. The results can support the prevention and evaluation of the freezing injury of tasty kiwifruit during overwintering period, and also can provide reference for industrial layout optimization and introduction of tasty kiwifruit in China.