Cold Hardiness of Prunus mume ‘Xiang Ruibai’ and Its Parents Based on Biological Indexes and Physical Parameters

Low temperature is a primary factor limiting the distribution of Prunus mume. In order to produce a variety that has both cold tolerance and the characteristic fragrance of true mume, previous researchers crossbred a strong-tolerance variety apricot mei, P. mume ‘DF’ (‘Dan Fenghou’) and the weak-tolerance variety of true mume, P. mume ‘BY’ (‘Beijing Yudie’). They gained an offspring variety named P. mume ‘XR’ (‘Xiang Ruibai’), but its cold tolerance is unknown at this point. Here, three varieties (XR, BY, and DF) were selected as the materials, and different low-temperature treatments were used, with temperature as the only variable. Conventional biological methods, such as ion leakage rate, different tissues, and plant viability statistics, were used, as well as an innovative use of infrared engineering and moisture monitoring for dynamic observation of the water-to-ice process in tissues. The results were as follows: DF cold tolerance was the highest, followed by XR and then BY. The LT50 of XR was increased by 6 °C after five days of cold priming at 4 °C, which indicated a stronger cold acclimation ability than the parent varieties. The XR variety enhanced the antioxidant capacity by increasing SOD and POD enzyme activities during low temperature treatment, thus enhancing the cold tolerance. The antioxidant enzyme genes PmSOD3, PmPOD2, PmPOD19, and PmPOD22 had important regulatory roles in XR’s cold acclimation process.