Comprehensive evaluation of physiological and biochemical indices of P. mume during cold hardening. (A) The genetic relationship of five P. mume cultivars. (B) Changes in soluble sugar content, soluble protein content, free proline content, POD and SOD activity of P. mume following cold hardening treatment at 4 °C for 0, 1, 3, 5, 7, 9, and 11 d. R² represents the degree of fitting of the trend line. (C) The relative electrolyte leakage of P. mume following freezing treatment. NF, freezing; CF, freezing after cold hardening. *P<0.05 (Student’s t-test).