Analysis of evaluation indexes of cold resistance of apple trees at flowering stage

Abstract:【Objective】Field survey are usually used for cold resistance of different fruit trees, but this method has difficulty to exclude the interference of external factors such as different ecological conditions and cultivation mode, and can not really objectively evaluate the cold resistance of a certain variety. By comparing the correlation between supercooling point and freezing point temperature of apple flower ovary and freezing rate under specific conditions, combining with the investigation of flower growth and development indexes of different varieties, a multiple linear regression model was established to determine the objective evaluation indexes of cold resistance of apple trees at flowering stage. The reasons for differences in cold resistance at flowering stage of some apple varieties were discussed. 【Methods】The Malus species Malus robusta and Darwin, apple varieties Nagafu 2, Rockfu 10, Danxia, Gala, Gold Delicious, OPAL were used as test materials. The supercooling and freezing temperatures of ovaries were measured using a simulated frost box system. The differences among varieties in the above  indexes were analyzed. A two-factors experiment based on cooling gradient and cooling time was used to determine the ovary freezing rate of different varieties. The linear fitting was carried out to analyze the relationship between the flower supercooling point, freezing point temperature and flower ovary freezing rate. The flowering duration, number of flowers per unit length, and the percentage of fully opened flowers at full bloom period were also investigated and analyzed to further clarify the influence of flower growth and development characteristics on freezing rate. The factors influencing the difference of cold resistance of apple trees during flowering stage were comprehensively discussed.【Results】There was no significant difference in the temperature of supercooling point and freezing point of central flowers and side flowers of the same variety. The supercooling point of M. robusta was the lowest (-4.27±0.33 ℃), It was significantly different from those of the other varieties (p＜0.05). Among the apple varieties, Danxia and Nagafu 2 had the lowest supercooling point temperature, which was significantly different from those of Rockfu 10 and OPAL. The freezing point temperature of OPAL was lower than those of most cultivars (p＜0.05). The freezing rate of M. robusta and apple variety Danxia was lower, followed by Darwin, Nagafu 2 and Gala. Rockfu 10 had the highest freezing rate under the different temperature gradients and cooling duration. With the extension of cooling time, the ovary freezing rate of flowers increased under the same temperature gradient, even under mild freezing stress (-1 ℃). Regardless of mild or severe freezing stress, the freezing rate of apple flowers was affected by two factors. The first factor was the degree of cooling, and the other factor was the duration of cooling. After 1h treatment at -2, -3 and -4 ℃, there was a significant positive correlation between the freezing rate and the supercooling point of all varieties, and the linear correlation between them was the strongest at -3 ℃ (R2 = 0.93). but there was no significant correlation between the freezing point and ovary freezing rate. The flowering time of M. robusta was the longest, followed by Darwin and Golden Delicious, and the other varieties lasted about 16 days. The flower quantity of branches per unit length of M. robusta was the largest, indicating that it had the flower quantity advantage, followed by Darwin and Golden Delicious, and Nagafu 2 had the least flower quantity. The ratios of fully opened flowers of M. robusta and Darwin were 82.33% and 80.67%, respectively, while the ratios of fully opened flowers of Golden Delicious and OPAL were less than 60% . The multiple linear regression model showed that there were multiple linear correlations (R2 =0.89) between supercooling point (X1), flowering duration (X2), flowering quantity (X3), full flowering rate in full blooming period (X4) and freezing rate of ovaries (Y). According to the standardized coefficients and significance p values of different independent variables, it could be found that the supercooling point temperature of the flowers was the main factor causing the difference of the ovary freezing rate, followed by the flower quantity and the other flower growth and development indicators.【Conclusion】The supercooling point temperature was an important index to measure the cold resistance ability of apple flowers. The normalization coefficient of the supercooling point temperature was 0.87, which had the greatest influence on the freezing rate of ovary. It would be necessary to comprehensively judge the cold resistance ability of different apple varieties at flowering stage by combining the flower quantity, flowering duration, full blossom ratio and other flower growth and development indexes.