A study on the cold resistance and the semi- lethal temperatures for branches of five apple rootstock cultivars

Abstract：【Objective】Heald invented the electrical conductance method in 1902 and Dertez used it in cold resistance identification in 1932. It has been recognized that this method is relatively reliable and has been widely used in the identification of cold resistance in fruit trees such as grapes, pears, citrus and apples. The damage caused by low temperature to fruit trees is one of the main natural disasters affecting fruit tree production in many parts of the world today. Cold resistance breeding is one of the goals of apple breeders. For the identification of cold resistance, many methods were used in the past, including electrical conductivity method, growth recovery method, tissue browning method, electrical impedance diagram method and Logistic equation method. Exploring the method for evaluating the cold resistance of apple rootstocks and in order to combine the LT50 and the freezing damage index to quickly and accurately determine the cold resistance of plants. It was suggested that 50% of electrolyte osmatic ratio could be used to determine the critical semi-lethal temperature for apple cultivars. In order to compare the cold resistance of branches of five apple rootstocks and select highly cold-resistant resources for breeding, and the electrical conductivity and freezing damage index were used to evaluate their cold resistance.【Methods】The experiments took branches of five apple rootstock cultivars including SH1,Y-1, B9,M9T337 andM26 as materials. Changesin cell membrane permeability at -15℃, -20 ℃, -25 ℃, -30 ℃, -35 ℃ and -40 ℃ were studied with one-year-old branches using electrical conduc-tance method and the semi-lethal temperatures (LT50) was obtained from the Logistic equations of electrical conductivity vs temperature. They were compared among the rootstock cultivars in relation to freezing injury index under different low temperatures.【Results】The results showed that the relationships between the relative electrolyte exudation rate and the temperature in the tested branches fitted well with the Logistic curve equation. The electrolyte leakage ratio of apple branches increased in an “S”curve pattern with the decrease in temperature. The semi-lethal temperature of the five apple cultivars ranged from -23.4 ℃ to -40.1 ℃. With the ability to recover from injury damage in some cells, the conductivity of the whole tissue always followed a“S”pattern in different temperatures. The fitting coefficient ranged from 0.899 to 0.973, which was higher than the correlation coefficient (r0.01=0.874, p < 0.01), indicating that the regression curve was highly reliable. The semilethal temperature of M9T337 was in -23.4 ℃; it was is -40.1 ℃ of B9, and around -30 ℃ in SH1, Y-1 and M26. According to lethal low temperature, it can be inferred that the order of cold resistance among the five varieties was B9> SH1>Y-1>M26>M9T337. The freezing damage index at each temperature was calculated. It was highest in M9T337, lowest in B9, and similar among SH1, Y-1 and M26. The semi-lethal low temperature of branches was positively correlated with the freezing index at each temperature. The correlation coefficients were in the range from 0.820 to 0.979, which were all greater than the correlation coefficient (p0.05=0.811). With the decrease of temperature, the freezing damage index showed an upward trend, and a sudden jump occurred in a low temperature range. This indicates that the level of the freeze damage index became higher when the branches were exposed to the critical point of lethal temperature. After low temperature stress, the browning degree and time sequence of each rootstock varieties were obviously different, indicating that the medulla, xylem and phloem of cell structure were different. The sensitivity and reaction time of the branches to bear freezing damage were related to the characteristics of the varieties. According to freezing damage index, it could be inferred that the order of cold resistance of the five apple rootstock was B9>SH1>Y- 1>M26>M9T337. With the decrease in temperature, the freezing damage index showed a rising trend with a sudden upsurge occurred at certain low level. The cold resistance of apple stock can be reflected objectively and accurately with semi-lethal low temperature combined with freezing damage index.【Conclusion】The cold resistance of apple rootstocks can be easily and quickly evaluated using the semi-lethal temperature and the freezing damage index methods. The cold resistance of the five apple rootstock cultivars was in the order of B9 > SH1 > Y-1 > M26 > M9T337.