Study On Cold Resistance Of New And Excellent Citrus Cultivars In Guangxi

China is one of the important citrus origins, in which citrus has been planting for more than 4000 years with abundant citrus resources and admirable cultivars. Guangxi is one of the most suitable areas for citrus production in China because it locates in the central of Asian tropical and subtropical monsoon climate zone, and has enough light, temperature and rainfall. In recent years, the global climate has been variable, low temperature and freezing weather has occurred uncertainly, which have seriously impacted the development of citrus production worldwide. Though Guangxi is located in Subtropical, production and quality have been affected to different extents by chilling injury. It will be of importance for the stable and healthy development of Guangxi citrus industry that comparing the cold resistance of new and excellent citrus cultivars in Guangxi and then choosing and breeding those with good cold hardiness, combining with the rational distribution.This study compared the cold resistance of 21 citrus cultivars in Guangxi by the electrical conductivity method. We also preliminarily studied the effects of low temperature stress on physiological and biochemical indicators of citrus. The main results are as follows:1. Optimization of determination conditions by electrical conductivity method. The test conditions of electrical conductivity method which was used to determine the semi-lethal temperature (LT50) of citrus were studied by using spring and autumn leaves of Shangtangju, autumn leaves of Gonggan and Wanlingqicheng as raw material, high-low temperature test chamber as treating equipment. The results showed that the leaves'cells were completely destroyed after boiling 10min; Extracting and setting for more than 18h at the constant temperature of 20℃, the cells electrolyte leaked fully and the relative electric conductivity kept stable. The operation of punching after pretreating the citrus leaves at low temperature could reduce the effects of mechanical damage on relative electric conductivity and hence keep the dates'stability. The semi-lethal temperature increased with the extension of treating time at low temperature, which tended to stability between 6 to 10h.2. Comparison of cold resistance for citrus cultivars. In order to compare with the cold resistance of different citrus cultivars, the changes of the leakage ratio of electrolyte which of autumn leaves collected from 21 citrus cultivars of Guangxi citrus research institute at different low temperature were studied by electrical conductivity method. The inflection point temperature named semi-lethal temperature was obtained on the basis of relative electric conductivity and Logistic equation. The results showed that the relative electric conductivity increased like "S" curve concomitant with a decrease of low temperature. The cold resistance of different cultivars were different, and the specific order on the cold resistance are as follows:Mandarin cultivars: Zaowen 8-1〉Shanxiahong〉Shanchuan〉Gongchuan〉Gongben〉Nangan No.20〉Xingjin〉Rinan No.1, Tangerine cultivars:Penggan No.79-2〉Penggan No.260〉Shangtangju〉Mingliutianju〉Nangfengmiju, Orange cultivars:Guicheng No.1〉Hongrouqicheng〉Ziyuanniuheer〉Qingyuanniuheer〉Bingtangcheng, Hybrid cultivars: Shaohemokete〉Maogugan〉Gonggan. The same citrus cultivar in different stocks has different cold resistance, Shangtangju—Zhike> Shangtangju—Suanju, Mingliutianju—Zhike〉Mingliutianju—Suanju.3. Effects of low temperature stress on relevant physiological and biochemical indices of citrus leaves. We preliminarily studied on some index of Shangtangju's spring and autumn leaves in different treatment time at -8℃, which included the changes of relative electrical conductivity, the contents of soluble protein, reducing sugar, soluble total sugar, pro and MDA. The results showed that the relative electrical conductivity firstly increased and then tended to stability with treatment time increasing. Under the same treatment time, the relative electrical conductivity of spring leaves was lower than that of autumn leaves of which cold resistance was weaker. With treatment time increasing, the contents of soluble protein, reducing sugar and soluble total sugar increased firstly and then decreased. During the whole stress process, the contents of three substances above in spring leaves were higher than that of autumn leaves. When being treated 2h, the content of pro in spring leaves increased to maximum and then decreased, and it tended to stability 4h later, but the content of pro in autumn leaves did not significantly change with the extension of treatment time. In the whole stress process, the contents of MDA changed irregularly, however, were significantly lower than that treated at room temperature on the whole.