Comprehensive evaluation of cold tolerance in litchi germplasm resources

Abstract:【Objective】In recent years, extreme low-temperature occurred frequently in winter, affecting the growth and yield of litchi in the following year. Therefore, low temperature is one of the main factors limiting the yield of litchi. Screening chilling-resistant litchi germplasm and breeding low-temperature-tolerant varieties is the most effective way to solve this problem. Litchi germplasm resources are important for litchi breeding, scientific research and production. The research on chilling- resistant germplasm resources of litchi started late, and there has been no report about comprehensive cold-resistant evaluation on existing litchi germplasm resources. Litchi is a subtropical evergreen fruit tree and has some cold-resistant resources. Most of the cold-resistant resources may remain undiscovered due to few related studies. In this study, 91 litchi varieties were comprehensively evaluated for cold resistance. The aim was to screen out litchi germplasm with relatively strong cold resistance, which is important for low-temperature breeding and introduction of litchi in various regions.【Methods】In this study, we selected and numbered litchi varieties with uniform growth status and monitored leaves. At the same time, shoot tips were labeled with hanging tags in the four sides of the canopy. Varietal information was counted on 91 litchi varieties, and five physiological and biochemical indexes were measured before and after low temperature treatments i.e. the maximum photochemical quantum yield of PSII (Fv/Fm), relative water content (RWC), relative electrical conductivity (REC), malondialdehyde (MDA) and pro-line (proline, Pro). A comprehensive evaluation of these five physiological and biochemical indicators were performed through affiliation function analysis and principal component analysis, and comprehensive evaluation value (D-value) related to cold tolerance was obtained. Then, the cold resistance of litchi germplasm resources was classified through gray-scale correlation calculation and cluster analysis, and finally the cold resistance classification registration was obtained.【Results】Fv/Fm and RWC showed a decreasing trend under low temperatures. In contrast, the REC, MDA and Pro showed an increasing trend, suggesting that litchi trees were affected by the chilling stress in the cold winter. Xuehuaizi litchi trees had the lowest Fv/Fm and the highest REC after low temperature, and were more susceptible to damage caused by low temperature. Among the tested litchi varieties, the average value of Fv/Fm was 0.12, and the coefficient of variation was 19.01 after low temperatures. The average value of REC was 14.77% , and coefficient of variation was 40.85 after low temperatures. The average value of RWC remained around 55% before low temperature, and decreased by 13% after low temperature, while the average value of MDA content was 35.02 μmol·mg-1 before low temperature, and increased to 46.39 μmol·mg-1 after low temperature. The average value of Pro content was 404.40 μg · g-1 after low temperature, and increased by nearly 10 times compared to that before low temperature. Hence, low temperature significantly affected Fv/Fm, RWC, REC, MDA and Pro, indicating that these five indexes are desirable physiological indexes for evaluating the cold resistance of litchi varieties. Through systematic clustering analysis, a total of 91 litchi varieties were evaluated and graded for cold resistance. The cold resistance levels of litchi germplasm were categorized into 5 levels, including highly cold sensitive (Ⅰ), cold sensitive (Ⅱ), moderately cold tolerant (Ⅲ), cold tolerant (Ⅳ), and highly cold tolerant (Ⅴ) levels. The highly cold-sensitive varieties accounted for 7.69% of the total varieties, including Jinxianguo, Xiafanzhi, Lanzhu, Da zao, Shangshuhuai, Xijiaozi, and Wuyejiu. The second class was cold-sensitive varieties, which accounted for 10.99% of the total, including Guanyinlv, Shuidong, Xuehuaizi, Sanyuehong, Salathiel, Kwai May Pink, Heifeihong, Lingfengnuo, Niannianhong No. 1 and Shiyueli. The third class was moderately cold-tolerant ones, which accounted for 25.27% of the total. The fourth class was cold-tolerant germplasm, which accounted for 43.96% of the total. The fifth class was highly cold- tolerant germplasm, which accounted for 12.09% of the total, including Ziniangxi, Guilin, Jinzhong, Jide, Dongliu No. 1, Zhuangyuanhong, Dahongpao, Wuye, Songjiaxiang, Chenziand Dajinzhong. Among the varieties tested, most fell into the category of cold tolerance (Ⅳ), with relatively few in the categories of high cold sensitivity (Ⅰ) and high cold tolerance (Ⅴ), indicating that the cold resistance in litchi germplasm is highly concentrated.【Conclusion】The comprehensive evaluation of litchi varieties was carried out and it was concluded that Ziniangxi, Guilin, Jinzhong, Jidi, Dongliu No. 1, Zhuangyuanhong, Dahongpao, Wuye, Songjiaxiang, Chenzi and Dajinzhong belong to the extreme cold tolerant germplasm and have outstanding cold resistance. These varieties can be used as germplasm resources for subsequent low temperature breeding.