Changes in photosynthetic physiological characteristics of three citrus species during occurrence of strong cold air and comprehensive evaluation of cold resistance

Abstract:【Objective】The study investigated the physiological effects of strong cold air flow on three major local citrus species in Jiangxi (Citrus reticulata‘Nanfengmiju’, C. grandis‘Jinshayou’, C. sinen-sis‘Newhall Navel Orange’), and evaluated the cold resistance among the species in order to provide reference for the introduction, production and cultivation management of citrus.【Methods】This experi-ment was carried out in Jiangxi Academy of Agricultural Sciences from December 23, 2021 to Decem-ber 30, 2021, with 5-year-old citrus trees used as the experimental materials. The effects of strong cold air flow on the chlorophyll content, photosynthetic fluorescence characteristics, reactive oxygen species and antioxidant enzyme activities in leaves of the three citrus species were studied, and principal compo-nent analysis was performed on the above physiological parameters, and the cold resistance of three cit-rus species was finally determined.【Results】The daily minimum temperature on December 24 and De-cember 26 was 7.2 ℃ and -1.5 ℃, respectively. The difference between the daily minimum temperature within 48 hours was greater than 8 ℃ , and the daily minimum temperature dropped below 8 ℃. There-fore, according to the national standard of Cold Air Grade (GB/T 20484—2017), it was classified as a strong cold air flow. As a result of this strong cold air flow, the daily maximum temperature and daily minimum temperature had the same trend. They declined continuously from December 24 to 26, reach-ing a minimum value on December 26, after which temperature gradually recovered. The content of chlo-rophyll in the leaves of the three citrus species changed greatly with a similar change pattern. From De-cember 24 to 26, the contents of Chla and Chlb gradually decreased, but slowly increased after Decem-ber 26. At the end of the experiment on December 30, Chla content in the three citrus species was slight-ly lower than that at the beginning, but Chlb decreased significantly. The change in carotenoid content varied among citrus species. With the decrease in air temperature, the maximum photosynthetic rate (Pmax), apparent quantum efficiency (AQY), PSⅡ primary light energy conversion efficiency (Fv/Fm) and light saturation point (LSP) showed a downward trend. During the temperature recovery period, the val- ues of chlorophyll content, Pmax, AQY, Fv/Fm and LSP increased. The change trends of light compensation point (LCP) and non- photochemical quenching (NPQ) were similar. With the temperature drop, they first increased and then decreased, and reached a highest value on December 26. The change trends of MDA and H2O2 contents in the three citrus species were basically the same, showing a trend of first ris-ing and then falling. Their values were the highest on December 26, and then showed a downward trend.However, at the end of the experiment, MDA and H2O2 contents maintained higher than those at the ini-tial day. During the whole process of strong cold air flow, the contents of MDA and H2O2 in C. sinensis‘Newhall Navel Orange’were the highest, followed by those in C. grandis‘Jinshayou’, and those in C. reticulata‘Nanfengmiju’were the lowest. The trend of POD content in the three citrus varieties was ba-sically the same, and all showed a trend of first rising and then decreasing. The POD values of the threecitrus cultivars were highest on December 26, increasing by 34.29% (Nanfengmiju), 30.77% (Jinshay-ou) and 39.02% (Newhall Navel Orange) compared to the values on December 24, respectively, and then showed a downward trend, but the POD value on the end day of the experiment was still higher than the initial day. The change trend of SOD activity was consistent with that of POD. During the temperature drop, the POD and SOD activities in Newhall navel orange were the highest, followed by those in Jinsha-you, and Nanfengmiju had the lowest enzyme activity.【Conclusion】Through the principal component analysis of chlorophyll content, photosynthetic fluorescence characteristics, reactive oxygen species and antioxidant enzyme activities in leaves of the three citrus species, the order of cold resistance among the three citrus species from strong to weak was Nanfengmiju＞Jinshayou＞Newhall Navel Orange.