Effects of different low night temperature conditions on the photosynthetic parameters and morphological characteristics in navel orange

Abstract:【Objective】Temperature is an important environmental factor affecting the growth and development of plants. Photosynthesis is the basis for plant growth, high yield and top quality. More than 90% of the dry weight of plants comes from the photosynthetic products of leaves, and photosynthesis is extremely sensitive to temperature. Low temperature condition is not conducive to the photosynthesis of citrus, but the responses of photosynthetic performance to different low temperatures, photosynthetic mechanism, and the criteria of freezing injury are unclear.【Methods】In this experiment, one-year-old branches (with 3 fruits) of over 8 years old Newhall navel oranges during the overwintering period were conducted in the temperature control boxes for 10 consecutive days. The low temperature experiment was carried out in two phases, with five temperature treatments, namely -3 ℃, -4 ℃, -5 ℃, -6 ℃ and -7 ℃. The first phase test (-5 ℃ and -7 ℃) was from December 25, 2020 to January 4, 2021, and the second phase test (-3 ℃, -4 ℃ and -6 ℃) was from January 5, 2021 to January 15, 2021. The fruit branches were placed in a portable temperature control box. The base of the fruit branches was connected to the navel orange tree through a small hole on one side of the temperature control box. After fixing the temperature control box with a height-adjustable tripod, low temperature treatment was performed in the box, and gradually reached the target temperature. The low temperature treatment period was from 23:00 at night to 7:00 in the next morning, and there was no light during this period. The temperature control box was removed, and the fruit branches grew naturally from 7:00 in the morning to 23:00 at night. According to this cycle, the experiment was processed continuously for 10 d. The effects of five low night temperatures (-3 ℃, -4 ℃, -5 ℃, -6 ℃, and -7 ℃) on photosynthetic parameters (photosynthetic rate, transpiration rate, stomatal conductance, and intercellular CO2 concentration) and chlorophyll content of navel orange were investigated, and the mechanism and standard for freezing injury were studied in combination with the observation of morphological characteristics in the fruit, leaf and branch of navel orange. The temperature under natural conditions of the same period was taken as the control. According to the meteorological and disaster data of typical years (1991, 1999, 2009 and 2021) and typical planting counties (Ningdu, Xingguo, Yudu, Anyuan, Huichang, Xinfeng and Xunwu), the index accuracy rate of navel orange was verified.【Results】Compared to the control, the net photosynthetic rate, transpiration rate and stomatal conductance of navel orange leaves decreased with the low temperature treatments, and the average decrease of these three parameters increased with the decrease of temperatures. The net photosynthetic rate decreased by 9.4% with -3 ℃ treatment, 21.9% with -4 ℃ treatment, 61.0% with -5 ℃ treatment, 61.1% with -6 ℃ treatment, and 99.8% with -7 ℃ treatment, respectively. In addition, the intercellular CO2 concentration increased, and the chlorophyll content of navel orange leaves decreased under low temperature conditions. The intercellular CO2 concentration increased by 8.2%, but the chlorophyll content decreased by 3.8% with -3 ℃ treatment, respectively. The fruits of navel orange showed water-soaked, and the leaves were curled, wilted, and even withered or dead after low temperature treatments. The grading index of freezing injury of navel orange is classified as follows: -3 ℃＜daily minimum temperature (Td) is seen no freezing injury; -4 ℃＜Td≤-3 ℃ (1-2 d) is seen no freezing injury; -4 ℃＜Td≤ -3 ℃ (≥3 d) is seen mild injury; -5 ℃＜Td≤-4 ℃ (1-4 d) is seen mild injury; -5 ℃＜Td≤-4 ℃ (≥5 d) is seen moderate injury; -7 ℃＜Td≤-5 ℃ (≥1 d) is seen moderate injury; Td≤-7 ℃ (≥1 d) is seen severe injury. According to the verification of typical years and typical planting counties, we found that the index accuracy rate of navel orange was 83.7%.【Con-clusion】The chlorophyll content, net photosynthesis rate, transpiration rate, and stomatal conductance decreased, and the intercellular CO₂ concentration of navel orange leaves increased under low tempera-ture conditions. Temperature condition of -3 ℃ for 3 d, or -4 ℃ for 1 d would inhibit the accumulation of photosynthetic products and the growth of fruits and leaves, and cause freezing injury to navel orange. According to the photosynthetic rate and morphological characteristics corresponding to the daily minimum temperature and the number of consecutive days, the grading index of freezing injury of navel orange was clarified, and the accuracy rate of index verification was 83.7%. Thus, the disaster degree of navel orange can be classified according to the grading index of freezing injury. The findings of the present study can provide a scientific reference for the indicator establishment for freezing injury, citrus disaster prevention and disaster mitigation. Key words: Navel orange; Low temperature; Photosynthetic rate; Chlorophyll content; Injury index