Disaster mechanism and disaster grade index of frost damage of Nanfeng mandarin orange with golden autumn sugar orange
Jin Guohua and so on
Funds: Jiangxi Meteorological Science and Technology Project (JX2021M02)
Citrus fruit trees have poor tolerance to low temperatures, and the lowest temperature can reach -12~-7 °C, or even lower in abnormally low temperature years, and it is not uncommon for citrus to suffer from frost damage. The area and degree of frost damage are the first meteorological disasters, and they are the main meteorological factors limiting the development of citrus. In addition to the low temperature intensity, low temperature duration and freezing, the meteorological factors are also related to the speed of temperature change, the occurrence time of low temperature, the diurnal temperature range and wind speed. Chen Shangmo et al. identified the daily minimum temperature (Td) as the causative factor of citrus during the wintering period; Huang Shoubo's study found that the frost damage index of mandarin oranges in Wenzhou was related to the duration of extreme minimum temperature and low temperature; Wang Jinghong et al. divided the degree of winter frost damage of citrus in southern Shaanxi into three grades: severe, moderate and mild based on the extreme minimum temperature value or harmful cold accumulation in the process; Zhang Yangcai et al. applied negative accumulated temperature (daily minimum temperature<0 °C), ≤ 90 °C and extreme minimum temperature ≤-8 °C were used as indicators of severe freezing damage in citrus. Generally, the temperature of citrus frost damage is divided into four grades: -5, -7, -9 and -11, which represent the beginning of frost damage, light frost damage, frost damage and severe frost damage, respectively. Under the same low temperature conditions, due to the influence of variety characteristics, rootstocks, tree age, topography, cultivation technology and site conditions, the frost damage indexes often lead to large differences. Different citrus species and varieties have different critical low temperatures, and among the wide-skinned oranges, Wenzhou mandarins have stronger cold tolerance than Nanfeng mandarins, and the cold-hardiness of broad-skinned oranges is about 2 °C stronger than that of sweet oranges. The strength of cold tolerance is closely related to the rootstock, and the general rule is that the rootstock with strong cold tolerance will enhance the cold tolerance of the scion variety. Frost damage mainly affects the yield of adult fruiting trees in the current year or the second year, while high inoculation can lead to the death of the whole tree. There are few studies on the frost damage index of citrus high grafting, citrus planting has a strong regionality, farmers consider more economic benefits in the process of high grafting, lack of correct understanding of the frost damage index of the introduced high grafting varieties, and various uncontrollable changes in natural climatic conditions will increase the uncertainty of production and operation after high grafting. From 2020 to 2021, citrus in Jiangxi Province suffered frost damage, especially high-grafted varieties. In early to mid-January 2021, Fuzhou experienced three rounds of low-temperature frost damage, with the extreme minimum temperature of -11 °C in Nanfeng County, and more than 70% of the orange trees in the county suffered varying degrees of frost damage after frost damage, and the damage to high-grafting orchards was even more serious, with some of the whole orchard freezing to death, and the planting area of tangerines dropped sharply from 46,700 hm 2 to 26,700 hm 2. Therefore, it is of great significance to find out the mechanism of frost damage and the disaster level index of citrus with high replacement varieties for rational introduction, disaster early warning and promotion of citrus yield.
1 Materials and methods
1.1 Test Materials
The experiment was carried out in the orchard of Shangdian Village, Qiawan Town, Nanfeng County, Jiangxi Province (116°57′E, 27°27′N). The test materials were grafted in 2019, with Citrus aurantium as the base anvil and 20-year-old Nanfeng mandarin in the middle anvil. The orchard is flat and under regular management, and the growth and development of fruit trees is normal.
1.2 Experimental design and test area environment
During the overwintering period of sugar orange in golden autumn, 3 plants with relatively consistent fruit tree size and growth were selected, and 3-year-old branches with moderate growth and normal leaves were randomly selected on the periphery of the tree waist for listing, and the branches with consistent growth were marked with red rope.
The low temperature test was conducted from January 16 to 20, 2023, and a total of 5 temperatures were designed for the golden autumn sugar orange, which were -5, -6, -7, -8, and -9 °C. Put the tagged branches into the portable artificial thermostatic control box (RR-CTC102C type, Beijing Yugen Technology Co., Ltd.), the base of the branch is connected with the tangerine tree through the small hole on one side of the temperature control box, and the temperature control box is fixed with a height-adjustable tripod for low temperature treatment, and gradually reach the set target temperature. The temperature chamber is available in sizes 42 cm (length), × 29 cm (width) × 35 cm (height) with a temperature accuracy of ±0.2 °C. The low-temperature treatment period for autumn sugar oranges is from 22:00 to 7:00 the next day, and there is no light during this period. At the end of the experiment, remove the temperature control box and allow the shoots to grow naturally. The listed shoots of the same experimental tree that were not treated with low temperature (naturally growing) were used as the control (CK). Each trial is treated with 2 replicates.
During the experiment, the temperature data of the outside world were obtained from the Qiawan Town Regional Observatory near the orchard. Figure 1 shows the changes in extreme minimum temperature at the regional station of Qiawan Town and the national reference station of Nanfeng County. As can be seen from Figure 1, the extreme minimum temperature reached -4.0 °C at the Qiawan Town Regional Station and -3.6 °C at the Nanfeng County National Reference Station on December 18, 2022, and -5.5 °C at the Qiawan Town Regional Station and Nanfeng County National Base Station on January 29, 2023. In addition, during the wintering period from December 16, 2022 to January 31, 2023, the extreme minimum temperature in Qiawan Town was -5.5 °C (1 d), the number of days below -3 °C was 5 days, and the number of days below 0 °C was 12 days.
Figure 1 Extreme minimum temperature in Heqiawan Town, Nanfeng County from December 16, 2022 to January 31, 2023
1.3 Analysis of air temperature elements at weather stations
The experiment was carried out during the overwintering period of golden autumn sugar orange, and the outside temperature was generally low. The minimum temperature elements of the regional station in Qiawan Town near the experimental orchard were basically the same as those of the national reference station in Nanfeng County. The daily minimum temperature of the national reference station and the township regional station in Nanfeng County is shown in Table 1. According to the extreme minimum temperature data of the regional stations, the extreme minimum temperature recorded by the regional station of Zhonghe Township was -8.0 °C, and that of Taiyuan Township was -4.2 °C, with a difference of 3.8 °C between the two regional stations and 2.5 °C and 1.3 °C respectively from the national reference station in Nanfeng County. The difference between the extreme minimum temperature of the regional station in Zhonghe Township and the national reference station in Nanfeng County was more than 1 °C. In terms of the number of days with minimum temperature lower than 0 °C, there was a large difference between the stations in different regions, among which Zhonghe Township reached 19 days, which was 6 days longer than the national reference station in Nanfeng County, and in terms of the number of days with the minimum temperature lower than -5 °C, Qiawan Town, Sangtian Town, Baishe Town, Taihe Town, Taiyuan Township, Laixi Township, Fufang Township and Zixiao Town were the same or 1 day different from the national reference station in Nanfeng County, and the other stations were 4 days or 7 days apart.
Table 1 Meteorological elements of frost damage in Nanfeng County from December 16, 2022 to January 31, 2023
1.4 Observation of morphological characteristics
During the experiment and every 15 days after the end of the experiment, the morphological characteristics of the leaves and shoots of the control and treatment were observed and recorded.
1.5 Frost damage level indicators and verification
According to the morphological characteristics of sugar orange in golden autumn, the disaster level index of low temperature freezing damage was analyzed. Combined with the investigation of the frost damage of sugar oranges in January 2021 and January 2023 in Nanfeng County, the frost damage level indicators were compared and verified.
2 Results and Analysis
2.1 Morphological characteristics of frozen oranges
Different low temperature conditions had different effects on the morphological characteristics of golden autumn sugar orange. The golden autumn sugar orange was treated with low temperature of -5, -5 (2 d), -6, -7, -8 and -9 °C, of which -5 °C was treated for 1 d and 2 d, respectively, and other temperatures were treated for 1 d. After 1 day of -5 °C treatment, there was basically no freezing, and the rest of the leaves of low temperature treatment had different degrees of frostbite, curling, yellowing, and leaf loss, and with the decrease of temperature, the branches lost water, dried up and died, and when the temperature dropped to -9 °C, the whole branches dried up and died. The trial began on January 16, 2023, with treatment in a temperature-controlled chamber at night.
The frost damage grade of citrus was preliminarily determined based on the classification of single-station frost damage of wide-skinned citrus in QX/T 197-2013, combined with the field disaster investigation of golden autumn sugar orange in 2022 and 2023 (Table 2 and Table 3). According to the test results, the unmature apical leaves were the first to fall off when they were subjected to freezing and wilting due to low cell fluid concentration and poor cold tolerance. Late-growing, immature shoots are the first to suffer from frost damage to brown, dry up and die due to incomplete tissues. The morphological manifestations of golden autumn sugar orange treated at low temperature showed a lag, especially the symptoms of frost damage to the branches.
Table 2 Classification of frost damage of sugar oranges in golden autumn
Table 3 Classification of frost damage degree of sugar orange in golden autumn
2.2 Frost damage level index of golden autumn sugar orange
The level of frost damage of sugar orange in golden autumn was as follows: -6 °C< daily minimum temperature (Td) ≤-5 °C or ≤-5 °C (2 d) was mild frost damage, -7 °C < daily minimum temperature (Td) ≤-6 °C was moderate frost damage, -8 °C < daily minimum temperature (Td) ≤-7 °C was severe frost damage, -9 °C ≤ daily minimum temperature (Td) ≤-8 °C was severe frost damage. Limited to the test conditions, the test of heavy frost damage needs to be studied in the next step.
According to Table 4, the extreme minimum temperature of the national reference station in Nanfeng County in January 2016 was -6.2 °C, which reached the critical temperature of frost damage of mandarins, and due to the impact of continuous rain and frost damage in January 2016 during the harvest period in 2015, the annual branches and leaves of mandarins were frozen, and the defoliation was serious, and the yield and efficiency of mandarins decreased significantly. From 2017 to 2020, the extreme minimum temperature of the national reference station in Nanfeng County was higher than -5 °C, and there was no obvious adverse effect of extreme weather on golden autumn sugar oranges in winter. In January 2021, the extreme low temperature of the national reference station in Nanfeng County was -8.0 °C, and the extreme minimum temperature of the Wangtian Regional Station in Baishe Town was -11.0 °C, and due to the low rainfall in the winter of 2020, the soil was excessively dry, which aggravated the frost damage of tangerine, resulting in the freezing of 1/3 of the tangerines in Nanfeng County, the severe frost damage of 1~2-year-old young trees, and the mild-moderate frost damage of adult trees. Due to the frost damage in 2021 and the drought in the summer and autumn of 2022, the recovery of tangerine trees was slow and the growth was weak. The investigation found that on December 18, 2022, the minimum temperature of the national reference station in Nanfeng County was -3.6 °C, and the minimum temperature at the regional station of Qiawan Town was -4.0 °C. In addition, during the investigation, it was found that all the golden autumn sugar orange saplings in Qiawan Town (-5.5 °C) and Sangtian Town (-6.0 °C) without anti-freezing measures were frozen to death. Seedlings covered or bandaged with straw are hardly exposed to frost.
Table 4 Daily extreme minimum temperature from December 2015 to early February of the following year
3 Discussion
3.1 Low temperature affects the frost tolerance of autumn sugar oranges
Citrus frost damage is affected by a variety of factors, not only determined by the intensity of low temperature and the duration of low temperature, the weather conditions before and after the occurrence of low temperature, the temperature before the occurrence of low temperature, the citrus has not yet entered the eye rest period; the earlier the low temperature appears, the citrus is in a semi-dormant state, the later the low temperature appears, the citrus is in a deeper dormant state; it is also determined whether the plant grows robustly, whether it has suffered from pests and diseases, the combination of rootstock varieties and rootstocks, maturity (such as tree age, branch tips, leaf maturity), the fruit of the previous year, Whether the fruit is harvested at the right time or not affects the occurrence and severity of frost damage. The meteorological data of Nanfeng County are used in this meteorological data, and the extreme temperature data are similar to those of the regional station in Qiawan Town, which avoids the difference between the data of the national reference station and the regional station, and at the same time tries to avoid the influence of tree potential, environment and other factors on the test results.
3.2 Frost damage level index of golden autumn sugar orange
The frost damage level of golden autumn sugar orange was as follows: -6 °C< daily minimum temperature (Td) ≤-5 °C or ≤-5 °C (2 d) was mild frost damage, -7 °C < daily minimum temperature (Td) ≤-6 °C was moderate frost damage, -8 °C < daily minimum temperature (Td) ≤-7 °C was severe frost damage, -9 °C ≤ daily minimum temperature (Td) ≤-8 °C was severe frost damage. In addition, it was also observed that the morphological characteristics of the saplings (annuals) in the orchard showed the severity of frost damage when they encountered a low temperature of ≤-5.5 °C.
The 1~3-year-old shoots of golden autumn sugar orange were used in the experiment, and different low temperature treatments were carried out, and the morphological expression of golden autumn sugar orange after low temperature treatment had a certain lag. In the future, orchard experiments with different low temperature intensities and durations should be carried out, and relevant frost damage experiments should be carried out in combination with different tree ages and different anti-freezing measures, so as to provide a scientific basis for the study of citrus frost injury mechanism, index selection and disaster prevention and mitigation.
4 Conclusion
In the old orange orchard, Nanfeng mandarin orange was grafted highly, which caused freezing damage at a critical extreme low temperature of -5 °C, -9~-5 °C would cause different degrees of frost damage, and -9 °C would cause all the three-year-old branches of golden autumn sugar orange to freeze to death. However, the whole sapling of autumn sugar orange trees will freeze to death at low temperatures below -5.5 °C. In early to mid-December (in the shallow dormant period), the low temperature of -4.0 °C caused frostbite to some leaves at the top of the shoot tips, and in early January (in the dormant period), there was no obvious frost damage when the low temperature duration was extended or the temperature continued to decrease, and when the temperature dropped to -9 °C, the frost damage degree was gradually obvious, and when the temperature dropped to -9 °C, the shoot died. The degree of frost damage can be judged according to the frost damage grade index, and the research results can provide a scientific reference for citrus disaster zoning, early warning, assessment, disaster prevention and mitigation.
Combined with citrus varieties and low-temperature freezing damage indicators, there are differences between the extreme minimum temperature of the national meteorological reference station and the regional meteorological station, and the low-lying land of the same orchard is relatively seriously frost damaged, so it is of great significance to release the early warning information of low-temperature frost damage for citrus to reduce or avoid the disaster caused by low temperature.