Temperature increasing effect of cold-prevention treatments in micro-environment of Tunisian soft seed pomegranate trees in South Xinjiang

【Objective】The demand for soft seed pomegranate in market is rising year by year. The Tunisian soft seed pomegranate has the widest commercial area for cultivation. However, due to the special geographical climate of Xinjiang and the freezing damage temperature threshold of Tunisian soft seed pomegranates, traditional soil covering methods cannot ensure safe overwintering, resulting in plant damage and low survival rate. Therefore, it is important to explore cold- prevention treatment methods that can maximize the safe overwintering of Tunisian soft seed pomegranate seedlings, providing technical support for the application of cold- prevention measures in winter for Tunisian soft seed pomegranate in the open field in Xinjiang production areas.【Methods】The test pomegranate orchard was located in the 12th Company of the 51st Regiment of the Third Division of Xinjiang Production and Construction Corps, with an altitude of 1 046.20 meters. It belongs to a warm temperate continental arid climate with an average temperature of -6.6 ℃ to -7.3 ℃ in the coldest month (January). The test plants were 2-year-old Tunisian soft seed pomegranate cuttings, planted in a creeping manner at a 40° inclination in a south-north direction with a planting distance of 2.0 m×5.0 m. The soil was sandy with a deep layer and moderate fertility. During the winter from 2021 to 2022, the open field pomegranateplants without cover were used as the control (control), seven cold-prevention treatments were designed [T0: soil cover at one of three thicknesses (15 cm, 20 cm, and 30 cm); T1: straw curtain + soil cover; T2: double- bar holder + straw curtain + soil cover; T3: EVA plastic film cover + straw curtain + soil cover; T4: covering with colored fabric cloth strips+ soil cover; T5: single sheet cover + soil cover; T6: double sheet cover + soil cover]. The experiment used IoT dynamic monitoring technology to monitor in real-time the temperature changes in the orchard and in the micro-environment under each treatment. The data were collected every 5 minutes from November 7, 2021, to March 15, 2022. The daily minimum temperature, the longest duration of ≤-10 ℃, thermal insulation performance, and negative accumulated temperature in the micro-environment under each treatment with the three soil cover thicknesses were analyzed to explore the relationship between the temperature collected from local weather station and near-surface temperature of the pomegranate orchard and the temperature changes of each treatment micro-environment under the treatments.【Results】There were significant differences in the daily maximum and minimum values between the near surface area in the orchard and local weather report (p＜0.05), which displayed an upward trend and a downward trend respectively, and there was an obvious lag in the appearance of the peak temperature value compared to the local weather station. There were significant differences in the temperature variations among treatments with the three soil thicknesses (p＜0.05). With the freezing threshold as the reference, it was found that compared with the control group (control), the 15 cm soil cover thickness in T1 (grass cover + soil cover) had the best effect in raising the micro-environment temperature. The daily minimum temperature was increased by 14.4 ℃, with no ≤- 10 ℃ low temperature, and the negative accumulated temperature was increased by 769.6 ℃; the thermal insulation performance was good. However, temperature elevation effects in T0 and T6 were poor and the longest duration of ≤-10 ℃ temperature (11 hours). Under the 20 cm soil cover thickness, T3 (EVA plastic film cover + grass cover + soil cover) had the best effect in raising the micro-environment temperature, with the daily minimum temperature increased by 15.4 ℃, no ≤-10 ℃ temperature, and the negative accumulated temperature increased by 780 ℃, indicating that the thermal insulation performance was good. The temperature elevation effects of T4 and T5 were poor, and the longest duration of ≤-10 ℃ temperature could reach 8 hours. Under the 30 cm soil cover thickness, T2 (steel frame + grass cover + soil cover) had the best effect in raising the micro-environment temperature, with the daily minimum temperature increased by 17 ℃ , good thermal insulation performance, and the negative accumulated temperature increased by 880.4 ℃. The temperature elevation effects of the other treatments were not significant.【Conclusion】Compared with control, all the treatments had a positive effect in raising the micro- environment temperature to different degrees under the three soil cover thicknesses. Comprehensively T2 (double bar holder + straw curtain + 30 cm soil cover) had the best cold prevention effect in the orchard of Tunisian soft seed pomegranate.