Effect of environment- friendly cold- proof fabric film covering on overwintering ability of Harumi tangor

Abstract:【Objective】Late- ripening citrus has had a significant impact on the citrus market in recent years due to the benefits of excellent quality, high profit, and enormous potential. The development of late-ripening citrus in the Sichuan Basin and surrounding areas has been promoted due to abundant annual rainfall, and few frosts and snowfalls. The area of late-ripening citrus increased to 153 300 hm2 in 2021. Harumi tangor [Citrus reticulate ×(C. reticulata× C. sinenesis)‘Harumi tangor’] has taken a significant role in late-ripening citrus industry. However, the late-ripening citrus industry is currently facing some problems, especially freezing damage. Low temperature would cause the citrus fruits granulation and dehydration, and decrease quality. Many measures for protecting late-ripening citrus trees and fruits from frost have been proposed, including covering film on tree canopy, bagging fruit, sod-culture, painting tree trunk with white coating, generating smoke in orchard, irrigation and so on. A new environmentally friendly non-woven cold-proof fabric film has been used for protecting citrus plant and fruits from low temperature in recent years to replace plastic film. This study aimed to investigate the mechanism of cold-proofing ability of the environmentally friendly cold-proof fabric materials on Harumi tangor, and its possibility to replace plastic film as a new type of cold protection.【Methods】The study was conducted in the orchard in Dongpo District, Meishan City, Sichuan Province from November 30, 2020, to March 28, 2021. 20 trees with relatively consistent growth and fruit number were selected for eachtreatment. There were three treatments: covering the canopy with non-woven fabric film only (FH), covering the canopy with non-woven fabric film + bagging the fruit (FHT), covering the canopy with nonwoven fabric film + bagging the fruit + covering the ground with reflective fabric (FHTF), and covering the canopy with plastic film + bagging the fruit (CK) was used as control. The temperature and moisture of the canopy, fruit drop rate, degree of freezing damage, and number of spring shoots and flowers were observed. The fruit quality was analyzed.【Results】In this study, the extremely low temperature of the field in the orchard reached -4.5 ℃, and duration of the low temperature below -2 ℃ excessed 6 hours. The rank of the extremely low temperature in the mid of the canopy of different treatments from low to high was FHverse. Compared with natural conditions, the fruit surface temperature of the each treatment increased by 0.7-1.0 ℃, and the duration of the low temperature was shortened. A canopy covered with coldproof fabric, as opposed to plastic film, could maintain a relatively stable temperature and humidity conditions. The temperature and humidity would change dramatically due to the excessive temperature rise, so the long-term covering of plastic film would not be conducive to the storage of the fruits on the trees of Harumi tangor. All the trees of the three treatments had no freezing damage, but the fruits had varying degrees of freezing damage. The freezing damage index of CK, FH, FHT, and FHTF was 38.33%, 50%, 36.67%, and 31.67%, respectively. The same trend also appeared in the fruit drop rate, and the result was consistent with the previous trend of the freezing damage index above. According to the data of freezing damage, FHTF had the best cold protection effect. All the treatments increased peel brightness and redness of the fruits, but the juice rate decreased by about 10% in the end of the study. The TSS of FHT increased by 1.13, whereas the TSS of CK decreased. In March, the TA of FH was the lowest and the solidity-acid ratio was the highest. The Fruit quality was ranked as follows by principal component analysis: FH＞CK＞FHT＞FHTF. The results revealed that the fruit quality of the cold- proof fabric covering was better than that of plastic film covering and that the fruit quality of cold-proof fabric covering was better than that of plastic film covering.【Conclusion】The canopy covering combined with fruit bagging could effectively protect the plants and fruits from the low-temperature damage. The effect of the new type of non-woven fabric film covering with fruit bagging was better than that of plastic film covering with fruit bagging. The results showed that the non-woven fabric film could be used to replace the plastic film to preserve the plants and fruits of late-maturing citrus.