Studies On Temperature Changes Of Tree Canopy In Winter And Fruit Freeze Protection Of Late-mature Citrus In Hilly Orchards In Changshou District Of Chongqing

Most of the citrus in China are mid-season varieties which are ripening from October to December. Their mature periods are so intensive that periodic unmarketable problem has become more and more seriously every year. Therefore, it’s very important to adjust the structure of mature stage for prolonging the fresh fruit supply period and improving citrus industrial structure in China. The late-mature varieties which bloom from March to April and mature from next January to May have long harvesting time of fresh fruit and good quality with high price. So they’re important varieties to prolong the supply period of fresh fruit and to improve business efficiency. However, the fruits of late-mature citrus have to suffer the cold weather of winter so they are only suitable for the region where the lowest temperature is higher than -2℃ in winter (The temperature causing fruit freezing damage). Most of the regions in the Three Gorges reservoir area of Chongqing are satisfied to this climatic condition. Therefore, late-mature citrus were widely grown in Chongqing since 2008. Through the rapid development of years, the places such as Changshou, Kaixian and Yunyang counties have become important late-mature citrus production bases. However, with the production of late-mature citrus, freezing damage has been frequently found in shallow hills, especially in lowland of Changshou, Kaixian and other counties. Interestingly, in these regions the temperatures from local meteorological station were not be inferior to the cold injury temperature (-2℃). Hence, presumably, the actual temperatures in orchards may be very different from those measured in meteorological station. It is necessary to study the temperature varying patterns of citrus canopy at different positions of the hilly orchards in the winter with comparison with those temperatures from meteorological stations and predicted temperatures and to establish the relation models between the actual temperatures and predicted temperatures for providing the basis for freeze protection. Meanwhile, we would like to compare the temperature changes and anti-freezing effects under different measures of freeze protection to provide references for passing winter safely of late-mature citrus.This study contains two parts. The first part is ’the differences of temperatures of citrus canopy at different positions of shallow hills in winter with comparison with Changshou district meteorological station’. The second part is’the study of freeze protection techniques of late-mature citrus in winter’. The results of each part are as follows.1. The differences of temperatures of citrus canopy at different position of shallow hills in winter with comparison with Changshou district meteorological station.In the modern agricultural area of Changshou district, automatic temperature and humidity recorders were installed on north of citrus canopy at the upper, middle and lower position of the orchard (3 orchard repeated), and meteorological observation stations were installed at the representative positions of orchard. The temperatures were recorded every 30 minutes from December 2013 to February 2014. After comparing temperatures from automatic temperature and humidity recorders, meteorological observation station in orchard and Changshou district as well as predicted temperatures, the relationship models among these temperatures were constructed using linear regression analysis. The results show that:(1) The daily average temperature and daily minimum temperature were meteorological station> meteorological observation station> upper of shallow hill> middle of shallow hill> the bottom of shallow hill, and the maximum temperature were always meteorological station< meteorological observing station< the canopy of hilly orchards. There was no obvious regularity in maximum temperatures of the canopy at different locations of the shallow hill, but generally the maximum temperatures at the upper of the shallow hill were higher than those at middle or bottom of the shallow hill on clear and calm days.(2) The differences of daily average temperatures and daily minimum temperatures of meteorological station, meteorological observing station and different positions of tree canopy in shallow hill were least in February, followed in December and January. As to the weather, differences were smaller on rainy day compared with clear day. The smallest differences appeared on cold wave (the differences were only 0.1-0.3℃), while the largest differences appeared on the first sunny day after cold wave (1.9-4.2℃).(3) During the experiment period, the number of days with tempreture below-2℃ are:0 day at meteorological station,0 day at meteorological observation station,1 day at the canopy of tree on the upper shallow hill,3.3 days at the canopy of tree on the middle shallow hill, and 6.7 days at the canopy of tree on the bottom shallow hill. The measured lowest temperature,-0.1℃ at the meteorological station,-2.0℃ at the meteorological observation station,-3.4℃ at the canopy of tree on the upper shallow hill,-3.7℃ at the canopy of tree on the middle shallow hill and -4.3℃ at the canopy of tree on bottom shallow hill, appeared on February 13,2014 (the first day after a big cold wave).(4) The relationship models between the temperatures from the tree canopy at different locations of orchards in the shallow hill (Tx) and the temperatures measured by meteorological station (Ta) were T1=1.104Ta-2.6437, T2=1.1398Ta-3.2671, and T3=1.1883Ta-3.8931 for the tree canopy of tree on the upper, middle, and bottom shallow hills, respectively. Therefore, when the temperature measured by meteorological station reached to 1.6℃, the temperature of the canopy of tree on the bottom shallow hills may have been reached to-2℃ which is the temperature of cold injury. These fruits should be harvested as soon as possible.(5) The forecast daily maximum temperatures mostly were lower than those measured on tree canopy, and the forecast daily minimum temperatures mostly were higher than those measured on tree canopy. The relationship models between the temperatures from the tree canopy of trees on different locations of orchards in the shallow hill (Tx) and the temperature forecast by meteorological station (Tx) were T1=1.1292Tc-2.7592, T2=1.1629Tc-3.3639, and T3=1.2023Tc-3.9365 for the canopy of trees on the upper, middle, and bottom shallow hills, respectively. Therefore, when the temperature forecast by meteorological stations reached to 1.6℃, the temperatures of the canopy of trees on the bottom shallow hills may have been reached to -2℃. Some protective measures should be applied, especially on the first sunny morning after the cold wave.(6) The quality analysis and storage tests of the citrus fruits from different location of orchards in the shallow hills showed that there were no obvious differences and rules. But with the extension of storage time, the fruit granulation indexes became higher for the middle and bottom parts of the orchards, and which indicates that they had suffered from cold injury.2. The study of freeze protection techniques of late-mature citrus in winter(1) The experiments of sun-shade net complete coverage, plastic film complete coverage, and plastic film half coverage on citrus canopy were conducted at different locations of hilly orchards in Longhe town of Changshou district from December 2013 to February 2014,and in citrus research institute of Southwest University, Beibei district of Congqing from December 2014 to February 2014. The temperatures of citrus canopy were recorded every 30 minutes. The results show that:① The average temperatures of canopy covered by film were significantly higher than those covered by sun-shade net and CK, the film complete coverage was slightly higher than the half coverage; Being covered by sun-shade net had small effect on the daily average temperatures of canopy, but reduced the daily maximum temperatures. The daily maximum temperatures of canopy covered by film were significantly higher than CK and those covered by sun-shade net, and it reached highest on sunny day followed by cloudy and rainy day during the experiments. The daily maximum temperature of canopy covered by film reached 40℃, and some leaves and branches touched by film were burned.② The plastic film complete coverage and half coverage improved the daily minimum temperatures. When the minimum temperatures occurred, plastic film complete coverage and half coverage increased canopy temperatures of different positions in shallow hilly citrus orchards by 0.3℃ to 0.9℃ and 0.3℃ to 0.5℃, respectively, and them also shorted the duration of low temperature. However, sun-shade net coverage decreased canopy temperature by 0.1℃ to 0.4℃.③ Canopy coverage in winter had no significant effect on fruit quality of citrus, while reduced the fruit cold injury and the occurrence of fruit granulation during postharvest storage in a certain degree. Sun-shade net coverage had no effect on reducing fruit cold injury, but increased granulation index of postharvest storage.(2) The fruit bagging experiments were conducted in the Powell navel orange orchards at low-lying land of Longhe town, Chongshou district from December 2013 to February 2014. There were three different fruit bagging treatments:black double-layer bag (bag 1), white double-layer bag (bag 2) and white single-layer bag (bag 3). Results are as follows:① Fruit bagging slightly increased daily average temperatures and average daily minimum temperatures of fruit surface. For the daily average temperature of fruit surface, the bag 1 and bag 2 treatments increased by 0.2℃, and bag 3 increased by 0.1℃; for the daily average minimum temperature of fruit surface, the bag 1, bag 2 and bag 3 treatments increased by 0.4℃,0.2℃ and 0℃, respectively. But the treatment of double-layer bag slightly reduced average daily maximum temperature by 0.3℃癈 for bag 1 and 0.2℃ for bag 2. On the contrary, single-layer bag treatment increased average daily maximum temperature by 0.1℃.② Fruit bagging had no significant or regularity effect on most of the indicators of fruit quality, but it reduced the fruit granulation indexes during storage. With the extension of storage time, this effect was more obviously, and it reached a significant level after 50 days. These results suggest that the fruit bagging can alleviate the freezing injury of fruits.