Study On Tree Canopy Environmental Factors And Physiological Changes Of Kiyomi Tangor Trees Covered By Polyethylene Film In Winter

At present,the early and mid-season citrus varieties occupy more than 90% citrus growing area in China,which leads to over-centralization for citrus fruit market season.In order to avoid citrus fruit market competition,developing the late-season citrus varieties or delaying the harvest of mid-season varieties are necessary.Due to the rain,windless,heavy fog and high relative humidity,weak sunshine,few frost and snow(minimum temperatures-1℃～2℃)in winter in Sichuan basin and Chongqing area,citrus fruit on-tree usually drop seriously in winter.However,the two areas are suitable for citrus simple protected cultivation,which can effectively prevent citrus fruit from drop in winter.In recent years,a citrus simple protected cultivation method--tree canopy-film-covering,which is effective for preventing citrus fruit from dropping in winter,has been rapidly extended in Sichuan and Chongqing.By the tree canopy-film-covering,the late-season fruits of Kiyomi tangor and Shiranui Tangor are harvested in Mar.or Apr.,and both the fruit quality and economy benefits are outstanding.However,no research has been focused on canopy environmental factor and physiological changes under tree canopy-film-covering condition,and the mechanism of canopy-film-covering to stop fruit dropping is still not clear.With a view to providing the theoretical basis and practical guide for citrus tree canopy-film-covering,the tree canopy environmental factors,fruit drop and quality, photosynthetic characteristics of Kiyomi Tangor(Citrus sinensis×Citrus Unshiu Marc.) trees directly covered by polyethylene film from December to March have been studied in this paper.1.The effccts of canopy-film-covcring on tree ternperatures Experiments were conducted during December,2007 to March,2008.Results showed that the temperatures in the centers of canopy-film-covered(CFC) trees were significant higher than those in the uncovered trees(control).For CFC trees,the temperatures were highest in canopy tops and lowest in bottoms.The trend of daily temperature change pattern is as the same as the control.Comparing to the control,the average temperatures in the centers of CFC trees were 0.8℃and 2.8℃higher in rainy and sunny days respectively,and the average Max.and Min.temperature is 5.3℃and 0.3℃higher,respectively.During the experiment period,the coldest sunny day was Feb.15,2008,and coldest rainy day was Jan.29,2008.On the coldest sunny day,the max.average temperature in CFC trees(the average of the daily highest temperatures in top,center and bottom canopy) was 12.9℃higher than the control,and the min.average temperature was 0.3℃higher than the control,and average daily temperature was 3.5℃higher than the control.On the coldest rainy day,the max.and the min. temperature in CFC trees was 5℃and 0.3℃higher than the control,respectively, and daily average temperature was 1.2℃higher than the control.According to the results,canopy-film can be covered till early March because no branch burn was found in late February even if the temperature at top of CFC tree canopy reached 42℃.However,canopy-film must be removed in mid-March,and at that time the temperature at top of CFC tree canopy got 55℃,which led to shoot burn。2.The effects of canopy-film-covering on tree relative humidityBoth in CFC trees and controls,the max.and rain.relative humidity(RH) happened at the same time.The RH dropped from the morning and got the lowest value during 14:00-16:00,and then rose and got the max.value during 2:00-8:00. However,the CFC trees had much great changes of daily temperatures.From Dec to Feb,RH in the CFC and control trees was almost the same(over 95%RH) during 20:00-10:00,and significant higher in the tops and centers of CFC trees than in the control trees during 10:00-16:00,but RH in bottoms of CFC trees was slightly lower than that in the control trees and occasionally showed significant during 10:00-16:00.After 16:00.RH in the tops and centers of CFC trees increased quickly since 16:00 and was almost the same as in the bottoms of CFC trees or the control after 20:00.RH of CFC and control trees decreased since 8:00 in Mar..but the max.RH decrease happened in control,and the min.RH decrease happened in the tops of CFC trees.The main reason was that,the RH of atmosphere was not high in Mar.and RH of control trees dropped quickly at noon,but for CFC trees,evaporation water was limited in tree canopy,which resulted in higher RH.3.The effects of canopy-film-covering on tree light radiation and photosynthesisSignificant decrease of light radiation was found in CFC trees,expecially at the bottoms of the CFC trees.After sun rose the tempreture inside the film increased but the tempreture outside the film was much lower,which resulted in water vapor condensd inside the film,and sunlight was partly stopped by film and small water drops on the film.The photosynthetically active radiation(PAR) in the tops and bottoms of CFC trees was 50%and 25%of the control,respectively.After a few hours of sunshine,the temperature inside the film got higher and the small water drops on the film evaporated and the film transmittance improved.The PAR in the tops and bottoms of CFC trees was 75%and 50%of the control,respectively.Both the light intensity and temperature inside the film reached the Max.at 14:00 and decreased from then on.Both the net photosynthetic rate(Pn) of CFC tree leaves and control increased gradually from morning and got the max.during 14:00-15:00,and then decreasd.Usually,the Pn of controls decreased a little fast.The control Pn was significantly higher than CFC trees' Pn in Jan,but lower in Feb.This maybe caused by weak dormancy in CFC trees due to their higher temperature than the control's.In Mar, temperature increased gradually and the PAR reached 1200μmol.m-2·s-1,and the max.Pn of the control was higher than that of the CFC trees.Both the Pn of CFC and control trees decreased as the leaf temperature reached 30℃at noon but increseased at 15:30.Both CFC and control trees' Pn-PAR response curves and the daily change trend had the same style.Pn value to PAR decreased significantly in Feb and increased in Mar.In addition,the light compensation point and light saturation point in CFC trees were lower than those in control.The ability of leaf using weaken light was enhanced by canopy-film-covering.The dark respiration rate(Rd) of tree leaves declined both in the control and CFC trees as the time went by since sunset.4h after sunset,the leaf Rd was only about half of that at 1h after sunset,and the Rd of control leaves declined more quikly.Generally, the leaf Rd was significantly higher in the tops than in the centers of CFC trees or the control,but at other test times there was nosignificant difference,Leaf Rd increasd as the spring temperature increased.In Mar,the max.leaf Rd at night was 20.23%and 38.4%higher than that in Jan and Feb,respectively,4.The effects of canopy-film-covering on fruit dropKiyomi tangor fruit drop was reduced significantly by canopy-film-coveting in winter.Results of two years from two orchards in Sichuan and Chongqing showed that canopy-film-covering reduced fi-uit drop very significantly.The accumulative fruit drop rates of the control were 73.1%at the Sichuan orchard and 18.4%at the Chongqing orchard,but only 2.8%and 6.1%for CFC trees at the same two orchards during 2007-08 season.During 2008-09 season,the accumulative fruit drop rate was 8.78%for the control and 3.6%for the CFC trees at the Chongqing orchard,and almost no fruit drop for both the control and CFC trees at the Sichuan orchard.Fruit drop occurred mainly in winter with frost,snow and rainy weather.Two reasons might account for the effects of tree canopy-film-coveting.Firstly,according to the results,the min.temperature in the CFC trees was about 0.3℃higher than the control,which played a role in reducing fruit drop.Secondly,by covering with the film, the rain and snow accumulation can be avoided in fruit pedicel,which prevented fungi from infecting fruits.5.The effects of canopy-film-covering on fruit qualityDuring the experimental period,the Kiyomi tangor fruit decreased on bright and yellow color,and increased on red color.Canopy film covering had no significant effects on fruit bright and color.After fully matured,Kiyomi tangor fruits of CFC trees showed the trend of peel growth,which resulted in fruit puffy and significantly increasing the ratio of peel weight.From Dec.to Mar.,the total soluble solids(TSS) of fruits from both the CFC and control trees increased gradually,and finally had 13.1%～16.0%increase.The titratable acid contents of fruits from both the CFC and control trees decreased gradually,and finally had 26.0%～16.8%decrease,which resulted in 27.4%～38.2% increase in TSS to acid ratios.Vc contents slightly decreased as fruit matured. Canopy-film-coveting had no significant effects on fruit TSS,titratable acid,TSS to acid ratio and Vc contents.