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Abstract In order to optimize and transform closed mature apple orchards with standard rootstocks and improve the quality of fruit, taking a closed Red Fuji apple orchard as the test object, the effects of different densityreducing methods (deinterlacing, removing every other plant in each row, removing every other plant every other row) on the canopy microenvironment, tree structure, leaf photosynthesis and fruit quality were studied. The results showed that different densityreducing methods significantly reduced the orchard coverage and increased the crown transmittance. Among them, the deinterlacing treatment was the best in improving the population structure of the closed orchard, as it reduced the orchard coverage rate by 55.68 percentage points and the canopy transmittance by 82.38 percentage points, compared with the control (CK). Different densityreducing methods all could significantly reduce the branch amount in the closed orchard and optimized the branch composition. The three densityreducing methods decreased the number of branches per plant by 18.96%, 12.41% and 19.58%, respectively, compared with the CK. And compared with the CK, the proportion of short branches and leafy branches to the total branches was increased by 17.13%, 14.27% and 7.37%, respectively, and the proportion of long branches and developmental branches to the total branches was decreased by 24.47%, 18.04% and 10.79%, respectively. The effects of the different densityreducing methods on the temperature, relative light intensity, SPAD and leaf photosynthetic rate in canopies all followed an order of deinterlacing＞removing every other plant in each row＞removing every other plant every other row＞CK, while those on the relative humidity showed an order of deinterlacing＜removing every other plant in each row＜removing every other plant every other row＜CK. The average single fruit weight (238.3 g), coloring index (89.2), smoothness index (83.2), soluble solid content (15.1%) and high quality fruit rate (82.4%) of the deinterlacing treatment were higher than those of other treatments, and the values were 18.2%, 11.4%, 5.85%, 26.9% and 25.2% higher than the CK, respectively. The use of deinterlacing to reduce density is the best for improving the microenvironment of closed apple orchards and improving the photosynthetic efficiency and fruit quality.
Key words Apple; Closed orchard; Density reduction method; Canopy microenvironment; Tree growth; Fruit quality Received: July 23, 2019Accepted: September 25, 2019
Supported by Key Research and Development Program of Shandong Province (2017CXGC0210); Agricultural Science and Technology Innovation Project of Shandong Academy of Agricultural Sciences (CXGC2017D01); National Key R&D Program of China (2016YFD0201100); National Natural Science Foundation of China (31600021); Modern Agricultural Industry Technology System of China for Apple (CARS27); Dongying Science and Technology Program (2015GG0104).
Ru CHEN (1985-), female, P. R. China, assistant research, PhD, devoted to research about Orchard microbes and cultivation physiology.
*Corresponding author. Email: wjz992001@163.com.
At present, the apple cultivation area in Shandong Province is more than 300 000 hm2, most of which are planted in the 1990s, and more than 90% of the orchards are planted in a highdensity mode. Now these orchards have been extremely closed［1］, resulting in poor ventilation, poor fruit quality, serious biennial fruiting, weak tree vigor and difficult production management, which seriously restrict highquality efficient apple production and sustainable development of apple industry［2］. Thinning is one of the main measures often used to transform mature closed apple orchards. Reasonable thinning can reduce the overlapping rate between canopies, improve the population structure and ventilation condition of orchards, enhance tree vigor and leaf photosynthetic capacity, and improve the yield per plant and fruit quality［3-7］. In this study, the effects of different densityreducing methods on canopy microenvironment, tree growth and fruit quality in a closed apple orchard were investigated, with an attempt to provide a theoretical basis for the transformation of highdensity mature apple orchards with standard rootstocks.
Materials and Methods
General situation of experiment field
The experiment was carried out in the orchard in Gejia town, Wendeng District, Weihai City. The apple orchard was closed and had an area of 12 hm2. The main apple variety was Red Fuji/crabapple of 10 years old, and the pollination variety was Gala. The tree shape was of the small and sparse canopy type. The plant spacing was 3 m×5 m. The soil was sandy loam with a pH of 6.5 and an organic matter content of 1.2%.
Experimental methods
Experimental design
In the closed apple orchard, the areas where the tree vigor and the tree shape were basically the same were randomly selected, and thinning was carried out in three densityreducing ways: deinterlacing, removing every other plant in each row, removing every other plant every other row and no thinning as the CK (Fig. 1). Determination items and methods
Orchard population structure, tree branch number and branch composition
Five plants were selected from each plot by random sampling method, in three replicates. The trunk height, trunk perimeter and crown width were measured with a meter ruler. The number of main branches and the numbers of leafy branches, short branches, medium branches, long branches and developmental branches of each plant were investigated, and the branch ratio, the number of branches per plant, and the canopy overlapping rate and canopy coverage were calculated.
The projected area of canopy per plant (m2) = π r2
In the formula, r is the radius of the canopy. When the projected area of a canopy per plant is larger than the area occupied by single plant (i.e., (plant spacing + row spacing)/4］, the area occupied by single plant is used［8］.
Orchard canopy coverage (%)=The projected area of canopy per plant×Number of planted trees/Total plant area×100%
Interplant canopy overlapping rate (%)=(Crown width-Plant spacing)/Plant spacing×100%
Interrow canopy overlapping rate (%)=(Crown width-Row spacing)/Row spacing×100%
Canopy light transmittance
From the end of September to the beginning of October 2018, 8:00-18:00 of a sunny day was chosen, and the light transmission area of each test tree was measured every 2 h by the grid method, which served as the light transmittance in the canopy. Five plants with the same growth vigor were determined in each treatment with three repetitions.
Canopy microclimate parameters, leaf SPAD and photosynthetic rate
The canopy was divided in the horizontal direction into the bore (<1.0 m from the trunk) and the periphery (>1.5 m from the trunk), and divided in the vertical direction into the upper layer (3.0 m from the ground), the middle layer (2.0 m from the ground) and the lower layer (1.0 m from the ground). Five trees with the same growth vigor were selected in each treatment by the random sampling method, and determined with three repetitions for the light intensity, temperature and humidity parameters in the upper, middle and lower layers of the bore and periphery. Among them, the light intensity was measured by a photometer, the temperature was measured by a thermometer, and the humidity was measured by a hygrometer.
The mature leaves on short branches or medium branches of the canopy in the upper, middle and lower layers of the bore and periphery were randomly selected, and three trees were selected in each treatment. Leaf SPAD values were determined using a Chlorophyll502 (Minolta, Japan), and 25 leaves were determined for each treatment with three repetitions. The net photosynthetic rate of the leaves was measured using a CIRA SII portable photosynthetic system tester (PPSystems, UK), and five leaves were determined for each treatment with three repetitions. Fruit quality indexes
After fruit ripening, 30 fruit were randomly collected for each treatment. The fruit weight was measured by a 1/100 electronic balance; the longitudinal and transverse diameters of the fruit were measured with a digital vernier caliper, and the fruit shape index (fruit transverse diameter/fruit longitudinal diameter) was calculated; the firmness without skin was measured with a GY1 fruit hardness tester; the soluble solid content was determined with a digital saccharimeter; and the highquality fruit rate was calculated according to apple fruit grading and testing standards DB 37/T 0561990［9］. The colored area and smoothness of fruit surface were observed, and according to their grading standards (Table 1), the number of fruits at each level was counted. And the coloring index［∑ (Number of fruits at each level ×Value of the representative level) / (Total number of fruits ×Value of the highest level) × 100］and the smoothness index［∑ (Number of fruits at each level ×Value of the representative level)/(Total number of fruits ×Value of the highest level) × 100］were calculated.
Results and Analysis
Effects of different densityreducing methods on population structure parameters, numbers of tree branches and branch composition of the closed apple orchard
Effects of different densityreducing methods on population structure of the closed apple orchard
The investigation results (Table 2) showed that different densityreducing methods all can reduce the orchard coverage and increase the light transmittance of canopies. Among them, the deinterlacing treatment had the best effect in improving the population structure of the closed orchard. Specifically, the deinterlacing treatment reduced the orchard coverage by more than 50%, and nearly doubled the light transmittance of canopies, compared with CK.
Effects of different densityreducing methods on branch amount and branch composition of trees
The investigation results (Table 3) showed that such three types of densityreducing treatments as deinterlacing, removing every other plant in each row and removing every other plant every other row can reduce the branch amount and optimize the branch composition of the closed apple orchard. Compared with the CK, the three methods reduced the branch amount by 18.96%, 12.41% and 19.58% respectively; the proportion of short branches and leafy branches in total branches was improved by 17.13%, 14.27% and 7.37%, respectively; and the proportion of long branches and developmental branches in the total branches were reduced by 24.47%, 18.04% and 10.79%, respectively. It can be seen that the deinterlacing method for reducing the density was the best for reducing the branch amount and optimizing the branch composition of the closed apple orchard. Ru CHEN et al. Effects of Different Densityreducing Methods on Canopy Microenvironment, Tree Growth and Fruit Quality in Closed Apple Orchard
Effects of different densityreducing methods on canopy microenvironment in closed apple orchard
Effects of different densityreducing methods on canopy temperature and humidity
The investigation results (Table 4) showed that such three types of densityreducing treatments as deinterlacing, removing every other plant in each row and removing every other plant every other row all affected the temperature and humidity in the upper, middle, and lower layers of the bore and periphery of canopies. The temperature in the bore and periphery of canopies gradually decreased from top to bottom, and the temperature of each canopy followed an order of deinterlacing>removing every other plant in each row>removing every other plant every other row>CK. The differences between the upper and lower layers showed an order of deinterlacing (0.4-1.6 ℃)<removing every other plant in each row (0.9-2.3 ℃)<removing every other plant every other row (0.9-2.9 ℃)<CK (0.6-3.3 ℃). In the same canopy, the temperature of canopies was higher in the periphery than in the bore, and gradually decreased from top to bottom, but the variation was small.
The variation trend of the relative humidity inside and outside canopies was opposite to the variation trend of the temperature. The relative humidity of the various layers in the bore and periphery of canopies showed an order of deinterlacing<removing every other plant in each row <removing every other plant every other row<CK. The relative humidity gradually increased from the upper layer to the lower layer of canopies, and the differences in the relative humidity between the upper and lower layers followed an order of deinterlacing (0.6%-1.9%)<removing every other plant in each row (1.5%-2.8%)<removing every other plant every other row (0.9 %-3.4%)<CK (1.3%-4.2%). In the same canopy, the relative humidity of the bore was higher than that of the periphery, but the variation was not large.
Effects of different densityreducing methods on the relative light intensity of canopy
The investigation results (Table 5) showed that the relative light intensity of the canopy was affected by all the three densityreducing methods, i.e., deinterlacing, removing every other plant in each row and removing every other plant every other row. The relative light intensities in the various layers of the bore and periphery ranked as deinterlacing>removing every other plant in each row>removing every other plant every other row>CK. In the same canopy, the relative light intensity was higher in the periphery than in the bore, and decreased from top to bottom sequentially in the periphery and bore, which meant that the relative light intensity was the highest in the upper layer of the periphery, and the lowest in the lower layer of the bore. Effects of different densityreducing methods on SPAD and photosynthetic rate of canopy leaves
The investigation results (Table 6) showed that the three densityreducing treatments all affected the SPAD and photosynthetic rate of the canopy leaves. The SPAD and leaf photosynthetic rates of the inner and outer canopy showed an order of deinterlacing>removing every other plant in each row>removing every other plant every other row>CK, indicating that the density reduction can promote leaf growth and development and increase net photosynthetic rate. The SPAD and leaf photosynthetic rates in the inner and outer canopy both gradually decreased from top to bottom, and the SPAD and net photosynthetic rate of the same canopy ranked as periphery>bore.
Effects of different densityreducing methods on fruit quality
The investigation results (Table 7) showed that different densityreducing treatments all could improve fruit quality. Among them, the deinterlacing treatment exhibited single fruit weight (238.3 g), coloring index (89.2), smoothness index (83.2), soluble solid content (15.1%) and highquality fruit rate (82.4%) all higher than other densityreducing treatments, and the values were 18.2%, 11.4%, 5.85%, 26.9% and 25.2% higher than the CK, respectively.
Conclusions and Discussion
Reasonable population structure and branch spatial distribution and good light environment are the key to achieving high quality and high yield of fruit trees［10-12］. After the transformation of closed orchards, the orchard coverage and the light transmittance in the canopy are improved, laying a foundation for the highquality highyield fruit production of orchards［2,13］. In this study, the three densityreducing methods, i.e., deinterlacing, removing every other plant in each row and removing every other plant every other row, could significantly reduce the orchard coverage and improve the light transmittance of canopies, and the effect of the deinterlacing treatment in improving the population structure of the closed orchard was the best. Thinning can effectively reduce the total branch amount of orchards, which is an effective way to improve the population structure of orchards and solve the problem of orchard closure［14］. After thinning treatment, the ventilation and light transmission of orchards are improved, the composition of the fruit branches is optimized, and new shoots grow and stop growth early, which is conducive to flower bud differentiation［15-16］. The results of this study showed that the different densityreducing methods could significantly reduce the branch amount in the closed orchard and optimize the branch composition, and could increase the proportion of short branches and leafy branches and reduce the proportion of long branches and developmental branches. This is consistent with the conclusions of Wu et al.［13］. Apple canopy temperature and humidity are factors directly affecting fruit growth and determining fruit quality and yield［17］. The results of this study showed that the effects of the different densityreducing methods on the temperature in canopies followed an order of deinterlacing>removing every other plant in each row>removing every other plant every other row>CK, and the effects on the relative humidity in canopies ranked as deinterlacing<removing every other plant in each row<removing every other plant every other row<CK. The canopy temperature gradually decreased from the upper layer to the lower layer, and the peripheral temperature of the same canopy was higher than the inner temperature, while the relative humidity of the canopy was opposite to the variation trend of the temperature, which is consistent with previous research results［18-19］.
The distribution of light in canopies is closely related to the shape of canopy, the number of branches and leaves, the density of branches and leaves and the spatial distribution of different branches, and directly affects flower bud formation, fruit development and fruit quality［12］. The light distribution of apple tree crowns is of guiding significance for improving high and stableyield stable cultivation management techniques of apples and improving the yield and quality of apples［20］. The results of this study showed that the effects of different densityreducing methods on the relative light intensity in canopies were as follows: deinterlacing>removing every other plant in each row>removing every other plant every other row>CK, and the relative light intensity distribution showed the law of decreasing from top to bottom and from outside to inside sequentially.
The leaves are important parts of photosynthesis of fruit trees. Their growth and development status is the basis of high and stable yield and high quality of fruit trees, and welldeveloped leaves can promote the robust growth of trees and vigorous metabolic activities, and are an important indicator of the growth and development of fruit trees. After thinning and reshaping mature closed orchards, the population structure and canopy environment of trees are improved, which can promote the growth and development of the leaves and improve the photosynthetic capacity of the leaves［3］. The results of this study showed that different densityreducing methods could effectively improve the ventilation and light transmission conditions of trees, promote the growth and development of leaves, and increase the net photosynthetic rate of leaves. The SPAD and photosynthetic rate of canopy leaves showed an order of upper layer>middle layer>lower layer from top to bottom, and the leaf net photosynthetic rate of the same canopy followed an order of periphery >bore of the canopy. The SPAD and photosynthetic rate values of leaves treated by different densityreducing methods ranked as deinterlacing>removing every other plant in each row>removing every other plant every other row>CK. Structural adjustment of closed orchards by thinning measures can improve the light condition of canopies and significantly improve the photosynthetic capacity of leaves, thereby promoting the improvement of fruit quality［5-6］. For closed orchards, the population structure can be directly improved through thinning, and the tree structure can be optimized through trimming and removing dense branches, so that the improvement of apple single fruit weight, coloring index, smoothness index, soluble solid content and other fruit quality indicators can be promoted［6,14］. The results of this study showed that the single fruit weight, coloring index, smoothness index, soluble solid content and highquality fruit rate of the deinterlacing treatment were higher than other densityreducing methods.
In summary, it can be seen that the use of deinterlacing to reduce density is the best for improving the microenvironment of closed apple orchards and improving the photosynthetic efficiency and fruit quality.
References
［1］ WANG JZ. Key Techniques for optimizing and reconstructing lightefficient structure of closed mature apple orchards in Shandong Province［J］. Yantai Fruits, 2018, 144(4): 30-31.
［2］ LI PH, WU JS, DONG XY, et al. Effects of different thinning methods on light, photosynthesis and growth results in closed apple orchards［J］. Scientia Agricultura Sinica, 2012, 45(11): 2217-2223.
［3］ SUN WT, NIU JQ, DONG T, et al. Effect of thinning and reshaping on the canopy structure and leaf quality at late growth stage in dense apple orchard in Loess Plateau of eastern Gansu, China［J］. Chinese Journal of Applied Ecology, 2018, 29(9): 3008-3016.
［4］ LIU XL, SUN WT, NIU JQ, et al. Canopy structure and leaf physiological characteristics of closed apple orchards subjected to intermediate cutting in east Gansu Province［J］. Gansu Agricultural Science and Technology, 2018, (1): 21-24.
［5］ NIU JQ, SUN WT, YIN XN, et al. Effects of thinning on photosynthetic characteristics and fruit yield and quality of dwarfing? red Fuji apple［J］. Forest Science and Technology, 2018, (10): 67-70.
［6］ CHEN R, WANG JZ, XUE XM, et al. Effects of different degree of canopy density on photosynthetic capacity and fruit quality in apple orchard［J］. Tianjin Agricultural Sciences, 2014, 20(7): 65-68.
［7］ YUAN BL, LIU JH, LI XW, et al. Effects of different thinning systems on light penetration, leaf characteristics and fruit quality in canopy overcrowd ‘Fuji’ apple orchard with standard rootstocks［J］. Scientia Agricultura Sinica, 2011, 44(18): 3805-3811. ［8］ WANG LP, XUE XM, NIE PX, et al. Effect of structural optimization and modification of old inefficient apple orchards in Yantai［J］. Journal of Hebei Agricultural Sciences, 2018, 22(5): 15-19.
［9］ DB 37/T 0561990, Apple fruit grading and testing standards［S］.
［10］ ZHANG Q, WEI XP, WANG XW, et al. Effect of shoot numbers and distribution in canopy on yields and qualities of ‘Fuji’apple with standard rootstock［J］. Acta Horticulturae Sinica, 2010, 37(8): 1205-1211.
［11］ WANG Y, FAN CH, JIANG DW, et al. Comparison on crown characteristics and fruit quality of different tree canopy shapes［J］. Acta Agriculturae Borealioccidentalis Sinica, 2011, 20(12): 93-97.
［12］ WEI QP, LU RQ, ZHANG XC, et al. Relationships between distribution of relative light intensity and yield and quality in different tree canopy shapes for ‘Fuji’ apple［J］. Acta Horticulturae Sinica, 2004, 31(3): 291-296.
［13］ WU JS, DONG XY, DUAN YX, et al. Effects of different thinning methods on group structure and fruit quality in airtight apple orchard［J］. Chinese Agricultural Science Bulletin, 2012, 28(19): 135-140.
［14］ YUAN JJ, ZHAO ZY, WAN YZ, et al. Effects of treethinning andreshaping on production and fruit quality of Red ‘Fuji’ apples in the midaged and dense orchards［J］. Journal of Northwest A & F University: Natural Science Edition, 2010, 38(4): 133-137, 142.
［15］ YUAN JJ, ZHANG LS, ZHAO ZY, et al. Effect of modifying tree form on growth and fruiting in Fuji apple highdensity orchards［J］. Journal of Northwest Forestry University, 2003, 18(4): 60-62.
［16］ SHANG ZH, WEI QP, SUN LZ, et al. Judgment parameters of canopy overcrowed for reformative high trunk open centre shape of Fuji apple with standard rootstock［J］. Scientia Agricultura Sinica, 2010, 43(1): 132-139.
［17］ GUO XM, FAN JC, ZHOU GM, et al. Spatial and temporal distribution rule of temperature and humidity in apple tree canopy on sunny days［J］. Chinese Agricultural Science Bulletin, 2016, 32(35): 188-192.
［18］ SUN ZH, WEI QP, YANG CX, et al. Relationships between distribution of shoots, leaves and temperature, relative humidity in the canopy of Red Fuji apple trees［J］. Journal of Fruit Science, 2008, 25(1): 6-11.
［19］ CHEN R, WANG JZ, XUE XM, et al. Effects of different canopy densities on group structure and canopy microclimate of apple orchard［J］. Shandong Agricultural Sciences, 2014, 46(9): 53-56.
［20］ LI BG, ZHANG YQ, ZHANG XM, et al. Studies on light distribution of different group strcture in apple orchards［J］. Hebei Journal of Forestry and Orchard Research, 2012, 27(3): 316-319.
Key words Apple; Closed orchard; Density reduction method; Canopy microenvironment; Tree growth; Fruit quality Received: July 23, 2019Accepted: September 25, 2019
Supported by Key Research and Development Program of Shandong Province (2017CXGC0210); Agricultural Science and Technology Innovation Project of Shandong Academy of Agricultural Sciences (CXGC2017D01); National Key R&D Program of China (2016YFD0201100); National Natural Science Foundation of China (31600021); Modern Agricultural Industry Technology System of China for Apple (CARS27); Dongying Science and Technology Program (2015GG0104).
Ru CHEN (1985-), female, P. R. China, assistant research, PhD, devoted to research about Orchard microbes and cultivation physiology.
*Corresponding author. Email: wjz992001@163.com.
At present, the apple cultivation area in Shandong Province is more than 300 000 hm2, most of which are planted in the 1990s, and more than 90% of the orchards are planted in a highdensity mode. Now these orchards have been extremely closed［1］, resulting in poor ventilation, poor fruit quality, serious biennial fruiting, weak tree vigor and difficult production management, which seriously restrict highquality efficient apple production and sustainable development of apple industry［2］. Thinning is one of the main measures often used to transform mature closed apple orchards. Reasonable thinning can reduce the overlapping rate between canopies, improve the population structure and ventilation condition of orchards, enhance tree vigor and leaf photosynthetic capacity, and improve the yield per plant and fruit quality［3-7］. In this study, the effects of different densityreducing methods on canopy microenvironment, tree growth and fruit quality in a closed apple orchard were investigated, with an attempt to provide a theoretical basis for the transformation of highdensity mature apple orchards with standard rootstocks.
Materials and Methods
General situation of experiment field
The experiment was carried out in the orchard in Gejia town, Wendeng District, Weihai City. The apple orchard was closed and had an area of 12 hm2. The main apple variety was Red Fuji/crabapple of 10 years old, and the pollination variety was Gala. The tree shape was of the small and sparse canopy type. The plant spacing was 3 m×5 m. The soil was sandy loam with a pH of 6.5 and an organic matter content of 1.2%.
Experimental methods
Experimental design
In the closed apple orchard, the areas where the tree vigor and the tree shape were basically the same were randomly selected, and thinning was carried out in three densityreducing ways: deinterlacing, removing every other plant in each row, removing every other plant every other row and no thinning as the CK (Fig. 1). Determination items and methods
Orchard population structure, tree branch number and branch composition
Five plants were selected from each plot by random sampling method, in three replicates. The trunk height, trunk perimeter and crown width were measured with a meter ruler. The number of main branches and the numbers of leafy branches, short branches, medium branches, long branches and developmental branches of each plant were investigated, and the branch ratio, the number of branches per plant, and the canopy overlapping rate and canopy coverage were calculated.
The projected area of canopy per plant (m2) = π r2
In the formula, r is the radius of the canopy. When the projected area of a canopy per plant is larger than the area occupied by single plant (i.e., (plant spacing + row spacing)/4］, the area occupied by single plant is used［8］.
Orchard canopy coverage (%)=The projected area of canopy per plant×Number of planted trees/Total plant area×100%
Interplant canopy overlapping rate (%)=(Crown width-Plant spacing)/Plant spacing×100%
Interrow canopy overlapping rate (%)=(Crown width-Row spacing)/Row spacing×100%
Canopy light transmittance
From the end of September to the beginning of October 2018, 8:00-18:00 of a sunny day was chosen, and the light transmission area of each test tree was measured every 2 h by the grid method, which served as the light transmittance in the canopy. Five plants with the same growth vigor were determined in each treatment with three repetitions.
Canopy microclimate parameters, leaf SPAD and photosynthetic rate
The canopy was divided in the horizontal direction into the bore (<1.0 m from the trunk) and the periphery (>1.5 m from the trunk), and divided in the vertical direction into the upper layer (3.0 m from the ground), the middle layer (2.0 m from the ground) and the lower layer (1.0 m from the ground). Five trees with the same growth vigor were selected in each treatment by the random sampling method, and determined with three repetitions for the light intensity, temperature and humidity parameters in the upper, middle and lower layers of the bore and periphery. Among them, the light intensity was measured by a photometer, the temperature was measured by a thermometer, and the humidity was measured by a hygrometer.
The mature leaves on short branches or medium branches of the canopy in the upper, middle and lower layers of the bore and periphery were randomly selected, and three trees were selected in each treatment. Leaf SPAD values were determined using a Chlorophyll502 (Minolta, Japan), and 25 leaves were determined for each treatment with three repetitions. The net photosynthetic rate of the leaves was measured using a CIRA SII portable photosynthetic system tester (PPSystems, UK), and five leaves were determined for each treatment with three repetitions. Fruit quality indexes
After fruit ripening, 30 fruit were randomly collected for each treatment. The fruit weight was measured by a 1/100 electronic balance; the longitudinal and transverse diameters of the fruit were measured with a digital vernier caliper, and the fruit shape index (fruit transverse diameter/fruit longitudinal diameter) was calculated; the firmness without skin was measured with a GY1 fruit hardness tester; the soluble solid content was determined with a digital saccharimeter; and the highquality fruit rate was calculated according to apple fruit grading and testing standards DB 37/T 0561990［9］. The colored area and smoothness of fruit surface were observed, and according to their grading standards (Table 1), the number of fruits at each level was counted. And the coloring index［∑ (Number of fruits at each level ×Value of the representative level) / (Total number of fruits ×Value of the highest level) × 100］and the smoothness index［∑ (Number of fruits at each level ×Value of the representative level)/(Total number of fruits ×Value of the highest level) × 100］were calculated.
Results and Analysis
Effects of different densityreducing methods on population structure parameters, numbers of tree branches and branch composition of the closed apple orchard
Effects of different densityreducing methods on population structure of the closed apple orchard
The investigation results (Table 2) showed that different densityreducing methods all can reduce the orchard coverage and increase the light transmittance of canopies. Among them, the deinterlacing treatment had the best effect in improving the population structure of the closed orchard. Specifically, the deinterlacing treatment reduced the orchard coverage by more than 50%, and nearly doubled the light transmittance of canopies, compared with CK.
Effects of different densityreducing methods on branch amount and branch composition of trees
The investigation results (Table 3) showed that such three types of densityreducing treatments as deinterlacing, removing every other plant in each row and removing every other plant every other row can reduce the branch amount and optimize the branch composition of the closed apple orchard. Compared with the CK, the three methods reduced the branch amount by 18.96%, 12.41% and 19.58% respectively; the proportion of short branches and leafy branches in total branches was improved by 17.13%, 14.27% and 7.37%, respectively; and the proportion of long branches and developmental branches in the total branches were reduced by 24.47%, 18.04% and 10.79%, respectively. It can be seen that the deinterlacing method for reducing the density was the best for reducing the branch amount and optimizing the branch composition of the closed apple orchard. Ru CHEN et al. Effects of Different Densityreducing Methods on Canopy Microenvironment, Tree Growth and Fruit Quality in Closed Apple Orchard
Effects of different densityreducing methods on canopy microenvironment in closed apple orchard
Effects of different densityreducing methods on canopy temperature and humidity
The investigation results (Table 4) showed that such three types of densityreducing treatments as deinterlacing, removing every other plant in each row and removing every other plant every other row all affected the temperature and humidity in the upper, middle, and lower layers of the bore and periphery of canopies. The temperature in the bore and periphery of canopies gradually decreased from top to bottom, and the temperature of each canopy followed an order of deinterlacing>removing every other plant in each row>removing every other plant every other row>CK. The differences between the upper and lower layers showed an order of deinterlacing (0.4-1.6 ℃)<removing every other plant in each row (0.9-2.3 ℃)<removing every other plant every other row (0.9-2.9 ℃)<CK (0.6-3.3 ℃). In the same canopy, the temperature of canopies was higher in the periphery than in the bore, and gradually decreased from top to bottom, but the variation was small.
The variation trend of the relative humidity inside and outside canopies was opposite to the variation trend of the temperature. The relative humidity of the various layers in the bore and periphery of canopies showed an order of deinterlacing<removing every other plant in each row <removing every other plant every other row<CK. The relative humidity gradually increased from the upper layer to the lower layer of canopies, and the differences in the relative humidity between the upper and lower layers followed an order of deinterlacing (0.6%-1.9%)<removing every other plant in each row (1.5%-2.8%)<removing every other plant every other row (0.9 %-3.4%)<CK (1.3%-4.2%). In the same canopy, the relative humidity of the bore was higher than that of the periphery, but the variation was not large.
Effects of different densityreducing methods on the relative light intensity of canopy
The investigation results (Table 5) showed that the relative light intensity of the canopy was affected by all the three densityreducing methods, i.e., deinterlacing, removing every other plant in each row and removing every other plant every other row. The relative light intensities in the various layers of the bore and periphery ranked as deinterlacing>removing every other plant in each row>removing every other plant every other row>CK. In the same canopy, the relative light intensity was higher in the periphery than in the bore, and decreased from top to bottom sequentially in the periphery and bore, which meant that the relative light intensity was the highest in the upper layer of the periphery, and the lowest in the lower layer of the bore. Effects of different densityreducing methods on SPAD and photosynthetic rate of canopy leaves
The investigation results (Table 6) showed that the three densityreducing treatments all affected the SPAD and photosynthetic rate of the canopy leaves. The SPAD and leaf photosynthetic rates of the inner and outer canopy showed an order of deinterlacing>removing every other plant in each row>removing every other plant every other row>CK, indicating that the density reduction can promote leaf growth and development and increase net photosynthetic rate. The SPAD and leaf photosynthetic rates in the inner and outer canopy both gradually decreased from top to bottom, and the SPAD and net photosynthetic rate of the same canopy ranked as periphery>bore.
Effects of different densityreducing methods on fruit quality
The investigation results (Table 7) showed that different densityreducing treatments all could improve fruit quality. Among them, the deinterlacing treatment exhibited single fruit weight (238.3 g), coloring index (89.2), smoothness index (83.2), soluble solid content (15.1%) and highquality fruit rate (82.4%) all higher than other densityreducing treatments, and the values were 18.2%, 11.4%, 5.85%, 26.9% and 25.2% higher than the CK, respectively.
Conclusions and Discussion
Reasonable population structure and branch spatial distribution and good light environment are the key to achieving high quality and high yield of fruit trees［10-12］. After the transformation of closed orchards, the orchard coverage and the light transmittance in the canopy are improved, laying a foundation for the highquality highyield fruit production of orchards［2,13］. In this study, the three densityreducing methods, i.e., deinterlacing, removing every other plant in each row and removing every other plant every other row, could significantly reduce the orchard coverage and improve the light transmittance of canopies, and the effect of the deinterlacing treatment in improving the population structure of the closed orchard was the best. Thinning can effectively reduce the total branch amount of orchards, which is an effective way to improve the population structure of orchards and solve the problem of orchard closure［14］. After thinning treatment, the ventilation and light transmission of orchards are improved, the composition of the fruit branches is optimized, and new shoots grow and stop growth early, which is conducive to flower bud differentiation［15-16］. The results of this study showed that the different densityreducing methods could significantly reduce the branch amount in the closed orchard and optimize the branch composition, and could increase the proportion of short branches and leafy branches and reduce the proportion of long branches and developmental branches. This is consistent with the conclusions of Wu et al.［13］. Apple canopy temperature and humidity are factors directly affecting fruit growth and determining fruit quality and yield［17］. The results of this study showed that the effects of the different densityreducing methods on the temperature in canopies followed an order of deinterlacing>removing every other plant in each row>removing every other plant every other row>CK, and the effects on the relative humidity in canopies ranked as deinterlacing<removing every other plant in each row<removing every other plant every other row<CK. The canopy temperature gradually decreased from the upper layer to the lower layer, and the peripheral temperature of the same canopy was higher than the inner temperature, while the relative humidity of the canopy was opposite to the variation trend of the temperature, which is consistent with previous research results［18-19］.
The distribution of light in canopies is closely related to the shape of canopy, the number of branches and leaves, the density of branches and leaves and the spatial distribution of different branches, and directly affects flower bud formation, fruit development and fruit quality［12］. The light distribution of apple tree crowns is of guiding significance for improving high and stableyield stable cultivation management techniques of apples and improving the yield and quality of apples［20］. The results of this study showed that the effects of different densityreducing methods on the relative light intensity in canopies were as follows: deinterlacing>removing every other plant in each row>removing every other plant every other row>CK, and the relative light intensity distribution showed the law of decreasing from top to bottom and from outside to inside sequentially.
The leaves are important parts of photosynthesis of fruit trees. Their growth and development status is the basis of high and stable yield and high quality of fruit trees, and welldeveloped leaves can promote the robust growth of trees and vigorous metabolic activities, and are an important indicator of the growth and development of fruit trees. After thinning and reshaping mature closed orchards, the population structure and canopy environment of trees are improved, which can promote the growth and development of the leaves and improve the photosynthetic capacity of the leaves［3］. The results of this study showed that different densityreducing methods could effectively improve the ventilation and light transmission conditions of trees, promote the growth and development of leaves, and increase the net photosynthetic rate of leaves. The SPAD and photosynthetic rate of canopy leaves showed an order of upper layer>middle layer>lower layer from top to bottom, and the leaf net photosynthetic rate of the same canopy followed an order of periphery >bore of the canopy. The SPAD and photosynthetic rate values of leaves treated by different densityreducing methods ranked as deinterlacing>removing every other plant in each row>removing every other plant every other row>CK. Structural adjustment of closed orchards by thinning measures can improve the light condition of canopies and significantly improve the photosynthetic capacity of leaves, thereby promoting the improvement of fruit quality［5-6］. For closed orchards, the population structure can be directly improved through thinning, and the tree structure can be optimized through trimming and removing dense branches, so that the improvement of apple single fruit weight, coloring index, smoothness index, soluble solid content and other fruit quality indicators can be promoted［6,14］. The results of this study showed that the single fruit weight, coloring index, smoothness index, soluble solid content and highquality fruit rate of the deinterlacing treatment were higher than other densityreducing methods.
In summary, it can be seen that the use of deinterlacing to reduce density is the best for improving the microenvironment of closed apple orchards and improving the photosynthetic efficiency and fruit quality.
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