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1. Shijiazhuang Pomology Institute, Hebei Academy of Agricultural and Forestry Sciences, Shijiazhuang 050061, China; 2. Hebei Provincial Department of Land and Resources, Shijiazhuang 050051, China

Abstract In order to clarify the correlations between the free spindle pruning mode and various factors in Fuji apple, the growth data of the free spindle-shaped short branch Fuji apple Tianhong No.2 and the free spindle-shaped long branch Fuji apple Changfu No.2 in the Apple Demonstration Park of Xingtang County, Hebei Province were investigated. Using SPSS statistical software, the correlations between the stem tapering grade and various growth traits were analyzed. The results showed that in Tianhong No.2, the stem tapering grade was negatively correlated with various growth traits of the central shaft, and the correlation coefficients between the stem tapering grade and the central shaft total thickness, the central shaft average thickness, the central shaft total length and the central shaft average length were -0.228, -0.033, -0.411 and -0.430, respectively; and in Changfu No.2, the stem tapering grade was positively correlated with various growth traits of the central shaft, and the correlation coefficients between the stem tapering grade and the central shaft total thickness, the central shaft average thickness, the central shaft total length and the central shaft average length were 0.475, 0.181, 0.120 and 0.258, respectively. This paper can provide a scientific basis for guiding the construction of free spindle pruning mode in densely planted dwarf apple trees.

Key words Fuji apple, Free spindle, Tree structure, Correlation factor analysis

1 Introduction

Currently, at home and abroad, dwarf self-rooted rootstock and dwarf intermediate stock are commonly used for close planting of apple[1-2]. According to the culture characteristics of the tree structure and the characteristics of canopy formation of dwarf rootstock apple trees, the shaping of young trees should be completed in about 3 years, and the first two to three years of shaping is the key. Therefore, the pruning modes of slender spindle or free spindle are commonly used[3-4]. The difference between the upper and lower diameters of the trunk of the three within the unit length is called tapering grade[4]. How to carry out appropriate trimming and pruning according to the characteristics of seedlings of different varieties is the key to ensure the formation of young trees. At present, the determination of the trimming and pruning index of densely-planted dwarf rootstock apple saplings is mainly based on some cultivation experience[5-6], lacking in-depth research and relevant data support. To this end, the correlations between the stem tapering grade and various growth traits of the central shaft in densely-planted dwarf rootstock Fuji apple saplings were studied to systematically explore the techniques related to the trimming and pruning of Fuji apple saplings, so as to provide a scientific basis for the development of densely-planted dwarf rootstock apples.

2 Materials and methods

2.1MaterialsThe test was conducted at the Apple Demonstration Park in Dongantaizhuang Village and Jialuoying Village (apple-producing areas in central and southern Hebei) in Xingtang County, Hebei Province in 2014. The Fuji apple trees with healthy growth, similar age, excellent cultivation conditions, and adjacent planting locations were selected. The test varieties were 6- to 8-year-old free spindle-shaped short-branch fruit-bearing Fuji apple Tianhong No.2 and free spindle-shaped long-branch fruit-bearing Fuji apple Changfu No.2. The plant and row spacing was 2 m×3 m. The grafting stock wasMalusmicromalus, and the dwarf intermediate rootstock was SH38.

2.2Testmethods

2.2.1Test subject. The subjects were selected randomly, labeled, and counted by individual plants. A total of 30 replicates were arranged. All the plants were arranged randomly. The trees of Tianhong No.2 and Changfu No.2 were selected in five directions: east, south, west, north and center. Total six trees were selected randomly from each direction, thus a total of 30 plants were selected for each Fuji apple tree variety for investigation.

2.2.2Test investigation. This test was carried out after the tree body completely entered the deciduous period in early December 2014. For each index, 30 repetitions were measured. For each tree, three measurements were carried out: the diameter of the base of the grafting interface (Dbase), the diameter of the dip (Ddip) and the length from the grafting interface to the tip (L, cm).

2.2.3Calculation method of tapering grade. The trees of the two varieties of free spindle-shaped Fuji apple were measured and analyzed. According to the following formula, the tapering grade of the Fuji apple under the tree structure was calculated.

Tapering grade (%)=(Dbase-Dtip)/L×100% (cm).

2.2.4Correlation analysis. The measured data of the stem tapering grade and the growth traits of the central shaft of the free spindle-shaped Fuji apple was analyzed statistically using the Duncan module of One-way ANOVA, and theLSDmethod was used to conduct significant (bilateral) analysis[6-7].

3 Results and analysis

3.1Correlationsbetweenstemtaperinggradeandvariousgrowthtraitsofcentralshaftinfreespindle-shapedshort-branchFujiappleThe stem tapering grade, central shaft total thickness, central shaft average thickness, central shaft total length and central shaft average length of the free spindle-shaped short-branch Fuji apple Tianhong No.2 were measured. The measured data was processed and analyzed using SPSS13.0 software. The correlations between the stem tapering grade and the growth traits of the central shaft were analyzed (Table 1).

As shown in Table 1, the stem tapering grade of Tianhong No.2 was negatively correlated with the total thickness, average thickness, total length and average length of the central shaft, with correlation coefficients of -0.228, -0.033, -0.411 and -0.430, respectively (P>0.05). The total thickness, average thickness, total length and average length of the central shaft were positively correlated with each other (P<0.01).

Table1Correlationanalysisbetweenstemtaperinggradeandgrowthtraitsofcentralshaftoffreespindle-shapedshort-branchFujiappleTianhongNo.2

Measured itemAnalyzed itemStemtaperinggradeCentralshaft totalthickness∥cmCentral shaftaveragethickness∥cmCentral shafttotallength∥cmCentral shaftaveragelength∥cmStem tapering gradePerson correlation analysis1.000 - - - -Significant difference analysis (bilateral) - - - - -Central shaft total thickness∥cmPerson correlation analysis-0.2281.000 - - -Significant difference analysis (bilateral)0.434 - - - -Central shaft average thickness∥cmPerson correlation analysis-0.0330.829**1.000 - -Significant difference analysis (bilateral)0.9100 - - -Central shaft total length∥cmPerson correlation analysis-0.4110.932**0.719**1.000 -Significant difference analysis (bilateral)0.1441.21E-060.004 - -Central shaft average length∥cmPerson correlation analysis-0.4300.720**0.701**0.886**1.000Significant difference analysis (bilateral)0.1250.0040.0052.44E-05 -

Note:**indicates a significant correlation at the 0.01 level (bilateral).

3.2Correlationsbetweenstemtaperinggradeandvariousgrowthtraitsofcentralshaftinfreespindle-shapedlong-branchFujiappleThe stem tapering grade and the total thickness, average thickness, total length and average length of the central shaft of free spindle-shaped long-branch Fuji apple Changfu No.2 were measured, and the measured data was processed and analyzed statistically using SPSS13.0 software (Table 2).

As shown in Table 2, the stem tapering grade of Changfu No.2 was positively correlated with the total thickness, average thickness, total length and average length of the central shaft, with correlation coefficients of 0.475, 0.181, 0.120 and 0.258, respectively (P>0.05). The total thickness, average thickness, total length and average length of the central shaft were positively correlated with each other (P<0.01).

Table2Correlationanalysisbetweenstemtaperinggradeandthegrowthtraitsofthecentralshaftoffreespindle-shapedlong-branchFujiappleChangfuNo.2

Measured itemAnalyzed itemStemtaperinggradeCentralshaft totalthickness∥cmCentral shaftaveragethickness∥cmCentral shafttotallength∥cmCentral shaftaveragelength∥cmStem tapering gradePerson correlation analysis1.000 - - --significant difference analysis (bilateral)- - - --Central shaft total thickness∥cmPerson correlation analysis0.4751.000 - --significant difference analysis (bilateral)0.847 - - --Central shaft average thickness∥cmPerson correlation analysis0.1810.843**1.000 --significant difference analysis (bilateral)0.4575.93E-06 - --Central shaft total length∥cmPerson correlation analysis0.1200.860**0.603**1.000-significant difference analysis (bilateral)0.6242.38E-060.006 --Central shaft average length∥cmPerson correlation analysis0.2580.717**0.679**0.896**1.000significant difference analysis (bilateral)0.2870.0010.0012.07E-07-

Note:**indicates a significant correlation at the 0.01 level (bilateral).

4 Conclusions and discussions

In the research field of tree structure, domestic and foreign studies focus on making full use of light energy to achieve high quality, emphasizing appropriate density, reasonable group structure, reasonable individual spatial distribution and good lighting system, which are the key to achieving high quality and high yield. Dwarf tree shape emphasizes the advantages of the center. In order to maintain the high quality and high yield, lateral strong branches must be controlled in a timely manner to maintain the advantages of the trunk. It is required that the thickness of the base of lateral branches is less than 1/3 of that of the trunk where they grow out, and 1/4 to 1/5 is optimal. In addition, the height of trees must be controlled strictly, and the elevation angle between the outer edge and top of the crown of two adjacent rows is not more than 49°. There are working channels more than 1.5 m wide between rows[7-8, 11]. At present, apple trees have been more closely planted with dwarf rootstocks or dwarf intermediate stocks. In China, dwarf intermediate stocks are used more commonly. In terms of tree structure, slender spindle or free spindle are commonly used[9-10]. The characteristics of these tree structures are strong central trunk, numerous small main branches, large opening angle, even distribution, no lateral branches on the main branches, uniaxial extension, fruiting branches and fruiting branch clusters growing on the central main branch or small main branches, long and narrow crown, large upper and small lower. For the cultivation of tree structure, the first two to three years of trimming and pruning is the key[10]. This study found that the stem tapering grade of the free spindle-shaped short-branch Fuji apple was negatively correlated with the total thickness, average thickness, total length and average length of the central shaft, with correlation coefficients of -0.228, -0.033, -0.411 and -0.430, respectively (P>0.05). It could be concluded that for the trimming and pruning of the trunk-shaped trees under close planting conditions, the central trunk must be maintained erect and strong (i.e. high tapering grade). In order to obtain ideal high-yield tree structure, the number of main branches is required to be large, that is, the growth thickness is relatively small, and the total length and the average length are both small. Therefore, in addition to appropriately increasing the height of the trunk, it is necessary to bud many times before germination to promote branching. Simultaneously, the main branch axis should be opened earlier in the growing season, from the original September to May, to ease the growth potential. The stem tapering grade of the free spindle-shaped long-branch Fuji apple was positively correlated with the total thickness, average thickness, total length and average length of the central shaft, with correlation coefficients of 0.475, 0.181, 0.120 and 0.258, respectively (P>0.05). The results showed that if the tree structure of free spindle is selected for long-branch Fuji apple, the number of main branches needs to be reduced appropriately to maintain a high tapering grade, thereby increasing the thickness and length of the main branches, and further achieving high yield. Fundamentally, the transformation of different tree structures is all to produce the best lighting conditions required to product high-quality fruit. Studies have shown that if trees have good lighting conditions, the fruit quality will be good, and the flower buds formed will be also increased significantly. Therefore, for dwarf dense planting orchards, the lighting conditions inside the crown must be good enough, and it should be no les than 3% of natural light[11]. Good lighting conditions can be achieved by bud notching, expanding the branch angle in the early growing season, adjusting branching time, increasing the height of the trunk, and other shaping measures[10, 12-13].