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Article

The Effects of the Tree Structure of Zaosu Pear on the Transport and Distribution of Photosynthetic Assimilates and Fruit Quality under Desert-Area Conditions

by
Mingxin Zhao
1,2,
Wentai Sun
2,
Hongxu Li
2,
Wei Wang
2,
Gang Cao
2 and
Falin Wang
1,2,*
1
College of Horticulture, Gansu Agricultural University, Lanzhou 730070, China
2
Institute of Fruit and Floriculture Research, Gansu Academy of Agricultural Sciences, Lanzhou 730070, China
*
Author to whom correspondence should be addressed.
Agronomy 2022, 12(10), 2440; https://doi.org/10.3390/agronomy12102440
Submission received: 24 August 2022 / Revised: 1 October 2022 / Accepted: 3 October 2022 / Published: 8 October 2022
(This article belongs to the Special Issue Nutrient Management in Orchards)
Browse Figures
Versions Notes

Abstract

:
Pear is an important fruit tree in China, and the Hexi area is the main pear-planting area in Gansu Province. Tree shapes have different effects on photosynthesis that influence fruit quality and development. Thus, in the current study, five tree shapes of Zaosu pear, including the Y-shaped, trunk, single-arm, double-arm, and spindle tree shapes, were selected, and their effects on photosynthesis and fruit quality were investigated. The thickness and length of shoot branches were highest in the single-, double-arm, and spindle tree shapes. The level of photosynthetically active radiation (PAR) varied by tree shape; from highest to lowest, the order was double-arm > single-arm > spindle > Y-shaped > trunk tree shapes. Leaf area and chlorophyll content were highest in the single- and double-arm tree shapes, with higher increases in the net photosynthetic rate to light intensity (Pn-PAR), the net photosynthetic rate to CO2 (Pn-CO2), the relative variable fluorescence (Vj), PSII maximum photochemical efficiency (Fv/Fm), and the light energy absorbed per unit reaction centre (ABS/RC). For fruit quality, the fruit shape index, fruit colour parameters, and content of soluble solids increased significantly in the single- and double-arm tree shapes, while the content of total acids, malic acid, and citric acid in the single- and double-arm tree shapes was lower than in the other tree shapes. All these results demonstrated that the single- and double-arm tree shapes are well ventilated and light-transmitting, which can promote fruit growth and quality.

1. Introduction

Pear (Pyrus spp.), belonging to the Rosaceae family, is cultivated worldwide in various countries and has become the third most dominant fruit tree in China [1,2]. According to FAQ data published by the UN Food and Agriculture Organization, the pear planting area in China accounts for 67.30% of the global area (http://www.fao.org/faostat/en/#data/QC (accessed on 2 October 2022) FAO 2020). Gansu Province is the main and most advantageous pear planting region in China; it has geographical and environmental advantages, such as its location in the upper region of the Yellow River, sufficient sunlight, and a great temperature difference between day and night, which all contribute to fruit growth quality. In Gansu Province, the pear planting areas are mainly distributed in six regions: Longzhong, Hexi, Longdong, Longnan, the Longnan Mountains, and Longxinan. The cultivars planted in these six regions differ. For example, ‘Dongguo’ and ‘Ruaner’ pears are mainly planted in the Longzhong region. The Hexi region is dominated by imported and local traditional cultivars, mainly ‘Pingguo’, ‘Zaosu’, ‘Huangguan’, and ‘Xinli No. 7’ pears. Among them, the ‘Zaosu’ pear, one of the main cultivars planted in northwest China, has a beautiful fruit shape, a smooth green peel, tender flesh, good flavour, and a high water content; it is also widely planted in Gansu Province. However, many factors affect the quality of pear fruits, including fertilizer, temperature, light, management, etc.
With the development of pear planting technology, labour savings, high production, and efficient cultivation have become the primary aims of the pear industry. The pear cultivation system has changed from the traditional mode with a large canopy to a dense planting mode accompanied by wide-row cultivation and a small canopy. In addition to environmental factors, tree space, tree shape and height, crop load, and soil nutrient levels also play important roles in maintaining the life of the orchard, affecting the balance between vegetation growth and pear fruit growth [3,4,5]. Good tree structures based on pruning can reduce the canopy size and increase light interception [6,7,8,9,10]. Thus, good tree structure and efficient management can also increase fruit quality and yield, which are related to light interception. Tree structure also has a direct effect on the density and spatial distribution of branches and leaves [11,12]. For example, a higher tree density with small canopy structures can intercept a high level of light and increase tree productivity in apple [13,14]. However, pear cultivars planted in different regions should use specific pruning technologies to form good tree structures correlated with high yield [15]. For example, in pear orchards, the trunk, spindle, and modified spindle tree shapes are widely planted in northern China. The Y-shaped and single- and double-arm shapes are mainly applied in south China. Currently, the cylindrical, inverted umbrella, Y-shaped, and horizontal trellis tree shapes, which feature high light efficiency, have become mainstream in pear-planting regions; with these tree structures, the quality and economic benefits of pears have been significantly improved [16]. Moreover, since 2012, trunk-shaped tree pruning has expanded rapidly in China, especially in Hebei and Gansu Provinces, as it has a series of advantages, including early fruit, high yield, simple tree structure, and convenient mechanical operation. Compared with the traditional cultivation mode, pear orchards of this type can achieve the full-fruit period three years earlier and show an increased average yield of 18 tonnes per hectare; both of which play important roles in promoting the recovery of the early cost of orchard establishment and increasing the enthusiasm of farmers [17]. Compared to the trunk and spindle tree shapes, light interception levels and solar radiation transmission are greater in the Y-shaped and single- and double-arm tree shapes [18,19]. In addition, the trunk, spindle, and modified spindle tree shapes need more specific management to achieve high-yield production.
The Hexi area is flat and sparsely populated, which is suitable for large-scale pear planting. The trunk tree has been widely promoted and developed in this area. However, due to improper management and weather effects, the cultivation mode did not show dwarf density, labour-saving, mechanisation, or standardisation characteristics. It is difficult to manage and has a long picking cycle.
Thus, in recent years, the Y-shaped, single-arm, and double-arm tree shapes have gradually been developed in this region. However, the relationships between tree shape, light utilisation, and fruit quality have not been systematically studied. Thus, in the current study, Zaosu pear was taken as the experimental material, and the tree characteristics, light intensity, photosynthesis, and fruit quality parameters were measured for the Y-shaped, trunk, single-arm, double-arm, and spindle tree shapes. The objective of this study was to propose a reasonable tree structure that is conducive to pear production in the Longdong region, which could provide a theoretical basis and practical guidance for high-quality, high-efficiency production and labour-saving cultivation of Zaosu pear.

2. Materials and Methods

2.1. Plant Materials

The experiments were carried out in the core demonstration area of the Lanzhou Experimental Station in Nongken Tiaoshan, Jingtai County, Baiyin City, Gansu Province. The experimental station is located on the southern edge of the Tengger Desert at an altitude of 1620 m. The annual frost-free period lasts 141 days. The soil of the orchard is sandy limestone soil, and the soil pH is 8.0. The soil layer is deep, and the organic element content is 1.4%. The terrain of the station is flat, and irrigation was performed by drip irrigation; management and fertilisation were consistent among the different tree shapes. The climate and site conditions in this region are suitable for pear planting.
Using Duli pear as rootstock, eight years of Zaosu pear trees were selected. Five tree shapes, including Y-shaped, trunk, single-arm, double-arm, and spindle, were selected to investigate the relationship between tree shapes and light utilisation and fruit quality parameters. For Y-shaped trees, the height of the tree is 2.5–3.0 m, and the height of the main trunk is 40–50 cm. Two main branches on the main trunk extend to the rows, with the base angle at 40–50° from the vertical. Groups of medium and short fruiting shoots grow on each main branch. For the trunk tree shape, the height of the tree is 3.0 m. Below 60 cm, there are no shoots on the trunk. Above 60 cm, 24 to 28 fruiting branches are distributed on the main trunk. The base angle of the branches is 70°–90°. Small fruiting shoots and short fruiting shoots are directly attached to the branches. The plant row spacing is 1 m × 4 m. For the single-arm tree shape, the plant row spacing is 2 m × 4 m, and a fruiting branch is cultivated at a distance of 30–40 cm on both sides. The double-arm tree shape has the same fruiting branch distribution, with a different plant-row spacing of 4 m × 4 m. For the spindle tree shape, the height of the tree is 3.5 m. Below 65 cm, there are no shoots on the trunk. The trunk is evenly distributed with 8–12 slender main branches. The opening angle of the main branches is 70–80°. The plant row spacing is 1.5–2.0 m × 4.0–4.5 m (Figure 1). In this study, for each tree shape, five trees of similar size, with the same growth pattern and no disease or pests, were selected at the station for further analysis. The experiments were performed for two consecutive years, in 2020 and 2021.

2.2. Determination of Tree Shape Parameters and Chlorophyll Content

The height, crown width, length and thickness of the main branch, and length of new shoots were measured by a metre ruler or slide calliper ruler for five trees of each tree shape. The thickness of the main branch was measured at a position five centimetres away from the main trunk. The crown width was measured at the projection of the crown from the east–west and north–south directions of the crown. The number of branches, branches per 1/15 hectare (ten thousand), number of long fruit branches (>15 cm), number of middle fruit branches (5~15 cm), number of short fruit branches (<5 cm), and number of axillary flower buds were counted for five trees of each tree shape.
Three mature leaves of annual branches were selected from four locations in the east, west, south, and north of the central periphery of the canopy of Zaosu pear grown in different tree shapes. Then, the dirt on the surface of the leaves was wiped off. The fresh leaves (0.1 g) were cut into pieces with a width of 0.2 cm and placed in 20 mL of ethanol:acetone (1:1) extraction liquid, and the absorbance was measured at different wavelengths (663 nm and 645 nm) by a UV-2550 spectrophotometer (Shimadzu, Japan). Total chlorophyll was calculated using the formula (FW·mg/g) = (20.2 A645 + 8.02 A663) × V/(1000 × FW), where FW stands for fresh weight and V stands for the volume (mL) of extracting solution.

2.3. Determination of the Canopy Characteristics of the Five Tree Shapes

For each tree shape, 100 mature leaves of new shoots at different positions in the middle of the canopy were selected. Then, the area of the leaves was measured using a CI-203 portable laser leaf area meter (CID Bio-Science, Inc., Camas, WA, USA).
The canopy characteristics of the five tree shapes were tested by a CI-110 canopy meter when the weather was cloudy or in the evening in the middle of August. The CI-110 canopy meter was placed at a distance of 50 cm from the main trunk under the canopy to take canopy images from the east, south, west, and north directions for five trees of each tree shape. Then, canopy analysis software (Plant Canopy Analysis System V6.0, CID Bio-Science, Inc.) was used to process the images and calculate the leaf area, leaf area index (LAI), and mean leaf inclination angle (MFIA).

2.4. Determination of Photosynthesis Parameters

At the beginning and middle of August, when the weather was sunny, the LI-6400XT portable photosynthesis instrument (LI-COR, Inc., Lincoln, NE, USA) was used to measure the net photosynthetic rate (Pn), transpiration rate (Tr), stomatal conductance (Gs), intercellular concentration of CO2 (Ci), light intensity, and concentration of CO2 in air (Ca) from 7:00 to 20:00. In addition, a Handy PEA (Hansatech, Pentney, UK) continuous excitation fluorometer was applied to determine the kinetic parameters of chlorophyll fluorescence, such as the initial fluorescence (Fo), maximum fluorescence (Fm), variable fluorescence at time t (Fv), and relative variable fluorescence (Vj). Then, the PSII maximum photochemical efficiency (Fv/Fm) = (Fm − Fo)/Fm, photosynthetic performance index (PIabs) = (RC/ABC) × [φPo/(1 − φPo)] × [ψo/(1 − ψo)], light energy absorbed per unit reaction centre (ABS/RC) = Mo × (1/VJ) × (1/φPo), and light energy dissipated by heat (DIo/RC) = (ABS/RC) − (TRo/RC) were calculated. Three annual new shoot middle leaves at the same height on the south side of the outer canopy were selected from five trees of each tree shape to perform this measurement, with at least three replications.

2.5. Determination of the Fruit Quality Index

In mid-August, the fruits were picked at the maturity stage. A total of 75 fruits from five trees of each tree shape were randomly selected to determine the fruit quality index. The weight of a single fruit was weighed with a 1/1000 electronic balance. The fruit shape index was measured by a slide calliper ruler. A GY-1 fruit firmness tester was used to measure fruit firmness [20]; a PAL-1 hand-held refractometer (ATAGO, Tokyo, Japan) was used to measure fruit soluble solid content, and the acid–base titration method was used to measure the fruit titratable acid content [21]. The soluble sugar content was determined by Fehling’s reagent method [22].
For the organic acid content, 3 g of pulp was extracted from each quadrant along the equatorial part of each fruit; it was then chopped and mixed. Then, 1 g of pulp was weighed and extracted in a 45 °C water bath for 25 min with 80% ethanol. After repeated extraction 3 times, the extracts were mixed, and the volume was brought to 8 mL. Then, 1 mL of the mixed extracts was evaporated and redissolved in water for liquid chromatography analysis using an Agilent 1260 and Agilent Zorbax Eclipse XBD-C18 chromatographic column (Agilent Technologies, Santa Clara, CA, USA) (4.6 mm × 250 mm, 5 μm) with a column temperature of 30 °C, a 0.02 mol/L disodium hydrogen phosphate buffer mobile phase (pH adjusted to 2.6 with orthophosphoric acid), a flow rate of 0.5 mL/min, and an Agilent 1260 VWD detector with a wavelength of 210 nm. The input sample volume was 10 μL. Standard samples of malic acid and citric acid were purchased from Yuanye Company (Shanghai, China).

2.6. Determination of Fruit Colour

A total of 75 pear fruits were randomly selected from five trees of each tree shape to measure the fruit gloss brightness with a Minolta portable chromameter (Konica Minolta, Tokyo, Japan, Model CR-400), which provided the CIE L*, a*, and b* values. The values of L*, a*, and b* were measured for each fruit at the equatorial position. L* represents the gloss brightness, and the value range is from 1 to 100. a* and b* represent colour components, and the value ranges are from −60 to +60. The value of a* represents the green to red changes in the fruit peel. A positive value represents red, and a negative value represents green. In addition, the value of b* represents yellow when it is positive. When the value is higher, the fruit surface is more yellow [23,24].

2.7. Statistical Analysis

All the data were statistically analysed with SPSS 19.0 software (SPSS Inc., Chicago, IL, USA). Significant differences were analysed by Duncan’s test with p values < 0.05 and <0.01. Photosynthetic response curves were fitted with SPSS 19.

3. Results

3.1. Different Tree Shapes Have Different Tree Parameters

To understand the characteristics of the different tree shapes, the tree parameters were investigated. As shown in Table 1, the trunk height was 480.0 cm and 467.7 cm in the trunk and spindle tree shapes, respectively. The other three frame shapes have no obvious trunk. The crown width was highest in the double-arm tree shape and lowest in the trunk shape, demonstrating that the trunk tree shape is suitable for dense planting. The number of branches was highest in the double-arm tree shape, at 27; in contrast, there were only 14 branches for the single-arm tree shape. Although the number of branches was lowest in the single-arm tree shape, the thickness and length of branches were highest in the single- and double-arm and spindle tree shapes. In addition, the length of new shoots was lowest in the trunk tree shape, at 45.61 cm; in the other four tree shapes, it was above 50 cm, and it was highest in the spindle tree shape. The number of axillary flower buds was also high in the double-arm and spindle tree shapes. Moreover, the number of long, middle, and short fruit branches was highest in the double-arm tree shape; it was the lowest in the trunk tree shape.

3.2. Different Tree Shapes Have Different Canopy Structure Indices

The canopy structure has an effect on photosynthesis, which ultimately affects fruit quality. Thus, the canopy structure index was measured for the five tree shapes. As shown in Table 2, the leaf area index (LAI) was highest in the trunk shape, reaching 4.19, and the LAI was lowest in the double-arm shape, at 3.55. The mean leaf inclination angle (MFIA) in the single-arm tree shape was 22.65, and MIFA was lowest in the trunk tree shape. In addition, the direct transmission coefficient (TC) was highest in the double-arm tree shape; it was also high in the single-arm tree shape, reaching 0.255, and it was lowest in the trunk tree shape. The TC in the double-arm tree shape was 1.59-fold that of the trunk tree shape. Moreover, the level of photosynthetically active radiation (PAR) varied by tree shape; the order from highest to lowest was double-arm > single-arm > spindle > Y-shaped > trunk shape. The PAR in the double-arm tree shape was 76.12% higher than in the trunk shape.

3.3. Effects of Different Tree Shapes on Leaf Area, Chlorophyll Content, Net Photosynthetic Rate, Light Intensity, and CO2

Tree shape had a direct effect on the canopy structure indices, as shown by the investigation of the leaf area and chlorophyll content in the five tree shapes. As shown in Figure 2A,B, the leaf area of the single- and double-arm tree shapes was significantly greater compared to that of the trunk tree shape, but there was no significant difference compared to the Y-shaped and spindle tree shapes. The content of total chlorophyll was also highest in the single- and double-arm tree shapes, while it was lowest in the Y-shaped trees.
In addition, the response of the net photosynthetic rate of Zaosu pear to light intensity and related parameters in different tree shapes was tested. As shown in Figure 2C, the curve of the net photosynthetic rate with light intensity (Pn-PAR) had the same variation trends among the five tree shapes. When PAR was between 0 and 200 μmol·m−2·s−1, Pn had a linear relationship with PAR and increased with increasing PAR. When PAR increased from 200 to 2000 μmol·m−2·s−1, the Pn grew slowly and reached the maximal level when PAR arrived at the light saturation point. Within the range of effective light intensity, the average value of Pn was greatly increased in the single- and double-arm tree shapes. In addition, according to the Pn-PAR curve data, we calculated the light compensation point (LCP), light saturation point (LSP), Pnmax, apparent quantum yield (AQY), and dark respiration rate (Rd). As shown in Table 3, the values of LCP and LSP were highest in the double-arm tree shape, demonstrating that the double-arm tree shape was adaptable to the light environment. Moreover, the value of Pnmax in the single- and double-arm tree shapes was significantly different compared to the other three tree shapes, while the value of Rd was higher in the double-arm tree shape than that of the spindle tree shape, which further shows the high efficiency of the light energy utilisation and physiological activity of leaves in these two tree shapes.
Furthermore, the response of the net photosynthetic rate of Zaosu pear grown in different tree shapes to CO2 and related parameters was tested. As shown in Figure 2D, the curve of the net photosynthetic rate to CO2 (Pn-CO2) had the same variation trends as Pn-PAR. When the CO2 concentration was 0 to 800 μmol·mol−1, Pn increased rapidly, and this stage was called the initial linear response stage. When the CO2 concentration increased from 800 to 1400 μmol·mol−1, the increment of Pn slowed; when it reached the CO2 saturation point, Pn stopped increasing. Within the range of effective CO2 concentrations, the average value of Pn was greatly increased in the double- and single-arm tree shapes. In addition, according to the Pn-CO2 curve data, we calculated the CO2 compensation point (CCP), CO2 saturation point (CSP), Pnmax, and CE. As shown in Table 4, the value of CCP was lowest in the single- and double-arm tree shapes, but there was no significant difference compared to the Y-shaped and trunk tree shapes. The value of CSP was also highest in the single- and double-arm tree shapes compared to the Y-shaped and trunk tree shapes, demonstrating that these two tree shapes can perform effective photosynthesis under low concentrations of CO2. Moreover, the Pnmax of the double-arm tree shape was highest, while the value of CE did not significantly differ among the five tree shapes.

3.4. Effects of Different Tree Shapes on Photosynthesis Parameters

To gain insight into the effects of different tree shapes on photosynthesis parameters, the photosynthetic rate (Pn), stomatal conductance (Gs), intercellular CO2 concentration (Ci), and transpiration rate (Tr) were determined. As shown in Figure 3, the diurnal variation in Pn was bimodal. With an increase in light intensity, the leaf Pn rose rapidly, and the peak values occurred at approximately 11:00 and 16:00. At approximately 12:00 o’clock, it reached its lowest value. The same variation trend was observed for Gs, Ci, and Tr. The appearance of the peaks on one day showed a slight difference, and two peaks appeared at approximately 10:00 and 15:00. Interestingly, the values of these photosynthetic parameters were highest in the double- and single-arm tree shapes, which further verified that these two tree shapes had a higher effective photosynthetic rate in Zaosu pear.

3.5. The Effects of Different Tree Shapes on Chlorophyll Fluorescence Parameters, Quantum Yield, and Activity of the PSII Reaction Centre

Chlorophyll fluorescence parameters, the quantum yield, and the activity of the PSII reaction centre are other factors that can reflect photosynthetic activity. As shown in Figure 4, the relative variable fluorescence (Vj) and PSII maximum photochemical efficiency (Fv/Fm) were highest in the Y-shaped, single-, and double-arm tree shapes and lowest in the trunk tree shape. The leaf φPo (maximum photochemical efficiency) and φEo (PSII reaction quantum yield of light energy absorbed by the heart for electron transfer) were lowest in the single-arm tree shape, but they showed no significant difference compared to the Y-shaped and double-arm tree shapes. In addition, the light energy absorbed per unit reaction centre (ABS/RC) and light energy dissipated by heat (DIO/RC) were also lowest in the double-arm tree shape. The above results demonstrate that higher levels of Fv/Fm and ABS/RC in the double-arm tree shape promoted electron transfer.

3.6. The Effects of Different Tree Shapes on the Picking Time and Number of Fruits

In Gansu Province, Zaosu pears are generally picked from the tree at three times during the ripening period. In this study, the first pick took place at the beginning of August, when the weight of a single fruit reached 280 g. The second pick started in mid-August, which is the best fruit-ripening period, when the weight of a single fruit was more than 225 g. All the remaining fruits on the tree were harvested during the third pick, which is approximately at the end of August. As shown in Table 5, the total amounts of fruit on the Y-shaped, trunk, single-arm, double-arm, and spindle trees were 263.33, 128.67, 138.33, 364.00, and 163.00, respectively. The highest yield was observed for the double-arm shape. In this study, the three fruit picking days were 9, 18, and 28 August for all five tree shapes. At the first picking time, 40.67 and 50.67 fruits were picked from the single- and double-arm tree shapes, respectively, which was much higher than the yield from the other three tree shapes. In addition, the lightest single fruit weight for the single-arm tree shape was 321.61 g, and fruit picked from trees of this shape accounted for 29.40% of total fruits. Six fruits were picked from the Y-shaped tree shape at the first picking time, and the lightest single fruit weight was 280.00 g. At the second picking time, the proportion of fruits picked from the single-arm shape was the highest, reaching 35.42%. The picking proportion for the trunk shape was the lowest, and its single fruit weight was the lightest, at approximately 247.36 g. Moreover, most fruits were harvested during the third pick, except for the single-arm shape. For example, more than 50% of fruits were picked during the third pick for the Y-shaped, trunk, double-arm, and spindle tree shapes, and the weight of the lightest fruit was lower compared to the first two picking times for all five tree shapes.

3.7. Effects of Different Tree Shapes on Fruit Firmness, Shape, and Colour

Different planting tree shapes affect the fruit characteristics of Zaosu pear. The average fruit firmness during the ripening period was highest in the spindle tree shape, at about 16.74 N, which was significantly higher than that of the Y-shaped, trunk, and double-arm tree shapes (Figure 5A). The fruit shape of Zaosu pear is usually oval or long oval and is related to the horizontal and vertical diameters of the fruit. Thus, the fruit shape, including horizontal and vertical diameters, was analysed for the five tree shapes. As shown in Figure 5B, there was no significant difference in the vertical diameter among the five tree shapes, but there was a significant difference in the horizontal diameter. The highest fruit horizontal diameter was 76.78 cm in the double-arm tree shape; this was not significantly different from that of the single-arm tree shape. In addition, the highest ratio of the vertical diameter to the horizontal diameter was observed in the trunk shape, and the lowest was in the single- and double-arm tree shapes.
Different planting tree shapes have an effect on the colour formation of fruits. Thus, the fruit colour parameters of the five tree shapes were analysed by fruit colorimetry. As shown in Figure 5C, the fruit L* value in the single- and double-arm tree shapes was the highest, while that of the trunk shape was the lowest; however, there were no significant differences among the Y-shaped, trunk, or spindle tree shapes. The L* of the frame tree shapes, including the Y-shaped and single- and double-arm tree shapes, was significantly higher than that of the other two tree shapes. However, the value of a* showed no significant differences among these five tree shapes. The fruit b* value was highest in the single- and double-arm tree shapes, and the trunk tree shape had the lowest value, but there were no significant differences among the Y-shaped, double-arm, and spindle tree shapes, indicating that the fruits in the single- and double-arm shapes had more yellow colour than those of the other tree shapes. The fruit colour changes in the five tree shapes were closely related to the proportion of picking quantity and fruit weight at different times, which may indicate that the frame tree shapes, especially the single- and double-arm shapes, were well ventilated and light-transmitting, promoting rapid fruit development and colour formation.

3.8. The Effects of Different Tree Shapes on Fruit Internal Quality

To gain insight into the effects of different tree shapes on fruit nutrition, the content of soluble solids, total acid, malic acid, and citric acid was measured. As shown in Figure 6, the content of soluble solids was the lowest in the Y-shaped and spindle tree shapes. There was no significant difference among the other three tree shapes, but levels were highest in the single- and double-arm tree shapes. The content of total acids showed significant differences between the Y-shaped and trunk tree shapes and the single-arm, double-arm, and spindle tree shapes. The content levels of total acids in the single- and double-arm shapes were 2.147 mg/g and 2.099 mg/g, respectively, and it was 2.575 mg/g in the fruits of the trunk tree shape. In addition, the organic acids in pear fruit are mainly aliphatic organic acids, and the most abundant acids in edible fruit tissues are malic acid and citric acid. The acid content and its relative ratio are related to fruit maturity. The content of malic acid and citric acid differed significantly among the five tree shapes. The content of malic acid was highest in fruits from trunk-shaped trees and lowest in the single- and double-arm shaped trees, while there were no significant differences among the Y-shaped, single- and double-arm, or spindle tree shapes. However, the citric acid content was highest in fruit from the Y-shaped and trunk tree shapes and lower in the other tree shapes.

4. Discussion

Light is the necessary “driving force” for fruit trees to perform photosynthesis. Through training and pruning, shoot/spur balance and light exposure are altered in different tree shapes [14,24]. In addition, light interception and distribution have great effects on productivity and fruit quality [25,26]. Here, we systematically studied the behaviour of five tree shapes of Zaosu pear in the Longdong area and the related effects on photosynthesis and fruit quality.

4.1. Effects of the Tree Shape on Photosynthetic Parameters

Under the same planting conditions, the tree shape determines the planting density and canopy structure. Different planting tree shapes lead to different effects on light intensity, temperature, and relative humidity. Among them, the light intensity in fruit orchards is an important factor that affects photosynthetic efficiency. The fruit tree canopy structure, including backbone branches, branch groups, and leaf curtains, has a great impact on light energy interception and energy flow among different organs [27,28]. The crown width of different tree shapes has a strong relationship with the planting density and branch types. Various tree structures are characterized by either long branches or short branches. Generally, long branches are concentrated in the middle and upper parts of the tree, and the branches in the lower part of the branch group. These different tree parameters have different effects on light transmission. For instance, open-centre trees have higher photosynthetic efficiency than trunk trees. In C. oleifera, light transmission in the open-centre tree shape was 15% higher than in the round-head tree shape [29]. Hao found that an open-centre canopy also produces advantageous leaf parameters that improve the photosynthetic capabilities of Camellia saplings [30]. In peach, greater light penetration was detected in the open-centre tree type [19]. The photosynthetic rate of plants is related to the nature of leaves. In addition to leaves, a sufficient amount of light and high light efficiency favour photosynthesis, resulting in an increase in fruit quality. In apples, DIFN, LAI, and K can be used as important parameters to evaluate canopy optical properties [28]. A reasonable LAI is the main condition for fruit trees to make full use of light energy. If it is too large or too small, the density of the leaf curtain is high, and light penetration is weak, which affects photosynthetic production or reduces the accumulation of fruit tree assimilation products. A reasonable LAI ranges from 3 to 4 [31]. In this study, the LAI values in the single-arm, double-arm, and spindle tree shapes ranged from 3 to 4, while it was above 4 in the Y-shaped and trunk tree shapes. This demonstrates that the single-arm, double-arm, and spindle tree shapes effectively avoided dense foliage aggregation that would cause canopy closure in the middle and lower parts of the canopy. In addition, if the incidence angle is small and the leaf inclination angle is large, or if the incidence angle is large and the leaf inclination angle is small, it can facilitate the utilisation of light energy in leaves [32]. In this study, the MFIA, TC, and PAR values in the single- and double-arm tree shapes were significantly higher than those of the other three tree shapes, which demonstrates that the single- and double-arm tree shapes are conducive to photosynthetic synthesis within the crown and improved energy utilisation. In addition, the leaf area and chlorophyll content were highest in the single- and double-arm tree shapes, with higher increases in Pn-PAR and Pn-CO2. Meanwhile, the values of the photosynthetic parameters and Vj and Fv/Fm were higher in the double- and single-arm tree shapes compared to the trunk shape. These results further elucidate that the light compensation point and range of light saturation points were enhanced within the adaptability of the light environment in the single- and double-arm tree shapes, which improved the utilisation rate of light energy and increased the speed of photosynthesis.

4.2. Effects of Tree Shapes on Fruit Quality

Light is the source of energy for photosynthesis, and 95% of dry matter is synthesized by photosynthesis. Thus, the canopy structure and spatial distribution characteristics of branches and leaves directly affect the production of biomass [29,33]. Thus, light is known to be a main factor affecting fruit quality [34,35]. Training and reshaping tree structure to improve canopy characteristics can lead to the production of higher quality fruit. At present, research on tree structure mainly focuses on improving the light energy utilisation efficiency. Different tree shapes directly affect the plant density, individual spatial distribution, and lighting system in orchards, which all affect the yield and fruit quality of pears [36]. In apple, the fruit size, firmness, and soluble solids were also affected by planting density [3]. In peach, the fruit soluble solid content and firmness showed higher variability than the fruit size, and the within-tree variability was higher in delayed vase trees than in Y-shaped trees [37]. Compared to the spindle and Y-shaped trees, the cylindrical, V-shaped, and horizontal trellis tree shapes showed higher yield and better fruit quality [17,38]. In this study, fruit firmness was highest in the spindle tree shape, and the fruit shape index and fruit colour parameters were highest in the single- and double-arm tree shapes. The differences in fruit firmness, shape, and colour are likely related to poor light distribution with higher planting density, such as in the trunk tree shape. Higher planting density features closer plant spacing, leading to less space among branches, which results in decreased fruit colour [39]. Furthermore, the content of soluble solids was highest in the single-arm tree shape. The levels of total acids, malic acid, and citric acid in the single- and double-arm shapes were the lowest. Thus, compared with the other three tree shapes, the single- and double-arm tree shapes have a reasonable tree structure, with smaller numbers of branches and leaves; this reduces their nutrient consumption, leaving more nutrients for fruit development, which may also increase the fruit quality parameters.

5. Conclusions

The number of shoot branches was lowest in the single-arm tree shape, but the thickness and length of the shoot branches were highest in the single-arm, double-arm, and spindle tree shapes. In addition, the leaf area and chlorophyll content were highest in the single- and double-arm tree shapes, with higher increases in Pn-PAR and Pn-CO2. Moreover, the fruit picking time differed among these five tree shapes, and more fruits were harvested during the first pick in the single-arm tree shape. Regarding the fruit quality, the fruit firmness was highest in the spindle tree shape, and the fruit shape index and fruit colour parameters were highest in the single- and double-arm tree shapes. Furthermore, the single- and double-arm tree shapes had the highest content of soluble solids and the lowest content of total acids, malic acid, and citric acid. All the results demonstrated that the single- and double-arm tree shapes are well ventilated and light-transmitting, which can promote fruit growth and quality.

Author Contributions

Conceptualisation, M.Z. and F.W.; methodology, F.W.; software, W.S.; validation, H.L., M.Z. and W.W.; formal analysis, M.Z.; investigation, G.C. and M.Z.; writing—original draft preparation, M.Z.; writing—review and editing, M.Z.; supervision, F.W. All authors have read and agreed to the published version of the manuscript.

Funding

This study received funding from the China Agriculture Research System, supported by the earmarked fund for CARS (CARS-28-47). National Natural Science Foundation of China Regional Science Foundation Project (31860532) and (31460118).

Conflicts of Interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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Agronomy 12 02440 g001 550
Figure 1. Different shapes of Zaosu pear trees. (A) Y-shaped; (B) trunk; (C) single-arm; (D) spindle; (E) double-arm.
Figure 1. Different shapes of Zaosu pear trees. (A) Y-shaped; (B) trunk; (C) single-arm; (D) spindle; (E) double-arm.
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Figure 2. The effects of different tree shapes on the leaf area (A), chlorophyll content (B), net photosynthetic rate with the light intensity (C), and net photosynthetic rate with CO2 (D). The error bar represents the standard deviation (SD). Different lowercase letters indicate statistically significant (p < 0.05) differences between tree shapes.
Figure 2. The effects of different tree shapes on the leaf area (A), chlorophyll content (B), net photosynthetic rate with the light intensity (C), and net photosynthetic rate with CO2 (D). The error bar represents the standard deviation (SD). Different lowercase letters indicate statistically significant (p < 0.05) differences between tree shapes.
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Figure 3. The effects of different tree shapes on the Pn (A), Gs (B), Ci (C), and Tr (D). The error bars represent the standard deviation (SD).
Figure 3. The effects of different tree shapes on the Pn (A), Gs (B), Ci (C), and Tr (D). The error bars represent the standard deviation (SD).
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Figure 4. The effects of different tree shapes of Zaosu pear on the chlorophyll fluorescence parameters (A), quantum yield (B), and activity of the PSII reaction centre (C). The error bars represent the standard deviation (SD). The lowercase letters indicate statistically significant (p < 0.05) differences between tree shapes.
Figure 4. The effects of different tree shapes of Zaosu pear on the chlorophyll fluorescence parameters (A), quantum yield (B), and activity of the PSII reaction centre (C). The error bars represent the standard deviation (SD). The lowercase letters indicate statistically significant (p < 0.05) differences between tree shapes.
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Figure 5. The effects of different tree shapes on fruit firmness (A), fruit diameter (B), and fruit peel colour (C). The error bars represent the standard deviation (SD), and the lowercase letters indicate statistically significant (p < 0.05) differences between tree shapes.
Figure 5. The effects of different tree shapes on fruit firmness (A), fruit diameter (B), and fruit peel colour (C). The error bars represent the standard deviation (SD), and the lowercase letters indicate statistically significant (p < 0.05) differences between tree shapes.
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Figure 6. The effects of different tree shapes on the content of soluble solids (A), total acid (B), malic acid (C), and citric acid (D). The error bars represent the standard deviation (SD), and the lowercase letters indicate statistically significant (p < 0.05) differences between each tree shape.
Figure 6. The effects of different tree shapes on the content of soluble solids (A), total acid (B), malic acid (C), and citric acid (D). The error bars represent the standard deviation (SD), and the lowercase letters indicate statistically significant (p < 0.05) differences between each tree shape.
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Table
Table 1. Investigation of the tree indices of Zaosu pear trees with different shapes.
Table 1. Investigation of the tree indices of Zaosu pear trees with different shapes.
Tree ShapesY-ShapedTrunkSingle-ArmDouble-ArmSpindle
Height (cm)-480--467.7
Crown width (cm)3.6 bB ± 0.101.7 dD ± 0.232.8 cC ± 0.155.5 aA ± 0.173.3 bB ± 0.17
Number of branches 19.0 bB ± 0.9920.7 bB ± 1.2214.0 ± cC1.8727.0 aA ± 1.8018.3 bB ± 1.75
Length of main branch (cm)141.5 cB ± 4.05128.8 dB ± 7.30163.6 bA ± 7.75177.1 aA ± 1.95179.3 aA ± 7.75
Thickness (mm)25.4 cB ± 0.8022.9 dB ± 1.0532.7 aA ± 0.9529.8 bA ± 1.3530.8 abA ± 2.00
Length of new shoots (cm)52.5 bB ± 0.8545.6 cC ± 2.2553.5 bB ± 2.3552.2 bB ± 1.3563.3 aA ± 1.10
Branches per ha (Ten thousand × 15)4.4 aA ± 0.154.8 aA ± 0.203.7 bB ± 0.353.4 bB ± 0.254.4 bB ± 0.20
Number of long fruit branches27.5 cC ± 1.0525.3 cC ± 1.4525.4 cC ± 1.4556.7 aA ± 1.8542.0 bB ± 1.05
Number of middle fruit branches25.7 bB ± 1.1016.7 dD ± 0.7513.6 eE ± 1.0535.3 aA ± 1.0622.0 cC ± 0.93
Number of short fruit branches393.5 bB ± 2.75186.0 dD ± 8.75345.6 bB ± 5.20597.3 cC ± 8.80397.0 bB ± 8.75
Number of axillary flower buds42.0 dD ± 7.6568.3 cC ± 7.3577.6 cC ± 4.95136.0 bB ± 5.35165.0 aA ± 3.01
Note: Within the same row, the capital letters indicate significant differences at the p < 0.01 level, and the lowercase letters indicate significant differences at the p < 0.05 level.
Table
Table 2. Comparison of the canopy structure indices of Zaosu pear trees with different shapes.
Table 2. Comparison of the canopy structure indices of Zaosu pear trees with different shapes.
Tree
Shapes		Leaf Area Index(LAI)Mean Leaf Inclination Angle
(MFIA)		Direct Transmission Coefficient
(TC)		Photosynthetically Active Radiation
(PAR)
Y-Shaped4.07 aA ± 0.10513.02 dCD ± 0.5600.203 bB ± 0.00544.29 cC ± 2.070
Trunk4.19 aA ± 0.13511.07 dD ± 0.3950.165 cC ± 0.00838.99 dC ± 2.545
Single-arm3.68 bB ± 0.11022.65 aA ± 2.0050.255 aA ± 0.02560.36 bB ± 2.055
Double-arm3.55 bB ± 0.15518.22 bB ± 1.1750.262 aA ± 0.01268.67 aA ± 2.450
Spindle3.74 bB ± 0.11015.75 cBC ± 1.4950.240 aA ± 0.01157.59 bB ± 2.030
Note: Within the same column, the capital letters indicate significant differences at the p < 0.01 level, and the lowercase letters indicate significant differences at the p < 0.05 level.
Table
Table 3. Parameters of the photosynthetic light-response curve.
Table 3. Parameters of the photosynthetic light-response curve.
Tree ShapesLCP
/(μmol·m−2·s−1)		LSP
/(μmol·m−2·s−1)		Pnmax
/(μmol·m−2·s−1)		AQYRd
/(μmol·m−2·s−1)
Y-Shaped35.92 bB ± 2.0401669.37 dC ± 20.3320.30 cC ± 0.2250.0292 abA ± 0.0031.0250 bcA ± 0.015
Trunk31.19 cC ± 1.2051528.24 eD ± 28.0719.89 cC ± 0.8300.0333 aA ± 0.0031.0398 abcA ± 0.008
Single-arm36.69 bB ± 2.2301825.70 bB ± 24.3123.69 bB ± 0.9050.0285 abA ± 0.0051.0460 abA ± 0.013
Double-arm42.12 aA ± 1.8151942.37 aA ± 41.2226.55 aA ± 1.1900.0249 bA ± 0.0041.0490 aA ± 0.013
Spindle36.71 bB ± 2.0651711.58 cC ± 19.8020.62 cC ± 1.3800.0277 abA ± 0.0031.0170 cA ± 0.009
Note: Within the same column, the capital letters indicate significant differences at the p < 0.01 level, and the lowercase letters indicate significant differences at the p < 0.05 level.
Table
Table 4. CO2 response curve parameters.
Table 4. CO2 response curve parameters.
Tree ShapesCCP
/(μmol·mol−1)		CSP
/(μmol·mol−1)		Pnmax
/(μmol·mol−1)		CE
/(μmol·mol−1)
Y-Shaped56.55 abAB ± 1.9951790.89 cB ± 32.5340.01 cBC ± 0.9150.051 a ± 0.003
Trunk55.68 cbAB ± 1.0151690.89 dC ± 25.5237.33 dC ± 1.1250.05 a ± 0.002
Single-arm53.26 bcB ± 2.1351910.89 bA ± 35.4943.35 bB ± 1.0550.05 a ± 0.004
Double-arm52.66 Bc ± 1.5201969.10 aA ± 34.3354.99 aA ± 1.4650.055 a ± 0.003
Spindle59.70 aA ± 1.7851892.60 bA ± 29.4340.89 cB ± 1.6750.056 a ± 0.004
Note: Within the same column, the capital letters indicate significant differences at the p < 0.01 level, and the lowercase letters indicate significant differences at the p < 0.05 level.
Table
Table 5. Variation in picking times of Zaosu pears with different tree shapes.
Table 5. Variation in picking times of Zaosu pears with different tree shapes.
Tree
Shapes		Amount of FruitPicking
Time		NumberSingle Fruit Weight (g)Picking Percentage (%)
Y-Shaped263.339 August6.0280.002.28
18 August39.6258.3815.06
28 August208.0208.5678.99
Trunk128.679 August13.0296.1510.10
18 August12.3247.369.58
28 August85.7152.6866.58
Single-arm138.339 August40.7321.6129.40
18 August49.0287.7635.42
28 August38.3251.2427.71
Double-arm364.009 August50.7299.3913.92
18 August97.3295.3926.74
28 August199.0261.2154.67
Spindle163.009 August21.0281.9812.90
18 August17.7257.5010.84
28 August110.3209.1967.69
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Zhao, M.; Sun, W.; Li, H.; Wang, W.; Cao, G.; Wang, F. The Effects of the Tree Structure of Zaosu Pear on the Transport and Distribution of Photosynthetic Assimilates and Fruit Quality under Desert-Area Conditions. Agronomy 2022, 12, 2440. https://doi.org/10.3390/agronomy12102440

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Zhao M, Sun W, Li H, Wang W, Cao G, Wang F. The Effects of the Tree Structure of Zaosu Pear on the Transport and Distribution of Photosynthetic Assimilates and Fruit Quality under Desert-Area Conditions. Agronomy. 2022; 12(10):2440. https://doi.org/10.3390/agronomy12102440

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Zhao, Mingxin, Wentai Sun, Hongxu Li, Wei Wang, Gang Cao, and Falin Wang. 2022. "The Effects of the Tree Structure of Zaosu Pear on the Transport and Distribution of Photosynthetic Assimilates and Fruit Quality under Desert-Area Conditions" Agronomy 12, no. 10: 2440. https://doi.org/10.3390/agronomy12102440

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