Branch Growth, Leaf Canopies and Photosynthetic Responses of Zizyphus jujube cv. “Huizao” to Nutrient Addition in the Arid Areas of Northwest China ^†

Abstract

Jujube is one of the main tree species found in the arid areas of Xinjiang, China. However, the nutritional requirements of central leader jujube trees are not understood. Our aim was to explore the effects of different fertilization gradients on the growth, development, and canopy formation of jujube trees to provide a basis for efficient cultivation. We measured and compared various suitable indices of jujube trees under 16 different fertilization treatments, and we confirmed the treatments suitable for cultivation by correlation and principal component analyses. The jujube tree growth indices under different fertilization treatments significantly differed (p < 0.05). The application of nitrogen fertilizer promoted the growth of branches and leaves. The soil and plant analyzer development value, light, and other traits significantly differed (p < 0.05). The leaf area index and direct transmittance coefficient significantly differed (p < 0.05). Compared with single-fertilizer application, mixed-fertilizer application significantly increased the leaf area index. Correlation analysis showed that the net photosynthetic rate of jujube leaves significantly correlated with the stomatal conductance, transpiration rate, and leaf SPAD content (p < 0.01). We obtained three principal components with a cumulative variance contribution rate of 82.192%. The final ranking showed that the mixed treatment (N 460.77 g/tree, P₂O₅ 460.77 g/tree, and K₂O 588.23 g/tree) performed better. To ensure branch and leaf growth, this treatment promoted photosynthesis, enabling the growth and development of fruit trees.

1. Introduction

Jujube (Zizyphus jujube Mill.) belongs to the genus Ziziphus of the family Rhamnaceae. It is native to China, has a long history of cultivation, and was cultivated as early as 4000 years ago [1] as “Shijing and August peeled jujube” [2]. Jujube trees are resistant to drought, cold, salt, and alkali conditions, and hence can easily grow in poor land. Xinjiang, China has rich light and heat resources but poor-quality soil, making it suitable for jujube planting. In 2019, the planted area of jujube in Xinjiang was 480,000 ha, and the yield was 372.8 million tons, accounting for nearly half (49.9%) of the total output of the country [3]. The income from jujube accounts for approximately 71.3% of the per capita net income of farmers in southern Xinjiang. With the increase in the jujube planting area, the rainfall has increased, dusty weather has decreased, and the ecological environment has substantially improved.

Mineral nutrition forms the basis of the growth, yield, and quality of fruit trees. Three essential mineral elements that considerably impact the growth and development of fruit trees are N, P, and K. N deficiency affects the synthesis of organic matter in trees, resulting in tree weakness and flower and fruit drop, leading to a decline in the yields of apples [4], jujube [5], pear [6], kiwifruit [7], and other fruit trees. Deficiency in K can disrupt the metabolism of trees, destroy chlorophyll, inhibit photosynthesis, and reduce cold and drought resistance. Phosphorus deficiency is characterized by late flowering, slender branches, and sparse leaves, resulting in smaller and poor-quality fruits. The leaf canopy, as the main factor determining photosynthetic efficiency, directly determines the fruit yield and quality as well as economic profit. The leaf area index (LAI), which refers to the leaf area per unit land area, is an important index of the canopy coefficient. It is used to measure the thickness of leaf canopies, which can reflect the growth and development of tree branches [8]. In studies on grapes [9], jacaranda mimosifolia [10], jujube [11], and other fruit trees, the leaf canopy size positively correlated with the fruit yield and quality. Photosynthesis is an important physiological process in the growth of fruit trees and is closely related to light intensity [12]. Increasing light intensity increases the photosynthetic rate and accumulation of dry matter. Direct transmittance (TCRP) effectively explains the use of light in fruit trees [13].

Plant photosynthesis is the basis of dry matter accumulation and yield formation, and a higher photosynthetic area and photosynthetic rate are the bases for high yields [14]. The leaf is the main source of plant photosynthesis, and its structure and function are closely related to plant photosynthesis [15]. The morphological structure change of plant leaves is closely related to their physiological function and external environment [16]. Under the influence of different fertilization treatments, the morphological structure of leaves undergoes physiological adaptation. Different N application rates have remarkable effects on the cotton leaf morphology, photosynthetic rate, and population quality [17]. Nitrogen application can promote an increase in leaf area, thereby increasing the area available for light capture [18]. P and K directly affect plant photosynthesis and are strongly associated with multiple leaf morphological indicators in deciduous broad-leaved and evergreen needles [19].

At present, studies on the precise fertilization of jujube (Zizyphus jujube cv. Huizao) have mainly focused on soil nutrient utilization, internal and external fruit quality and yield, and its photosynthetic characteristics. However, only a few studies have been conducted on the changes in the leaves of Z. jujube cv. Huizao, the relationship between leaf canopy formation and photosynthesis, and canopy formation after fertilization treatment. In this study, we used 16 fertilization treatments with different nutrient (N, P, and K) ratios to measure and compare the growth, canopy, light intensity, photosynthetic characteristics, and relative chlorophyll content of jujube trees. We controlled the growth and development process of jujube trees with fertilization, thereby affecting their leaf canopies, improving the photosynthetic utilization rate of the canopy, and optimizing the nutrient structure of the trees. Our results provide a reference for a central leader jujube to enable the establishment of a reasonable fertilization system and provide a theoretical basis for improving the use rate of N and P fertilizers, reducing fertilizer pollution.

2. Materials and Methods

2.1. Selection of the Test Site

We conducted the experiment in a Xinliulian jujube plantation in the ninth regiment of the first division of Xinjiang (Figure 1). The site was located at 81°10′23″ E, 40°57′36″ N and an altitude of 1011 m. The study area is characterized by a continental arid desert climate, less snow in winter, and strong surface evaporation. The average annual sunshine is 2556.3–2991.8 h, average annual precipitation is 49 mm, average temperature is 10.7 °C, and frost-free period is 207–220 d. The photothermal resources are abundant, and the temperature difference between day and night is large. The test terrain was flat, and the soil layer was deep.

2.2. Test Materials and Design

We selected gray jujube in a jujube orchard as the study object, which had a plant spacing of 1.5 × 3 m and were planted in the north–south direction. The grafted rootstock was sour jujube, and the soil type was sandy. For these orchard jujube trees, flood irrigation was uniformly adopted. We set 16 fertilization treatments with 3 replicates per treatment, including individual N, P, K, and mixed fertilizers.

Table 1. Single-fertilization group.

Treatment (Code)		Group of Single Fertilization Amount (g·Tree)
		CH4N2O	P2O5	K2O
1	h1	564.12	326.08	588.23
2	h2	460.77	489.13	588.23
3	h3	396.97	652.17	588.23
4	h4	524.57	326.08	588.23
5	h5	524.57	326.08	1029.41
6	h6	524.57	326.08	1176.46
7	h7	524.57	326.08	441.17
8	n1	326.08
9	n2	652.17
10	n3	978.26
11	p1		326.08
12	p2		652.17
13	p3		978.26
14	k1			294.12
15	k2			588.23
16	k3			882.35

describes the experimental scheme. The field management of the tested plants was consistent with that of the farmers at the test site. We used urea (N ≥ 46%), potassium sulfate (K₂O ≥ 51%), diamine phosphate (P₂O₅ ≥ 46%), and the N, K, and P fertilizers, which are all water-soluble fertilizers. We fertilized the field from April to August 2021 during the early germination stage (late April), full flowering stage (middle June), and fruit expansion stage (early August) of the jujube trees. The three fertilizations are listed in Table 1. We performed furrow fertilization after water solubility.

2.3. Determination Method

2.3.1. Determination of Growth Indicators

For each treatment plant, we selected 5 perennial secondary branches from the middle of the jujube stock, and we numbered and registered 15 fruit-bearing shoots in Z. jujube plots. We measured the growth indices of all registered jujube fruiting branches in early June and September 2021.

We used an A ruler (Deli group, Ningbo, China) with an accuracy of 0.1 cm to measure the jujube fruiting branch length.

To determine the jujube fruiting branch diameter, we directly measured the diameter (coarseness) of the jujube fruiting branch 1 cm away from its base using an electronic digital display Vernier caliper IP54 (Dasqua, Chengdu, China) with an accuracy of 0.01 cm.

To determine the leaf number of the jujube fruiting branches, we used the direct counting method to count the number of leaves [20].

2.3.2. Determination of the Tree Canopy and Light Intensity

We measured the canopy characteristics of the trees using a CI-110 digital canopy analyzer (CID, Inc., Washington, DC, USA). We performed measurements under cloudy or evening conditions in mid-July 2021. We divided the zenith angle into 5 rings with angles of 8.50°, 25.50°, 42.50°, 59.50°, and 76.50°. We divided the azimuth angle into 4 parts with angles of 0–90°, 90–180°, 180–270°, and 270–360° [21]. During the measurement, we took the trunk as the center, and we placed the fish eyes 50 cm below the canopy from the trunk. We collected canopy images in four directions: east, south, west, and north. We analyzed and processed these images using the canopy analysis software that came equipped with the instrument. The measured parameters included the direct transmission coefficient (TC), LAI, and average leaf angle (MLA). Under sunny weather in mid-July, we used TES-1332a illuminance scoring (TES, Taipei, China) to collect data in eight directions: southeast, northwest, inner, central, outer, upper, middle, and lower. We collected nine data points in each direction, and we obtained the average value of three readings each time.

2.3.3. Determination of Photosynthetic Traits

We selected mature functional leaves from the middle part of the annual new branch hanging from the middle of the southward crown of the trees in each treatment. We measured the net photosynthetic rate (Pn, μmol·m⁻²·s⁻¹), stomatal conductance (Gs, mmol·m⁻²·s⁻¹), intercellular CO₂ concentration (Ci, μmol·mol⁻¹), and transpiration rate (Tr, mmol·m⁻²·s⁻¹) using an Li-6400 photosynthetic apparatus (LI-COR Biosciences, Inc., Lincoln, NE, USA). We recorded three sets of stable readings for each plant, which we repeated three times. We calculated the water use efficiency (WUE) according to the recorded parameters using the formula WUE = Pn/Tr [22].

2.3.4. Determination of the Soil and Plant Analyzer Development (SPAD) Value of the Leaves

When measuring the photosynthetic traits of the leaves, we selected 10 functional leaves from a new jujube fruiting branch in four directions—east, west, north, and south—outside of the crowns of 3 trees in each treatment. We measured the relative chlorophyll content using an SPAD-502 handheld chlorophyll meter (Konica Minolta Holdings, Inc. Tokyo, Japan), and we recorded the average values.

2.4. Data Processing

We used Excel 2016 for routine analysis, DPS 7.05 software for statistical analysis, and the new complex range method to analyze the differences. In the level of significance test, a lowercase letter indicates that the p < 0.05 level denotes significant correlation, and an uppercase letter indicates that the p < 0.01 level denotes extremely significant correlation. We used SPSS 25 software for correlation analysis between the canopy and photosynthetic traits and CI-110 software for statistical analysis of the canopy characteristic parameters.

3. Results

3.1. Effects of Different Fertilization Treatments on the Growth Indices of the Fruit-Bearing Shoot in Ziziphus jujuba

We found significant differences in the length, diameter, and number of expanded leaves on the jujube trees under different fertilization treatments (p < 0.05). Treatment n1 produced the highest length increase of 3.47 cm, followed by k1 at 3.08 cm. The maximum diameter increase in the jujube fruiting branch length in treatment p1 was 0.66 cm, followed by that in n2. The largest number of jujube leaves in mixed fertilization treatment h1 was 14.1. The N and P fertilizer application rates remained unchanged among the mixed application treatments (h1, h4, h5, h6, and h7), and when the amount of K fertilizer increased, the length, diameter, and leaf number of the fruit-bearing shoots in Z. jujuba first increased and then decreased (

Table 2. Growth index of jujube fruiting branch under different fertilization treatments (mean ± SD, n = 45). One tree was measured for 15 fruit branches, with 3 trees having 45 replicates. Mean totals with a common letter are not different (p < 0.05) by ANOVA for the ranking test.

Treatments	Length Growth (LG) (cm)	Diameter Growth (DG) (cm)	Leaf Number (Leaves) (Leaf)
h1	2.96 ± 0.38 bc	0.49 ± 0.01 bc	14.07 ± 1.03 a
h2	2.24 ± 0.55 bcd	0.22 ± 0.04 e	13.47 ± 1.17 ab
h3	1.86 ± 0.31 d	0.36 ± 0.09 d	13.60 ± 1.06 ab
h4	1.90 ± 0.67 cd	0.49 ± 0.14 bc	12.80 ± 0.92 abc
h5	1.91 ± 0.77 cd	0.46 ± 0.07 c	12.73 ± 2.04 abc
h6	1.64 ± 0.44 d	0.37 ± 0.08 d	10.45 ± 0.73 c
h7	2.65 ± 1.06 bcd	0.45 ± 0.03 c	13.07 ± 2.34 ab
n1	3.47 ± 0.92 a	0.51 ± 0.05 abc	12.20 ± 0.8 abc
n2	2.68 ± 0.49 bcd	0.63 ± 0.14 ab	13.20 ± 1.25 ab
n3	1.97 ± 0.12 cd	0.46 ± 0.04 c	12.30 ± 1.51 abc
p1	2.65 ± 0.41 bcd	0.66 ± 0.07 a	11.50 ± 0.95 bc
p2	2.38 ± 0.23 bcd	0.59 ± 0.18 abc	12.50 ± 1.48 abc
p3	2.19 ± 0.45 bcd	0.55 ± 0.09 abc	13.07 ± 0.91 ab
k1	3.08 ± 0.36 b	0.57 ± 0.13 abc	11.73 ± 1.01 bc
k2	2.08 ± 0.24 bcd	0.48 ± 0.14 bc	13.53 ± 0.76 ab
k3	1.78 ± 0.38 d	0.47 ± 0.05 c	11.53 ± 1.91 bc

).

3.2. Effects of Different Fertilization Treatments on the Light Interception Ability of Jujube Trees

Table 3. Canopy characteristics of the trees in different fertilization treatments (mean ± SD, n = 12). Mean totals with a common letter are not different (p < 0.05) by ANOVA for the ranking test.

Treatments	Leaf Area Index (LAI)	Mean Leaf Angle (MLA)	Transmission Coefficient (TC)
h1	1.28 ± 0.14 a	27.53 ± 6.55 a	0.33 ± 0.05 bc
h2	0.94 ± 0.12 d	33.32 ± 6.08 a	0.40 ± 0.05 ab
h3	1.04 ± 0.16 bcd	30.08 ± 8.18 a	0.36 ± 0.07 abc
h4	1.23 ± 0.21 abc	29.51 ± 0.65 a	0.36 ± 0.01 abc
h5	1.12 ± 0.05 abcd	28.62 ± 1.91 a	0.35 ± 0.02 bc
h6	1.01 ± 0.19 cd	30.55 ± 5.99 a	0.37 ± 0.05 abc
h7	1.26 ± 0.04 ab	25.63 ± 0.12 a	0.31 ± 0.01 c
n1	1.06 ± 0.03 abcd	29.87 ± 1.64 a	0.36 ± 0.02 abc
n2	1.12 ± 0.01 abcd	28.42 ± 0.52 a	0.33 ± 0.01 c
n3	0.97 ± 0.025 d	35.22 ± 2.37 a	0.42 ± 0.02 a
p1	1.03 ± 0.09 cd	31.37 ± 4.83 a	0.37 ± 0.04 abc
p2	0.97 ± 0.12 d	31.25 ± 6.14 a	0.38 ± 0.05 abc
p3	0.93 ± 0.13 d	35.23 ± 0.26 a	0.43 ± 0.01 a

shows that the LAIs of the jujube trees in different fertilization treatments were significantly different (p < 0.05), and we found the highest LAI (1.28) in the mixed N, P, and K treatment (h1). The LAI of the h1 trees was 27.34% higher than that of the trees treated with pure P fertilizer (p3: 0.93). The TC of the p3 trees was 27.91% higher than that of the h7 trees. We found no significant difference in the MLA among the treatments.

3.3. Light Distribution Characteristics

The leaf canopy refers to the total leaf area in the area where the distribution of leaves in the canopy is concentrated. The fruiting jujube in the test area was the central leader tree, and the canopy formed in early June. Figure 2a,b shows the three-dimensional distribution of light after leaf canopy formation. Although the light in the upper canopy was stronger than that in the lower canopy, and the light in the outer canopy was stronger than that in the inner canopy, the three-dimensional distribution was relatively balanced. As shown in Figure 2c,d, the light intensities at the interior and exterior of the fruit trees under different fertilization treatments significantly differed (p < 0.05). The light intensity at the interior and exterior of the canopy for the h2 treatment tree was the highest, and the light intensities at the interior and exterior of the trees in treatments h1 and h7 were lower, indicating that the branches grew more densely.

3.4. Effect of Different Fertilization Treatments on the SPAD Value of the Jujube Leaves

The SPAD value of the jujube leaves widely fluctuated with the application of different N, P, and K fertilizer amounts to the soil, varying from 31.5 to 42.3 (Figure 3). The SPAD value of the jujube leaves increased with the increased application of different N, P, and K fertilizers to the soil. The maximum value (42.3) occurred in the trees treated with pure N (n3), and the minimum value (31.5) occurred in the trees treated with pure K fertilizer (k1). Except for the treatment with a larger proportion of K fertilizer (h6), which was lower, the fluctuations in the treatments were not significantly different from those of the treatments with full applications of N, P, and K. The order of the strength of the effect of fertilization on the SPAD values of the jujube leaves was N > P > K.

3.5. Effects of Different Fertilization Treatments on the Jujube Leaf’s Photosynthetic Traits

The Pn reflects the photosynthetic capacity of the fruit trees.

Table 4. Photosynthetic traits of jujube trees under different fertilization treatments (mean ± SD, n = 48). Mean totals with a common letter are not different (p < 0.05) by ANOVA for the ranking test. Pn = net photosynthetic rate, Gs = stomatal conductance, Ci = intercellular CO₂ concentration, Tr = transpiration rate, and WUE = water use efficiency. Temperature in sample cell is 31.05 °C. Temperature of leaf thermocouple is 30.52 °C. Reference cell CO₂ is 397.86 μmol CO₂ mol⁻¹. Sample cell CO₂ is 392.04 μmol CO₂ mol⁻¹. Relative humidity in the reference cell is 41.67%. Relative humidity in the sample cell is 41.67%.

Treatment	Pn (μmol·m−2·s−1)	Gs (mmol·m−2·s−1)	Ci (μmol·mol−1)	Tr (mmol·m−2·s−1)	WUE (μmol·mmol−1)
h1	9.06 ± 0.02 e	96.8 ± 0.11 de	292.01 ± 2.01 c	3.90 ± 0.23 bc	2.28 ± 0.13 cd
h2	10.06 ± 0.83 d	115 ± 1.88 c	232.52 ± 3.47 d	4.20 ± 0.34 ab	2.35 ± 0.32 cd
h3	9.56 ± 0.62 de	132.6 ± 1.05 bc	301.82 ± 14.26 bc	4.62 ± 0.33 a	2.07 ± 0.06 e
h4	8.07 ± 0.28 f	57.7 ± 0.53 f	313.89 ± 9.55 b	3.81 ± 0.12 cd	2.15 ± 0.07 de
h5	8.62 ± 0.51 ef	52.3 ± 0.37 fg	242.58 ± 3.52 d	3.76 ± 0.33 cd	2.29 ± 0.13 cd
h6	7.34 ± 0.29 j	56.1 ± 0.28 f	400.22 ± 6.03 a	3.46 ± 0.47 de	2.15 ± 0.28 de
h7	10.50 ± 0.06 cd	83.4 ± 0.29 e	196.82 ± 5.68 de	4.06 ± 0.25 bc	2.59 ± 0.16 c
n1	11.54 ± 0.72 b	153.4 ± 0.02 b	189.18 ± 0.17 e	4.06 ± 0.14 bc	2.85 ± 0.28 ab
n2	12.76 ± 0.14 a	186.4 ± 0.01 a	182.27 ± 0.37 e	4.27 ± 0.15 ab	2.98 ± 0.08 a
n3	11.10 ± 0.49 bc	93.6 ± 0.01 d	218.24 ± 2.97 de	4.01 ± 0.11 cd	2.77 ± 0.16 ab
p1	9.47 ± 0.37 e	32.5 ± 0.28 h	140.46 ± 2.22 f	3.63 ± 0.29 de	2.61 ± 0.11 bc
p2	9.68 ± 0.17 de	65.4 ± 0.50 f	240.77 ± 13.82 d	3.38 ± 0.48 e	2.92 ± 0.48 ab
p3	10.42 ± 0.33 cd	102.7 ± 0.05 cd	268.51 ± 9.82 cd	3.78 ± 0.23 cd	2.76 ± 0.13 ab

shows that the Pn of the jujube leaves significantly differed among the 13 fertilization treatments (p < 0.05). We observed the highest Pn in treatment n2 (pure N fertilizer) and the lowest value in treatment h6 (high K fertilizer application), indicating that a certain amount of N fertilizer is helpful for improving the Pn of the jujube leaves and that excessive K fertilizer inhibits the photosynthetic efficiency of leaves. The stomatal channel allows CO₂ and H₂O to enter and leave the leaves of plants, which directly affects plant growth and development. Table 4 demonstrates that the change in Gs value under different fertilization treatments was consistent with that of the Pn and substantially differed between treatments. The Ci of each treatment was between 141 (p1) and 400 (h6) μmol·mol⁻¹, and the Ci of the trees in the mixed total fertilization treatments was generally higher than that of trees in the single-fertilizer application treatments. Of the mixed treatments, the Ci of the h6 treatment trees was substantially higher than that of the h7 trees, indicating that the application of K fertilizer more strongly affected the Ci of the leaves. The Tr reflects the change in the ability of the crop to regulate its own water loss. Among the 13 treatments shown in Table 4, the Tr of the trees in the pure PO₄ fertilizer treatment (p2) was the lowest at 3.46 mmol·m⁻²·s⁻¹, and it was the highest in the mixed fertilizer treatment (h3), being 4.29 mmol·m⁻²·s⁻¹. The WUE of the different fertilization treatments was consistent, with no significant difference among the treatments.

3.6. Correlation Analysis of the Physiological Growth Indices of the Jujube Leaves

We analyzed the correlation between the photosynthetic characteristic parameters and canopy parameters of jujube leaves under 13 different fertilization treatments. As shown in Figure 4, the leaf Pn significantly correlated with the Gs and Tr, with correlation coefficients of 0.75 and 0.80, respectively. The leaf Pn was significantly positively correlated with the leaf SPAD (r = 0.63) and significantly negatively correlated with the Ci (r = −0.71). The Tr correlated positively with the Gs (r = 0.71) and negatively with the Ci (r = −0.69). The LAI of the trees in the 13 treatments were extremely significantly negatively correlated with the inner light (r = 0.97), and the inner light was significantly negatively correlated with the peripheral light (r = −0.67). The inner and peripheral light intensities were extremely significantly positively correlated (r = 0.72). Among the growth indices, the monthly increase in the jujube fruiting branch diameter in the 13 treatments was significantly positively correlated with the WUE (r = 0.63), and the monthly growth of the jujube fruiting branch length was significantly positively correlated with the Pn (r = 0.56) and significantly negatively correlated with the Ci (r = −0.61). The leaf number of the jujube fruiting branches was significantly positively correlated with the Tr (r = 0.65).

3.7. Principal Component Analysis of the Physiological Growth Indexes of Jujube Trees

The physiological growth indices of the jujube trees had different levels and orders of magnitude, and thus they influenced the results. We standardized the original data using the Z-score to obtain new data, and then we tested the applicability of factor analysis by the Kaiser–Meyer–Olkin (KMO) and Bartlett sphere tests. The KMO value was 0.602, indicating that a certain correlation existed between the indicators. The Bartlett sphere test result was 174.067 with a Sig value of 0.00, indicating that the correlation coefficient was a rejected unit matrix; that is, the indicators were related. Both results showed that we could apply principal component analysis to the data. Therefore, we standardized the data from the 13 indicators through principal component analysis to extract 3 principal components.

Table 5. Eigenvalues and contribution rates of three principal components extracted.

Total Variance Explained
Component	Initial Eigenvalue			Extract the Load Sum of Squares
	Total	Variance (%)	Accumulation (%)	Total	Variance (%)	Accumulation (%)
1	4.597	35.360	35.360	4.597	35.360	35.360
2	3.865	29.727	65.087	3.865	29.727	65.087
3	2.224	17.105	82.192	2.224	17.105	82.192
4	0.791	6.084	88.275

lists the initial eigenvalue, variance contribution rate, cumulative variance contribution rate, etc., where the variance of each principal component is the eigenvalue that represents the extent to which the corresponding component described the original information. Table 5 shows that the first three principal components—PC1, PC2, and PC3—explained 82.192% of the total variance, indicating that the 3 extracted principal components could represent 82.192% of the original 13 physiological growth indicators. The extracted principal components reflected most of the information of the jujube physiological growth indicators.

According to the three principal component coefficients, we obtained a linear combination of PC1, PC2, and PC3:

PC1 = 0.2131 × SPAD + 0.0629 × DG + 0.0172 × LG + 0.2467 × Pn + 0.1623 × Gs − 0.1441 × Ci + 0.2523 × WUE + 0.0592 × Tr − 0.3959 × LAI + 0.4043 × MLA + 0.3913 × TC + 0.42628 × ICLI + 0.3455 × ECLI.

PC2 = 0.2671 × SPAD + 0.1908 × DG + 0.1907 × LG + 0.4146 × Pn + 0.3215 × Gs − 0.3647 × Ci + 0.3072 × WUE + 0.2248 × Tr + 0.2009 × LAI − 0.2131 × MLA − 0.2304 × TC − 0.14598 × ICLI − 0.1607 × ECLI.

PC3 = 0.1649 × SPAD − 0.5452 × DG − 0.0892 × LG + 0.05029 × Pn + 0.4017 × Gs + 0.2407 × Ci − 0.3091 × WUE + 0.5344 × Tr + 0.0396 × LAI − 0.0791 × MLA − 0.0986 × TC + 0.0396 × ICLI + 0.2127 × ECLI.

It can be seen from the above formula that in the principal component PC1, the absolute values of the coefficients of the LAI, MLA, TC, ICLI, and ECLI were greater than those of the other variables, so the principal component PC1 was a comprehensive reflection of these five physiological growth indexes, representing factors such as light transmittance.

In the principal component PC2, the SPAD, LG, Pn, and Ci coefficients were greater than the other variables, and thus the principal component PC2 was mainly reflected by the four physiological growth indicators. These four data represent the light reaction process of fruit trees.

Taking the relative variance contribution rate of each principal component as the weight, the scores of the first three principal components and the corresponding weights were linearly weighted and summed to construct a comprehensive evaluation function for the physiological growth index of jujube trees:

$$\mathrm{F}=0.3536\times \mathrm{PC}1+0.29727\times \mathrm{PC}2+0.17105\times \mathrm{PC}3.$$

According to

Table 6. Principal component scores, comprehensive scores, and ranking of physiological growth of jujube trees under 13 treatments.

Treatment	PC1	PC2	PC3	F	Ranking
h1	0.23	0.74	−2.87	−0.19	7
h2	2.26	−1.09	2.53	0.91	1
h3	2.78	−0.72	−0.32	0.72	3
h4	0.23	−1.02	−1.03	−0.39	9
h5	−0.55	−0.78	−1.56	−0.69	10
h6	0.31	−3.43	−1.49	−1.16	13
h7	−0.25	1.17	−3.06	−0.26	8
n1	0.19	1.39	1.38	0.71	4
n2	0.90	1.44	0.54	0.84	2
n3	−0.55	0.18	3.58	0.47	5
p1	−2.72	−0.06	−0.13	−1.00	12
p2	−1.99	−0.92	0.91	−0.82	11
p3	−0.86	−0.89	2.53	−0.14	6

, we calculated and sorted the comprehensive scores of each physiological growth index using the model, and we obtained the scores and ranking results of the 13 treatments for the 3 principal components. The results showed that the h2 and h3 (mixed application) treatments were more effective, and N was more effective than P in the pure fertilization treatments.

4. Discussion

With the acceleration of urbanization, the amount of land suitable for cultivation has continuously reduced. The food needs of the existing world population can only be met by increasing crop yields per unit area [23]. Applying fertilizers has become a method to reduce the effects of pests and diseases and quickly increase crop yields. However, the overapplication of chemical fertilizers by farmers has extremely negatively impacted the ecological environment [24]. In light of the serious pollution of the agricultural ecological environment caused by incorrect fertilization application methods, optimizing the fertilization methods, ensuring crop yield, and clarifying the amount of chemical fertilizer are required to ensure reasonable and precise fertilization.

The high quality and stable yield of fruit trees must be based on adequate tree nutrition, and the accumulation of nutrients in the orchard should be achieved based on adequate water and fertilizer management [25]. In this study, we observed significant differences in the lengths and diameters of the fruit-bearing shoots and the number of leaves on the fruit-bearing shoots of Z. jujuba under different fertilization treatments. N significantly affects plant growth and development. Increasing the amount of N applied within a certain threshold range can promote plant growth. When this threshold is exceeded, the physiological growth of the plant is excessive, and the branches and leaves grow excessively [26], which is consistent with our results in this study. The application of a certain amount of N has a stronger effect on the increase in the length of the jujube fruiting branch than P and K because N can promote cell division and elongation [27]. According to Erisman, applying a certain amount of P can increase the stoutness of the jujube fruiting branch, while applying mixed fertilizer in certain proportions increases the number of jujube leaves. When other factors remained unchanged, excessive K application inhibited tree growth. Apple [28], melon [29], watermelon [30], and other agricultural crops also showed this state.

In this study, we investigated the different light intensities in jujube tree canopies. The light intensity is affected by the leaf canopy environment. Regardless of the direction, the light intensity increased with an increasing crown height and decreased with a decreasing distance from the trunk, which is consistent with the conclusion reached by Ma and Yue [31]. The LAI of the fruit tree canopy is an important parameter for determining the biological characteristics of a fruit tree canopy [32]. The canopy size represents the effective leaf area used for photosynthesis by plant leaves and is an important indicator for evaluating the growth of plants. The number of branches and leaves and the LAI differed among the treatments, which resulted in different abilities to block light. According to the correlation analysis results, we found that the light intensity at the interior and periphery of the tree was significantly negatively correlated with the LAI, indicating that with an increasing leaf area, the light-blocking ability of the tree body strengthened. The leaf area coefficient of the N, P, and K mixed fertilizer treatment was significantly higher than those of the N or P fertilizer treatments, whereas the direct light transmittance coefficient was the opposite, indicating that the application of mixed fertilizer effectively increased the growth of tree branches but also led to canopy depression, higher radiation energy intercepted by the canopy, and a lower direct light transmittance coefficient. Chlorophyll is central to light absorption and transmission during photosynthesis, and its content positively correlates with photosynthesis. The chlorophyll content is positively correlated with the SPAD value [33]. In this study, the SPAD value of the leaves of trees treated with pure N (n3) was the highest (42.3). The results showed that increasing the N fertilizer application effectively increased the N and photosynthetic pigment contents in plant leaves, thereby increasing the photosynthetic performance of the plants and physiological activity of the leaves. Moreover, applying an appropriate N amount can effectively increase the chlorophyll content and photosynthetic intensity of jujube trees.

Photosynthesis is a decisive factor for crop growth and development [34]. N, P, and K are essential for plant growth and development. For example, N is a constituent element of various proteins involved in chlorophyll production and photosynthetic reactions. P is an important component of the cell and photosynthetic membranes, nucleic acids, and other important substances. P is directly involved in photosynthetic phosphorylation. K is an important element in stomatal regulation and an activator of various enzymes. Reasonable N, P, and K nutrition can increase plant growth and stomatal conductance to a certain extent, thereby increasing photosynthetic efficiency. However, excessive N, P, and K levels lead to a decline in photosynthetic capacity. In this study, with an increase in N fertilizer application, the net photosynthetic rate and stomatal conductance first increased and then decreased. Stomata are channels for CO₂ entry and water outflow [35], and the size of the stomatal aperture directly affects the photosynthetic rate and Tr of leaves [36]. The studies by Zhu Zu and Wu [37,38] showed that the application of more than a certain amount of K fertilizer inhibits the photosynthesis of jujube, which is consistent with our results.

Chlorophyll is the central pigment of photosynthesis in higher green plants, and its content has a notable influence on the photosynthetic capacity and yield formation [39]. Nitrogen is a component of several plant compounds. Excessive and insufficient N can inhibit the increases in chlorophyll content and root activity, reduce photosynthetic capacity, and lead to decreases in crop yield and quality. An appropriate N content is conducive to the synthesis of chlorophyll and an increase in root activity, promotes the improvement of photosynthetic capacity, and produces high-quality and yield crops [40]. Potassium can promote the uptake of N by plants and increase the efficiency of N use. The results of this study show that the application of a certain amount of N fertilizer could effectively increase the SPAD value of jujube leaves. The results of correlation analysis showed that the net photosynthetic rate of jujube leaves was significantly correlated with the stomatal conductance, transpiration rate, and SPAD value, indicating that the higher the chlorophyll content of jujube leaves, the stronger the photosynthetic potential. The application of P and K fertilizers did not significantly increase the SPAD value of the jujube leaves.

Principal component analysis has been widely used to comprehensively evaluate the quality of edible fungi [41], rice [42], and fruits [43]. In this study, we performed principal component analysis on 13 growth and development and photosynthetic physiological indices of fruit trees. The cumulative contribution rate of the first three principal components was 82.192%, effectively reducing the number of variables and extracting representative principal components. Based on the principal component score and load value, we concluded that the positive growth of the first principal component was conducive to increasing the ability of jujube trees to intercept light, and the positive growth of the second principal component was conducive to increasing the chlorophyll content of fruit tree leaves and accelerating the light reaction process. The positive increase in the third principal component increased the growth and development of the fruit tree branches. We comprehensively and objectively evaluated the effects of the 13 fertilization treatments on the growth and development of jujube trees.

5. Conclusions

In this study, we regulated the three components of fertilizer (N, P, and K) in the field to examine the changes in the growth dynamics and photosynthetic traits of jujube leaves under different nutrient stresses. The results showed that N application substantially contributed to the growth and development of fruit trees and canopy formation. In the mixed fertilizer treatments (h4–h7), under the premise of constant N and P fertilizer applications, the net photosynthetic rate decreased with an increase in K fertilizer application. The fertilization effect of each component was as follows: N > P > K. We established a comprehensive evaluation model of the physiological growth and development of jujube trees by principal component analysis: F = 0.3536 × PC1 + 0.29727 × PC2 + 0.17105 × PC3. In summary, we found that the h2 mixed fertilizer treatment (N = 460.77 g/tree, P₂O₅ = 480.13 g/tree, and K₂O = 588.23 g/tree) was more effective, and the photosynthetic efficiency and SPAD value were maintained at a high level while ensuring a certain leaf area. In the future fertilization management of jujube trees, more N fertilizer should be applied, appropriate P fertilizer should be applied, and less K fertilizer should be applied.