Cultivar Differences in the Response of the Fruiting Characteristics of Camellia oleifera Abel to the Concentration of Potassium Dihydrogen Phosphate during Foliar Spraying

Abstract

One of the main reasons for the low yield of Camellia oleifera Abel is the large number of flowers and fruits that fall off before ripening. The aim of this study was to investigate the effect of foliar spraying of potassium dihydrogen phosphate (KH₂PO₄) on the fruiting characteristics of C. oleifera, and to provide technical support for its flower and fruit preservation and yield increasing. Three C. oleifera cultivars, ‘Changlin 18’, ‘Changlin 166’, and ‘Changlin 40’, were used as experimental materials to conduct foliar spraying experiments with different water concentrations of KH₂PO₄. The effects of KH₂PO₄ on the fruit retention rate, fruit properties, and seed oil quality of each cultivar were measured and analyzed. The application of the optimal concentration of KH₂PO₄ significantly enhanced various fruit quality metrics of three C. oleifera cultivars. Specifically, the total fruit retention rate was increased by 30.02~87.53%, the transverse diameter by 7.36~21.21%, and the longitudinal diameter by 18.56%, and the fruit weight of ‘Changlin 40’ could increase by 83.63%. It also increased dry seed yield by 27.87~80.81%, dry kernel rate by 10.29~30.12%, dry seed oil content by 28.00~29.77%, total unsaturated fatty acids (TUFAs) by 0.63~5.3%, monounsaturated fatty acids (MUFAs) by 0.30~5.37%, and squalene by 0.09~0.14% during the maturing stage. However, camellia cultivars had the different responses to KH₂PO₄ concentrations. To promote the fruiting of C. oleifera, improve the economic traits of fruits, and enhance the quality of tea oil, the recommended concentrations of KH₂PO₄ solution are 4.50 g·L⁻¹, 1.50 g·L⁻¹, and 1.50 g·L⁻¹, for mist spraying on the trees of ‘Changlin 18’, ‘Changlin 166’, and ‘Changlin 40’, respectively. For mixed cultivars of C. oleifera which planted randomly, the recommended concentration range of KH₂PO₄ solution for spraying is 1.50 to 4.50 g·L⁻¹.

1. Introduction

Camellia oleifera Abel is a small tree or shrub of the Camellia genus in the Theaceae family, it is a unique woody oil tree in China, with a cultivation and utilization history of over 2000 years [1]. Along with olive, oil palm, and coconut, Camellia oleifera is known as one of the world’s four major woody oil plants [2]. Because of its high economic value and mainly commercial demand in a high edible woody-oil quality, Camellia oleifera is extensively cultivated in the southern regions of the Yangtze River in China. As of 2020, the planting area reached approximately 4.53 million ha, and it is projected to extend to 6 million ha by 2025 [3].

The phenomenon of falling flowers and fruits is common in C. oleifera [4], which is one of the main reasons for the low yield of C. oleifera. The fruit trait is an important indicator that is related to the oil quality during artificial production for measuring the economic value of C. oleifera [5,6]. At present, foliar fertilization has become the most direct and efficient fertilization technique in modern agricultural and forestry production [7,8,9,10]. Potassium dihydrogen phosphate (KH₂PO₄), as a water-soluble and economically effective foliar fertilizer, contains P and K, and the plant utilization rate is high, which can promote the absorption of N and P by plants; plus, it has good water solubility, meaning is the first choice for a foliar fertilizer [11]. KH₂PO₄ has been widely used in many plants; spraying KH₂PO₄ on the leaves can improve the fruit yield and quality of chillis [12], tomatoes [13], apples [14], grapes [15], and other fruits. Foliar spraying of KH₂PO₄ can also be an effective way to increase the yield of C. oleifera. Studies have shown that foliar spraying of KH₂PO₄ has a certain promoting effect on the growth of C. oleifera seedlings [16]. Spraying KH₂PO₄ or a KH₂PO₄ mixture during the peak flowering period can increase the fruit setting rate of C. oleifera [17,18]. However, the impact of foliar spraying of KH₂PO₄ on the fruiting characteristics of different cultivars of C. oleifera is currently unclear.

The purpose of this study was to reveal the effects of different concentrations of KH₂PO₄ on the fruit-bearing characteristics of C. oleifera. Three C. oleifera cultivars—’Changlin 18’, ‘Changlin 166’, and ‘Changlin 40’—were selected as experimental materials, and leaf spraying experiments with different concentrations of KH₂PO₄ (0.00, 1.50, 3.00, and 4.50 g·L⁻¹) were carried out. The current research focused on the fruit retention rate, fruit characteristics, and seed oil quality to test the following hypotheses: (1) Different cultivars of C. oleifera may have different responses to the concentration of KH₂PO₄ sprayed on the leaf surface. (2) Different C. oleifera cultivars may have different optimal leaf spraying concentrations of KH₂PO₄. The purpose of this paper is to provide a basis for the application of foliar fertilizer with KH₂PO₄ solution on the leaf surface of C. oleifera during the fruit developing period, so that might be promoted in the future technological development for increasing production.

2. Materials and Methods

2.1. Overview of the Experimental Site

The experimental site is located at the Jiangxi Forestry Breeding Center in Yongxiu County (29°04′ N, 115°49′ W), belonging to the subtropical monsoon climate zone. It has abundant rainfall and sufficient sunlight throughout the year, with an average annual temperature of 17.40 °C and an average annual rainfall of 1580.00 mm. The frost-free period is 239–266 days. The soil type is red soil developed from Quaternary red clay, with an average soil layer thickness of about 50.00 cm. The experimental slope is gentle (9 degrees). The contents of ammonium nitrogen, available phosphorus, and available potassium in the soil of the C. oleifera trees were 38.00 mg·L⁻¹, 6.30 mg·L⁻¹, and 50.00 mg·L⁻¹, respectively.

2.2. Plant Materials and Site Conditions

Three camellia cultivars with significant differences in morphological characteristics planted in the same year (January 2012) and that 96 trees were selected as the test materials. The same trees cultivation measurement were adopted before the experiment treated in 2020, which all C. oleifera trees were fertilized once a year with 300 g of “Shi Kefeng” fertilizer per tree, by 4-point holing with 40 cm depth around each tree. The fruit-setting rates were caculated on 30 March 2020. Three cultivars and their tree statuses are shown in Figure 1 and Supplementary Materials Table S1. The main characteristics of each cultivar are as follows:

Changlin 18 (National R-SC-CO-007-2008), is a compact tree shape, with many short branches and dense branches and leaves; the leaves are elliptical in shape; the fruit is spherical to orange shaped, moderately large, and orange–red in color [18]. Changlin 166 (National R-SC-CO-008-2011), is also a compact tree shape, multiple and slender branches, and dense branches and leaves, with the narrow and long leaves; is the oval shaped of fruit, slightly small, and hold a dark-red color [18]. Changlin 40 (National R-SC-CO-011-2008), is an upright tree, semi-open, and has the sparse branches and leaves; the leaves are elliptical in shape; the fruit is irregularly pear shaped, slightly small in size, and hold a yellowish color, which color is not large difference between the lit and backlit surfaces [19].

2.3. Experimental Design

Both cultivars and the concentrations of KH₂PO₄ spraying were the experimental factors, a two-factor experimental design was adopted. Three cultivars ‘Changlin 18’, ‘Changlin 166’, and ‘Changlin 40’, were sprayed by four KH₂PO₄ solutions of 0.00, 1.50, 3.00, and 4.50 g·L⁻¹. Each cultivar had treated in 8 trees for each concentration of foliar spraying. After the fruit picked on all trees of C. oleifera, in October 2019, 8 healthy and well-developed trees of each cultivar were selected randomly for each spraying concentration treatment. One standard branch was selected and listed in four different directions as east, west, south, and north, so 32 standard branches of 8 trees for every concentration in each cultivar. Due to the drought in the summer and autumn of 2019, the flowering phenology of C. oleifera. was delayed about 20 days. According to the flowering status, the foliar spraying dates of KH₂PO₄ solution were on 18 August 2019 (before the initial flowering period), 21 December 2019 (before the peak flowering period), 20 December 2019 (before the final flowering period), 15 April 2020 (before the first fruit dropping), 25 July 2020 (before the second fruit dropping), and 6 October 2020 (before fruit ripening). The spraying time was after 4:00 p.m. on non-windy and rainy days, and the entire tree leaf surface was sprayed, until no water dripping from the leaves surface (about 3 L solutions for each tree). These practices were consistent with our other management measures.

2.4. Measurement Method

2.4.1. Flower and Fruit Setting Investigation

In October 2019, the number of flower buds on each standard branche was counted and recorded [20]. On 30 March 2020 (before physiological fruit dropping) and 17 October 2020 (before fruit harvesting), the number of fruits was recorded, and the fruit setting rate (the proportion of the fruits’ number on the tree at the end of physiological drop to the number of flowers at flowering) and fruit retention rate were calculated [21].

Fruit setting rate (%) = number of fruits/number of flower buds × 100%;

Fruit retention rate (%) = number of fruits in October/number of fruits in March × 100%.

2.4.2. Fruit Characteristics Determination

On 23 October 2020, fruits at the maturity stage were collected from each individual plants, and 15 fruits from each plant were randomly mixed, totaling 120 fruits. Then, 30 fruits were randomly selected from the 120 fruits to determine the transverse and longitudinal diameters with vernier calipers, and the fruit shape coefficient (longitudinal/transverse) was calculated.

We harvested the swollen fruits on 6 August 2020. We randomly mixed 15 fruits from each plant, totaling 120 fruits. Then, we measured the fresh weights of 30 fruits from the 120 fruits. We peeled the fruit shells and weighed the fresh seeds. We dried the fresh seeds in an 80 °C oven until they reached a constant weight. Then, we weighed the dry seeds (the percentage of dried seeds extracted from fresh fruit) and calculated the dry seed yield and water content of the fresh seeds.

We harvested the mature fruit on 23 October 2020, with samples taken and handled in the same way as before. After weighing the dry seeds, we continued to peel the seed coat and ascertained the kernel dry weight. Finally, we used nuclear magnetic resonance technology to determine the kernel oil content. We then calculated the dry seed yield, kernel rate of dry seeds, and oil content of dry seeds as follows:

Dry seed yield = dry seed weight/fresh fruit weight × 100% [6];

Kernel rate of dry seeds = dry kernel weight/dry seed weight × 100% [6];

Oil content of dry seeds = kernel rate of dry seeds × dry seed kernel yield × 100% [6].

2.4.3. Seed Oil Quality Determination

We took 100 mg of each oil sample and placed it in a 10 mL stoppered test tube. We next added 2 mL of 1 mol/L KOH-methanol solution, and the contents were shaken at room temperature; then, we added 2 mL of n-hexane, and we let it stand for 10 min; finally, we took the supernatant, added a small amount of anhydrous sodium sulfate, and normalized the area. The chromatographic conditions were analyzed using an Agilent 7890B-7000B gas chromatography machine (Agilent Technology Corp., Beijing, China) equipped with an Elite-5MS capillary column (30 m × 0.25 mm × 0.25 μm). The heating program was maintained at 100 °C for 10 min, rising to 280 °C at a rate of 10 °C/min for 5 min. The carrier gas was helium, with a total flow rate of 6.0 mL/min, a pressure of 52.5 kPa, and an injection volume of 0.2 μL. The levels of fatty acids were reported as relative proportions [22].

2.5. Data Statistics and Analysis

We used Excel 2010 software for data processing, Origin (ver. 9.0; Origin Lab Corp., Northampton, MA, USA) and Photoshop (ver. CS6; Adobe Systems., San Jose, CA, USA) for chart making, and SPSS 20.0 software (IBM Corp., Armonk, NY, USA) for analysis of variance. Since we used two-factor analysis of variance, data are presented as the mean ± standard error, where uppercase letters indicate differences between varieties, and lowercase letters indicate differences between treatments, p < 0.05. The F-test and LSD method were used for multiple comparisons.

3. Results

3.1. The Effect of the KH₂PO₄ Spraying Concentration on the Fruit Setting Rate among Cultivars

The results of the two-factor analysis of variance showed that the application of KH₂PO₄ had a significant impact on the fruit setting rate and the total fruit retention rate of C. oleifera, but it had little effect on the fruit retention rate in the fruit period (

Table 1. Two-way ANOVA of the effects of KH₂PO₄ spraying on the fruit retention rate.

Source of Variation	P
	Fruit Setting Rate	Fruit Retention Rate in Fruit Period	Total Fruit Retention Rate
Cultivar	0.723	0.348	0.681
Concentration	0.018	0.839	0.015
Cultivar × Concentration	0	0.685	0

). Different cultivars also had different responses to the concentration, and the differences in the fruit setting rate and total fruit retention rate between cultivars were closely related to the concentration of KH₂PO₄ sprayed (

Table 2. The effect of KH₂PO₄ spraying on the fruit retention rate.

Variable	Cultivar	KH2PO4 (g/L)
		0.00	1.50	3.00	4.50
Fruit setting rate (%)	Changlin18	6.39 ± 0.97 Ab	11.39 ± 1.14 Aa	4.57 ± 0.93 Ab	4.02 ± 1.26 Bb
	Changlin166	5.46 ± 1.07 Aa	5.77 ± 1.23 Ba	5.04 ± 0.89 Aa	8.19 ± 1.24 Aa
	Changlin40	3.71 ± 0.86 Ab	6.64 ± 1.31 Bab	7.68 ± 1.38 Aa	8.97 ± 1.50 Aa
Fruit retention in fruit period (%)	Changlin18	61.27 ± 8.75 Aa	62.38 ± 4.44 Aa	59.51 ± 10.55 Aa	53.89 ± 13.58 Aa
	Changlin166	46.05 ± 7.68 Aa	56.66 ± 8.72 Aa	56.79 ± 8.63 Aa	70.82 ± 8.20 Aa
	Changlin40	49.56 ± 9.53 Aa	51.93 ± 7.32 Aa	50.93 ± 7.58 Aa	50.92 ± 8.89 Aa
Total fruit retention rate (%)	Changlin18	3.37 ± 0.59 Ab	7.18 ± 0.89 Aa	2.70 ± 0.65 Ab	2.61 ± 1.05 Ab
	Changlin166	2.62 ± 0.61 Ab	2.88 ± 0.54 Bb	3.06 ± 0.63 Ab	5.28 ± 0.91 Aa
	Changlin40	1.91 ± 0.53 Ab	3.38 ± 0.68 Bab	4.27 ± 0.85 Aab	5.28 ± 1.38 Aa

Data are presented as mean ± standard error; uppercase letters indicate differences between varieties, and lowercase letters indicate differences between treatments, p < 0.05.

). Without the application of KH₂PO₄, the fruit setting rate and total fruit retention rate of the three cultivars took the order of ‘Changlin 18’ > ‘Changlin 166’ > ‘Changlin 40’, but the difference did not reach a significant level. After the application of KH₂PO₄, the fruit setting rate and total fruit retention rate of ‘Changlin 18’ reached the highest values of 11.39% and 7.18%, respectively, at a minimum concentration of 1.5 g·L⁻¹ of KH₂PO₄, which were 78.12% and 112.73% higher than those without spraying KH₂PO₄. Spraying higher concentrations of KH₂PO₄ was not conducive to fruit retention in ‘Changlin 18’, resulting in a decrease in its fruit retention rate. Meanwhile, ‘Changlin 40’ and ‘Changlin 166’ responded well to higher concentrations of KH₂PO₄, which were beneficial for flower and fruit preservation. The response of ‘Changlin 40’ improved with the increase in KH₂PO₄ concentration, and its fruit setting rate and total fruit preservation rate were best at the maximum concentration of 4.5 g·L⁻¹, reaching 8.97% and 5.28%, respectively, which were 142.03% and 1.76% higher than those without a spraying treatment. ‘Changlin 166’ had the best fruit setting rate and total fruit preservation rate at the maximum concentration of 4.5 g·L⁻¹, reaching 8.19% and 5.28%, respectively, which were 50.02% and 101.95% higher than those without a spraying treatment.

3.2. Effect of the KH₂PO₄ Spraying Concentration on the Fruit Size and Weight among Cultivars

Table 3. Two-way ANOVA of the effects of KH₂PO₄ spraying on the fruit size and weight during the ripening period.

Source of Variation	P
	Transverse Diameter	Longitudinal Diameter	Longitudinal/Transverse	Single Fresh Fruit Weight
Cultivar	0	0	0	0
Concentration	0	0.01	0.062	0
Cultivars × Concentration	0	0	0.491	0

and

Table 4. Effect of KH₂PO₄ spraying on the fruit size and weight during the ripening period.

Variable	Cultivar	KH2PO4 (g/L)
		0.00	1.50	3.00	4.50
Transverse diameter (mm)	Changlin18	29.49 ± 0.75 Aa	30.37 ± 0.83 Aa	28.71 ± 0.87 Ba	29.26 ± 0.77 Ba
	Changlin166	26.43 ± 0.97 Ba	26.36 ± 0.83 Ba	26.61 ± 0.93 Ba	28.53 ± 0.89 Ba
	Changlin40	27.68 ± 0.60 ABb	28.15 ± 0.58 ABb	32.56 ± 0.90 Aa	35.13 ± 0.80 Aa
Longitudinal diameter (mm)	Changlin18	32.52 ± 0.88 Aab	33.49 ± 0.66 Aa	29.79 ± 0.77 Bb	31.66 ± 0.89 Bab
	Changlin166	35.15 ± 0.97 Aa	34.55 ± 0.92 Aa	34.25 ± 0.75 Aa	35.26 ± 0.72 Aa
	Changlin40	29.01 ± 0.64 Bb	29.41 ± 0.62 Bb	33.79 ± 0.79 Aa	35.62 ± 0.63 Aa
Fruit shape coefficient	Changlin18	1.11 ± 0.02 Ba	1.11 ± 0.02 Ba	1.04 ± 0.02 Ba	1.09 ± 0.02 Ba
	Changlin166	1.35 ± 0.04 Aa	1.32 ± 0.04 Aa	1.30 ± 0.03 Aa	1.25 ± 0.03 Aa
	Changlin40	1.05 ± 0.02 Ba	1.05 ± 0.01 Ba	1.05 ± 0.03 Ba	1.02 ± 0.02 Ba
Single fresh fruit weight (g)	Changlin18	13.68 ± 0.70 Aa	16.83 ± 0.82 Aa	13.84 ± 1.53 Ba	15.12 ± 1.27 Ba
	Changlin166	11.18 ± 0.36 Aa	11.36 ± 1.17 Ba	13.16 ± 1.06 Ba	14.32 ± 0.96 Ba
	Changlin40	11.24 ± 0.91 Ab	13.13 ± 0.86 Bb	20.64 ± 0.93 Aa	20.74 ± 1.18 Aa

Data are presented as mean ± standard error; uppercase letters indicate differences between varieties, and lowercase letters indicate differences between treatments, p < 0.05.

show the statistical analysis results for the morphology and fruit weight. From Table 3, it can be seen that the application of KH₂PO₄ had a significant impact on the horizontal and vertical diameter and single fruit weight of C. oleifera, but it had little effect on the fruit shape coefficient. Different cultivars also had different responses to the concentration (Table 4). The longitudinal and transverse diameters of ‘Changlin 18’ showed a trend of first increasing and then decreasing with the increase in KH₂PO₄ solution concentration. The maximum values under 1.50 g·L⁻¹ spraying were 30.37 mm and 33.49 mm, respectively, which were 3.00% higher than those of CK. The transverse and longitudinal diameters of ‘Changlin 40’ increased with the concentration of KH₂PO₄ solution, with the maximum values of 35.13 mm and 35.62 mm under 4.50 g·L⁻¹ treatment, which were increased by 26.92% and 22.79% compared to CK, respectively. The transverse and longitudinal diameters of ‘Changlin 40’ responded more significantly to the KH₂PO₄ solution, and they significantly increased under the 4.50 g·L⁻¹ treatment, more so than those of ‘Changlin 18’ and ‘Changlin 166’. Meanwhile, the responses of the transverse and longitudinal diameters of ‘Changlin 166’ to the changes in KH₂PO₄ solution were not significant. Beyond this, the fruit shape coefficient showed significant differences between cultivars, with the fruit shape coefficients of ‘Changlin 18’ and ‘Changlin 40’ significantly rounder than those of ‘Changlin 166’. The single fresh fruit weight showed extremely significant differences between cultivars and their interactions with the spraying concentrations. The maximum single fresh fruit weight of ‘Changlin 40’ was 20.74 mg under 3.00 and 4.50 g·L⁻¹ spraying, which was 83.63% higher than that of CK. The single fresh fruit weight of ‘Changlin 40’ increased significantly under the high concentration of KH₂PO₄, while for ‘Changlin 18’ and ‘Changlin 166’, we observed no significant effect under different concentrations of KH₂PO₄ spraying.

3.3. Effect of the KH₂PO₄ Spraying Concentration on the Fruit Kernel Percentage among Cultivars

Table 5. Two-way ANOVA on the effects of KH₂PO₄ spraying on the kernel percentages of the fruits.

Source of Variation	P
	Dry Seed Yield	Water Content of Fresh Seeds	Dry Seed Yield	Kernel Rate of Dry Seeds
	Swelling Period (6 August 2020)		Ripening Period (23 October 2020)
Cultivar	0.056	0.883	0.373	0.796
Concentration	0.907	0.833	0	0.053
Cultivar × Concentration	0.383	0.83	0	0.222

and

Table 6. Effect of KH₂PO₄ spraying on the kernel percentages of the fruits.

Variable	Harvest Time	Cultivar	KH2PO4 (g/L)
			0.00	1.50	3.00	4.50
Dry seed yield (%)	Swelling period (6 August 2020)	Changlin18	8.53 ± 2.26 Aa	4.88 ± 1.17 Aa	4.32 ± 0.52 Ba	3.98 ± 1.31 Aa
		Changlin166	7.05 ± 2.72 Aa	9.11 ± 3.43 Aa	8.03 ± 1.46 Aa	6.17 ± 2.03 Aa
		Changlin40	3.40 ± 1.00 Aa	3.51 ± 0.85 Aa	2.91 ± 0.58 Ba	7.08 ± 2.97 Aa
Water content of fresh seeds (%)		Changlin18	70.07 ± 9.28 Aa	76.28 ± 8.90 Aa	83.56 ± 1.79 Aa	83.95 ± 5.93 Aa
		Changlin166	80.06 ± 5.40 Aa	73.02 ± 8.17 Aa	73.97 ± 5.21 Aa	80.65 ± 4.09 Aa
		Changlin40	73.98 ± 8.95 Aa	79.57 ± 4.03 Aa	74.07 ± 9.55 Aa	75.77 ± 6.94 Aa
Dry seed yield (%)	Ripening period (23 October 2020)	Changlin18	15.20 ± 1.38 Aab	14.40 ± 0.83 Bb	19.44 ± 1.71 Aa	16.56 ± 2.00 Aab
		Changlin166	11.73 ± 0.81 Ab	21.20 ± 1.33 Aa	18.66 ± 1.18 Aa	17.83 ± 1.32 Aa
		Changlin40	15.58 ± 1.05 Ab	22.28 ± 2.12 Aa	21.55 ± 2.00 Aa	12.04 ± 1.37 Bb
Kernel rate of dry seeds (%)		Changlin18	55.51 ± 2.11 ABb	56.24 ± 1.75 Ab	64.68 ± 1.04 Aa	64.76 ± 1.86 Aa
		Changlin166	48.65 ± 4.20 Bb	63.09 ± 5.04 Aa	58.95 ± 3.71 Aab	63.31 ± 1.25 Aa
		Changlin40	57.06 ± 2.91 Aa	62.93 ± 2.02 Aa	60.20 ± 8.03 Aa	56.49 ± 2.58 Ba

Data are presented as mean ± standard error; uppercase letters indicate differences between varieties, and lowercase letters indicate differences between treatments, p < 0.05.

show the statistical analysis results for the seed kernel percentages of fruits during the swelling period (6 August 2020). From the graph, it can be seen that when conducting our analysis of variance on the dry seed yields and water contents of fresh seeds during the swelling period (Table 5), there were no significant differences among different cultivars, concentrations, and their interactions. The application of KH₂PO₄ had no significant effect on the fresh seed yields and water contents of fresh seeds during the swelling period in the three cultivars of C. oleifera.

Analysis of variance was also conducted on the seed kernel percentages of fruits during the ripening period (23 October 2020). The application of KH₂PO₄ had a significant impact on the dry seed yields and kernel rates of dry seeds during the mature stage of C. oleifera (Table 5). The differences in seed yields among the different cultivars were closely related to the concentration of KH₂PO₄ applied. In the absence of KH₂PO₄ application, the dry seed yields and kernel rates of dry seeds of the three cultivars followed the order of ‘Changlin 40’ > ‘Changlin 18’ > ‘Changlin 166’, but the difference did not reach a significant level. After the application of KH₂PO₄, the dry seed yields in ‘Changlin 18’, ‘Changlin 166’, and ‘Changlin 40’ all followed a trend of first increasing and then decreasing with the increase in KH₂PO₄ solution concentration (Table 6). The maximum dry seed yields were 19.44%, 21.20%, and 22.28% under spraying at 3.00, 1.50~4.50, and 1.50~3.00 g·L⁻¹, respectively. When compared with CK, these were increased by 27.87%, 80.81%, and 57.09%, respectively. The kernel yield of dried seeds in ‘Changlin 18’ increased with the increase in KH₂PO₄ solution concentration. The kernel yield of dried seeds in Changlin 40 showed a trend of first increasing and then decreasing with the increase in KH₂PO₄ solution concentration. The maximum kernel yields of dried seeds in ‘Changlin 18’, ‘Changlin 166’, and ‘Changlin 40’ were 64.75%, 63.31%, and 62.93% under spraying at 3.00~4.50, 4.50, and 1.50 g·L⁻¹, respectively, which were 16.66%, 30.12%, and 10.29% higher than CK, respectively.

3.4. Effect of the KH₂PO₄ Spraying Concentration on the Seed Oil Content and Quality among Cultivars

The results of the two-factor analysis of variance showed that the application of KH₂PO₄ had a significant impact on the oil content of dry seeds at different concentrations (

Table 7. Two-way ANOVA of the effects of KH₂PO₄ spraying on the seed oil content and quality among cultivars of C. oleifera oil during the ripening period.

Source of Variation	P
	Oil Content of Dry Seeds	TUFA	MUFA	Squalene
Cultivar	0.48	0	0	0
Concentration	0	0	0	0
Cultivar × Concentration	0.116	0	0	0

), which different responses even to the same concentration were shown between cultivars (

Table 8. Effect of KH₂PO₄ spraying on the seed oil content and quality among cultivars of C. oleifera oil during the ripening period.

Variable	Cultivar	KH2PO4 (g/L)
		0.00	1.50	3.00	4.50
Oil content of dry seeds (%)	Changlin18	21.03 ± 0.80 Ad	24.15 ± 0.75 Ac	29.77 ± 0.48 Aa	27.31 ± 0.79 Ab
	Changlin166	21.40 ± 1.85 Ab	28.74 ± 2.30 Aa	28.17 ± 1.77 Aa	29.00 ± 0.57 Aa
	Changlin40	23.30 ± 1.19 Aa	28.00 ± 0.90 Aa	26.95 ± 3.60 Aa	23.83 ± 1.09 Ba
TUFA (%)	Changlin18	81.86 ± 0.03 Bd	83.47 ± 0.03 Cb	83.29 ± 0.03 Cc	85.19 ± 0.00 Aa
	Changlin166	79.51 ± 0.05 Cd	84.05 ± 0.03 Ab	83.65 ± 0.01 Bc	84.81 ± 0.04 Ba
	Changlin40	83.59 ± 0.05 Ac	83.69 ± 0.02 Bb	84.22 ± 0.01 Aa	83.32 ± 0.00 Cd
MUFA (%)	Changlin18	81.16 ± 0.00 Bd	83.04 ± 0.01 Bb	82.72 ± 0.01 Bc	84.69 ± 0.02 Aa
	Changlin166	78.73 ± 0.00 Cc	83.30 ± 0.06 Ab	83.27 ± 0.03 Ab	84.10 ± 0.01 Ba
	Changlin40	82.92 ± 0.00 Ab	82.89 ± 0.01 Cc	83.22 ± 0.01 Aa	82.52 ± 0.00 Cd
Squalene (%)	Changlin18	0.10 ± 0.00 Bd	0.12 ± 0.01 Bc	0.17 ± 0.00 Bb	0.20 ± 0.00 Aa
	Changlin166	0.01 ± 0.01 Cc	0.08 ± 0.00 Cb	0.15 ± 0.00 Ca	0.14 ± 0.00 Ca
	Changlin40	0.12 ± 0.00 Ad	0.14 ± 0.00 Ac	0.22 ± 0.01 Aa	0.16 ± 0.00 Bb

Data are presented as mean ± standard error; uppercase letters indicate differences between varieties, and lowercase letters indicate differences between treatments, p < 0.05.

). The oil content of dry seeds in ‘Changlin 18’ and ‘Changlin 40’ showed a trend of first increasing and then decreasing with the increase in KH₂PO₄ solution concentration, while in ‘Changlin 166’, it increased with the increase in KH₂PO₄ solution concentration. The maximum oil contents were 29.77%, 28.00%, and 29.00% under 3.00, 1.50, and 4.50 g·L⁻¹ spraying, respectively, which were 41.52%, 20.16%, and 35.48% higher than CK. The highest contents of TUFA, MUFA, and squalene (Table 8) in ‘Changlin 18’ C. oleifera oil under 4.50 g·L⁻¹ treatment were 85.19%, 84.69%, and 0.20%, respectively, which were 3.33%, 3.53%, and 0.10% more than CK. The highest contents of TUFA and MUFA in ‘Changlin 166’ C. oleifera oil were 84.81% and 84.10% under 4.50 g·L⁻¹ treatment, which were 5.30% and 5.37% more than CK, respectively. The maximum content of squalene under 3.00 g·L⁻¹ treatment was 0.15%, which was 0.14% higher than CK. The highest contents of TUFA, MUFA, and squalene in ‘Changlin 40’ C. oleifera oil were 84.22%, 83.22%, and 0.22% under 3.00 g·L⁻¹ treatment, which were 0.63%, 0.3%, and 0.09% more than CK, respectively.

3.5. Comprehensive Evaluation of Fruit Effects of KH₂PO₄ Foliar Spraying

To objectively evaluate the effect of different concentrations of KH₂PO₄ spraying on the fruiting characteristics of C. oleifera, nine representative indicators of C. oleifera yield and quality were selected: fruit setting rate, fruit retention rate in fruit period, total fruit retention rate, oil content of kernels, TUFA, dry seed yield, kernel rate of dry seeds, oil content of dry seeds, and oil content of fresh fruits. When we used principal component analysis to extract three principal components (F1, F2, F3) that affect yield and quality indicators, their cumulative contribution rate was 80.50%. When four common factors were extracted, they could explain the nine characteristics, and the extracted common factors were representative. The initial factor loading matrix was subjected to variance maximization orthogonal rotation, and the factor analysis of the output value of C. oleifera after spraying KH₂PO₄ was established (Figure 2). It can be seen that parameters such as the fruit retention rate in the fruit period, total fruit retention rate, oil content of kernels, TUFA, dry seed yield, kernel rate of dry seeds, oil content of kernels, and oil content of fresh fruits were heavily loaded in common factor F1. The parameters of fruit retention rate in the fruit period and total fruit retention rate were heavily loaded in common factor F2, while parameters such as the fruit setting rate, total fruit retention rate, TUFA, and kernel rate of dry seeds were heavily loaded in common factor F3.

4. Discussion

4.1. Effect of the KH₂PO₄ Spraying Concentration on the Fruit Characteristics among Cultivars

There are many factors that affect the fruit drop of C. oleifera, among which the nutritional status of the tree and the distribution of nutrients are among the most important. Insufficient nutrients can affect the formation, growth, and enlargement of the fruit [23,24]. In this experiment, spraying different concentrations of KH₂PO₄ solution during the fruiting period of C. oleifera had no effect on the fruit retention rate. However, throughout the entire flowering and fruiting period of C. oleifera, foliar spraying of KH₂PO₄ had a significant impact on the total fruit retention rate. In particular, the fruit setting rate and total fruit retention rate of ‘Changlin 18’ were the highest when sprayed with a concentration of KH₂PO₄ of 1.50 g·L⁻¹; here, spraying a higher concentration of KH₂PO₄ was not conducive to the fruit retention rate of ‘Changlin 18’, instead leading to a decrease in its fruit retention rate. Meanwhile, spraying 4.50 g·L⁻¹ of KH₂PO₄ solution could significantly improve the fruit setting rates and total fruit retention rates of ‘Changlin 40’and ‘Changlin 166’. As such, ‘Changlin 40’ and ‘Changlin 166’ required a higher concentration of KH₂PO₄ for flower and fruit preservation. A key takeaway is that different cultivars had different responses to the same concentration. If the production is only aimed at increasing the yield, and the cultivars of C. oleifera are planted with clearly separated, then different concentrations of KH₂PO₄ solution should be sprayed during the flowering period of C. oleifera. For ‘Changlin 18’, ‘Changlin 40’, and ‘Changlin 166’, it is recommended to spray 1.50, 4.50, and 4.50 g·L⁻¹ KH₂PO₄ solution, respectively. However, there are many mixed forests of C. oleifera with unknown C. oleifera cultivars [25]. For mixed forests with unknown cultivars, 1.50 to 4.50 g·L⁻¹ KH₂PO₄ solution can be sprayed during the flowering period of C. oleifera. At present, research on the potential for increasing the yields of fruit trees through foliar spraying of KH₂PO₄ mainly focuses on whether the fruit setting rate can be improved during the flowering period, while there is relatively little research on the total fruit retention rate and fruit retention rate during the fruit period.

Insufficient nutrients can affect the growth and swelling of fruits. Studies on hazelnuts have shown that fertilization during the fruit growth period can increase the hazelnut yield (increase the transverse and longitudinal diameters of hazelnut fruit), leading to a sustained increase in its quality [26]. Studies of cherries have shown that as the size of the fruit increases, the hardness decreases. The color value of the fruit varies according to the size of the fruit. The results showed that increasing the fruit size decreased the color value. The soluble solid content and vitamin C value vary with the fruit size [27]. Although the fruit size is an inherent characteristic of a variety, scientific cultivation techniques can increase the ratio of large fruits within a certain range. Conversely, the ratio of small fruits increases [28]. Reasonable nutrient regulation during the growth-sensitive fruit development period is also one of the means to increase the fruit size. The experimental variance results showed that the horizontal and vertical diameters of the fruit were influenced by the concentration and the interaction between the concentration and variety under KH₂PO₄ spraying. In this experiment, when KH₂PO₄ was sprayed on the C. oleifera tree, the response of the transverse and longitudinal diameters of ‘Changlin 40’ to the KH₂PO₄ solution was significant (that is, significantly increasing under the 4.50 g·L⁻¹ treatment) and greater than those of ‘Changlin 18’ and ‘Changlin 166’. Meanwhile, the responses of the transverse and longitudinal diameters of ‘Changlin 166’ to the changes in the KH₂PO₄ solution were not significant. The transverse and longitudinal diameters of ‘Changlin 18’ were the largest under 1.50 g·L⁻¹ spraying, and CK increased by 3.00%. The fruit shape coefficient only showed significant differences under the influence of the cultivar (Table 4), and the fruit shape coefficients of ‘Changlin 18’ and ‘Changlin 40’ were significantly rounder than those of ‘Changlin 166’. The maximum single fruit weight of ‘Changlin 40’ was 20.74 mg (3.00~4.50 g·L⁻¹), representing an increase of 83.63% (compared to clear water). The single fresh fruit weight of ‘Changlin 40’ underwent a more significant increase under the high concentration of KH₂PO₄, and different concentrations of KH₂PO₄ spraying had no significant effect on the single fruit weights of ‘Changlin 18’ and ‘Changlin 166’. Studies on fruits such as apples, pomelo, and sweet cherries have shown that spraying KH₂PO₄ can increase the weights of individual fruits or grains [13,14]. In this experiment, ‘Changlin 40’ exhibited a similar phenomenon after foliar spraying of KH₂PO₄.

Fertilization can promote a better nutritional growth of C. oleifera trees, thereby increase fruit volume so it has a better economic value [5]. One of the fertilization methods for C. oleifera trees is to use KH₂PO₄ for foliar spraying. During the swelling period of C. oleifera fruit, the dry seed yield and water content of fresh seeds were not significantly affected by KH₂PO₄ foliar spraying. During the fruit ripening period, spraying KH₂PO₄ has a significant effect on the dry seed and kernel yields of fresh C. oleifera fruits. In this study, the maximum dry seed yields of fresh fruits in ‘Changlin 18’, ‘Changlin 166’, and ‘Changlin 40’ were 19.44% (3.00 g·L⁻¹), 21.20% (1.50 g·L⁻¹), 22.28% (1.50~3.00 g·L⁻¹), and 22.28% (1.50~3.00 g·L⁻¹), respectively, with an increase of 27.87~80.81%. Meanwhile, the maximum dry seed yields of ‘Changlin 18’, ‘Changlin 166’, and ‘Changlin 40’ were 19.43% (3.00 g·L⁻¹), 21.20% (1.50 g·L⁻¹), and 22.27% (1.50~3.00 g·L⁻¹), respectively, with an increase of 27.83%~81.35% (compared to water). This demonstrates that foliar application of KH₂PO₄ solution has a significant impact on the seed kernel percentage of C. oleifera fruits during the mature stage, but there are differences in the responses of the three cultivars to different concentrations.

4.2. Effect of the KH₂PO₄ Spraying Concentration on the Seed Oil Content and Quality among Cultivars

Multiple research results have confirmed that a sufficient nutrient supply has a significant positive impact on the oil yield of plants [29,30,31]. In this experiment, the maximum oil contents of kernels of ‘Changlin 18’, ‘Changlin 166’, and ‘Changlin 40’ were 46.02%, 47.78%, and 44.77%, respectively, under the application of 3.00 g·L⁻¹, with an increase of 7.93~13.32% (compared to water). The maximum oil contents of dry seeds in ‘Changlin 18’, ‘Changlin 166’, and ‘Changlin 40’ were 29.77% (3.00 g·L⁻¹), 29.00% (4.50 g·L⁻¹), and 28.00% (1.50~3.00 g·L⁻¹), respectively, with an increase of 20.16~41.52% (compared to water). The maximum oil contents of fresh fruits in ‘Changlin 18’, ‘Changlin 166’, and ‘Changlin 40’ were 5.81% (4.50 g·L⁻¹), 6.05% (1.50 g·L⁻¹), and 6.26% (1.50 g·L⁻¹), respectively, with an increase of 71.94~146.03% (compared to water). The experimental results show that foliar spraying with an appropriate concentration of KH₂PO₄ solution can improve the oil content of C. oleifera fruits, and the optimal spraying concentration required for the three cultivars is different.

Camellia oil is beneficial to the health and is considered a dietary oil [32]. Research has indicated that tea oil contains abundant unsaturated FAs (fatty acids), including oleic acid, monounsaturated fatty acid, and linoleic acid [33]. Regarding the quality of C. oleifera, studies have shown that different site conditions will lead to differences in C. oleifera quality [22], and the nutrient contents vary in different soils. The impacts of different cultivation modes on the quality of oil also differ [34,35]. In this experiment, the highest TUFA contents of ‘Changlin 18’ and ‘Changlin 166’ were 85.19% and 84.81%, respectively, obtained under the 4.50 g·L⁻¹ treatment, while the highest TUFA content of ‘Changlin 40’ (obtained under the 3.00 g·L⁻¹ treatment) was 84.22%. The optimal concentration of KH₂PO₄ can increase the TUFA content by 0.63~5.30% (compared with water). The contents of MUFA in ‘Changlin 18’, ‘Changlin 166’, and ‘Changlin 40’ were 84.69% (4.50 g·L⁻¹), 84.09% (4.50 g·L⁻¹), and 83.22% (3.00 g·L⁻¹), respectively. The optimal concentration of KH₂PO₄ can increase the MUFA content by 0.3~5.37% (compared with water). The maximum squalene contents in ‘Changlin 18’, ‘Changlin 166’, and ‘Changlin 40’ were 0.20% (4.50 g·L⁻¹), 0.15% (1.50 g·L⁻¹), and 0.22% (3.00 g·L⁻¹), respectively, and the optimal concentration of KH₂PO₄ increased the squalene content by 0.09~0.14% (compared with water). To increase the contents of TUFA and MUFA, a KH₂PO₄ concentration of 4.50 g·L⁻¹ was optimal for ‘Changlin 18’ and ‘Changlin 166’, while 3.00 g·L⁻¹ was best for ‘Changlin 40’. Under the optimal concentration of treatment, ‘Changlin 40’ showed the smallest increases in their contents, while ‘Changlin 166’ had the largest increases. For different cultivars of C. oleifera oil, the required concentration of KH₂PO₄ differs, and the effect on increasing the fatty acid contents also varies.

5. Conclusions

This study provides a practical basis for the simple transformation of nutrients to reduce the flower and fruit drop in C. oleifera forests, and it provides a reference and scientific basis for the concentration and spraying range of C. oleifera surface fertilizer. It also provides a scientific impetus for promoting the development of the C. oleifera industry and driving economic growth. Moderate foliar spraying of KH₂PO₄ during the flowering period can increase the fruit setting rate and fruit setting of C. oleifera. Before the fruit ripening period, appropriate foliar spraying of KH₂PO₄ can improve the economic characteristics and oil quality of the fruit. However, different cultivars require different concentrations of KH₂PO₄. It is recommended to use 1.50 g·L⁻¹ for ‘Changlin 40’ and ‘Changlin 166’, and the spraying effect of ‘Changlin 166’ is more prominent. Meanwhile, for ‘Changlin 18’, the spraying effect is ideal at 4.50 g·L⁻¹. If different cultivars can be sprayed in production, it is recommended to choose the appropriate concentration for each. Alternatively, spraying KH₂PO₄ with a concentration of 1.50~4.5 g/L⁻¹ on the leaves of C. oleifera can significantly promote and improve the yield and quality of multiple cultivars of C. oleifera. In sum, this article has a certain reference value for the foliar spraying of KH₂PO₄ in C. oleifera forests with single, separated, or mixed cultivars.