Optimal Dark Tea Fertilization Enhances the Growth and Flower Quality of Tea Chrysanthemum by Improving the Soil Nutrient Availability in Simultaneous Precipitation and High-Temperature Regions

Abstract

The simplex strategies of fertilizer management and problems caused by simultaneous precipitation and high-temperature (SPH) climate were the main factors that led to yield loss and quality decline in the continuous cropping of tea chrysanthemum (Dendranthema morifolium ‘Jinsi Huang’). In this study, with sustainable biofertilizers being proposed as a potential solution. However, their effects under such constraints are underexplored. In this study, we compared different proportions of a sustainable dark tea biofertilizer, made with two commonly used fertilizers, by their contributions to the morphological, photosynthetic, and flowering traits of D. morifolium ‘Jinsi Huang’. The results showed that increasing the dark tea biofertilizer application to 4.5 kg·m⁻² significantly enhanced the soil alkali hydrolyzed nitrogen (596.53% increase), available phosphorus (64.11%), and rapidly available potassium (75.56%) compared to the levels in yellow soil. This nutrient enrichment in soil caused D. morifolium ‘Jinsi Huang’ to produce more leaves (272.84% increase) and flower buds (1041.67%), along with a strengthened photosynthetic capacity (higher Fv/Fm values and light saturation point). These improvements alleviated the photoinhibition caused by SPH climate conditions, ultimately leading to significantly higher contents of chlorogenic acid (38.23% increase) and total flavonoids (80.28%) in the harvested flowers compared to the control group. Thus, dark tea biofertilizer is a cost-effective and efficient additive for growing tea chrysanthemum in SPH regions due to improving soil quality and causing nutritional and functional components to accumulate in harvest flowers, which greatly promotes the commercial value of rural revitalization industries centered around tea chrysanthemum.

1. Introduction

Dendranthema morifolium ‘Jinsi Huang’, a traditional tea chrysanthemum cultivar, has achieved an industrialized cultivation scale in numerous rural areas of China in the rural revitalization industry due to its high ornamental and medicinal value [1]. D. morifolium ‘Jinsi Huang’, originally growing in regions with a 16.7 °C yearly average temperature and 1600 mm average yearly precipitation [2], prefers cool temperatures and fertile cultural substrates with high soil porosity for good drainage and air permeability in the soil. As the increasing demand for this plant leads to rapid consumption, its region of cultivation has expanded to numerous areas of the countryside from southeast China to the southwest. Further, it is now produced on an industrial scale and has its production has become one of the mainstay industries in rural revitalization regions. Especially in Xiushui prefecture, China, the original place of the D. morifolium ‘Jinsi Huang’ cultivation industry, the annual yield exceeds 7,500,000 kg and the yearly output value has increased to 4182 dollars per acre, which helps farmers get out of poverty and become well-off [3]. Its edible flowers contain variable effective and active components, such as flavonoids, polysaccharides, phenolics, and terpenoids, which have been reported to have antioxidation effects, lower blood lipids, anti-tumor effects, lower blood sugar, and hepatoprotective effects, as well as being used in the treatment of cardiovascular diseases [4,5,6,7,8,9,10,11]. Furthermore, some phenolics, such as isochlorogenic acid, cymaroside, and isochlorogenic acid A, are considered the main antioxidant components of D. morifolium ‘Jinsi Huang’ [12,13]. The comprehensive output value of its blooms positively rises with the nutrient content and size.

However, it was reported that simultaneous precipitation and high-temperature (SPH) climates can result in a string of soil and environment problems in the main cultivation regions of this plant, such as soil hardening and eluviation, and humid and hot conditions cause slow growth and a sharp drop in the yield of edible chrysanthemums [14]. For example, during its anthesis with high temperatures and plenty of rain, soil hardening and ponding conditions smother its roots, eventually causing the roots to rot and the stems to wither, which strongly reduces the production and quality of its flowers [15]. SPH climates are marked by continuing high temperatures, either with abundant rain or with drought in summer and the start of autumn, which easily contributes to photoinhibition, resulting in leaf variations of the salix leaf shape and the dormancy of chrysanthemums, which leads to stunted growth and a significant reduction in flower yield [16]. This climatic pattern has become increasingly frequent in the Hunan province during the summer and autumn seasons, with the post-autumn heatwave exacerbating these effects. Currently, studies have shown that chrysanthemum cultivation relies heavily on chemical fertilizers, which accumulate in the soil and disrupt the nutrient balance, contributing to soil nutrition disorders, overnutrition, and environmental harm, which are exacerbated by SPH weather [17,18]. Farmers used to focus on promoting soil fertility while ignoring and destroying the soil structure by constantly and incorrectly using chemical fertilizers. Furthermore, soil-borne diseases gradually take place over time when chrysanthemums are cultivated using a monoculture and vegetative propagation mode, and this causes a continuous cropping obstacle which inflicts huge economic damage on farmers [19]. Therefore, novel and sustainable biofertilizers have attracted attention for their ability to enhance the nutrient uptake and availability in plants, which promotes their growth and production and strengthens stress tolerance [20]. Through some eco-friendly soil amelioration measures, the negative effects of various unfavorable conditions in the cultivation environment are minimized, so that plant growth and yield are directly improved through the application of biochar, biofertilizers, and cellulose-based fertilizers [21,22,23,24,25]. SPH weather triggers chrysanthemum leaves to change into salix leaf shapes, while biofertilizers, abundant in nutrients along with an appropriate soil permeability and drainage, help plants thrive in abiotic stress environments by optimizing their leaf photosynthetic system reaction and adjusting the gas and water exchange to reduce the occurrence of photoinhibition and result in higher flower yields [26,27,28]. Thus, the application of biofertilizers offers a promising solution for mitigating plant damage during simultaneous precipitation and high-temperature conditions, thereby significantly supporting the sustainable production of chrysanthemum-related industries.

Previous studies have established the value of biofertilizers in crop production: rapeseed cake, for instance, has been shown to enhance the yield and quality of rice through its rich nutrient content [29]. Meanwhile, tea residue waste—easily accessible in key cultivation regions of Dendranthema morifolium ‘Jinsi Huang’ due to local tea-drinking traditions [30]—has demonstrated positive effects when applied as a biofertilizer, boosting growth in crops like tomatoes [31,32,33,34]. Notably, dark tea residue waste holds unique promise. Its production involves high-temperature processing, which simplifies the sanitization steps compared to those of other organic fertilizers (e.g., manure, sewage sludge) that require elaborate pathogen removal. This makes it a particularly sustainable candidate for agricultural use, with the potential to improve soil nutrients and physical properties while addressing waste. Despite these advantages, the specific impact of dark tea residue on Dendranthema morifolium ‘Jinsi Huang’—a crop with distinct growth requirements—remains unexamined. Given the substantial annual volume of tea residue waste generated in China (2.66 million tons) [35] and the need to optimize biofertilizer applications for target crops, filling this gap is critical to validating the utility of our developed biofertilizer in chrysanthemum cultivation.

Conclusively, the production of D. morifolium ‘Jinsi Huang’ currently faces pressing challenges including soil issues from long-term monoculture, conventional fertilizer management, and adverse weather in SPH regions, with underutilized tea residue—although it has potential for use as a biofertilizer, which could mitigate such issues—exacerbating waste burdens while leaving a valuable resource untapped. Our prior research has substantiated that applying 3 kg m⁻² of dark tea biofertilizer substantially enriches the nutrient content of soil and enhances the growth and photosynthetic characteristics of D. morifolium ‘Jinsi Huang’ within a greenhouse. In this study, we delved into the effects of the application of dark tea biofertilizer on the structure and nutrient levels of soil in SPH regions, as well as how to promote plant growth, how to improve flower agronomic traits, and, especially, how to notably stimulate the accumulation of flower-effective components during SPH weather. This study could offer valuable guidance on the optimal biofertilizer type and dosage for revitalizing D. morifolium ‘Jinsi Huang’ production in rural areas, which would greatly increase the income of farmers who are struggling with simultaneous precipitation and high-temperature climates, sparking interest from the pharmaceutical industry and augmenting its commercial value. Moreover, this approach presents an eco-friendly means of managing dark tea residue waste.

2. Materials and Methods

2.1. Plant Materials and Substrates

In this study, the seedlings of D. morifolium ‘Jinsi Huang’ were used when they reached 5–6 cm in height and had 5 fully expanded leaves after 2 months of cutting. Four cultural substrate treatments, dark tea biofertilizer (DT), commercial organic fertilizer, rapeseed cake fertilizer, and yellow soil, were respectively selected from the dark tea instant tea factory (Hunan Meishan Dark Tea Co., Ltd., Yiyang, China) after composting, Hunan Xiangmu Biotechnology Co., Ltd. (Changsha, China), the rapeseed oil factory (Hunan Jinhao Grain and Oil Industry Co., Ltd., Changsha, China), and reclaimed flower base soil. Based on seasonal meteorological variations in Hunan, China, the growth periods for D. morifolium ‘Jinsi Huang’ in 2021 were divided into three growth stages: spring (11 April–10 June, 0–60 d post-cultivation), summer (11 June–23 September, 61–165 d) and autumn (24 September–7 December, 166–240 d). The monthly average temperatures are shown in

Table 1. Monthly maximum temperature of Changsha, Hunan Province, China in 2021.

Month	Jan.	Feb.	Mar.	Apr.	May	Jun.	Jul.	Aug.	Sep.	Oct.	Nov.	Dec.
Average daily high temperature	10 °C	15 °C	15 °C	18 °C	25 °C	31 °C	34 °C	32 °C	33 °C	22 °C	18 °C	13 °C
Monthly extreme high temperature	20 °C	26 °C	29 °C	32 °C	34 °C	37 °C	37 °C	40 °C	39 °C	37 °C	23 °C	21 °C

Note: data sources: www.tianqi.com (accessed on 1 January 2022).

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2.2. Experimental Site and Design

This study was carried out in the open field at Hunan Agricultural University (28°10′ N, 113°04′ E), which exhibits a typical subtropical monsoon climate with simultaneous precipitation and high temperatures. Regarding the optimal application rate of commercial organic fertilizers in the cultivation of D. morifolium ‘Jinsi Huang’ [3], six treatments and a control group were set up, respectively: 1.5 kg·m⁻² dark tea biofertilizer group(0.5 × DT), 3 kg·m⁻² dark tea biofertilizer group (1 × DT), 4.5 kg·m⁻² dark tea biofertilizer group (1.5 × DT), 6 kg·m⁻² dark tea biofertilizer group (2 × DT), 3 kg·m⁻² commercial organic fertilizer group (CF), 3 kg·m⁻² rapeseed cake fertilizer group (RC), and yellow soil group (CK). Each group included 10 pots (1 plant pot⁻¹).

2.3. Determination Items and Methods

2.3.1. Determination of Soil Physicochemical Properties

During the 15-day open field experiment, 7 distinct cultural substrates were exposed to natural conditions without shelter to stabilize the soil properties. Three pots were randomly selected from each group for testing. The bulk density, total porosity, and water-holding capacity of the substrates were measured using a sterilized stainless-steel cutting ring [36]. Furthermore, the pH and EC values were determined using an acidity meter and a conductivity meter with 1:5 (w/v) soil–H₂O extracts, respectively. The methods utilized to ascertain effective nutrition contents are delineated in

Table 2. Methods of effective nutrition contents determination.

Nutrients	Methods	Reference
Total nitrogen (N)	Elemental analyzer	[37]
Total potassium (K)	The flame photometry method after melting with sodium hydroxide	[38]
Total phosphorus (TP)	The molybdate colorimetric method after perchloric acid digestion	[39]
Available phosphorus (AP)	Ammonium fluoride extraction molybdenum antimony colorimetry	[39]
Available potassium (AK)	Ammonium acetate extraction atomic absorption spectrometry	[38]
Alkali hydrolyzed nitrogen (AHN)	Alkali hydrolysis diffusion method	[38]
Organic carbon (C)	Potassium dichromate volumetric method and external heating method	[38]
Organic matter	Soil organic matter (g·kg−1) = soil organic carbon (g·kg−1) × 1.724	[40]

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2.3.2. Measurement Method of Morphological Indexes

The morphological indexes were measured following potting at the end of every growth period (60 d, 165 d, and 240 d), including plant height, crown width, branch number, stem diameter, and leaf number. Plant height was measured at the growth point of the highest branch; the average value of the maximum and minimum crown width of the whole plant was taken as the crown width; a leaf area of over 2 cm² was counted as the number of leaves; and stem diameter was measured at the lignified node 3–5 cm upward from the root neck. All morphological indexes were measured with a ruler (100 cm, 0.1 cm) or vernier caliper (150 mm, 0.01 mm).

2.3.3. Determination Method of Photosynthetic Characteristics

Mature leaves were collected and their photosynthetic pigment content was determined by ethanol extraction method using an ultraviolet spectrophotometer (TSD-599, Shanghai, China) [41] in the middle of the spring (37 d), summer (100 d), and autumn (187 d) growth periods. The formulas were as follows.

Chl a (mg·L⁻¹) = 13.95 × A₆₆₅ − 6.8 × A₆₄₉,

(1)

Chl b (m·L⁻¹) = 24.96 × A₆₄₉ − 7.32 × A₆₆₅,

(2)

Car (mg·L⁻¹) = (1000 × A₄₇₀ − 2.05 × Chl a − 114.8 × Chl b)/248,

(3)

Chlorophyll fluorescence parameters, OJIP, non-photochemical quenching (NPQ) and photochemical quenching (qP), were measured in the middle of the summer growth period by FluorPen (FP110, Drásov, Czech Republic), after 20 min of dark adaptation with detachable leaf clips. As per the above 3 parameters, the ABS/RC, DIO/RC, and TRo/RC were calculated [42]. The photosynthetic characteristic parameters, net photosynthetic rate (P_n), stomatal conductance (G_s), intercellular CO₂ concentration (C_i), and transpiration rate (T_r) were measured in summer (101 d) using a Li-6400 portable photosynthetic apparatus (Li-Cor6400XT PSC-4817, Lincoln, Dearborn, MI, USA). Then, we fit the light response curve and stomatal conductance response curve [43]. Meanwhile, the maximum net photosynthetic rate (P_nmax), water use efficiency (WUE), light compensation point (LCP), light saturation point (LSP), dark respiration (Rd), apparent quantum efficiency (AQY), and stomatal limit-related value were calculated [44]. The leaves between the 8th and 10th leaves below the plant growth point were selected randomly and experiments were repeated in triplicate.

2.3.4. Methods for Determination of Florescence Traits

Flower quantity and character were measured once a day when buds appeared by a vernier caliper. The flower quantity index was estimated by the number of flower buds distinguished in the blooming period. Flower character indexes were shown as maximum flower diameter, middle round lingual flower diameter, inner round lingual flower diameter, outer round lingual flower length and width, and middle round lingual flower length and width. The specific sites were measured as shown in Figure 1.

2.3.5. Determination Methods of Flower Components

The 80% fully opened flowers were collected to measure components in each treatment. The samples were dried at 38 °C for 4 h and then raised to 50 °C for 4 h. According to the nutritional evaluation method for D. morifolium ‘Jinsi Huang’ [45] and the medicinal active ingredients of chrysanthemum [46], the contents of 4 basic nutritional components, including ascorbic acid, total protein, soluble sugar, and carotenoid, and 5 medicinal active components, including total polysaccharide, total flavone, chlorogenic acid, cymaroside, and isochlorogenic acid A, were determined. The total protein was determined with Bicinchoninic Acid Kit for Protein Determination (ZC-S0470, ZCBIO, Shanghai, China); the soluble sugar was determined with Soluble Sugar Assay Kit (ZC-S0697, ZCBIO, Shanghai, China); total flavonoids were determined with total Flavone Content Assay Kit (ZC-S0873, ZCBIO, Shanghai, China); ascorbic acid was determined with Ascorbic Acid Assay Kit (ZC-S0609, ZCBIO, Shanghai, China); the total polysaccharide was determined with Total Polysaccharide Assay Kit (ZC-S0885, ZCBIO, Shanghai, China). The above assay kits were provided by Zhuocai Company, Shanghai, China. Carotenoid in flowers was determined with the same method detailed in Section 2.3.3. Chlorogenic acid, cymaroside, and isochlorogenic acid A were determined by high-performance liquid chromatography (HPLC, Agilent Technologies, Santa Clara, CA, USA) using an Aglient1260 liquid chromatography kit [46].

2.4. Comprehensive Evaluation by Principal Component Analysis (PCA)

The compositions were selected according to the principle that the cumulative variance contribution rate should not be less than 85%. The eigenvector function expressions were calculated according to the eigenvalues and loadings of the principal components, and the comprehensive evaluation function model was constructed to analyze the principal components for the 36 indicators related to the data of growth, photosynthesis, flower traits, and effective components.

2.5. Statistical Analysis

All statistical analyses were performed by SPSS 26 software (IBM, Armonk, New York, NY, USA). One-way ANOVA was used to test for significant differences, and Duncan’s test was applied for multiple comparisons (p < 0.05). The Origin2021 (OriginLab, Northampton, MA, USA) and GraphPad Prism software (9.5) were used to construct the figures. The principal component analysis was performed by Origin2021 software.

3. Results

3.1. Different Fertilization Treatments Greatly Improved Soil Physicochemical Properties in SPH Regions

In this study, six treatments were applied to yellow soil (CK), including dark tea biofertilizer with four dosages (0.5 × DT, 1 × DT, 1.5 × DT, 2 × DT), commercial organic fertilizer (CF), and rapeseed cake fertilizer (RC). These six treatments significantly improved the physical and chemical properties of the soil, such as its bulk density, water-holding capacity, soil porosity, and nutrient levels, in comparison to CK, while maintaining a neutral pH (Figure 1). In various types of fertilizer substrates, the DT treatments had the highest total porosity (37.06% higher than CK), a higher water-holding capacity (57.07% higher than CK), and a lower bulk density (18.47% higher than CK). With the increasing dosage of DT, the total porosity, water-holding capacity, and EC were gradually improved with lower bulk density (Figure 2a–e). When the dosage of DT was increased to 6 kg·m⁻² (2 × DT), the total porosity and water-holding capacity reached the highest values, with the lowest bulk density, among the seven treatments. This treatment was followed by the 1.5 × DT, DT, RC, 0.5 × DT, CF and CK treatments. As a result, the higher availability of O₂, CO₂, and water in the DT substrates provided good soil conditions for root elongation and the extension of nutrient absorption areas. At the same time, the effective nutrient contents of six of the treatments were higher than that of CK (Figure 2f–m), especially the RC treatment. As shown in Figure 2i,l,m, the AHN, total organic matter, and organic carbon showed the same trend, with extremely significant differences between the six treatments and CK. Compared with the different fertilizers, 1 × RC showed the highest nutrient level, followed by 1 × DT, CF, and CK. In particular, the RC group exhibited a lower soil bulk density (21.30%), higher soil total porosity (32.21%), higher content of alkali hydrolyzed nitrogen (1957.66%), higher available phosphorus (287.21%), and higher rapidly available potassium (352.64%), respectively, than those of CK. Thus, abundant nutrients in the studied biofertilizers were released into the yellow, which soil strongly supported the rapid vegetative growth of D. morifolium ‘Jinsi Huang’. Furthermore, as the dosage of DT was increased from 1.5 kg·m⁻² to 6 kg·m⁻², all effective nutrient contents showed a gradual improvement.

3.2. Different Fertilization Treatments Enhanced Morphological Growth of Dendranthema Morifolium ‘Jinsi Huang’

After the application of the fertilizers, there was a significant improvement in the vegetative growth in each group over time, particularly in the DT groups (Figure 3). Abundant nutrients in RC treatment stimulated the vegetative growth of D. morifolium ‘Jinsi Huang’ in spring to the highest height and widest crown width among the six treatments. The RC treatment was followed by CF, 2 × DT, 1.5 × DT, 1 × DT, 0.5 × DT, and CK. However, its plant height, crown width, stem diameter, and number of leaves over time were lower than those of the DT groups as well as the commercial organic fertilizer group. Additionally, as the dosage of DT gradually increased, the plant height, crown width, stem diameter, and number of leaves were significantly improved. In particular, the 1.5 × DT treatment showed considerable benefits, with a 101.71% increase in plant height, a 75.74% wider crown width, a 64.14% thicker stem diameter, and a substantial surge in the number of leaves by 272.84% compared with the CK in autumn. Abundant nutrients in the dark tea biofertilizer substrates assisted D. morifolium ‘Jinsi Huang’ in extending its photosynthetic area to potentially promote photosynthesis and thrive in simultaneous precipitation and high-temperature climates.

3.3. Different Fertilization Treatments Promoted the Photosynthetic Efficiency of Dendranthema Morifolium ‘Jinsi Huang’ Leaves in Summer

3.3.1. Changes in the Content of Photosynthetic Pigments of Dendranthema Morifolium ‘Jinsi Huang’ Leaves

The contents of chlorophyll a, chlorophyll b, and carotene showed no significant difference in the middle of spring (37 d) in each group (Figure 4b,e,h), while they were significantly affected by the biofertilizers over time (Figure 4c,d,f,g,i,j). Photosynthetic pigments play an essential role in the growth of plants, directly impacting the efficiency of photosynthesis and influencing the color of the leaves. After entering the summer growth period (100 d), the DT treatments showed significantly higher chlorophyll a (39.97–63.28%) and carotene (64–68%) contents than CK, with varying increases among the DT groups. Meanwhile, the content of photosynthetic pigments continued to increase, reaching an extremely significant difference in the middle of autumn. Compared with CK (7.07 mg·g⁻¹) in autumn, the DT groups exhibited significantly increased total photosynthetic pigment contents (from 9.38–11.34 mg·g⁻¹) that were 32.64–60.40% higher than CK, CF (10.61 mg·g⁻¹), and RC groups (9.11 mg·g⁻¹) and 49.97% and 28.86% higher than the CK group, respectively. Moreover, the leaf color in the six treatment groups deepened compared with that in CK and, in the four DT treatments, it showed a gradual trend towards a deeper green color with increasing dosages (Figure 4a).

3.3.2. Changes in Photosynthesis and Chlorophyll Fluorescence Parameters of Dendranthema Morifolium ‘Jinsi Huang’ Leaves

The original fluorescence (F_o) represents the portion of light energy radiation that does non-participate in the photochemical reaction of PSII. The maximal fluorescence (F_m) reflects the transferrable ability of photosynthesis-related electrons through PSII. Meanwhile, the maximum quantum efficiency (F_v/F_m) represents the maximum light energy conversion efficiency of the PSII, indicating the potential maximum photosynthetic capacity and the positive correlation with the stress resistance of plants.

According to the typical characteristics of simultaneous precipitation and high-temperature climates, the photosynthesis and chlorophyll fluorescence parameters were measured in the middle of summer. As shown in Figure 5a,b the OJIP curve of CK displayed the most deformation with the lowest value at each point compared to the other groups, which indicates that the CK group suffered higher levels of stress in SPH weather. There were no significant differences in the value of F_o and F_m among the six treatments and CK, except that the 1.5 × DT group (29.11%) had a significantly higher in F_m than CK (Figure 5c). Six biofertilization treatments had significant effects on the chlorophyll fluorescence parameters in the simultaneous precipitation and high-temperature climate (

Table 3. Changes in substrates in chlorophyll fluorescence parameters of Dendranthema morifolium ‘Jinsi Huang’ in simultaneous precipitation and high-temperature climate.

Treatment Groups	Fv/Fm	ABS/RC	TRo/RC	DIO/RC	ETo/RC
CK	0.788 ± 0.024 bc	2.42 ± 0.25 abc	1.90 ± 0.16 ab	0.51 ± 0.10 ab	1.19 ± 0.07 a
0.5 × DT	0.784 ± 0.017 c	2.54 ± 0.10 a	1.99 ± 0.10 a	0.54 ± 0.04 a	1.19 ± 0.06 a
1 × DT	0.803 ± 0.014 abc	2.46 ± 0.16 ab	1.97 ± 0.10 a	0.48 ± 0.06 abc	1.24 ± 0.07 a
1.5 × DT	0.818 ± 0.003 ab	2.14 ± 0.04 c	1.75 ± 0.03 b	0.38 ± 0.01 c	1.16 ± 0.04 a
2 × DT	0.823 ± 0.008 a	2.24 ± 0.09 bc	1.84 ± 0.09 ab	0.39 ± 0.01 c	1.22 ± 0.07 a
CF	0.815 ± 0.013 ab	2.22 ± 0.11 bc	1.81 ± 0.06 ab	0.41 ± 0.05 bc	1.14 ± 0.03 a
RC	0.801 ± 0.020 abc	2.34 ± 0.15 abc	1.87 ± 0.08 ab	0.45 ± 0.07 abc	1.21 ± 0.08 a

Note: different lowercase letters are significant differences based on one-way ANOVA (p < 0.05). CK: yellow soil group, 0.5 × DT: 1.5 kg·m⁻² dark tea biofertilizer group, 1 × DT: 3 kg·m⁻² dark tea biofertilizer group, 1.5 × DT: 4.5 kg·m⁻² dark tea biofertilizer group, 2 × DT: 6 kg·m⁻² dark tea biofertilizer group, CF: 3 kg·m⁻² commercial organic fertilizer group, and RC: 3 kg·m⁻² rapeseed cake fertilizer group.

). The F_v/F_m in the 2 × DT group had the highest value, followed by the 1.5 × DT, CF, 1 × DT, RC, 0.5 × DT, and CK, which suggests that 2 × DT experienced less stress than CK during SPH weather. Additionally, the 1.5 × DT group had the lowest values for the unit reaction center (RC), absorbed light energy (ABS/RC), dissipated light energy (DIo/RC), and energy captured for QA reduction (TRo/RC) of its leaves. Conversely, the 0.5 × DT group had the highest value, followed by the CK group, among the seven groups in the middle of summer. Moreover, there was no significant difference in the energy captured by RC for electron transport (ETO/RC) among the seven groups, nor in the F_v/F_m, ABS/RC, DIo/RC, and TRo/RC among the 1 × DT, CF, and RC groups when they were compared with CK. The 1.5 × DT group showed the lowest value of nonphotochemical chlorophyll fluorescence quenching (NPQ), which was 38.48% lower than that of CK, while the qP showed no significant differences in the six treatments compared with CK (Figure 5f), which suggests excessive available light energy, lower absorption efficiency, and some extent of photoinhibition in the leaves of D. morifolium ‘Jinsi Huang’ in CK. As a result, the 1.5 × DT group showed higher efficiency of light energy conversion and availability and less energy dissipation as heat, and thrived in the simultaneous precipitation and high-temperature climate, contrary to CK.

The response to the light intensity of D. morifolium ‘Jinsi Huang’ during SPH weather is shown in Figure 5d,e, and its parameters are shown in

Table 4. Changes in substrates in light response characteristic parameters of Dendranthema morifolium ‘Jinsi Huang’ leaves in simultaneous precipitation and high-temperature climate.

Treatment Groups	AQY μmol m−2 s−1	LSP μmol m−2 s−1	Pnmax μmol m−2 s−1	LCP μmol m−2 s−1	Rd μmol m−2 s−1
CK	0.107 ± 0.003 a	1687.37 ± 12.60 d	16.29 ± 0.47 c	41.13 ± 4.58 a	3.76 ± 0.28 a
0.5 × DT	0.072 ± 0.014 b	1765.65 ± 15.75 b	18.85 ± 0.39 b	51.09 ± 11.88 a	3.23 ± 0.21 ab
1 × DT	0.091 ± 0.015 ab	1772.71 ± 17.08 b	20.03 ± 1.35 b	40.41 ± 5.00 a	3.28 ± 0.27 ab
1.5 × DT	0.082 ± 0.009 ab	1817.24 ± 18.57 a	22.81 ± 1.29 a	41.58 ± 12.63 a	3.10 ± 0.65 ab
2 × DT	0.086 ± 0.009 ab	1819.66 ± 4.25 a	23.04 ± 1.84 a	35.64 ± 3.27 a	2.83 ± 0.19 b
CF	0.100 ± 0.021 a	1805.06 ± 13.36 a	20.01 ± 0.40 b	39.56 ± 13.40 a	3.37 ± 0.51 ab
RC	0.090 ± 0.004 ab	1739.54 ± 12.65 c	19.49 ± 0.88 b	38.62 ± 9.88 a	3.11 ± 0.74 ab

Note: different lowercase letters are significant differences based on one-way ANOVA (p < 0.05). CK: yellow soil group, 0.5 × DT: 1.5 kg·m⁻² dark tea biofertilizer group, 1 × DT: 3 kg·m⁻² dark tea biofertilizer group, 1.5 × DT: 4.5 kg·m⁻² dark tea biofertilizer group, 2 × DT: 6 kg·m⁻² dark tea biofertilizer group, CF: 3 kg·m⁻² commercial organic fertilizer group, and RC: 3 kg·m⁻² rapeseed cake fertilizer group.

. The maximum net photosynthetic rate (P_n) was significantly increased in all groups compared with CK. In particular, the P_nmax of the four DT treatments gradually increased with the increases in DT dosages, of which the 1.5 × DT (40.02%) and 2 × DT (41.43%) were significantly higher than CK (Table 4). The apparent quantum yield (AQY) showed no significant differences in each group compared with CK, with the exception of the significantly lower 0.5 × DT group. The light saturation point (LSP) reflects the availability of light energy of plants in bright light. The LSP of the six treatments was significantly higher than that of CK, with the highest value being observed in the 2 × DT, 1.5 × DT, and CF groups, which indicates that the biofertilizers were less inhibited in high temperatures with bright light conditions. The dark respiration rate (Rd) of the 2 × DT group was significantly lower than that of CK, with no significant difference being observed in the other fertilization treatments. Meanwhile, there were no significant differences in light compensation point (LCP). Therefore, the 1.5 × DT and 2 × DT groups showed a lower leaf Rd and a higher P_nmax and LSP, the opposite to the CK group.

As shown in

Table 5. Changes in substrates in photosynthetic characteristics of Dendranthema morifolium ‘Jinsi Huang’ leaves in simultaneous precipitation and high-temperature period.

Treatment Groups	Pn μmol m−2 s−1	Gs μmol m−2 s−1	Ci μmol mol−1	Tr μmol m−2 s−1	WUE μmol mol−1
CK	16.64 ± 1.15 c	0.13 ± 0.01 b	241.95 ± 10.10 c	3.38 ± 0.33 c	4.69 ± 0.45 a
0.5 × DT	19.62 ± 0.75 b	0.26 ± 0.03 a	261.53 ± 16.61 b	7.83 ± 0.71 a	2.01 ± 0.51 c
1 × DT	20.23 ± 1.66 ab	0.25 ± 0.03 a	273.67 ± 13.32 b	6.79 ± 1.61 ab	2.97 ± 1.03 abc
1.5 × DT	22.54 ± 1.21 a	0.27 ± 0.02 a	301.66 ± 8.02 a	6.00 ± 2.65 ab	4.07 ± 1.35 ab
2 × DT	22.64 ± 1.71 a	0.23 ± 0.05 a	303.77 ± 13.59 a	5.23 ± 0.45 abc	4.43 ± 0.57 a
CF	20.37 ± 1.03 ab	0.21 ± 0.07 a	271.25 ± 4.76 b	4.64 ± 1.63 bc	4.66 ± 1.37 a
RC	19.64 ± 1.77 b	0.23 ± 0.04 a	268.96 ± 5.40 b	7.48 ± 0.29 a	2.54 ± 0.20 bc

Note: Different lowercase letters are significant differences based on one-way ANOVA (p < 0.05). CK: yellow soil group, 0.5 × DT: 1.5 kg·m⁻² dark tea biofertilizer group, 1 × DT: 3 kg·m⁻² dark tea biofertilizer group, 1.5 × DT: 4.5 kg·m⁻² dark tea biofertilizer group, 2 × DT: 6 kg·m⁻² dark tea biofertilizer group, CF: 3 kg·m⁻² commercial organic fertilizer group, and RC: 3 kg·m⁻² rapeseed cake fertilizer group.

, the photosynthesis of D. morifolium ‘Jinsi Huang’ leaves was significantly affected by the application of biofertilizers. The Pn, stomatal conductance (G_s), and intracellular CO₂ concentration (C_i) of D. morifolium ‘Jinsi Huang’ were significantly increased in the six fertilization treatments compared with CK, which the 1.5 × DT and 2 × DT groups showed the highest level. The Gs and Ci of 1.5 × DT and 2 × DT groups were increased by 107.70% and 76.92%, and 24.68% and 25.56%, respectively, compared to CK, which suggests that these two groups increased the efficiency of light energy transformation by using intercellular CO₂ and water to promote photosynthesis. The transpiration rate (Tr) of the 0.5 × DT, 1 × DT, 1.5 × DT, and RC groups showed significant differences, being 131.66%, 100.89%, 77.51%, and 121.30% higher than CK, but no significant differences compared to the 2 × DT and CF groups. Compared with CK, the water use efficiency (WUE) of the 1 × DT, 1.5 × DT, 2 × DT, and CF groups showed no significant differences, but the 0.5 × DT (57.14%) and RC (45.84%) groups showed significantly lower values than that of CK. The higher P_n, G_s, C_i, and WUE of the 1.5 × DT and 2 × DT groups suggested that the application of dark tea biofertilizer significantly promoted the photosynthesis and reduced the photoinhibition of D. morifolium ‘Jinsi Huang’ during SPH weather.

3.4. Different Fertilization Treatments Boosted Flower Performance and Compositions Accumulation of Dendranthema Morifolium ‘Jinsi Huang’

3.4.1. Effects on Flower Performance of Dendranthema Morifolium ‘Jinsi Huang’

The corolla of D. morifolium ‘Jinsi Huang’ was mainly composed of outer ligulate flowers, middle ligulate flowers, and inner ligulate flowers, but its tubular flowers seriously degraded. The maximum diameter of the whole corolla was determined by the length of the longest outer ligulate flowers. The length of the middle ligulate flower was shorter than that of the outer ligulate flower, but the large number, small spacing, and tight arrangement determined the visual size of the whole corolla. As shown in Figure 6i, eight flower morphological traits of D. morifolium ‘Jinsi Huang’ were significantly improved by the application of biofertilizers. In particular, the number of flower buds, maximum flower diameter, middle round lingual flower diameter, outer round lingual flower length and width, and middle round lingual flower length and width showed extremely significant differences in the DT treatment groups, increasing by 29.70–40.20%, 25.26–44.31%, 24.40–36.40%, 25.18–36.70%, 29.80–49.84%, 26.07–58.24%, 764.58–1047.92% compared to CK, respectively, with no significant differences being observed between the 1.5 × DT and 2 × DT groups (Figure 6a–c,e–h). The inner ligulate flower diameter was significantly higher in the 1 × DT, 1.5 × DT, 2 × DT and CF groups, by 34.93%, 22.16%, 23.94%, and 34.93% compared to CK, with no significant differences being observed in the 0.5 × DT and RC groups (Figure 6d). In various types of fertilizers, the number of flower buds of the 1 × DT, CF, and RC groups was 7.88, 8.69, and 4.56 times higher than that of CK, respectively, but much lower than that of the 1.5 × DT group (10.42 times) (Figure 6a). The width of the outer ligulate flower of the 1 × DT, CF, and RC groups was 28.37%, 24.20%, and 14.63% higher than that of CK (Figure 6g), suggesting that the dark tea biofertilizer greatly increased the size of the corolla compared to the other types of fertilizer. The results showed that, with the amount of dark tea fertilization increasing, the flower morphological traits were gradually enhanced, with the 1.5 × DT and 2 × DT groups showing the highest benefits, which were much higher than those observed for 1 × DT, 0.5 × DT, CF, RC, and CK.

3.4.2. Effects on Basic Nutritional and Medicinal Active Components of Dendranthema Morifolium ‘Jinsi Huang’ Flowers

The content of three basic nutritional components, excluding the carotenoid content, and five medicinal active components of D. morifolium ‘Jinsi Huang’ flowers was significantly affected by the application of biofertilizers (Figure 7). The content of ascorbic acid of the 1.5 × DT, 2 × DT, CF, RC, and 1 × DT groups was significantly increased by 52.45%, 52.85%, 33.75%, 27.64%, and 17.80% compared to CK (Figure 7a). Compared with CK, six fertilization treatments were significantly higher in soluble sugar and total protein (Figure 7c,d). In the DT fertilization treatments, the content of soluble sugar and total protein was increased from 3.18 mg 100 g⁻¹ to 5.06 mg 100 g⁻¹ and from 333.97 mg 100 g⁻¹ to 510.90 mg 100 g⁻¹, respectively, being from 40.88% to 122.76% and from 37.65% to 110.57% higher than those of CK (2.27 mg 100 g⁻¹ and 242.63 mg 100 g⁻¹). The groups with various types of fertilizers, compared with the CK, 1 × DT, CF, and RC groups, had contents of soluble sugar that were 95.01%, 94.42%, and 55.07% higher and contents of total protein that were 89.43%, 92.07%, and 62.48% higher, but which were much lower than those of the 1.5 × DT, 2 × DT groups. However, there was no significant difference in carotenoid content (Figure 7b).

In addition, results based on five medicinal components are shown in Figure 6e-i. In the DT fertilization treatments, the contents of polysaccharide, isochlorogenic acid, and isochlorogenic acid A were significantly increased by from 24.87% to 50.31%, from 10.83% to 38.23%, and from 6.73% to 23.29%, respectively, compared with CK, with the increasing dosage of dark tea biofertilizer (Figure 7e,g,i). Among the various types of fertilizers, the 1 × DT, CF, and RC groups showed significant differences, being 43.16%, 43.99%, and 30.94% higher in their content of polysaccharide, 26.39%, 29.23%, and 23.85% higher in their content of isochlorogenic acid, and 18.86%, 9.49%, and 19.01% higher in their content of isochlorogenic acid A, respectively, compared with CK. The content of total flavonoids in the other five fertilization treatments, except the 0.5 × DT group, was significantly higher than in CK. The 2 × DT group had the highest value, up to 87.05 mg 100 g⁻¹, which was 86.54% higher than that of CK, followed by 1.5 × DT (80.28% higher than CK). The content of cynaroside in the other five fertilization treatments, except the 1 × DT group, was significantly higher than in CK; 1.5 × DT (32.87% higher than CK) > 2 × DT (29.13% higher than CK) > CF (28.16% higher than CK) > RC (23.46% higher than CK) > 0.5 × DT (9.84% higher than CK). All fertilization treatments showed higher content in terms of their accumulation of nutritional and medicinal active components at different levels, of which 1.5 × DT and 2 × DT showed the highest contents in all components, with the lowest being CK. This indicates that dark tea fertilizer slowly releases nutrients that enhance the growth of plants, giving them a higher height, wider crown width, stem diameter, and leaf area, improving the photosynthesis in their leaves, accelerating their conduction of needed elements and materials in plants, and promoting the accumulation of components in their flowers.

3.5. Principal Component Analysis of Effects on Substrate-Plant System

Principal component analysis (PCA) showed how the types and dosages of fertilizers affected D. morifolium ‘Jinsi Huang’ in terms of its morphology, photosynthesis, and flower traits and components (Figure 8). As shown in Figure 8A, 64.7% and 8.8% of the total variation was represented by two axes, PC1 and PC2. PC1 was principally composed of soluble sugar, polysaccharide, the width of outer round lingual flower, chlorophyll a, chlorogenic acid, and the light saturation point, while PC2 was mainly correlated with the transpiration rate, light compensation point, stomatal conductance, and width of the crown. The confidence ellipse of 1.5 × DT showed the highest proportion, especially in the quadrant (PC1+ and PC2+), followed by 2 × DT. The results showed that the 1.5 × DT group was the optimal fertilization treatment with the most beneficial effects on D. morifolium ‘Jinsi Huang’ in terms of enhancing its growth and promoting the accumulation of effective components on flowers in simultaneous precipitation and high-temperature regions.

4. Discussion

4.1. Dark Tea Biofertilizer Success in the Improvement of Soil Environment in SPH Regions

Waterlogged but hot conditions in SPH regions caused the high occurrence of pathogens and pests exacerbated the leaching and loss of organic matter, damaged the soil structure, and gradually accelerated the process of soil degradation, which were a great threat to the growth and development of plants [47]. Dark tea biofertilizers maintained good drainage and air permeability (high total porosity and low bulk density) of the soil in simultaneous precipitation and high-temperature periods, while maintaining a high water-holding capacity in drought periods because of cellulose’s porous structure, which ensured that D. morifolium ‘Jinsi Huang’ successfully survived high-temperature conditions with abundant rain or with drought. In contrast, a higher bulk density in yellow soil in SPH weather contributed to heavy soil compaction with a low water-holding capacity and soil porosity, which deeply inhibited the formation and extension of roots [48]. Meanwhile, in the late growth stage, the RC group, though rich in soil nutrients, showed poor growth performance and inferior flower quality, coupled with higher EC and pH values. This aligns with previous studies showing that elevated EC and pH levels inhibited the long-term growth and development of plants, even in nutrient-abundant substrates, whereas chrysanthemums prefer a near-neutral pH [49,50]. Cellulose-based dark tea biofertilizers not only fostered a well-aerated and light soil environment that was ideal for robust root development, but the rich nutrients maintained the balance in the consumption of soil nutrients by the plants, thereby setting the stage for a surge in above-ground biomass. Previous studies have reported that the application of biofertilizers, like tea seed meal, increased the concentration of soil organic carbon, available N, and available P by 14.0%, 20.9%, and 69.3%, respectively, compared CK treatment, maintaining the balance of nutrients in the soil as well as improving the relative abundance of dominant and beneficial microbial communities, which increased the nutrients supply and in the soil delayed the process of soil degradation [51,52]. Meanwhile, the application of bio-organic fertilizer and microbial agent mixture increased soil organic matter, total nitrogen, and alkali hydrolyzed nitrogen by 20.18%, 10.06%, and 10.28%, respectively [53], but the effects were significantly lower in the 4.5 kg·m⁻² dark tea biofertilizer treatment group than in the commercial organic fertilizer group, in which the N, AHN, and C were increased by 35.53%, 200.00%, and 69.19%, respectively. Tea biofertilizers, as a kind of controlled-release fertilizer, improved the crop yield and development through the biodegradation of cellulose over time, which continuously released nutrients into the soil, enhancing the infertility, porous structure, and microbial actions of the soil in SPH regions, all of which promoted aspects of the soil structure like its water-holding capacity and soil porosity [54,55]. Therefore, dark tea biofertilizer, as a controlled fertilizer, improved the moisture, nutrients, and air permeability of soil in SPH regions, which provided beneficial conditions for the root growth and absorption of water and nutrition.

4.2. Dark Tea Biofertilizer Stimulated Light Energy Utilization of Dendranthema Morifolium ‘Jinsi Huang’ Leaves in SPH Weather

Composting was an effective and sustainable process of recycling organic wastes to transform poorly soluble organic matter into easily available and absorbable matter for plants [34]. Previous studies have reported that the biodegradation of cellulose and polyphenol in the process of tea composting formed a higher concentration of humic substances, increasing the basic nutrition level of soil, which assisted plants in alleviating damages caused by high temperatures and water deficit [56,57,58]. Abundant N, P, K, and C in the 1.5 × DT treatment greatly increased the plant height, crown width, stem diameter, and number of leaves of D. morifolium ‘Jinsi Huang’, expanding the leaf areas and stimulating the synthesis and accumulation of chlorophyll pigments to form a deep green leaf color. In contrast, the nutrient-poor yellow soil, deficient in key elements like N (impaired chlorophyll synthesis), resulted in light green leaves and a small leaf size, as the limited nutrients restricted plant growth. This suggested that the application of biofertilizers stimulated D. morifolium ‘Jinsi Huang’ to enhance its efficiency of absorption and conduction of nutrients and water to maintain the homeostasis of photosynthesis and osmotic adjustment systems, to obtain a relatively stable growth in stress conditions [59]. Moreover, the high light saturation point (LSP) and low value of dark respiration (Rd) represented a high light-energy transforming and utilization efficiency, which accumulated more organic matter and consumed less, whereas the decreased F_v/F_m value indicated the photoinhibition of plants under stress [60,61]. In this study, the low stomatal conductance (G_s), F_v/F_m value, and LSP in the yellow soil group indicated photoinhibition with a low photosynthetic capacity and efficiency under SPH weather, which easily induced a short-term dormancy and stagnating growth stage in D. morifolium ‘Jinsi Huang’, and sharply dropped the yield of flowers. Conversely, the 4.5 kg·m⁻² dark tea biofertilizer treatment showed a high content of chlorophyll pigments that were activated the photosynthetic system of D. morifolium ‘Jinsi Huang’ leaves, which was achieved increasing the G_s and LSP, improving its utilization of CO₂ and water, and improving its conduction of nutrients to accelerate the utilization of light with a higher net photosynthesis rate (P_n). This alleviated the abnormal salix-leaf-shape variation and photoinhibition. Thus, the application of dark tea biofertilizer improved the absorption of soil nutrition and utilization of light energy, alleviated photoinhibition, reduced the change to the salix leaf shape, and accumulated nutrients for blooming.

4.3. Dark Tea Biofertilizer Greatly Promoted the Accumulation of Chlorogenic Acid in Dendranthema Morifolium ‘Jinsi Huang’ Flowers in SPH Weather

The nutrition content level of biofertilizers applied to the soil influenced the concentration of nutrients and bioactive components of leaves, fruits, and flowers [62,63]. One study found that the moderately combination of rapid-acting fertilizers with biofertilizers significantly improved the contents of chlorogenic acid (30.1%) and cymaroside (12.8%) of chrysanthemum compared to the use of rapid-acting fertilizer only [64], which suggested that biofertilizers enhanced the efficiency of nutrient absorption. These results supported the crucial role of soil nutrients in regulating metabolic processes and the accumulation of secondary metabolites. Meanwhile, the existing study has confirmed that N positively regulated the metabolic processes of carbon flow allocation, which negatively impacted the accumulation of flavonoids and polyphenols in plants with the increasing N concentration [65,66]. However, in this study, the higher nitrogen concentration in 4.5 kg m⁻² dark tea fertilizer substrate led to an increase in various compounds including the total flavonoids (80.28%), chlorogenic acid (38.23%), cynaroside (32.87%), isochlorogenic acid A (23.29%), polysaccharide (48.64%), and soluble sugar (122.76%) compared to CK. This was due to the better internal C–N–K balance of D. morifolium ‘Jinsi Huang’ in that substrate, which controlled the flow allocation of carbon in both the carbohydrate biosynthesis pathway and the flavonoid biosynthesis pathway, as well as the polyphenols-related biosynthesis [67,68]. Additionally, the rapeseed cake treatment, which had the highest content of initial nutrients in the soil, showed weaker effects on the performance of flower traits and the content of flower components than the 4.5 kg m⁻² DT treatment. This was attributed to the fact that the dark tea composting biofertilizer was well balanced in terms of its soil nutrients and microbial activity, which slowly but continuously released nutrients in the process of cellulose degradation and humus formation to accumulate more beneficial compounds, which enhanced its yield and could increase its economic benefits [69,70].

Furthermore, according to a previous study, spraying with foliar humic acid improved the bloom diameter of chrysanthemum to 10.47 cm and its stem diameter to 0.64 cm [71], but the effect was much less strong than that of dark tea fertilizers applied in the soil. At the same time, some liquid organic fertilizers, like seaweed extracted fertilizer, showed positive effects on the plant height of chrysanthemum due to providing an adequate supplement of N, but no effects on the flower [72]. However, the adequate content of P (from 12.28% to 26.99% higher than CK) in slow-release dark tea biofertilizer stimulated the blooms to bigger sizes (from 29.29% to 40.31%) and significantly promoted the flower size, exceeding first class flowers (12.5 cm), which was expected to greatly increase farmers’ income in the chrysanthemum-led rural revitalization areas. In this study, the application of dark tea biofertilizer, which caused a bigger flower size, exceeding first-class flowers, and the accumulation of higher basic nutritional and medical composition contents in D. morifolium ‘Jinsi Huang’, which strongly validates it as a feasible, practical approach to supporting chrysanthemum-based agricultural practices in rural revitalization efforts.

5. Conclusions

This study has confirmed the feasibility of the application of dark tea fertilizer in the cultivation and production of Dendranthema morifolium ‘Jinsi Huang’ in main cultivating regions. The 4.5 kg m⁻² DT treatment was the most effective, showing the strongest improvement in soil health, plant growth, leaf light energy utilization, and floral functional components. For one thing, this optimal fertilization of dark tea biofertilizer improved yellow soil to confer better water retention, make the soil lighter and more nutrient-rich, and assist roots to extend their absorption area of needed nutrients and water, which increased the above-ground biomass, especially the number of leaves and leaf area. Meanwhile, the expanding leaf area activated the photosynthesis system and enhanced the light utilization to minimize the damage caused by simultaneous precipitation and high-temperature weather, as well as to stimulate the accumulation of effective components in flowers, like chlorogenic acid, promoting the quality and economic value of the plant. Dark tea biofertilizer holds potential as an easily available, effective, and environmentally friendly fertilizer substitution that can be applied in the culture of Dendranthema morifolium ‘Jinsi Huang’, as it will reduce the costs of cultivation, boost the industrial development of Dendranthema morifolium ‘Jinsi Huang’, increase farmers’ income, and solve the problem of handling a large number of dark tea wastes annually.