Seasonal Variations in the Growth and Physiology of Acer miaotaiense subsp. yangjuechi Fang et P. L. Chiu Seedlings Under Shading Treatments

Abstract

This study investigated the seasonal variations in photosynthetic characteristics and adaptation mechanisms of the endangered Acer miaotaiense subsp. yangjuechi Fang et P. L. Chiu under different shading treatments, offering theoretical insights and technical support for its relocation and conservation strategies. Five shading intensity treatments were established: full light (CK), 30% (T1), 50% (T2), 70% (T3), and 90% (T4). The growth and physiological conditions of 5-year-old A. miaotaiense subsp. yangjuechi seedlings were monitored across seasons. The findings revealed that plants grown under 70% shade had the best growth performance and leaf morphological indices. In summer, the levels of osmotic adjustment substances and peroxidase activity were the highest, whereas malondialdehyde (MDA) levels increased progressively throughout the season. Notably, superoxide dismutase activity exhibited an opposite trend to MDA. Proline and MDA contents were the highest under full light conditions and lowest under 70% shade, whereas soluble sugars and starch showed the reverse pattern. Chlorophyll content and photosystem II efficiency peaked under 70% shade, with the highest net photosynthetic rate and light saturation point observed in the 70% and 90% shade treatments. A comprehensive evaluation suggests that 70% shade is more conducive to the normal growth and development of A. miaotaiense subsp. yangjuechi seedlings.

1. Introduction

Acer miaotaiense subsp. yangjuechi Fang et P. L. Chiu is an ancient relict species of wild plant under state-level protection, named by Mr. Wenpei Fang and Mr. Baolin Qiu, based on a specimen collected from Tianmu Mountain in Lin’an [1]. It is closely related to A. myabei, which originated from Japan, and its taxonomic status has been a subject of debate. In 2023, Fan et al. conducted phylogenetic, morphological, and ecological studies on A. campestre, A. myabei, A. miaotaiense, and A. miaotaiense subsp. yangjuechi. Their findings showed that coalescent species trees based on 544 and 77 single-copy nuclear genes (SCNGs) both supported series Campestria as monophyletic, with A. yangjuechi most closely related with A. miaotaiense. A. miyabei and A. miaotaiense are distinct species, while A. yangjuechi (endemic to Mt. Tianmu/East China) should be treated as a subspecies of A. miaotaiense, considering its unique plastome and disjunct distribution [2].

A. miaotaiense subsp. yangjuechi is native to the broad-leaved forests located in gullies of Tianmu Mountain in Lin’an, at altitudes of 800–870 m. Its natural habitat features high rock exposure and steep slopes, creating an extremely harsh survival environment. Notably, the native population of four wild plants perished in 2013 owing to the influence of snowstorms and geological disasters [3,4]. Currently, seedlings are being cultivated at several locations, including Tianmu Mountain Rare and Endangered Botanical Garden, Tianmu Ironwood Garden, Houshanmen Arboretum, Hengwu Afforestation, Huamei Villa, Martyrs’ Shrine, Zenyuan Temple, Yuhua Pavilion and Laodian. A. miaotaiense subsp. yangjuechi has also been introduced and cultivated at Zhejiang A&F University and Lin’an. In recent years, despite effective protection measures implemented by relevant departments for A. miaotaiense subsp. yangjuechi, this species faces challenges such as poor seed reproduction capabilities, high abortion rates, deep seed dormancy, and weak natural regeneration. Exploring their light and response mechanisms is essential to accurately understand their ecological and biological characteristics and to determine their potential for conservation and restoration in natural habitats. To date, only two naturally regenerated seedlings have been identified in the Painted Eyebrows Villa and Yu Hua Pavilion. Consequently, A. miaotaiense subsp. yangjuechi remains critically endangered, with its survival in an extremely precarious state. It is imperative to enhance conservation efforts to safeguard this subspecies, which holds significant scientific research value for understanding the origin and evolution of the East Asian flora and paleogeographic changes [5]. Furthermore, its aesthetically pleasing tree shape, upright crown, and unique winged fruits resembling ram’s horns make it valuable for ornamental gardening applications [3,6].

Our investigation into the introduction of A. miaotaiense subsp. yangjuechi revealed that its seedlings might be adversely affected by strong light. Generally, intense light exposure can damage A. miaotaiense subsp. yangjuechi seedlings, leading to diminished growth and development. Therefore, it is essential to study the effects of light intensity on the growth of A. miaotaiense subsp. yangjuechi seedlings. Plant growth is closely linked to environmental conditions, and light serves as a crucial factor for plant survival. It significantly influences plant growth and development as well as various physiological and biochemical processes, including the synthesis and accumulation of secondary metabolites [7,8,9]. Plants exhibit varying light intensity requirements and can be categorized as sun-loving, shade-loving, or shade-tolerant. Sun-loving plants experience stress-related morphological changes in low-light environments, whereas shade-loving plants face similar challenges in high-light conditions [10,11]. Investigating the photosynthetic and morphological responses of plants to varying light intensities can provide insights into their tolerance to different light conditions. This approach may also be employed to evaluate optimal habitat conditions for the conservation of endangered species, especially those with limited populations or in extremely complex habitats [9]. Studies have shown that endangered plants respond differently to light in terms of growth and development. Appropriate light intensity not only promotes the growth and development of endangered plants but also enhances their survival and reproductive success, which is crucial for maintaining the stability of endangered plant populations and ecosystems. Research has indicated that optimal light intensity holds the potential to significantly enhance the growth and development of endangered plant species and improve their survival and reproductive success, which is essential for maintaining the stability of endangered plant populations and ecosystems [12,13]. For example, low-light conditions severely restrict the growth and development of endangered species such as Manglietia ventii and Horsfieldia glabra, whereas Davidia involucrate demonstrates higher light utilization efficiency under similar conditions [14,15,16]. Additionally, moderate shading can alleviate the adverse effects of high temperatures and drought on Magnolia sinostellata seedlings during summer [17].

For endangered plant species with critically low population sizes, introgression is an essential conservation strategy. Furthermore, the establishment of suitable ex situ cultivation conditions is particularly crucial for the survival and growth of seedlings of A. miaotaiense subsp. yangjuechi. Currently, there is limited information available on A. miaotaiense subsp. yangjuechi, with research primarily focusing on its systematic characteristics, embryonic development, and dormancy lifting [18,19]. However, the effects of light conditions on growth, development, and physiological adaptations remain unreported. This study investigated the response patterns of growth, physiology, photosynthesis, and chlorophyll fluorescence in A. miaotaiense subsp. yangjuechi to varying light intensities. The aim was to determine the optimal light conditions for its growth and to assess whether light poses a potential threat to its survival. These findings will provide a theoretical foundation for the development of in situ and translocation conservation strategies and may also contribute to the protection and cultivation of other endangered plant species.

2. Materials and Methods

2.1. Plant Materials

The experiment was conducted at the Rare and Endangered Plants Specialized Garden of Zhejiang Agriculture and Forestry University (ZAFU), Donghu Campus (119°72′ E, 30°26′ N). This site is situated at the southern edge of the Central Subtropical Monsoon Climate Zone (CSCZ), which is characterized by a monsoon climate with warm temperatures year-round, four distinct seasons, and abundant rainfall. The average annual precipitation is 1613.9 mm, with an average annual temperature of 16 °C, and it is located at an elevation of 58.2 m above sea level. The study materials were sourced from ZAFU’s Endangered Plants Specialized Garden. Five-year-old A. miaotaiense subsp. yangjuechi seedlings, selected for their robust and uniform growth, were derived from a single mother tree and cultivated in a ‘Golden No. 3’ substrate, exhibiting an average height of 94.22 ± 2.25 cm and an average diameter of 13.28 ± 1.38 mm.

2.2. Shading Treatments

The shade treatment was implemented in mid-April 2022, using a steel frame to construct a 2.5 m high shade shed covered with shade nets of varying densities. A ZDS-10 illuminometer (Nanjing Wen Nuo I.E. Co., Ltd., Nanjing, China) measured the light intensity to ensure compliance with experimental shading requirements. The experiment included five treatments: CK (full light), T1 (approximately 30% shading), T2 (approximately 50% shading), T3 (approximately 70% shading), and T4 (approximately 90% shading), with a light intensity of 1800 μmol·m⁻²·s⁻¹ under CK. Growth and morphological indicators were measured in spring, whereas physiological indicators were assessed on 30 April (spring), 15 July (summer), and 15 October (autumn). The experiment used a completely randomized block design with four pots per treatment and three replicates. The potting substrate used for A. miaotaiense subsp. yangjuechi seedlings was “Golden No. 3”, and the seedlings were cultivated under natural light at ambient temperatures ranging from 15 to 35 °C. Seedlings were watered daily to ensure optimal growth.

2.3. Measurement Indicators and Methods

2.3.1. Measurement of Growth and Morphological Indicators

(1)

Leaf length, width, and area were measured using a YMJ-CHA3 leaf area meter (Zhejiang Top Cloud-Agri Technology Co., Ltd., Nanjing, China).

(2)

To determine leaf moisture content, fresh material was collected and heated in an oven at 96 °C for 30 min. The temperature was then reduced to 60 °C, and the material was baked until a constant weight was achieved. Dry weight was measured using an electronic balance with an accuracy of 0.001 g. Subsequently, leaf water content (%) was calculated using the following formula: (leaf fresh weight-leaf dry weight)/leaf fresh weight × 100.

(3)

The trunk height growth was measured using a tape measure with an accuracy of 0.1 cm to assess both the spring pre-treatment and post-treatment heights. The change in trunk height (∆trunk height) was calculated as the difference between post-treatment and pre-treatment heights.

2.3.2. Measurements of Physiological Indicators

To determine physiological indices, four pots were selected for each treatment, each containing fully expanded functional leaves from the upper portions of the plants that were free from pests and diseases, mature, healthy, and consistently positioned. Fifteen leaves were collected from each potted plant and quickly transported to the laboratory for physiological index assessment. The specific measurements were as follows.

The proline (Pro) content was determined using the ninhydrin method [20], while the soluble sugar (SS) content was measured using the sulfuric acid method [21]. The soluble protein (SP) was assessed using the indanedione method [22]. The malondialdehyde (MDA) content was quantified using the thiobarbituric acid method [23]. Superoxide dismutase (SOD) activity was measured using the nitrogen blue tetrazolium (NBT) method, and peroxidase (POD) activity was assessed using the guaiacol method [23].

2.3.3. Determination of Photosynthetic Pigment Levels

Fully expanded functional leaves from the upper parts of the plant, free of pests and diseases, well developed, and oriented similarly, were collected. The leaves of A. miaotaiense subsp. yangjuechi were punched with a 6.78 mm diameter perforator, with the midvein, leaf tip, and leaf base removed, and then preserved by quick-freezing in liquid nitrogen. The samples were homogenized with 4 mL of 96% ethanol and then centrifuged at 5000 rpm for 10 min. The supernatant was collected to measure the optical density (OD) at 470, 649, and 665 nm. The chlorophyll (Chl) a, Chl b, and carotenoid (C_x+c) contents in the supernatant were measured using the Amon method, as revised by Lichtenthaler and Welburn, and the photosynthetic pigment content per unit leaf area (μg·mm⁻²) was calculated [24].

2.3.4. Determination of Photosynthetic Light Response Curves

The light response curves for A. miaotaiense subsp. yangjuechi were measured hourly from 8:30 a.m. to 4:30 p.m., using a Li-6800 portable photosynthesizer (Li-COR Inc., Lincoln, NE, USA). The photosynthetically active radiation (PAR) intensity gradient was set at 0, 50, 100, 150, 200, 300, 400, 500, 600, 700, 800, 1000, and 1200 μmol·m⁻²·s⁻¹. In this experiment, each treatment involves four potted plants, with a total of five leaves, one to two leaves per pot, and the net photosynthetic rate (Pn), stomatal conductance (Gs), transpiration rate (Tr), and intercellular carbon dioxide concentration (Ci) were measured. The maximum net photosynthetic rate (Pn max), light compensation point (LCP), light saturation point (LSP), and dark respiration rate (Rd) were calculated using Ye Zhipiao’s software (Photosynthesis Calculator 4.1.1) [25].

2.3.5. Determination of Chl Fluorescence Kinetic Parameters

A YZQ-500 Chl dynamic fluorescence meter (Wing Manqi Technology Co., Ltd., Beijing, China) was used to obtain Chl fluorescence-induced kinetic curves [26]. Leaves were selected as previously described, with ten leaves assessed for each treatment. The Strasser method and other techniques were employed to calculate the fluorescence kinetic parameters, including the photosystem II (PSII) maximum quantum yield of primary photochemistry (φPo), quantum yield for excitation energy transfer by the reaction center (Ψo), quantum yield for electron transfer (φEo), and non-photochemical deexcitation (φDo) [27].

Formula:

$$\mathsf{\phi }\mathrm{Po}={\displaystyle \frac{\mathrm{Fm}-\mathrm{Fo}}{\mathrm{Fm}}}$$

(1)

$$\mathsf{\Psi }\mathrm{o}={\displaystyle \frac{\mathrm{ETo}}{\mathrm{TRo}}}$$

(2)

$$\mathsf{\phi }\mathrm{Eo}={\displaystyle \frac{\mathrm{ETo}}{\mathrm{ABS}}}=\left(1-{\displaystyle \frac{\mathrm{Fo}}{\mathrm{Fm}}}\right)\times \mathsf{\Psi }\mathrm{o}$$

(3)

$$\mathsf{\phi }\mathrm{Do}=1-\mathsf{\phi }\mathrm{Po}={\displaystyle \frac{\mathrm{Fo}}{\mathrm{Fm}}}$$

(4)

2.3.6. Comprehensive Evaluation

The affiliation function values for different indices were calculated using the following formula, with the average taken as the mean degree of affiliation. Principal component analysis (PCA) and affiliation function analysis (AFA) were employed to comprehensively evaluate the shade tolerance ability of A. miaotaiense subsp. yangjuechi seedlings [28,29]. The affiliation function values for different indices were calculated using the following formula, with the average taken as the mean degree of affiliation. The calculation formula is as follows:

$$\mathrm{R}({\mathrm{X}}_{\mathrm{i}})={\displaystyle \frac{{\mathrm{X}}_{\mathrm{i}}-{\mathrm{X}}_{\min }}{{\mathrm{X}}_{\max }-{\mathrm{X}}_{\min }}}$$

(5)

$$\mathrm{R}({\mathrm{X}}_{\mathrm{j}})=1-{\displaystyle \frac{{\mathrm{X}}_{\mathrm{i}}-{\mathrm{X}}_{\min }}{{\mathrm{X}}_{\max }-{\mathrm{X}}_{\min }}}$$

(6)

where ${\mathrm{X}}_{\mathrm{i}}$ represents the ith composite indicator, and X_min and X_max denote the minimum and maximum values of the ith composite indicator, respectively. When the ith indicator exhibited a positive correlation with shade tolerance capacity, Equation (5) was applied. When the ith indicator demonstrated an inverse correlation with shade tolerance capacity, Equation (6) was used.

2.4. Data Processing

Excel 2021 was used for statistical analysis of the data, and Ye Zhipiao software was employed for data processing. The indicators were reported as the mean ± standard error. Analysis of variance (ANOVA) and Duncan multiple comparisons were performed using SPSS version 23.0, with the significance level set at 0.05. Origin 2021 software was used for graphing.

3. Results

3.1. Effects of Shading on the Growth and Leaf Morphology of A. miaotaiense subsp. yangjuechi Seedlings Under Different Seasons

Different shading treatments significantly affected the leaf length, leaf width, leaf area, and trunk height of A. miaotaiense subsp. yangjuechi, with these parameters generally decreasing as light intensity increased. Under the T3 treatment, the leaf length, leaf width, and leaf area measured 10.64 cm, 8.95 cm, and 51.95 cm², respectively, exhibiting increases of 16.03%, 10.90%, and 26.68% compared to the CK. The trunk height growth in the T3 treatment was 17.33 cm, which was 2.28 times greater than that of the CK. There was no significant difference in leaf water content between treatments (

Table 1. Effects of different shading treatments on growth indices of A. miaotaiense subsp. yangjuechi seedlings.

Shading Treatments	Leaf Length (cm)	Leaf Width (cm)	Leaf Area (cm2)	Leaf Moisture Content (%)	Growth of Trunk Height (cm)
CK	9.17 ± 0.26 b	8.07 ± 0.65 ab	41.01 ± 1.27 b	67.77 ± 0.62 a	7.60 ± 0.94 b
T1	8.56 ± 0.04 b	6.93 ± 0.19 b	38.71 ± 0.88 b	69.57 ± 1.70 a	13.90 ± 2.08 ab
T2	9.01 ± 0.23 b	7.17 ± 0.15 b	41.56 ± 1.74 b	68.27 ± 0.63 a	13.73 ± 3.90 ab
T3	10.64 ± 0.12 a	8.95 ± 0.07 a	51.95 ± 0.65 a	71.09 ± 0.78 a	17.33 ± 1.86 a
T4	10.57 ± 0.48 a	8.87 ± 0.29 a	49.32 ± 0.70 a	69.25 ± 1.03 a	15.40 ± 3.54 a

Different lowercase letters indicate a significant difference at p < 0.05. Values represent mean ± SE (n = 5).

).

3.2. Effects of Shading on Osmoregulatory Substances in Leaves of A. miaotaiense subsp. yangjuechi Seedlings Under Different Seasons

The Pro content in the sample leaves across different shading treatments presented a seasonal pattern, with an initial increase, followed by a subsequent decrease and significant seasonal variation. Pro content was the highest in summer and lowest in autumn. Additionally, Pro content showed an increasing trend with higher light intensity, with the CK treatment exhibiting significantly higher levels than the other shading treatments. Significant differences were observed between the T3 and T4 treatments compared to CK in spring; between the T2, T3, and T4 treatments and CK in summer; and between all shading treatments and CK in autumn. Furthermore, a gradual decrease in the Pro content ratio of T3 relative to CK was observed over time, along with a significant reduction in Pro content in shade treatments.

The seasonal trend of SS content was consistent with that of Pro content, showing an initial increase followed by a decrease in response to constantly rising light intensities, with the peak observed in T3 and the lowest observed in CK. The SP content varied significantly between seasons, being the highest in summer and the lowest in spring, with significant differences between the shading treatments and CK in different seasons. In spring, the SP content decreased with increasing light intensity, with T4 showing the highest content, 1.63 times that of CK. In summer and autumn, the SP content initially increased and then decreased with increasing light intensity, ultimately aligning with the CK levels, with T3 exhibiting the highest content, 1.36 and 1.46 times that of CK, respectively (

Table 2. Effects of shading treatments on osmoregulatory substances in leaves of A. miaotaiense subsp. yangjuechi seedlings under different seasons.

Physiological Indicators	Shading Treatments	Shading Time (d)
		Spring	Summer	Autumn
Pro (μmol·g−1)	CK	40.27 ± 0.81 Ba	56.19 ± 2.49 Aa	30.20 ± 1.21 Ca
	T1	37.58 ± 1.58 Bab	46.97 ± 1.54 Aab	24.56 ± 1.43 Cb
	T2	29.96 ± 1.96 Bab	41.91 ± 3.42 Ab	19.33 ± 1.21 Cc
	T3	24.90 ± 1.17 Bb	30.91 ± 2.10 Ab	15.91 ± 1.07 Cc
	T4	25.92 ± 0.74 Bb	36.79 ± 0.52 Ab	17.75 ± 0.63 Cc
SS (U·g−1)	CK	19.96 ± 0.19 Bd	22.31 ± 0.62 Ac	17.20 ± 0.18 Cd
	T1	21.14 ± 0.96 Acd	22.69 ± 1.01 Ac	22.54 ± 0.16 Ac
	T2	21.85 ± 0.39 Bbc	30.63 ± 1.25 Ab	23.19 ± 1.54 Bc
	T3	24.14 ± 0.46 Ca	36.32 ± 0.11 Aa	29.56 ± 0.59 Ba
	T4	22.64 ± 0.51 Cb	34.23 ± 0.79 Aa	26.92 ± 0.26 Bb
SP (U·g−1·min−1)	CK	7.90 ± 0.14 Cd	12.36 ± 0.31 Ac	11.44 ± 0.36 Bd
	T1	9.09 ± 0.22 Bc	11.81 ± 0.07 Ad	12.02 ± 0.35 Ad
	T2	8.94 ± 0.22 Cc	14.86 ± 0.26 Ab	13.08 ± 0.23 Bc
	T3	11.73 ± 0.12 Bb	16.81 ± 0.33 Aa	16.66 ± 0.36 Aa
	T4	12.91 ± 0.21 Ba	13.18 ± 0.49 ABc	14.85 ± 0.18 Ab

Pro, proline; SS, soluble sugar; SP, soluble protein. Different capital letters represent significant differences (p < 0.05) between seasons for the same shading treatment, and different lowercase letters represent significant differences (p < 0.05) between shading treatments within the same season. Values represent mean ± SE (n = 5).

).

3.3. Effects of Shading on MDA and Antioxidant Enzymes in Leaves of A. miaotaiense subsp. yangjuechi Seedlings Under Different Seasons

Among the different shading treatments, the MDA content in A. miaotaiense subsp. yangjuechi leaves exhibited varying trends across seasons, with no significant differences observed between seasons except for CK and T1. MDA content increased with light intensity, with CK consistently showing the highest values. SOD activity varied significantly across seasons, being notably lower in autumn than in summer. SOD activity increased with increasing light intensity in different seasons, with CK displaying the highest levels. The T3 and T4 treatments differed significantly from the CK treatment. POD activity first increased and then decreased with seasonal changes, exhibiting significant differences among seasons, with the highest activity in summer and the lowest in spring, except for T3 POD activity, which was the highest in CK across all seasons. In spring and summer, POD activity increased with increasing light intensity, with significant differences observed between CK and T3 and T4 treatments in spring, and between the shading treatments and CK in summer. In autumn, POD activity first decreased and then increased with increasing light intensity, with significant differences observed between the shading treatments and CK (

Table 3. Effects of shading treatments on MDA and antioxidant enzymes in leaves of A. miaotaiense subsp. yangjuechi seedlings under different seasons.

Physiological Indicators	Shading Treatments	Shading Time (d)
		Spring	Summer	Autumn
MDA (umol·g−1)	CK	58.22 ± 2.49 Ba	70.72 ± 2.78 Aa	78.80 ± 6.17 Aa
	T1	53.68 ± 0.25 Bab	61.81 ± 4.44 Aab	71.66 ± 6.29 Aa
	T2	50.11 ± 0.58 Aab	54.20 ± 3.16 Abc	51.72 ± 4.11 Ab
	T3	44.46 ± 5.40 Ab	46.41 ± 1.36 Ac	39.35 ± 7.76 Ab
	T4	47.51 ± 0.34 Ab	51.95 ± 0.93 Ac	46.72 ± 3.17 Ab
SOD (U·g−1)	CK	719.44 ± 6.96 Aa	733.05 ± 39.64 Aa	563.18 ± 27.54 Ba
	T1	683.94 ± 18.39 Aa	686.98 ± 38.11 Aa	557.76 ± 22.73 Ba
	T2	680.33 ± 4.66 Aa	681.21 ± 25.67 Aa	521.49 ± 21.84 Ba
	T3	632.11 ± 9.71 Ab	639.03 ± 21.92 Ab	483.24 ± 6.84 Bb
	T4	567.04 ± 8.25 Ac	572.46 ± 9.55 Ac	511.60 ± 17.65 Bb
POD (U·g−1 min−1)	CK	545.11 ± 23.59 Ca	746.56 ± 15.34 Aa	652.78 ± 4.68 Ba
	T1	502.78 ± 37.07 Ca	668.84 ± 27.39 Aab	618.89 ± 9.56 Bab
	T2	483.89 ± 16.77 Ca	656.33 ± 7.18 Abc	582.33 ± 16.25 Bb
	T3	422.92 ± 10.76 Bb	548.11 ± 15.22 Ac	388.11 ± 7.85 Cd
	T4	408.06 ± 14.95 Cb	579.11 ± 8.34 Ac	506.89 ± 18.22 Bc

MDA, malondialdehyde; SOD, superoxide dismutase; POD, peroxidase. Different capital letters represent significant differences (p < 0.05) between seasons for the same shading treatment, and different lowercase letters represent significant differences (p < 0.05) between shading treatments within the same season. Values represent mean ± SE (n = 5).

).

3.4. Effects of Shading on Photosynthetic Pigment Contents in the Leaves of A. miaotaiense subsp. yangjuechi Seedlings Under Different Seasons

The contents of Chl a, Chl b, and C_x+c in the sample leaves generally increased and then decreased with seasonal changes. Significant differences in Chl a and Chl b were observed between seasons, except for Chl b in the T4 treatment, whereas C_x+c demonstrated no significant seasonal differences. In spring, Chl a and Chl b decreased with the increasing light intensity, whereas in summer and autumn, Chl a, Chl b, and C_x+c initially increased and then decreased with the increasing light intensity. Chl a, Chl b, and C_x+c levels were higher under medium to heavy shading (T3 and T4), with the shading treatments significantly enhancing the photosynthetic pigment content of A. miaotaiense subsp. yangjuechi seedlings (

Table 4. Effects of shading on chlorophyll content of A. miaotaiense subsp. yangjuechi seedlings under different seasons.

Physiological Indicators	Shading Treatments	Shading Time (d)
		Spring	Summer	Autumn
Chl a (μg·mm−2)	CK	0.26 ± 0.01 Ab	0.28 ± 0.02 Ac	0.23 ± 0.01 Bc
	T1	0.27 ± 0.01 Bb	0.30 ± 0.01 Abc	0.22 ± 0.0 Cc
	T2	0.33 ± 0.01 Aa	0.32 ± 0.02 Aabc	0.24 ± 0.01 Bbc
	T3	0.34 ± 0.02 Aa	0.36 ± 0.01 Aa	0.31 ± 0.01 Ba
	T4	0.36 ± 0.02 Aa	0.35 ± 0.02 Aab	0.29 ± 0.02 Bab
Chl b (μg·mm−2)	CK	0.07 ± 0.01 Bb	0.10 ± 0.03 Aa	0.07 ± 0.01 Bb
	T1	0.11 ± 0.01 Aa	0.12 ± 0.01 Aa	0.08 ± 0.02 Bb
	T2	0.12 ± 0.01 Aa	0.12 ± 0.01 Aa	0.07 ± 0.02 Bb
	T3	0.13 ± 0.01 Ba	0.16 ± 0.03 Aa	0.15 ± 0.01 ABa
	T4	0.13 ± 0.01 Aa	0.14 ± 0.03 Aa	0.14 ± 0.01 Aa
Cx+c (μg·mm−2)	CK	0.03 ± 0.01 Aa	0.05 ± 0.01 Aa	0.04 ± 0.01 Aa
	T1	0.04 ± 0.014 Aa	0.04 ± 0.01 Aa	0.04 ± 0.01 Aa
	T2	0.04 ± 0.01 Aa	0.05 ± 0.01 Aa	0.05 ± 0.01 Aa
	T3	0.05 ± 0.01 Aa	0.04 ± 0.02 Aa	0.04 ± 0.01 Aa
	T4	0.04 ± 0.01 Aa	0.05 ± 0.01 Aa	0.04 ± 0.01 Aa

Chl a, chlorophyll a; Chl b, chlorophyll b; C_x+c, carotenoid. Different capital letters represent significant differences (p < 0.05) between seasons for the same shading treatment, and different lowercase letters represent significant differences (p < 0.05) between shading treatments within the same season. Values represent mean ± SE (n = 5).

).

3.5. Effects of Shading on Light Response Curves of A. miaotaiense subsp. yangjuechi Seedlings Under Different Seasons

The light response of Pn in A. miaotaiense subsp. yangjuechi leaves exhibited a similar trend across different shading treatments. As PAR increased, Pn initially increased slowly and then plateaued once PAR reached LSP. In spring, Pn decreased with increasing light intensity, whereas in summer and autumn, it initially increased and then decreased with increasing light intensity. Shading treatments significantly enhanced Pn in A. miaotaiense subsp. yangjuechi seedlings (Figure 1).

Table 5. Light response fitting parameters of A. miaotaiense subsp. yangjuechi seedling leaves under different seasons.

Physiological Indicators	Shading Treatments	Shading Time (d)
		Spring	Summer	Autumn
Pn max (μmol·m−2·s−1)	CK	2.26 ± 0.18 Ac	2.19 ± 0.21 Ac	1.97 ± 0.09 Ae
	T1	4.58 ± 0.51 Aab	3.50 ± 0.43 ABb	2.61 ± 0.03 Bd
	T2	4.93 ± 0.69 Aab	3.78 ± 0.32 Bb	3.48 ± 0.07 Cc
	T3	6.18 ± 0.98 Aa	5.78 ± 0.58 Aa	4.61 ± 0.45 Ba
	T4	6.32 ± 0.05 Aa	5.29 ± 0.26 Ba	4.05 ± 0.12 Cb
LCP (μmol·m−2·s−1)	CK	19.17 ± 1.39 Ba	14.17 ± 0.43 Ca	25.88 ± 0.33 Aa
	T1	16.35 ± 1.30 Bb	13.12 ± 0.81 Cab	18.08 ± 0.49 Ab
	T2	13.39 ± 1.09 Abc	10.67 ± 0.13 Bbc	15.49 ± 1.55 Ab
	T3	12.09 ± 1.07 Bcd	9.84 ± 2.25 Cc	16.54 ± 0.16 Ab
	T4	9.31 ± 1.04 Bd	7.69 ± 0.59 Bc	16.53 ± 1.98 Ab
LSP (μmol·m−2·s−1)	CK	865.80 ± 6.00 Ac	709.46 ± 6.00 Bb	683.73 ± 87.25 Cc
	T1	892.93 ± 12.79 Ac	741.24 ± 86.87 Ab	895.49 ± 56.79 Ab
	T2	990.91 ± 44.37 Aab	981.13 ± 76.19 Aab	1022.59 ± 13.14 Aab
	T3	915.15 ± 18.11 Bbc	1066.66 ± 0.01 Aa	1043.63 ± 17.53 Aa
	T4	1066.67 ± 0.01 Aa	1076.00 ± 118.43 Aa	1033.70 ± 13.70 Aab
Rd (μmol·m−2·s−1)	CK	0.67 ± 0.04 Bab	0.57 ± 0.05 Ba	0.91 ± 0.05 Aa
	T1	0.75 ± 0.09 Aa	0.56 ± 0.01 Ba	0.85 ± 0.02 Aab
	T2	0.62 ± 0.05 ABab	0.55 ± 0.02 Bab	0.68 ± 0.05 Ac
	T3	0.51 ± 0.06 Bbc	0.38 ± 0.04 Cb	0.67 ± 0.01 Ac
	T4	0.40 ± 0.03 Bc	0.47 ± 0.06 Bab	0.72 ± 0.04 Abc

Pn max, maximum net photosynthetic rate; LCP, light compensation point; LSP, light saturation point; Rd, dark respiration rate. Different capital letters represent significant differences (p < 0.05) between seasons for the same shading treatment, and different lowercase letters represent significant differences (p < 0.05) between shading treatments within the same season. Values represent mean ± SE (n = 5).

presents that the Pn max was not significantly different among seasons under CK treatment, while the opposite was true for LSP. Moreover, LCP and Rd were autumn > spring > summer, with all of them significantly different. The Pn max decreased with the seasonal changes in shade treatments and was the highest in spring. LSP, LCP and Rd generally decreased and then increased with seasonal changes. The Pn max and LSP were significantly higher than CK under the T4 treatment in spring; LCP and Rd increased with increasing light intensity, and the differences between treatments were significant. In summer, the Pn max increased and then decreased with the increase in light intensity; LSP decreased with the increase in light intensity; and Pn max and LSP were the highest in the T3 treatment. The T3 and T4 treatments significantly reduced LCP and Rd in the seedlings of A. miaotaiense subsp. yangjuechi, which were significantly different from CK. In autumn, the Pn max and LSP were the highest in the T3 treatment, and there were significant differences between the shade treatments and CK. LCP and Rd decreased and then increased with the increase in the light intensity, and the lowest values were obtained in the T2 and T3 treatments, respectively. There were significant differences between the two treatments and CK.

3.6. Effects of Shading on Gas Exchange Parameters of A. miaotaiense subsp. yangjuechi Seedlings Under Different Seasons

The Gs of the sample leaves varied with season, showing significant differences across seasons, with the highest values in spring and the lowest in autumn. The overall Gs initially increased and then decreased with increasing light intensity and was the highest under the T3 treatment. In spring, T3 exhibited a significant difference from CK and was 1.50 times higher. In summer, the T2, T3, and T4 treatments significantly differed from CK, with T3 being 1.75 times higher. In autumn, the T3 and T4 treatments significantly differed from CK, with T3 being 1.36 times higher. Tr increased and then decreased with seasonal changes, presenting significant seasonal differences, with the highest values in summer and the lowest in autumn. Tr increased and then decreased with increasing light intensity across different seasons, reaching its peak under the T3 treatment. In spring, both T3 and T4 treatments were significantly different from CK, with T3 being 1.48 times higher than CK. In summer and autumn, the T2, T3, and T4 treatments significantly differed from CK, with T3 being 2.67 and 1.92 times higher than CK, respectively (Figure 2). Ci varied with season, with significant differences observed between seasons. CK consistently had the highest Ci levels, whereas T3 and T4 had lower values. In spring, the T3 and T4 treatments were significantly different from CK, and in summer and autumn, the T2, T3, and T4 treatments were significantly different from CK.

3.7. Effects of Shading on Chl Fluorescence of A. miaotaiense subsp. yangjuechi Seedlings Under Different Seasons

The fluorescence intensity of Chl fluorescence kinetic curves for A. miaotaiense subsp. yangjuechi under different shading treatments decreased from point O to point P with seasonal changes, demonstrating the highest intensity in spring and the lowest in summer. In spring, the Chl fluorescence intensity was the strongest under the T4 treatment, whereas in summer and autumn, it was the highest under the T3 treatment. The CK treatment consistently exhibited the lowest fluorescence intensity across all seasons (Figure 3).

The φPo and Ψo values of the seedlings varied with season, being highest in spring and lowest in autumn, with significant differences among seasons for all treatments, except for T2 and T3. In spring, φPo and Ψo decreased with increasing light intensity, with the T4 treatment significantly higher than CK, showing increases of 1.29 and 1.33 times compared to CK, respectively. In summer and autumn, φPo and Ψo initially increased and then decreased with increasing light intensity, with the T3 treatment exhibiting the highest values. Specifically, φPo was 1.41 and 1.43 times higher than that of CK, respectively, and Ψo was 1.45 and 2.45 times higher than that of CK. φDo exhibited the opposite trend to that of φPo. The trend of φEo for the sample leaves under different shading treatments varied with season, with significant differences observed between seasons for all treatments, except T3. In spring, φEo decreased with increasing light intensity, and was significantly higher in the T4 treatment than in CK, showing an increase of 1.65 times compared to CK. In summer and autumn, φEo initially increased and then decreased with increasing light intensity, with the T3 treatment being significantly higher than CK, being 2.15 and 3.95 times higher than CK, respectively (

Table 6. Effects of shading on chloroplast fluorescence parameters of A. miaotaiense subsp. yangjuechi seedlings under different seasons.

Physiological Indicators	Shading Treatments	Shading Time (d)
		Spring	Summer	Autumn
φPo	CK	0.53 ± 0.03 Ac	0.44 ±0.02 Bc	0.42 ± 0.02 Bd
	T1	0.55 ± 0.02 Ac	0.48 ± 0.02 Bc	0.51 ± 0.02 Bc
	T2	0.58 ± 0.01 Ac	0.55 ±0.02 Ab	0.53 ±0.03 Ac
	T3	0.63 ± 0.01 Ab	0.62 ± 0.01 Aa	0.60 ± 0.02 Aa
	T4	0.68 ± 0.01 Aa	0.59 ± 0.02 Bab	0.58 ± 0.01 Bb
Ψo	CK	0.42 ± 0.04 Ab	0.31 ± 0.02 Bb	0.20 ± 0.03 Cc
	T1	0.44 ± 0.04 Aab	0.35 ± 0.05 Aab	0.37 ± 0.03 Ab
	T2	0.48 ± 0.02 Aab	0.45 ± 0.02 ABa	0.40 ± 0.04 Bb
	T3	0.50 ± 0.02 Aab	0.45 ± 0.03 Aa	0.49 ± 0.03 Aa
	T4	0.56 ± 0.01 Aa	0.42 ± 0.03 Bab	0.40 ± 0.04 Bb
φEo	CK	0.23 ± 0.04 Ac	0.13 ±0.01 Bc	0.08 ± 0.02 Cc
	T1	0.25 ± 0.03 Abc	0.17 ± 0.03 Bbc	0.22 ± 0.02 Ab
	T2	0.28 ± 0.02 Abc	0.25 ± 0.02 ABab	0.22 ± 0.03 Bb
	T3	0.31 ± 0.01 Ab	0.28 ± 0.02 Aa	0.32 ± 0.02 Aa
	T4	0.38 ± 0.01 Aa	0.23 ± 0.02 Bab	0.23 ± 0.03 Bb
φDo	CK	0.47 ± 0.03 Ba	0.56 ± 0.02 Aa	0.58 ± 0.03 Aa
	T1	0.45 ± 0.01 Ba	0.52 ± 0.02 Aa	0.49 ± 0.02 Aab
	T2	0.42 ± 0.01 Ba	0.45 ±0.02 Ab	0.48 ± 0.03 Aab
	T3	0.37 ± 0.01 Ab	0.38 ±0.01 Ac	0.40 ±0.02 Ab
	T4	0.32 ± 0.01 Bc	0.41 ± 0.02 Abc	0.42 ± 0.01 Ab

φPo, photosystem II (PSII) maximum quantum yield of primary photochemistry; Ψo, quantum yield for excitation energy transfer by the reaction center; φEo, quantum yield for electron transfer; φDo, non-photochemical deexcitation. Different capital letters represent significant differences (p < 0.05) between seasons for the same shading treatment, and different lowercase letters represent significant differences (p < 0.05) between shading treatments within the same season. Values represent mean ± SE (n = 5).

).

3.8. PCA of Shadings on A. miaotaiense subsp. yangjuechi Seedlings Under Different Seasons

PCA was used to analyze the growth and physiological indicators of seedlings under the five shading treatments across different seasons. In spring, the cumulative contribution of the first three principal components was 77.60%, effectively reflecting growth and physiological data. PC1 contributed 60.13%, with Pro, SP, and Rd having the highest contributions. PC2 contributed 10.54%, with C_x+c, Ψo, and φEo being the most influential. In summer, the cumulative contribution of the first three principal components was 85.02%, with PC1 contributing 65.28% and PC2 contributing 11.86%. Gs, Tr, and Ci had the highest contributions to PC1, while SOD, Ψo, and φEo were more influential in PC2. In autumn, the cumulative contribution of the first three principal components was 83.01%, with PC1 contributing 59.38% and PC2 contributing 14.75%. SS, Pn max, and Tr contributed more to PC1, whereas φPo, Ψo, and φDo were more influential in PC2 (Figure 4).

3.9. Comprehensive Evaluation of Shadings on the Growth and Physiological Characteristics of A. miaotaiense subsp. yangjuechi Seedlings Under Different Seasons

Based on the indicators of osmoregulatory substances, MDA, antioxidant protective enzyme activities, photosynthetic pigments, photosynthetic parameters, and fluorescence parameters, the fuzzy affiliation function method was used to calculate the affiliation function values for each indicator and conduct a comprehensive evaluation of shade tolerance in seedlings under the five different shading treatments across various seasons. As shown in

Table 7. Comprehensive evaluation of different shading treatments on the physiological characteristics of A. miaotaiense subsp. yangjuechi seedlings.

Subordinate Function Values	Spring Shading Treatments					Summer Shading Treatments					Autumn Shading Treatments
	CK	T1	T2	T3	T4	CK	T1	T2	T3	T4	CK	T1	T2	T3	T4
Leaf length	0.219	0.064	0.177	0.613	0.595	-	-	-	-	-	-	-	-	-	-
Leaf width	0.400	0.111	0.173	0.622	0.604	-	-	-	-	-	-	-	-	-	-
Leaf area	0.271	0.141	0.299	0.888	0.740	-	-	-	-	-	-	-	-	-	-
Leaf moisture content	0.176	0.464	0.256	0.636	0.412	-	-	-	-	-	-	-	-	-	-
Growth of trunk height	0.141	0.481	0.510	0.666	0.562	-	-	-	-	-	-	-	-	-	-
Proline	0.086	0.182	0.612	0.756	0.818	0.145	0.475	0.583	0.667	0.652	0.099	0.413	0.704	0.895	0.792
Soluble sugars	0.072	0.092	0.352	0.871	0.584	0.080	0.104	0.619	0.987	0.851	0.019	0.422	0.472	0.953	0.754
Soluble protein	0.050	0.258	0.232	0.722	0.929	0.296	0.027	0.558	0.895	0.265	0.112	0.201	0.367	0.924	0.643
Malondialdehyde	0.148	0.308	0.433	0.632	0.525	0.136	0.421	0.664	0.914	0.736	0.139	0.257	0.584	0.788	0.666
Superoxide dismutase	0.041	0.241	0.262	0.534	0.901	0.300	0.476	0.498	0.660	0.915	0.291	0.331	0.600	0.883	0.673
Peroxidase	0.348	0.313	0.257	0.924	0.435	0.101	0.411	0.461	0.892	0.768	0.025	0.144	0.271	0.950	0.535
Chlorophyll a	0.075	0.075	0.550	0.582	0.697	0.257	0.419	0.536	0.756	0.706	0.211	0.146	0.280	0.747	0.599
Chlorophyll b	0.043	0.043	0.561	0.578	0.732	0.247	0.374	0.374	0.534	0.468	0.311	0.330	0.304	0.862	0.737
Carotenoid	0.236	0.236	0.614	0.603	0.530	0.508	0.418	0.557	0.494	0.559	0.517	0.505	0.685	0.642	0.431
Maximum net photosynthetic rate	0.063	0.506	0.573	0.825	0.840	0.109	0.389	0.449	0.875	0.770	0.049	0.262	0.552	0.926	0.741
Light Saturation Point	0.323	0.554	0.651	0.819	0.895	0.145	0.272	0.568	0.668	0.928	0.027	0.541	0.712	0.643	0.643
Light Compensation Point	0.150	0.333	0.671	0.561	1.000	0.187	0.241	0.643	0.786	0.802	0.253	0.669	0.918	0.960	0.935
Dark respiration rate	0.313	0.398	0.431	0.766	0.885	0.260	0.294	0.328	0.881	0.567	0.166	0.905	0.591	0.603	0.518
Stomatal conductance	0.177	0.590	0.568	0.888	0.888	0.011	0.120	0.416	0.786	0.492	0.085	0.133	0.283	0.825	0.514
Transpiration rate	0.120	0.401	0.496	0.961	0.671	0.015	0.191	0.483	0.812	0.610	0.039	0.252	0.439	0.843	0.460
Intercellular carbon dioxide concentration	0.231	0.358	0.384	0.627	0.882	0.356	0.633	0.742	0.969	0.992	0.162	0.179	0.669	0.828	0.621
φPo	0.361	0.449	0.559	0.742	0.931	0.195	0.340	0.585	0.814	0.702	0.354	0.497	0.641	0.839	0.774
Ψo	0.465	0.517	0.633	0.698	0.768	0.274	0.381	0.630	0.623	0.459	0.235	0.548	0.666	0.831	0.664
φEo	0.415	0.475	0.597	0.721	0.871	0.112	0.235	0.517	0.611	0.432	0.181	0.428	0.550	0.777	0.600
φDo	0.361	0.449	0.559	0.742	0.931	0.195	0.340	0.585	0.814	0.702	0.354	0.497	0.641	0.839	0.774
Mean value	0.211	0.322	0.456	0.719	0.745	0.197	0.328	0.540	0.772	0.669	0.181	0.383	0.546	0.828	0.654
Response evaluation	5	4	3	2	1	5	4	3	1	2	5	4	3	1	2

, shading treatments significantly affected the comprehensive shade tolerance scores of the seedlings. In spring, the average affiliation value increased with the degree of shading, with T4 having the highest value (0.745) and CK having the lowest (0.240). In summer and autumn, the average affiliation value first decreased and then increased with the degree of shading, with T3 showing the highest values (0.772 and 0.838, respectively) and CK showing the lowest (0.197 and 0.187, respectively).

4. Discussion

Light is essential for plant growth and metabolic activities and significantly affects plant development and differentiation. Insufficient light may stress plants by limiting photosynthesis, resulting in reduced net carbon gain and stunted growth. Conversely, excessive light can damage the photosynthetic apparatus. In response to such adverse conditions, plants adapt by altering their morphology and physiological state [30,31]. A. miaotaiense subsp. yangjuechi is recognized as a spring growth type, with an active growth period of approximately 30 to 40 d, after which growth either slows or ceases. In this study, significant differences were observed among the shading treatments during the high growth period in spring. Specifically, notable increases in leaf length, leaf width, and trunk height were observed in T3 and T4. These results indicated that the shading treatments positively affected seedling growth and development, whereas bright light inhibited seedling growth.

Important osmoregulatory substances in plants, such as Pro, SS, and SP, play crucial roles in lowering the water potential of plant cells, mitigating stress damage, regulating osmotic potential, and maintaining biofilm systems, thereby enhancing the ability of plants to adapt to stress [32,33,34]. This study observed that the Pro content in the seedlings increased with light intensity, peaking in the summer. Specifically, Pro content increased with increasing light intensity across different seasons, with CK exhibiting values 1.55 (40.27 umol·g⁻¹), 1.81 (56.19 umol·g⁻¹), and 1.89 (30.20 umol·g⁻¹) times higher than T3 in spring, summer, and autumn, respectively. This increase suggests that bright light has a significant adverse effect on seedlings, and that the degree of bright light stress on sheephorn maple increases with time. The accumulation of Pro helped plants mitigate the damage caused by intense light, which is consistent with the findings for Firmiana kwangsiensis [35]. Typically, plants accumulate carbohydrates to better adapt to adverse environments [36]. However, for A. miaotaiense subsp. yangjuechi, the SS and SP values were the lowest under CK conditions. On one hand, photosynthesis of A. miaotaiense subsp. yangjuechi was negatively affected by excessive light, leading to a reduction in the synthesis of SS and SP. On the other hand, this may be attributed to the diversion of carbohydrates toward non-structural compounds, thus limiting the accumulation of photosynthesized products. Consequently, shading was more favorable for nutrient accumulation, which aligns with similar findings in studies on Palygunaum cyrtonema [37].

MDA content reflects the extent of cell membrane disruption, whereas antioxidant enzyme systems help modify plant resistance and maintain the balance of reactive oxygen species [38,39,40]. Under light stress, plants accumulate free radicals, resulting in elevated levels of MDA. In this study, MDA content was observed to increase with rising light intensity across different seasons, with CK showing values 1.30 (58.22 umol·g⁻¹), 1.52 (70.72 umol·g⁻¹), and 2.00 (78.80 umol·g⁻¹) times higher than T3 in spring, summer, and autumn, respectively. This indicates progressive accumulation of hazardous substances over time. This indicates that the seedlings experienced the highest accumulation of membrane lipid peroxides and the most severe stress under CK. In contrast, the stress on plasma membranes in heavy shade was less pronounced, consistent with observations in Camellia nitidissima [41]. The synergistic action of antioxidant enzymes is crucial for maintaining the intracellular oxidative balance and mitigating the damage caused by reactive oxygen species in plants. This study observed that the antioxidant enzyme activities were the highest in the sample seedlings under CK conditions during the summer. In contrast, the shading treatments significantly reduced SOD and POD activities in these seedlings. Similar results were observed for the endangered plant Tetrastigma hemsleyanum [42]. SOD, a key component of the antioxidant defense system, plays a crucial role in plant protection. Both SOD and POD activities were the highest under CK, demonstrating consistent changes, indicating that these antioxidant enzymes work synergistically to help plants cope with adverse environmental stresses.

Chl in plant cells is a vital pigment involved in photosynthesis, and under shaded conditions, plants often enhance their Chl levels to capture more light energy [43]. There is an ongoing debate about the effect of shading on Chl content, with some studies indicating that shading can reduce Chl levels, while others suggest that Chl content increases with shading intensity [44,45,46]. In this study, the highest Chl content was observed in leaf samples under heavy shade conditions during spring. The Chl content in the T3 treatment was 1.44, 1.30 and 1.47 times higher than that in CK during spring, summer, and autumn, respectively. This indicated that A. miaotaiense subsp. yangjuechi increased the Chl content to capture more light energy under low–light conditions, a common adaptation among many plant species [47,48]. The highest Chl content in moderate shade during summer and autumn may be attributed to the higher light intensity during these seasons, suggesting that appropriate shading treatments can enhance Chl content in seedlings.

Light response eigenvalues are crucial for assessing plant photosynthetic efficiency and can be categorized into primary reactions, photosynthetic electron transport, photosynthetic phosphorylation, and carbon assimilation [49]. Parameters such as Pn max, LSP, and Rd reflect key physiological aspects of photosynthesis. Pn max indicates the capacity of the plants to utilize light energy, whereas the LSP and CO₂ saturation point represent the abilities of plants in the light and dark reactions, respectively [50]. For many plants, photosynthetic performance typically peaks in summer, as observed in Rhododendron catawbiense [51]. However, A. miaotaiense subsp. yangjuechi seedlings exhibited optimal photosynthetic performance in spring, with the highest Pn values and the lowest performance in autumn, likely due to the high temperatures in summer and autumn. The net Pn of the seedlings exhibited an initial increase, followed by a decrease with increasing shading intensity. Moderate shading significantly enhanced Pn max and LSP, optimizing the light energy utilization and resulting in the best performance. This ensured a lower LCP while achieving Pn max, thereby providing a competitive advantage. In contrast, the seedlings experienced stress in both high–light and heavy shading environments, with stronger light causing more stress than weaker light.

Plants can adjust their gas exchange parameters to improve their adaptability to environmental conditions. For the sample seedlings, Gs and Tr were the highest under medium–heavy shade and lowest under CK conditions. Conversely, Ci exhibited the opposite trend, indicating that both excessive and insufficient light were detrimental to the photosynthetic reactions of seedlings. Reduced photosynthetic capacity under adverse light conditions has been observed in other plant species such as D. involucrate [16], Macropanax rosthornii [52], and Castanopsis hystrix [53]. Chl fluorescence parameters can be used to assess the absorption, distribution, and utilization of light energy during photosynthesis. These parameters not only reflect the activities of PSII and PSI but also offer a comprehensive assessment of the photosynthetic status and performance of plants [54,55]. For the sample seedlings, the fluorescence intensity was higher in spring with increased shading intensity, with heavy shading proving to be most beneficial for growth during this period. In addition, short-term shading improved the photosynthetic capacity of seedlings. In summer and autumn, medium–heavy shading resulted in higher values of φPo, Ψo, and φEo, whereas φDo was lower, indicating optimized photochemical reaction efficiency of PSII and electron transfer under medium–heavy shade. These seedlings demonstrated some shade tolerance, suggesting that moderate shading during the seedling period supported better growth and development.

The experiment measured various indicators, including osmoregulatory substances, antioxidant enzyme activity, Chl content, and light-fitting parameters. Evaluating the adaptability of the sample seedlings to light intensity based on individual indicators or indicator types is challenging. The Pro, SS, and Ci indicators were found to significantly contribute to the principal components, suggesting that higher values of these physiological indicators reflected greater light stress experienced by the seedlings. The mean values of the affiliation functions for the different treatments were ranked as follows in spring: T4 > T3 > T2 > T1 > CK. In summer and autumn, the ranking of the mean values of the affiliation functions was T3 > T4 > T2 > T1 > CK. This indicates that for A. miaotaiense subsp. yangjuechi, which thrives in a natural habitat characterized by high rock exposure, steep slopes, and extremely harsh living conditions, the heavy shading during the peak growth period is beneficial for the growth and development of seedlings. However, prolonged shading may result in excessive stress for the seedlings.

In summary, the osmoregulatory substance Pro accumulated significantly in the sample seedlings under strong light stress, working in conjunction with SS and SP to maintain the osmotic pressure balance and alleviate stress on the cell membrane. The high accumulation of MDA, a cytotoxic substance, exacerbated membrane peroxidation and increased cell membrane permeability, with the MDA content peaking and stress being the greatest under CK conditions. SOD and POD activities effectively scavenged the reactive oxygen species in response to both strong and weak light stress, with these antioxidant enzymes working collectively to maintain the dynamic balance of free radicals. Additionally, seedlings enhanced light energy utilization by increasing Chl content, particularly in low-light environments during spring.

Photoinhibition occurred in seedlings under adverse stress, with both stomatal and non-stomatal factors contributing to the decrease in Pn. This decline was primarily due to strong light stress during the pre-shade period (spring) and a reduced photosynthesis rate in the mid-to late-shading periods (summer and autumn) resulting from excessive shading, which required the plant to expend significant energy to capture light. Chloroplast fluorescence parameters φPo, Ψo, and φEo increased with the degree of shading, whereas φDo decreased. This indicated that the seedlings adjusted the energy allocation ratio in the PSII reaction centers by increasing the quantum ratio of electron transfer and decreasing the quantum ratio of heat dissipation to mitigate stress-induced damage. The severity of growth inhibition in the sample plants correlated with the intensity of light stress. The physiological regulatory mechanisms of these seedlings in response to varying shading environments are outlined above, with changes in photosynthetic and physiological indices providing insights into their adaptation to different light conditions. This study found that the growth and development of A. miaotaiense subsp. yangjuechi seedlings were optimal at approximately 70% shade. This suggests that light may be a significant factor contributing to the endangerment of A. miaotaiense subsp. yangjuechi. Furthermore, the extremely harsh conditions in the species’ native habitat, where seedlings are fully exposed to intense light, are likely the primary reason for the extinction of the four wild plant species native to A. miaotaiense subsp. yangjuechi.

5. Conclusions

This study indicated that in terms of growth and leaf morphology indices, the 70% shade and 90% shade treatments (T3 and T4) were most favorable for the growth of A. miaotaiense subsp. yangjuechi seedlings. This suggests that medium to heavy shade is the most effective for the growth of A. miaotaiense subsp. yangjuechi seedlings during spring, which is their peak growth period. Over time, under natural conditions, the T3 shade treatment achieved the maximum light saturation point for the net photosynthetic rate of A. miaotaiense subsp. yangjuechi seedlings, demonstrating the best photosynthetic performance among all shade treatments evaluated in this study. The observed changes in Pro, SS, and other osmoregulatory substances, as well as the products of membrane lipid peroxidation (MDA), were consistent with the alterations in plant net photosynthesis. These findings indicate that the T3 treatment was the most favorable for the growth and development of A. miaotaiense subsp. Yangjuechi seedlings. In contrast, seedlings under full light conditions (CK) experienced significant stress, with notable reductions in growth and photosynthetic mechanisms. Chlorophyll content and chlorophyll fluorescence parameters, including φPo, Ψo, and φEo of photosystem II, were the highest under T3 treatment. Notably, as the season changed, seedlings subjected to CK and T4 treatments exhibited somewhat similar photosynthetic performance. Both strong light and heavy shading adversely affected the growth of A. miaotaiense subsp. yangjuechi seedlings; however, physiological limitations on their growth were more pronounced under strong light stress, resulting in poorer photosynthetic performance. In conclusion, appropriate shading conditions can enhance the growth and physiological characteristics of A. miaotaiense subsp. yangjuechi seedlings, with optimal growth occurring under approximately 70% shading.

The aim of this study was to identify the shade conditions conducive to the growth of A. miaotaiense subsp. yangjuechi seedlings and to investigate how light serves as a critical factor affecting their development. Both strong light and excessive shade can lead to a decline in the photosynthetic performance. This study provides a scientific foundation for the protection and cultivation of endangered A. miaotaiense subsp. yangjuechi species and aids in selecting the appropriate light intensity. Furthermore, it offers a theoretical basis for the development of relocation and conservation strategies for this species while contributing to the conservation and cultivation efforts of other endangered plant species.