Response of Four Shrubs to Drought Stress and Comprehensive Evaluation of Their Drought Resistance

Abstract

Drought stress is a crucial factor limiting plant survival and growth, especially during the seedling establishment stage. A deep understanding of different plants’ responses to drought stress and their drought resistance is of great significance for vegetation restoration under drought conditions. This study selected one-year-old seedlings of Winter Jasmine (Jasminum nudiflorum), Oleander (Nerium oleander), Privet (Ligustrum lucidum), and Redleaf Photinia (Photinia × fraseri) as research objects. Through pot experiments, we investigated the physiological and biochemical responses of these shrubs under different levels of drought stress (control, mild, moderate, and severe drought stress, corresponding to 75%, 60%, 45%, and 30% of field maximum water holding capacity) to comprehensively assess their drought resistance capabilities. The research results indicated that as the level of drought stress increased, significant changes (p < 0.05) occurred in the physiological and biochemical indicators of all four plant species. The chlorophyll content (Chla+b) of Winter Jasmine and Redleaf Photinia gradually decreased with the intensification of stress, while the Chla+b of Oleander showed the most significant decline under moderate stress and Privet was most affected under mild stress. The proline (Pro) and soluble sugar (SS) contents of all four plants exhibited an upward trend, suggesting that the plants coped with drought stress by accumulating these osmoregulatory substances. Drought stress led to damage to plant cell membranes, manifested by an increase in malondialdehyde content (MDA), with Winter Jasmine showing the most pronounced increase. The activities of peroxidase (POD) and superoxide dismutase (SOD) in the four plant species responded differently to drought stress: the POD activity of Oleander and Redleaf Photinia increased with the deepening of stress, while that of Winter Jasmine and Privet decreased. A comprehensive evaluation of the drought tolerance of the four plant species was performed using principal component analysis and affiliation function value methods. The drought tolerance of the four shrubs, from strongest to weakest, was as follows: Redleaf Photinia > Oleander > Privet > Winter Jasmine. This finding provides valuable insights for plant selection in ecological slope protection projects, and Redleaf Photinia and Oleander can be promoted for use in vegetation restoration work under drought conditions.

1. Introduction

Against the backdrop of global climate change, the incidence of extreme weather events is escalating, with drought emerging as a particularly alarming phenomenon that poses formidable challenges to plant growth and distribution across the globe [1]. The rising global average temperatures and shifting precipitation patterns have led to more frequent and intense drought periods in numerous regions, threatening the stability of ecosystems and agricultural productivity [2]. Consequently, investigating the adaptive mechanisms and ecological responses of plants under drought conditions holds immense importance for the development of effective ecological conservation strategies [3].

Currently, research on drought stress primarily focuses on the morphological and structural adaptations, physiological and biochemical responses, as well as the molecular genetic mechanisms of plants. Plants cope with water stress by regulating the accumulation of osmolytes such as proline and soluble sugars, modulating the activity of antioxidant enzymes like superoxide dismutase and peroxidase, and adjusting the efficiency of photosynthesis [4,5]. A common consequence of drought stress is a reduction in chlorophyll content in plant leaves, which disrupts the efficiency of photosynthesis and subsequently hampers plant growth and biomass accumulation [6]. Under drought conditions, plants exhibit morphological changes in their roots and leaves, such as increased root depth and reduced leaf thickness, which enhance water absorption efficiency and minimize water loss [6,7]. Furthermore, plants can also respond to the impacts of drought stress by regulating gene expression [8,9].

In ecologically fragile regions, especially those with soils that have poor water retention capabilities, vegetation restoration is particularly arduous. Hence, studying the drought resistance and ecological response mechanisms of plants in such areas is of paramount importance for regional ecological protection [10]. Additionally, shrubs, as vital components of ecosystems, hold immense value in ecological restoration and vegetation reconstruction, making research on their drought resistance particularly significant [11].

Nonetheless, the response mechanisms of different plants to drought stress exhibit considerable variability, necessitating a comprehensive evaluation from diverse angles [12]. The shrubs Winter Jasmine (Jasminum nudiflorum), Oleander (Nerium oleander), Privet (Ligustrum lucidum), and Redleaf Photinia (Photinia × fraseri) are widely distributed across numerous regions in China. Winter Jasmine plants survive more easily, grow faster, have better soil and water conservation abilities, and are usually planted as medicinal plants and landscape plants. Privet is often planted in factories and public roads as a hedge against pollution because of its strong adaptability, fast growth, and pruning resistance [13]. Oleander is highly adaptable, has good drought resistance and slope protection, and can be used as an alternative plant for vegetation restoration [14]. Redleaf Photinia is ecologically well-adapted, fast-growing, and pruning-resistant, with significant growth still occurring in winter [15]. In this study, we focused on these four shrub species, employing a pot-based water control method to simulate varying degrees of drought stress and investigate their physiological and biochemical response characteristics. We further utilized principal component analysis and affiliation function value methods for a comprehensive evaluation [16]. This study offers fresh perspectives and approaches to the study of plant physiological ecology under drought stress, enhancing our understanding of plant adaptation strategies in arid environments. Through this endeavor, we provide theoretical and technical support for the selection of drought-resistant shrub varieties. We posit the following hypotheses: (1) Drought stress triggers physiological and biochemical responses that are species-specific. (2) Redleaf Photinia and Oleander demonstrate superior drought tolerance, attributable to their enhanced osmoregulatory and antioxidant capabilities, when compared to Winter Jasmine and Privet.

2. Materials and Methods

2.1. Experimental Materials

According to the results of the ecological restoration vegetation survey of exposed rocky slopes in the field of Jiangsu Province, we selected one-year-old seedlings of four shrub species, namely, Winter Jasmine, Oleander, Privet, and Redleaf Photinia (

Table 1. Basic characteristics of experimental materials.

Plant Name	Family	Morphological Characteristics, Habits, and Ecological Functions
Winter Jasmine (Jasminum nudiflorum)	Oleaceae	Deciduous shrub, light-loving and slightly shade-tolerant; likes a warm, humid climate but also cold conditions; exhibits drought tolerance but not tolerance of water and humidity; the soil requirements are not strict; shallow roots, strong growth, and strong germination
Oleander (Nerium oleander)	Apocynaceae	Evergreen upright shrub; light-loving; prefers warm, humid conditions; hardy; strong drought resistance; the soil requirements are not strict; and it can grow on alkaline soil
Privet (Ligustrum lucidum)	Oleaceae	Evergreen shrubs or small trees with dense foliage; good cold resistance and water and wet resistance; prefers a warm and humid climate; light and shade tolerance; deep-rooted species with developed fibrous roots, fast growth, strong germination, and resistance to pruning
Redleaf Photinia (Photinia × fraseri)	Rosaceae	Evergreen shrub, light-tolerant, slightly shade-tolerant, warm and humid climate-tolerant; prefers dry and barren conditions and does not like wet conditions

), as experimental materials. The seedlings, pots, humus soil, peat moss, perlite and carbendazim were all purchased from the Xiaxi Seedling Market in Changzhou, Jiangsu, China. The saplings were transplanted into pots with a height of 40 cm and a diameter of 35 cm. The soil mixture for the experiment was composed of humus soil, peat moss, perlite, and local yellow-brown soil, mixed at a volume ratio of 2:2:1:10, and sterilized with carbendazim. The local yellow-brown loam soil used for the experiment was loamy in texture, with a pH of 7.03 and a soil density of 1.40 g·cm⁻³. After transplantation, the seedlings were regularly watered and tended to. Water was supplied every 3 days at 150 mL per plant. Seedlings that exhibited good growth and consistent vigor were selected as test materials for the drought resistance comparison experiment. The experiment was conducted in the greenhouse facilities at the Jurong Forestry Practice Base of Nanjing Forestry University.

2.2. Research Methods

2.2.1. Experimental Design

The drought resistance experiment employed a pot-based water control approach. All experimental plants were established and subjected to routine management practices. Following a three-month acclimation period for the plants, soil moisture content was meticulously measured for each pot to acquire precise water control data. Prior to the imposition of water control treatments, the plants were given a thorough watering, 500 mL per plant, and the soil moisture content was allowed to decrease naturally thereafter. Subsequently, four distinct soil moisture gradients were established for each plant species, corresponding to 75% (CK—control), 60% (T1—mild stress), 45% (T2—moderate stress), and 30% (T3—severe stress) of the maximum field capacity (determined to be 30.62% for the test soil). To ensure robust data collection throughout the experiment, three replicates were established for each of the four stress gradients across all four plant species, with each replicate comprising nine experimental plants. Upon reaching the predetermined soil moisture gradients, daily measurements were conducted in the afternoon using a TZS-1 soil moisture meter, with prompt replenishment of lost water to maintain soil moisture within the specified range until the conclusion of the drought stress experiment. The cumulative stress duration was 45 days.

The experiment was conducted during September and October, and the cumulative stress duration was 45 days for each of the four shrubs under four different soil moisture gradients (CK, T1, T2, and T3). The relative humidity of the air was controlled at 50 to 70%, and the air temperature was about 10–25 °C during the test period. An exhaust unit was used to promote air circulation and regulate temperature and humidity in all areas of the greenhouse. The above ranges of air temperature and humidity brought the test closer to the real conditions in the field. During sampling, mature leaves were randomly selected from the plants, with meticulous attention paid to maintaining consistency in leaf orientation and position. The fresh leaves were immediately placed in sealed bags and transported to the laboratory for the determination of physiological and biochemical indicators. Three replicates were consistently employed for each measurement to ensure accuracy and reliability (

Table 2. Four gradient treatments of drought stress (control, mild, moderate, and severe).

Processing Number	Stress Gradient	Soil Water Content (%)	Ratio of Soil Water Content to Field Water Capacity (%)
CK	Control	22~23	75
T1	Mild stress	18~19	60
T2	Moderate stress	13~14	45
T3	Severe stress	10~11	35

Note: The maximum field water holding capacity of the tested soil was 30.62%.

).

2.2.2. Measurement Indicators and Methods

Building upon previous research methodologies concerning plant drought resistance [4,13], this study carefully selected 11 physiological and biochemical indicators to serve as quantitative evaluations of plant drought tolerance. These indicators included the chlorophyll a content (Chla), chlorophyll b content (Chlb), total chlorophyll content (Chla+b), chlorophyll a/b ratio (Chla/b), relative water content (RWC), relative electrical conductivity (REC), proline content (Pro), malondialdehyde content (MDA), peroxidase activity (POD), superoxide dismutase activity (SOD), and soluble sugar content (SS). This comprehensive approach ensured a robust assessment of plant drought resistance capabilities.

Among the selected indicators, chlorophyll content was determined using the acetone–ethanol mixture method [17]; RWC was assessed through the immersion and drying procedure [18]; REC was measured by employing the conductivity method [19]; Pro was quantified using the acidic ninhydrin method [20]; MDA content was determined by the thiobarbituric acid (TBA) method [21]; POD activity was evaluated using the guaiacol colorimetric method [22]; SOD was analyzed via the nitroblue tetrazolium (NBT) method [19]; and SS content was measured using the anthrone method [19]. All of the aforementioned determinations were carried out with three replicates to ensure accuracy and reliability.

2.2.3. Data Processing and Evaluation Methods

Employing principal component analysis (PCA), we analyzed the indicators reflecting the drought resistance of four shrub species. Based on the principles of fuzzy mathematics, we used the membership function value method to comprehensively evaluate the drought resistance indicator values of these shrubs. Initially, we calculated the specific membership function values for each comprehensive drought resistance indicator within the shrub plants. Subsequently, we obtained a weighted average based on the weights of the principal components, resulting in a comprehensive evaluation score for drought resistance. A higher score indicates stronger drought resistance.

If the comprehensive indicators were positively correlated with drought resistance, their calculation was as follows:

$$L\left(S_j\right)={\displaystyle \frac{\left(S_j-S_{min}\right)}{\left(S_{max}-S_{min}\right)}}=\mathrm{1,2},\dots \dots n$$

(1)

If the comprehensive indicators were negatively correlated with drought resistance, their calculation was as follows:

$$L\left(S_j\right)=1-{\displaystyle \frac{\left(S_j-S_{min}\right)}{\left(S_{max}-S_{min}\right)}}=1, 2, \dots \dots n$$

(2)

The symbols were used as follows: $L\left(S_j\right)$represents the membership function value, $S_j$represents the j-th comprehensive indicator,${ S}_{min}$represents the minimum value of the j-th comprehensive indicator, and $S_{max}$represents the maximum value of the j-th comprehensive indicator.

The data were analyzed using SPSS 18.0 (IBM, Armonk, NY, USA) for variance, correlation, and principal component analysis [23]. Differences between different soil moisture gradients were assessed using one-way analysis of variance (ANOVA), and post hoc testing was performed using Tukey’s Honestly Significant Difference (HSD) test at p < 0.05. The charts and tables were created using Excel (Microsoft Corporation, Redmond, WA, USA) and OriginPro2021 (OriginLab, MA, USA).

3. Results

3.1. Characteristics of Changes in Physiological and Biochemical Indicators

3.1.1. Effects of Drought Stress on Chlorophyll Content and Chlorophyll a/b Value

Drought stress significantly influenced the chlorophyll content in the plants. As illustrated in Figure 1a, with escalating levels of stress, the chlorophyll content of both Winter Jasmine and Redleaf Photinia demonstrated a gradual decline. Specifically, the chlorophyll content of Winter Jasmine in the control group was measured at 2.70 mg/g, which diminished to 2.09 mg/g under severe stress, reflecting a substantial reduction of 22.59% (p < 0.05). Similarly, Redleaf Photinia exhibited a decrease of 25.73% in chlorophyll content under severe stress, with significant statistical differences noted (p < 0.05). In contrast, Oleander experienced the most pronounced reduction in chlorophyll content (52.10%) under moderate stress conditions, while Privet was predominantly affected by mild stress (18.94%). Normally, drought stress leads to water deficit in plant cells, which triggers reactive oxygen species (ROS) accumulation, membrane lipid peroxidation, and chloroplast structure disruption, thus accelerating chlorophyll decomposition. However, there are also plants that may maintain their chlorophyll content through different mechanisms, such as Privet under mild drought stress and Oleander under severe drought stress, by depleting antioxidant enzymes to scavenge reactive oxygen species (ROS) and delaying chlorophyll degradation, in which case the chlorophyll content increases slightly (Figure 1a). The impact of drought on the chlorophyll a/b ratio is depicted in Figure 1b; notably, the ratios for Winter Jasmine, Oleander, and Privet decreased under drought conditions (p < 0.05), indicating a potential impairment in their capacity to absorb red light and consequently affecting photosynthetic efficiency. Conversely, Redleaf Photinia showed a significant increase in its chlorophyll a/b ratio under drought conditions (p < 0.05), possibly suggesting an enhancement in its photosynthetic capability.

3.1.2. Effects of Drought Stress on Relative Water Content and Relative Electrical Conductivity of Leaves

Drought stress markedly influenced the relative water content (RWC) and relative electrical conductivity (REC) of plant leaves (Figure 2). As the intensity of drought stress escalated, the RWC in leaves from all four plant species demonstrated a declining trend, ultimately reaching its nadir under severe stress conditions (Figure 2a). In comparison to the control group, the RWC values for Winter Jasmine, Oleander, Privet, and Redleaf Photinia decreased by 28.19%, 27.67%, 60.22%, and 13.39%, respectively. Additionally, drought stress resulted in a significant elevation (p < 0.05) in REC for Oleander, Privet, and Redleaf Photinia leaves under moderate to severe stress levels; although the increase observed in Winter Jasmine was not statistically significant, it still exhibited an upward trend (Figure 2b).

3.1.3. Effects of Drought Stress on Proline Content and Soluble Sugar Content of Plants

Proline, a crucial osmoregulatory compound, accumulates significantly under drought stress (Figure 3a). As stress levels escalated, the proline content in all four plant species rose markedly (p < 0.05), yet the extent of this increase varied by plant species. The proline content in Winter Jasmine soared notably, with significant differences observed between various treatment levels (p < 0.05). This suggests that Winter Jasmine copes with drought stress by accumulating substantial amounts of proline. While Oleander’s proline content also increased with intensifying stress, the disparity between severe stress and the control group was less pronounced than in Winter Jasmine (p < 0.05). This may indicate that Oleander possesses a relatively more stable osmoregulatory mechanism under drought stress. The trend in proline accumulation in Privet mirrors that of Winter Jasmine, albeit with slightly lower overall levels. The proline content in Redleaf Photinia also rises under drought stress, but the increase is relatively modest. This could be attributed to its inherent drought resilience, enabling it to maintain lower osmoregulatory demands under dry conditions.

The effects of drought stress on the content of soluble sugar (SS) in plants are shown in Figure 3b. With the deepening of drought stress, the soluble sugar content of the four plants showed an increasing trend, and the soluble sugar content was significantly higher than that of the control group under moderate or severe stress (p < 0.05). Soluble sugar is an important osmoregulatory substance in plants, which indicates that when plants are under drought stress, the osmotic pressure of cells can be improved by increasing the content of soluble sugar in plant leaves so that plants can better absorb water from the outside world under drought conditions so as to maintain normal physiological functions. Under the same conditions, Privet had the strongest ability to adapt to drought stress by adjusting its soluble sugar content, followed by Redleaf Photinia, Winter Jasmine, and Oleander.

3.1.4. Effects of Drought Stress on the Activities of Peroxidase (POD) and Superoxide Dismutase (SOD) in Plants

The effects of drought stress on the activities of peroxidase (POD) and superoxide dismutase (SOD) in the plants are shown in Figure 4. As shown in Figure 4a, the POD activity of Winter Jasmine and Privet decreased with the deepening of drought stress and was significantly lower than that of the control group under severe stress (p < 0.05). Compared with the control group, POD activity decreased by 39.64% and Privet decreased by 33.61% under severe stress. Different from Winter Jasmine and Privet, the POD activity of Oleander and Redleaf Photinia showed an increasing trend with the deepening of drought stress. The POD activity of Oleander under severe stress was significantly higher than that of the control group (p < 0.05), while that of Redleaf Photinia under mild, moderate, and severe stress was significantly higher than that of the control group (p < 0.05). Under drought stress, the POD activity of Winter Jasmine and Privet decreased, while the POD activity of Oleander and Redleaf Photinia increased, indicating that the adaptive ability of Oleander and Redleaf Photinia was stronger than that of Winter Jasmine and Privet. On the whole, SOD activity showed a decreasing trend under drought stress, but this decreasing trend was not significant for Oleander and Privet (p > 0.05), while that of Winter Jasmine and Redleaf Photinia was significantly higher than that in the control group only under severe stress (p < 0.05, Figure 4b).

3.1.5. Effects of Drought Stress on the Content of Malondialdehyde in Plants

As an indicator of oxidative stress (Figure 5), malondialdehyde (MDA) content also exhibits an upward trend under drought stress, reflecting an increase in cell membrane damage. Winter Jasmine shows the most significant increase in MDA content under drought stress, indicating more severe cell membrane damage. The MDA content in Oleander also rises with increasing stress levels, but the magnitude of increase is relatively small, suggesting that Oleander can better maintain cell membrane stability under drought conditions. The trend in MDA content in Privet is similar to that in Oleander, but the overall accumulation is slightly higher. The MDA content in Redleaf Photinia remains relatively stable under mild and moderate stress, with a significant increase only observed under severe stress. This indicates that Redleaf Photinia can better maintain cell membrane integrity and reduce membrane lipid peroxidation damage under drought conditions.

3.2. Correlation Analysis and Principal Component Analysis of Each Index

According to

Table 3. Correlation coefficients for various indicators.

Index	Chla	Chlb	Chla+b	Chla/b	RWC	REC	Pro	MDA	POD	SOD
Chlb	0.919
Chla+b	0.994	0.956
Chla/b	0.608	0.258	0.522
RWC	−0.229	−0.294	−0.251	−0.005
REC	−0.417	−0.472	−0.439	−0.075	−0.329
Pro	−0.200	−0.145	−0.190	−0.267	−0.608	0.464
MDA	−0.299	−0.378	−0.328	−0.071	−0.470	0.533	0.526
POD	−0.031	0.057	−0.006	−0.105	−0.005	0.360	0.013	−0.426
SOD	−0.093	0.009	−0.065	−0.179	0.548	−0.287	−0.331	−0.798	0.387
SS	−0.088	−0.035	−0.075	−0.121	−0.632	0.369	0.713	0.253	0.306	−0.318

Note: Chla is chlorophyll a, Chlb is chlorophyll b, Chla+b is chlorophyll content, RWC is relative water content, REC is relative electrical conductivity, Pro is proline content, MDA is malondialdehyde content, POD is peroxidase content, SOD is superoxide dismutase content, SS is soluble sugar content, and the same below.

, chlorophyll a was positively correlated with chlorophyll b and total chlorophyll (p < 0.01), and the correlation coefficients were 0.919 and 0.994, respectively. It was positively correlated with the chlorophyll a/b ratio (p < 0.05), and the correlation coefficient was 0.608. Chla+b was negatively correlated with the leaf relative water content; the relative electrical conductivity; and the proline, malondialdehyde, peroxidase, superoxide dismutase, and soluble sugar contents, but the relationship was not significant (p > 0.05). There was a significant positive correlation between chlorophyll b and total chlorophyll (p < 0.01), and the correlation coefficient was 0.956. The leaf relative water content was significantly negatively correlated with proline and soluble sugar (p < 0.05), and the correlation coefficients were −0.608 and −0.632, respectively. There was a significant positive correlation between proline and soluble sugar (p < 0.01), and the correlation coefficient was 0.713. There was a significant positive correlation between malondialdehyde and superoxide dismutase (p < 0.01), and the correlation coefficient was 0.798.

The drought resistance indexes of four shrubs were analyzed by principal component analysis [23]. Principal components with eigenvalues greater than 1 should be extracted, and the cumulative contribution rate of variance of extracted principal components should be greater than 85%. Therefore, the original eleven physiological and biochemical indicators can be combined into a small number of four relatively independent principal component indicators that can fully reflect plant drought resistance, and the cumulative contribution rate of variance of these four principal components reaches 88.791% (

Table 4. Variance decomposition principal component extraction analysis table.

Principal Component	S1	S2	S3	S4
Eigenvalue	3.957	3.032	1.763	1.014
Contribution rate/%	35.977	27.563	16.03	9.22
Cumulative contribution rate/%	35.977	63.54	79.571	88.791

).

S1, S2, S3, and S4 represent the comprehensive indicators 1–4, respectively. Zx1~Zx11 respectively represent the data of drought resistance physiological and biochemical indexes after Z standardization. By dividing the principal component load matrix (

Table 5. Principal component load matrix.

Index	Chla	Chlb	Chla+b	Chla/b	RWC	REC	Pro	MDA	POD	SOD	SS
1	0.799	0.763	0.803	0.463	0.288	−0.721	−0.639	−0.708	−0.002	0.368	−0.478
2	0.585	0.533	0.580	0.340	−0.844	0.157	0.506	0.437	−0.122	−0.684	0.536
3	0.018	0.191	0.067	−0.254	−0.178	0.227	0.254	−0.491	0.883	0.481	0.508
4	0.080	−0.219	0.000	0.675	0.135	0.524	−0.256	0.057	0.350	−0.103	−0.096

) by the square root of the eigenvalues corresponding to the principal components, the coefficients corresponding to each index in the four principal component equations were obtained [23]. The four comprehensive index equations are as follows:

S1 = 0.402Zx1 + 0.384Zx2 + 0.404Zx3 + 0.233Zx4 + 0.145Zx5 − 0.363Zx6 − 0.321Zx7 − 0.356Zx8 − 0.001Zx9 + 0.185Zx10 − 0.240Zx11

(3)

S2 = 0.336Zx1 + 0.306Zx2 + 0.333Zx3 + 0.195Zx4 − 0.485Zx5 + 0.090Zx6 + 0.290Zx7 + 0.251Zx8 − 0.070Zx9 − 0.393Zx10 + 0.308Zx11

(4)

S3 = 0.013Zx1 + 0.144Zx2 + 0.051Zx3 − 0.191Zx4 − 0.134Zx5 + 0.171Zx6 + 0.191Zx7 − 0.370Zx8 + 0.665Zx9 + 0.362Zx10 + 0.383Zx11

(5)

S4 = 0.079Zx1 − 0.217Zx2 + 0.000Zx3 + 0.671Zx4 + 0.134Zx5 + 0.520Zx6 − 0.254Zx7 + 0.057Zx8 + 0.348Zx9 − 0.102Zx10 − 0.096Zx11

(6)

where x1 is the chlorophyll a content, x2 is the chlorophyll b content, x3 is the chlorophyll content, x4 is the chlorophyll a/b value, x5 is the relative water content, x6 is the relative electrical conductivity, x7 is the proline content, x8 is the malondialdehyde content, x9 is the peroxidase content, x10 is the superoxide dismutase content, and x11 is the soluble sugar content, and the same below.

As can be seen in Table 4, the variance contribution rate of the first principal components is about 36%, the variance contribution rate of the second principal component is about 28%, the variance contribution rate of the third principal component is about 16%, and the variance contribution rate of the fourth principal component is about 9%. As can be seen in Table 5, Chla+b was the most significant factor in principal component 1, with a coefficient of 0.803, followed closely by Chla (0.799). In principal component 2, the dominant factor was RWC, which had a coefficient of −0.844.

3.3. Fuzzy Comprehensive Evaluation of Drought Resistance Index

According to the average values of the comprehensive evaluation (

Table 6. Calculation of comprehensive evaluation values for each tree species.

Species of Trees	Drought Stress Gradient	Composite Indicator Values by Means of the Value of the Affiliation Function Method				Comprehensive Valuation		Rank
		Principal Component 1	Principal Component 2	Principal Component 3	Principal Component 4	D	Average D
Winter Jasmine	CK	0.486	0.729	0.607	0.000	0.533	0.407	4
	T1	0.356	0.536	0.591	0.016	0.419
	T2	0.196	0.511	0.610	0.499	0.400
	T3	0.000	0.235	0.765	0.615	0.275
Oleander	CK	0.483	0.519	0.625	0.535	0.525	0.575	2
	T1	1.000	0.000	0.936	0.423	0.618
	T2	0.612	0.382	1.000	0.553	0.605
	T3	0.453	0.500	0.970	0.964	0.614
Privet	CK	0.543	0.110	0.868	0.509	0.464	0.465	3
	T1	0.483	0.519	0.625	0.535	0.525
	T2	0.763	0.768	0.207	0.407	0.627
	T3	0.588	0.873	0.045	0.662	0.586
Redleaf Photinia	CK	0.583	0.222	0.128	0.654	0.396	0.654	1
	T1	0.343	0.100	0.000	0.783	0.251
	T2	0.483	0.519	0.625	0.535	0.525
	T3	0.580	0.960	0.825	1.000	0.786
Weight		40.52%	31.04%	18.05%	10.38%

Note: The drought tolerance of the four shrubs, from strongest to weakest, was as follows: Redleaf photinia > Oleander > Privet > Winter Jasmine.

), the drought resistance of the four shrubs was in the following order, from strong to weak: Redleaf Photinia > Oleander > Privet > Winter Jasmine, indicating that Redleaf Photinia showed the strongest drought resistance under drought stress, while the drought resistance of Winter Jasmine was relatively weak. With the increase in drought stress, the comprehensive evaluation value of Winter Jasmine decreased gradually, indicating that its drought resistance weakened gradually. Under severe stress, its comprehensive evaluation value was the lowest, indicating that the resistance to drought stress was poor. The comprehensive evaluation value of Oleander under mild stress was the highest, indicating that it showed better drought resistance under this condition. However, with the increase in the stress degree, its comprehensive evaluation value decreased but still remained at a high level, indicating that Oleander had a certain ability to adapt to drought stress. The comprehensive evaluation value of Privet was relatively stable under different stress gradients, but the overall value was at a low level, which indicated that although the drought resistance of Privet was not as good as that of Photinea and Oleander, it could still maintain a certain stability under drought stress. The comprehensive evaluation value of Redleaf Photinia under different stress gradients remained at a high level, and the comprehensive evaluation value increased with the increase in stress degree. This indicated that Redleaf Photinia showed strong tolerance and adaptability to drought stress.

4. Discussion

Drought stress is an unavoidable environmental factor which ignores boundaries and exists in various environments without warning, hindering biomass production, quality, and energy accumulation in plants. It is a key environmental stress caused by changes in temperature and light intensity and decreased rainfall [24]. In response to water scarcity, plants have evolved a variety of complex resistance and adaptation mechanisms, including physiological and biochemical reactions that vary at the species level. The study found that drought stress significantly reduced the chlorophyll content of the four plants, because drought can limit the water absorption and transport of the plants, thus affecting the synthesis and stability of chlorophyll. Deng et al. found a similar pattern: with the aggravation of drought stress, the relative content of chlorophyll decreased [5]. Liu et al. also concluded that with the increase in the drought stress cycle, chlorophyll content showed a trend of first decreasing and then increasing [25]. Under severe stress, the chlorophyll content of Winter Jasmine and Redleaf Photinia decreased by 22.59% and 25.73%, respectively, while the chlorophyll content of Oleander and Privet decreased more significantly under moderate or mild stress. In particular, the chlorophyll content of Oleander decreased by 52.10% under moderate stress, which may be related to the sensitive response of Oleander to drought stress. Drought stress will significantly reduce the photosynthetic efficiency of plants, photosynthesis being the process by which plants use light energy to convert carbon dioxide and water into organic matter and oxygen, while drought will lead to stomatal closure and restrict the entry of carbon dioxide, thus reducing the rate of photosynthesis [26]. The study found that drought stress affected chlorophyll a/b values, which reflected changes in the plants’ ability to absorb red and blue light. The chlorophyll a/b values of Winter Jasmine, Oleander, and Privet decreased under drought stress, indicating that the red light absorption capacity of these plants was weakened, which may have further affected their photosynthetic efficiency. However, the increase in chlorophyll a/b values under drought stress may mean that Redleaf Photinia exhibits stronger adaptability by adjusting photosynthetic pigment composition to enhance photosynthesis ability.

Under drought stress, plants respond to cell dehydration and maintain cell turgor by accumulating osmoregulatory substances such as proline and soluble sugar [27]. The results showed that the proline content of the four plants increased significantly with the increase in the stress degree, but the increase range varied with plant species. Proline accumulation in Winter Jasmine and Privet was relatively large, which may be related to their weak drought resistance and short-term drought; a large amount of proline accumulation was needed to maintain cell osmotic pressure [28]. Proline is considered to be a major component of osmoregulation, and this osmotic solute plays a key role in alleviating oxidative damage and stabilizing cell membranes. In contrast, the proline accumulation of Redleaf Photinia was small, indicating that it had a strong drought resistance and could maintain a low osmotic regulation requirement under drought conditions. Muhamma et al. [29] found that under drought stress, the accumulation of proline and other amino acids and the degradation of protein are inversely proportional to the water status of plants, that is, the production of proline and amino acids is related to the reduction in the water potential of leaves, indicating that these solutes play a role in osmotic regulation. Drought stress also led to the damage of plant cell membranes, which was manifested by the increase in MDA content. Drought-tolerant plants typically exhibit lower MDA levels by reducing oxidative damage through more efficient antioxidant systems and protective mechanisms [30]. The increase in malondialdehyde content in Winter Jasmine was the most obvious, indicating that its cell membrane was seriously damaged under drought stress. The content of MDA in Redleaf Photinia did not change much under mild and moderate stress, but increased significantly under severe stress, indicating that the membrane of Redleaf Photinia had good stability under drought conditions. All plants showed MDA levels higher than the control. This may be due to the short-term drought conditions caused by the plants due to the rapid activation of defense mechanisms. In the follow-up study, we will focus on the change and accumulation process of MDA under long-term stress.

Drought stress can induce the production of a large number of reactive oxygen species (ROS) in plants, causing damage to cell structure and function [29]. In response to this oxidative stress, plants increase the activity of antioxidant enzymes such as peroxidase (POD) and superoxide dismutase (SOD) [31]. However, in this study, there were differences in the response of POD and SOD activities of the four plants to drought stress. The POD activity of Winter Jasmine and Privet decreased under drought stress, while that of Oleander and Redleaf Photinia increased. This suggests that Oleander and Redleaf Photinia have stronger antioxidant capacity under drought stress and are better able to remove ROS produced in plant leaves [32]. In addition, although the SOD activity of the four plants showed a downward trend under drought stress, this trend was not significant for Oleander and Privet, indicating that these two plants could still maintain a certain antioxidant enzyme activity under drought stress.

SS is an important osmoregulatory substance in plants, and the increase in its accumulation under drought stress helps to improve the osmotic pressure of cells so that plants can better absorb water from the outside world [32]. In this study, the SS content of the four plants increased with the deepening of drought stress and was significantly higher than that of the control group under moderate or severe stress. This suggests that under drought stress, plants enhance cellular osmoregulation by increasing SS accumulation [24]. Under the same conditions, Privet had the strongest ability to adapt to drought stress by adjusting the soluble sugar content, followed by Redleaf Photinia, Winter Jasmine, and Oleander.

Through principal component analysis and the fuzzy comprehensive evaluation method, we were able to comprehensively evaluate the drought resistance of four shrubs [29]. The results showed that Redleaf Photinia showed the strongest drought resistance, followed by Oleander, Privet, and Winter Jasmine. Redleaf Photinia can maintain a high chlorophyll content, relatively stable cell membrane, strong antioxidant enzyme activity, and soluble sugar accumulation ability under drought stress, and the synergistic effects of these physiological and biochemical indexes enable it to maintain normal physiological functions under drought conditions [24]. In contrast, the drought resistance of Winter Jasmine was relatively weak, and the physiological and biochemical indexes showed great fluctuation and a decreasing trend under drought stress. In summary, drought stress had significant effects on physiological and biochemical indexes of the four shrubs, and the different plants had different response mechanisms and adaptive abilities with respect to drought stress. Further study of these differences and their underlying physiological and biochemical mechanisms could provide a scientific basis for genetic improvement and cultivation management of drought resistance.

5. Conclusions

As an unavoidable environmental factor, drought stress poses a serious threat to the biomass production, quality, and energy accumulation of plants. Drought stress significantly reduced the chlorophyll content of the four shrubs and affected their photosynthetic efficiency, mainly because drought limited the water absorption and transport of plants and then affected the synthesis and stability of chlorophyll. In addition, drought stress also resulted in plant cell membrane damage, which was manifested by the increase in malondialdehyde content and the change in antioxidant enzyme activity. Although all four shrubs were affected by drought stress, there were significant differences in their response mechanisms and adaptive abilities. Redleaf Photinia showed the strongest drought resistance under drought stress, and its chlorophyll content, cell membrane stability, antioxidant enzyme activity, and soluble sugar accumulation ability were maintained at a high level. Oleander also showed strong drought resistance, especially under moderate stress; the adjustment of physiological and biochemical indexes was more significant. In contrast, the drought resistance of Winter Jasmine and Privet was relatively weak, and their chlorophyll content, proline accumulation, and antioxidant enzyme activity fluctuated greatly under drought stress, reflecting their weak adaptability to drought environments. A comprehensive evaluation of the drought tolerance of four plant species was performed using principal component analysis and affiliation function value methods. The drought tolerance of the four shrubs, from strongest to weakest, was as follows: Redleaf Photinia > Oleander > Privet > Winter Jasmine. This finding provides valuable insights for plant selection in ecological slope protection projects, and Redleaf Photinia and Oleander can be promoted for use in vegetation restoration work under drought conditions.

The present study, a greenhouse pot experiment designed to compare the drought tolerance strength of the four shrubs, had some limitations, and the results should be verified in the future using field trial methods.