Influence of Exogenous Abscisic Acid on Germination and Physiological Traits of Sophora viciifolia Seedlings under Drought Conditions

Abstract

This study investigates the role of abscisic acid (ABA) in bolstering drought resistance in plants, employing “Panjiang Sophora viciifolia” as the subject. A simulated drought scenario was created using polyethylene glycol (PEG-6000) to examine the impact of varying drought intensities (0%, 5%, 20% PEG) and ABA concentrations (0, 10, 50, 100, 200 mg·L⁻¹) on the germination and physiological parameters of Sophora viciifolia. The results showed that in the absence of ABA, the germination rate (GR), germination potential (GP), and germination index (GI) of S. viciifolia seeds initially increased and then decreased with escalating PEG-induced drought stress. At PEG-induced drought stress levels of 5% and 20%, the activities of peroxidase (POD) and catalase (CAT), along with the malondialdehyde (MDA) content, were significantly higher than in the control (CK) (p < 0.05). In response to drought stress, S. viciifolia seeds adapted by modulating germination behavior, augmenting the content of osmoregulatory substances, and boosting the activity of protective enzymes. The addition of ABA markedly enhanced GR, GE, GI, activities of POD, superoxide dismutase (SOD), and CAT, as well as the levels of MDA and proline (Pro) under drought conditions (p < 0.05). Relative to CK, low ABA concentrations (10–100 mg·L⁻¹) resulted in increased GR, GP, GI, POD, SOD, CAT, MDA, and Pro levels; whereas, at a higher concentration (200 mg·L⁻¹), although GR, GP, and GI decreased, POD, SOD, CAT, MDA, and Pro levels increased. Through principal component analysis and membership function comprehensive evaluation, it was determined that administering 50 mg·L⁻¹ ABA was most effective in enhancing drought resistance in S. viciifolia seedlings.

1. Introduction

The Karst Mountains of Southwestern China, spanning approximately 540,000 km², represent one of the most extensive continuous karst landscapes globally [1]. This region has experienced profound losses in agricultural and forestry productivity, alongside drastic declines in plant diversity, soil physicochemical characteristics, microbial diversity, and carbon sequestration capacity [2,3,4]. These changes are attributed to a variety of natural and human-induced disturbances, rendering it one of the areas most severely impacted by rocky desertification [5]. Vegetation plays a fundamental role in the rehabilitation of these degraded ecosystems. Prior studies have highlighted that native karst plants significantly enhance soil microbial diversity and the ability to sequester carbon [2,3,6]. Conversely, introduced species, such as Pinus massoniana forests and Lolium perenne L. grasslands, fail to positively influence soil physicochemical properties, enzymatic activity, or microbial communities [7,8,9,10]. Therefore, leveraging native karst plants as primary resources and models for the transformation of rocky desertified lands, delving into the “genetic gene pool” of karst flora, and investigating their adaptive strategies are immensely significant endeavors.

Sophora viciifolia Hance, a critical leguminous shrub found across the karst mountains of Southwestern China, has gained prominence in vegetation restoration efforts within karst regions due to its exceptional drought resilience compared to other shrub species [11]. Additionally, S. viciifolia serves significant economic roles, often utilized in traditional Chinese medicine [12]. Its roots have been identified as sources of various compounds, including alkaloids like matrine, which exhibit analgesic properties akin to those of pethidine, underscoring its considerable medicinal potential [12]. While the drought resistance of mature S. viciifolia is well acknowledged, the challenges associated with seedling growth, particularly the constraints imposed by drought conditions on seedling development, profoundly hinder population regeneration [13,14,15]. Thus, examining the mechanisms through which S. viciifolia adapts to drought conditions is of paramount importance.

Seed germination and the seedling phase are critical junctures in a plant’s life cycle, markedly susceptible to environmental conditions [16]. Abscisic acid (ABA) significantly influences plant growth within optimal concentrations [17]. ABA is pivotal in seed dormancy and germination processes, managing the synthesis of seed storage proteins, triggering and preserving seed dormancy, and overseeing germination and seedling development [18]. It also plays a crucial role in mitigating abiotic stress by regulating stomatal dynamics, dormancy, and leaf aging [19]. Research indicates that the exogenous administration of ABA can extend chlorophyll longevity in various plants, including rice, kiwifruit, and black fungus [18,20,21]. It elevates proline levels and protective enzyme activities, boosts cold resilience and water retention, prompts stomatal closure, diminishes transpiration rates, augments photosynthesis, and enhances leaf water content, thereby mitigating the adverse effects of drought stress [18,20,21].

Nonetheless, investigations into the effects of exogenous ABA on the physiological traits of woody plants, such as S. viciifolia, remain sparse. Present research on the drought tolerance of S. viciifolia predominantly concentrates on physiological, biochemical, and transcriptomic metabolic mechanisms, alongside drought alleviation techniques like the application of exogenous calcium, arbuscular mycorrhizal fungi, microbial agents, and moisture conservers [13,22,23,24]. However, an exploration into the ramifications of exogenous ABA on seed germination and the physiological responses of seedlings under drought stress is notably absent. Accordingly, this study utilizes “Panjiang Sophora viciifolia” as the experimental subject to investigate the effects of exogenous ABA on seed germination and the physiological dynamics of seedlings under drought conditions. The aim is to furnish insights for the ecological restoration and reconstruction efforts in drought-prone karst regions.

2. Materials and Methods

2.1. Testing Material

The experiment was conducted in the Guizhou Academy of Agricultural Sciences. In this experiment, “Panjiang Sophora viciifolia”, the nationally approved variety in China, was obtained from the Prataculture Institute of Guizhou Academy of Agricultural Sciences. Previous experiments showed that Panjiang Sophora viciifolia is more tolerant to drought than the wild type of S. viciifolia. The primary reagents included abscisic acid (ABA) and polyethylene glycol (PEG-6000), both of analytical grade.

2.2. Experimental Design

S. viciifolia seeds, selected for uniform size and fullness, were cleansed with distilled water, sterilized using 75% alcohol for 30 s, and subsequently rinsed 3–5 times with distilled water. Excess surface moisture was removed with double-layer filter paper. Using sterile tweezers, the sterilized seeds were placed in 9 cm diameter Petri dishes at a density of 50 seeds per dish with four replicates per treatment, moistened with 8 mL of the respective treatment solution. The experiment incorporated five ABA concentration treatments: 0 (CK, with distilled water), 10, 50, 100, 200 mg·L⁻¹. Additionally, the experiment incorporated two levels of PEG-6000-induced drought conditions: mild drought (5% PEG) and severe drought (20% PEG). These employed a randomized block design (

Table 1. Experimental design of ABA concentration treatments and PEG-6000-induced drought conditions.

ABA/PEG Treats	0% PEG	5% PEG	20% PEG
0 mg·L−1 ABA	CK	P5	P20
10 mg·L−1 ABA	A10	A10P5	A10P20
50 mg·L−1 ABA	A50	A50P5	A50P20
100 mg·L−1 ABA	A100	A100P5	A100P20
200 mg·L−1 ABA	A200	A200P5	A200P20

Note: CK represents 0 mg·L⁻¹ ABA and 0% PEG-6000; A10 represents 10 mg·L⁻¹ ABA; A50 represents 50 mg·L⁻¹ ABA; A100 represents 100 mg·L⁻¹ ABA; A200 represents 200 mg·L⁻¹ ABA; P5 represents 5% PEG-6000; P20 represents 20% PEG-6000.

).

The Petri dishes containing the seeds were situated in a constant temperature light incubator set at a light intensity of 3000 lx, a photoperiod of 12 h, and a temperature of 25 °C. Seed germination was monitored daily throughout the experiment, with ABA solution of the respective concentrations applied as needed to maintain moisture in the filter paper. A seed was considered to have germinated when the emerging radicle elongated to 2 mm from the seed coat [11]. From the onset of germination, the count of germinated seeds was recorded daily across a 30-day germination cycle. The germination rate and potential were calculated once there was no increase in the number of germinated seeds for three consecutive days.

2.3. Index Measurement

2.3.1. Seed Germination Index Measurement

From the commencement of the experiment, daily germination counts were recorded, using the emergence of the radicle through the seed coat as the primary criterion. The germination phase was deemed complete when no additional seeds germinated for three successive days. The germination rate (GR), germination potential (GP), and germination index (GI) were calculated using the formulas below:

GR (Germination rate) = Final normal seedling number/Number of seeds tested × 100;

GP (Germination potential) = Maximum germination number/Number of seeds tested × 100

where the maximum germination number is the maximum number of seeds germinated on any day during the test period;

GI (Germination Index) = Σ(Gt/Dt),

where GT is the number of germinations on day t, and Dt is the corresponding germination days [14].

2.3.2. Physiological Index Measurement

After subjecting the seedlings to 30 days of drought stress, seedlings were dissected into shoot and root sections. A 0.1 g sample of the shoot was immediately weighed, placed in a 2 mL centrifuge tube, flash-frozen in liquid nitrogen, and stored at −80 °C for physiological indicator analysis [24]. The physiological indices for each group were measured as follows: peroxidase (POD) activity was gauged via the guaiacol method [25,26,27]; superoxide dismutase (SOD) activity was evaluated through the nitro blue tetrazolium (NBT) photochemical reduction method [25,26,27,28,29]; catalase (CAT) activity was determined by the ultraviolet absorption technique [25,26,27,28,29]; malondialdehyde (MDA) content was quantified using the thiobarbituric acid method [25,26,27,28,29]; and proline (Pro) content was measured employing the acidic ninhydrin colorimetric method [29].

2.3.3. Comprehensive Evaluation of ABA Application

The subordinate function method was utilized for a comprehensive assessment of the ABA application’s effectiveness. The method varied based on whether the indicator value (X) and the effect of ABA application (U) were positively (1) or negatively (2) correlated, employing respective formulas for calculation [24].

$$U(X_{ijk})=\frac{X_{ijk}-X_{k\min }}{X_{k\max }-X_{k\min }}$$

(1)

$$U(X_{ijk})=1-\frac{X_{ijk}-X_{k\min }}{X_{k\max }-X_{k\min }}$$

(2)

The subordinate function value U(X_ijk), ranging between [0, 1], represented the efficacy of the ABA application for the i-th treatment, j-th period, and k-th indicator. The mean subordinate value across indicators served as the basis for the comprehensive evaluation of the ABA application effect.

2.4. Data Analysis

The experimental data were processed and analyzed using Excel 2019 and SPSS22.0 software. All statistical procedures were performed using SPSS Version 22.0 for Windows (SPSS Inc., Chicago, IL, USA). The significance of differences in seed germination and physiological indices between ABA treatments and between drought conditions, and of the interaction between these two factors, was determined using two-way ANOVA, and the Tukey test for multiple comparisons.

3. Results

3.1. Effect of Exogenous ABA on Sophora viciifolia Seed Germination under Drought Stress Induced by PEG

Exogenous ABA and PEG had significant effects on seed germination in S. viciifolia (

Table 2. Effect of ABA on germination of Sophora viciifolia seeds under drought stress.

Index	PEG-6000 Concentration	ABA Concentration (mg·L−1)
		0	10	50	100	200
Germination rate (GR)	0%	30.00 ± 8.90 Aab	25.63 ± 8.98 Bab	35.63 ± 1.25 Ba	19.38 ± 11.25 Bb	30.00 ± 16.20 Aab
	5%	42.50 ± 23.18 Ab	29.38 ± 7.18 Bb	69.38 ± 21.25 Aa	51.88 ± 15.05 Aab	43.75 ± 11.27 Ab
	20%	3.75 ± 3.23 Bc	57.50 ± 9.13 Aa	43.75 ± 12.67 Bab	45.63 ± 10.08 Aab	25.63 ± 29.04 Abc
Germination potential (GP)	0%	29.38 ± 8.98 Aa	23.13 ± 7.74 Ba	27.50 ± 3.54 Ba	8.13 ± 10.08 Bb	1.88 ± 2.39 Ab
	5%	35.00 ± 17.44 Ab	28.13 ± 5.91 Bb	57.50 ± 24.92 Aa	21.25 ± 6.61 Abc	1.88 ± 2.39 Ac
	20%	3.13 ± 3.75 Bc	40.00 ± 2.04 Aa	13.75 ± 1.44 eBb	3.75 ± 3.23 Bc	0.63 ± 1.25 Ac
Germination Index (GI)	0%	13.74 ± 0.09 Bb	11.70 ± 0.20 Cc	16.18 ± 0.19 Ca	7.71 ± 0.33 Ce	10.54 ± 0.53 Ad
	5%	19.14 ± 0.69 Ac	13.37 ± 0.80 Bd	31.34 ± 0.50 Aa	20.35 ± 0.68 Ab	10.34 ± 0.78 Ae
	20%	1.28 ± 0.85 Ce	24.84 ± 0.82 Aa	17.94 ± 0.21 Bb	14.87 ± 0.44 Bc	5.74 ± 3.83 Bd

Note: Different uppercase letters indicate significant differences among the three PEG-6000 treatments within the same ABA treatment at p < 0.05 level; different lowercase letters indicate significant differences among different ABA treatments within the same PEG-6000 treatment at p < 0.05 level. All data are presented as mean ± SE (standard error) (n = 4).

and

Table 3. Results of two-way ANOVA on the effects of ABA, PEG and their interactions on germination and physiological traits of Sophora viciifolia seedlings.

Source of Variation	ABA		PEG		ABA × PEG
GR	F = 0.008	p = 0.931	F = 0.078	p = 0.781	F = 0.022	p = 0.882
GP	F = 27.915	p < 0.001	F = 4.351	p = 0.042	F = 0.556	p = 0.459
GI	F = 5.383	p = 0.024	F = 0.248	p = 0.621	F = 0.093	p = 0.762
POD	F = 38.099	p < 0.001	F = 139.480	p < 0.001	F = 11.622	p = 0.001
SOD	F = 30.235	p < 0.001	F = 31.627	p < 0.001	F = 0.124	p = 0.726
CAT	F = 33.548	p < 0.001	F = 55.129	p < 0.001	F = 0.590	p = 0.446
MDA	F = 26.261	p < 0.001	F = 21.266	p < 0.001	F = 0.508	p = 0.479
Pro	F = 40.767	p < 0.001	F = 56.761	p < 0.001	F = 0.011	p = 0.916

Note: GR, germination rate; GP, germination potential; GI, germination index; POD, peroxidase; SOD, superoxide dismutase; CAT, catalase; MDA, malondialdehyde; Pro, proline.

). In the absence of ABA, the germination rate (GR), germination potential (GP), and germination index (GI) of S. viciifolia seeds initially increased and subsequently decreased as the intensity of PEG-induced drought stress escalated. Specifically, at a 5% PEG-induced drought stress concentration, the GR, GP, and GI were all elevated in comparison to the control (CK). Conversely, at a 20% PEG-induced drought stress concentration, these metrics significantly diminished relative to CK (p < 0.05). The introduction of ABA in varying concentrations distinctly influenced the seed germination under PEG-induced drought stress. At a 5% PEG-induced drought stress concentration, administering 50 mg·L⁻¹ of ABA notably enhanced the GR, GP, and GI of S. viciifolia seeds over the control (p < 0.05), with increases of 1.632, 1.643, and 1.637 times, respectively, compared to CK. Furthermore, at a 20% PEG-induced drought stress concentration, treatment with 10 mg·L⁻¹ and 50 mg·L⁻¹ ABA significantly boosted the GR, GP, and GI relative to the control (p < 0.05), marking enhancements of 15.333, 10.667, and 19.406 times, and 11.667, 3.667, and 14.016 times that of CK, respectively. This reveals that an optimal ABA concentration can mitigate the adverse effects of PEG-simulated drought stress on S. viciifolia seed germination, thereby boosting their germination efficiency.

3.2. Effect of Exogenous ABA on the Antioxidant Enzyme Activities in Sophora viciifolia under Drought Stress

Exogenous ABA and PEG had significant effects on the activities of peroxidase (POD), superoxide dismutase (SOD) and catalase (CAT) of S. viciifolia (Figure 1 and Table 3). In the absence of ABA, POD and SOD in S. viciifolia seedlings initially surged and then declined under the PEG-induced drought stress, while the activity of CAT progressively increased. Specifically, at 5% and 20% PEG-induced drought stress concentrations, the activities of POD and CAT significantly exceeded those of CK (p < 0.05), though SOD activity showed no significant variance from CK (p > 0.05).

The introduction of ABA modulated the responses of SOD, POD, and CAT activities in the seedlings to the PEG-induced drought stress. As the ABA concentration increased across different PEG-induced drought stress, there was a corresponding enhancement in the activities of POD, CAT, and SOD (Figure 1). At a 5% PEG-induced drought stress concentration, the application of ABA at concentrations of 10 mg·L⁻¹, 50 mg·L⁻¹, 100 mg·L⁻¹, and 200 mg·L⁻¹ significantly fostered the activities of POD, CAT, and SOD (p < 0.05) compared to CK. The peak activities for SOD and CAT were observed at an ABA concentration of 100 mg·L⁻¹, whereas the apex of POD activity was noted at 200 mg·L⁻¹ ABA. In conditions of 20% PEG-induced drought stress concentration, varying concentrations of ABA markedly elevated the activities of POD, CAT, and SOD relative to the control (p < 0.05), with the maximal activities of POD and SOD recorded at an ABA concentration of 200 mg·L⁻¹ and the highest CAT activity observed at 100 mg·L⁻¹ ABA.

3.3. Effect of Exogenous ABA on Malondialdehyde and Proline Levels in Sophora viciifolia Seedlings under Drought Stress

Exogenous ABA and PEG had significant effects on the contents of malondialdehyde (MDA) and proline (Pro) of S. viciifolia (Figure 2 and Table 3). In the absence of exogenous ABA, MDA content in S. viciifolia seedlings escalated with the severity of drought stress, registering significantly higher levels at both 5% and 20% PEG-induced drought stress concentrations than in CK (p < 0.05). This indicates that PEG-induced drought stress prompted lipid peroxidation within the seedlings. Following the addition of ABA, there was a significant increase in MDA content across all PEG-induced drought stress treatments (p < 0.05), yet no notable difference was observed among the varying ABA concentrations (p > 0.05). This outcome suggests that the addition of ABA intensified the extent of lipid peroxidation in the cell membranes under drought stress, thereby leading to increased MDA production.

Without ABA treatment, Pro content in S. viciifolia seedlings trended upwards with increasing drought stress intensity, although this rise was not statistically significant when compared to CK (p > 0.05). The addition of ABA led to a pronounced increase in Pro content, correlating with the escalation in ABA concentration (Figure 2). At a 5% PEG-induced drought stress concentration, ABA treatment at concentrations of 10 mg·L⁻¹, 50 mg·L⁻¹, 100 mg·L⁻¹, and 200 mg·L⁻¹ markedly enhanced Pro levels compared to CK (p < 0.05), with significant variation observed among these concentrations (p < 0.05). At a 20% PEG-induced drought stress concentration, ABA treatments similarly boosted Pro content significantly relative to CK (p < 0.05); however, the differences between the various ABA concentrations were not statistically significant (p > 0.05).

3.4. Comprehensive Evaluation of Drought Resistance

3.4.1. Correlation Analysis of Different Indices

As shown in

Table 4. Correlation coefficients between seed germination and physiological indices of Sophora viciifolia.

Index	GP	GR	GI	MDA	SOD	Pro	CAT	POD
GP	1
GR	0.587 **	1
GI	0.752 **	0.750 **	1
MDA	−0.199	0.280 *	0.197	1
SOD	−0.110	0.412 **	0.289 *	0.913 **	1
Pro	−0.229	0.349 **	0.156	0.837 **	0.896 **	1
CAT	−0.189	0.384 **	0.238	0.889 **	0.950 **	0.910 **	1
POD	−0.236	0.277 *	0.111	0.832 **	0.864 **	0.881 **	0.890 **	1

Note: * represents p < 0.05; ** represents p < 0.01. GR, germination rate; GP, germination potential; GI, germination index; POD, peroxidase; SOD, superoxide dismutase; CAT, catalase; MDA, malondialdehyde; Pro, proline.

, the correlation analysis among drought resistance indices illustrates significant (p < 0.05) and highly significant (p < 0.01) positive correlations between germination indices (GR, GP, and GI) and physiological and biochemical indices (MDA, SOD, POD, CAT, Pro) in S. viciifolia. This relationship suggests that both growth and physiological and biochemical indices serve as reliable measures for evaluating the drought resistance capabilities of S. viciifolia.

3.4.2. Comprehensive Evaluation of Seed Germination and Physiological Indices in Sophora viciifolia Treated with Exogenous ABA

To analyze the results and explore the differences between the seed germination and physiological indices in S. viciifolia treated with exogenous ABA, we conducted principal component analysis (PCA) on 11 physiological indexes. As shown in

Table 5. Principal component results of seed germination and physiological indices in Sophora viciifolia.

Variance	Principal Component 1	Principal Component 2
Germination rate (GR)	−0.0466	0.6111
Germination potential (GP)	0.2153	0.5038
Germination index (GI)	0.1462	0.5799
POD activity	0.4187	−0.1250
SOD activity	0.4437	−0.0175
CAT activity	0.4429	−0.0589
MDA content	0.4217	−0.0890
Pro content	0.4288	−0.0957
Eigenvalue	4.804	2.331
Contribution rate (%)	60.05%	29.14%
Cumulative contribution rate (%)	60.05%	89.19%

, PCA related to S. viciifolia indicated that the combined contribution rate of the first two principal components amounted to 89.19%. In statistical terms, a cumulative contribution rate exceeding 85% for the first two components is sufficient for encapsulating the response of various indices of S. viciifolia under different ABA treatment concentrations. Thus, the indices associated with these principal components serve as comprehensive evaluators for the impact of ABA treatment. The first principal component (PC1) had an eigenvalue of 4.804, accounting for 60.05% of the variance, while the second principal component (PC2) had an eigenvalue of 2.331, contributing 29.14% to the variance (Table 5).

Leveraging the PCA results and employing the membership function formula to compute and average the values across eight indices, the findings (

Table 6. Comprehensive evaluation of the effect of exogenous ABA on drought resistance using membership function value.

Treatment	GP	GR	GI	MDA	SOD	Pro	CAT	POD	Membership Function Value	Rank
CK	0.375	0.533	0.350	0.553	0.542	0.455	0.440	0.602	0.481	9
A10	0.500	0.472	0.594	0.456	0.312	0.545	0.521	0.397	0.475	10
A50	0.565	0.625	0.542	0.466	0.574	0.301	0.562	0.544	0.522	4
A100	0.607	0.656	0.608	0.408	0.416	0.407	0.586	0.374	0.508	6
A200	0.538	0.529	0.561	0.406	0.516	0.412	0.433	0.364	0.470	11
P5	0.583	0.596	0.373	0.532	0.336	0.437	0.351	0.493	0.463	12
P5A10	0.361	0.417	0.539	0.569	0.542	0.621	0.505	0.471	0.503	8
P5A50	0.584	0.653	0.616	0.632	0.566	0.493	0.503	0.429	0.560	1
P5A100	0.417	0.501	0.737	0.440	0.463	0.485	0.534	0.462	0.505	7
P5A200	0.344	0.375	0.350	0.409	0.563	0.531	0.442	0.622	0.455	13
P20	0.375	0.501	0.547	0.366	0.463	0.338	0.450	0.395	0.429	14
P20A10	0.544	0.509	0.643	0.618	0.407	0.572	0.610	0.538	0.555	2
P20A50	0.513	0.517	0.732	0.604	0.498	0.515	0.525	0.435	0.542	3
P20A100	0.567	0.488	0.428	0.547	0.583	0.589	0.484	0.474	0.520	5
P20A200	0.372	0.519	0.566	0.365	0.346	0.316	0.397	0.405	0.411	15

Note: CK represents 0 mg·L⁻¹ ABA and 0% PEG-6000; A10 represents 10 mg·L⁻¹ ABA; A50 represents 50 mg·L⁻¹ ABA; A100 represents 100 mg·L⁻¹ ABA; A200 represents 200 mg·L⁻¹ ABA; P5 represents 5% PEG-6000; P20 represents 20% PEG-6000.

) established a comprehensive evaluation ranking for the 15 treatments as follows: P5A50 > P20A10 > P20A50 > A50 > P20A100 > A100 > P5A100 > P5A10 > CK > A10 > A200 > P5 > P5A200 > P20 > P20A200. This sequence underscores that administering 50 mg·L−1 of ABA optimally enhances drought resistance, benefiting both the germination process and seedling development in S. viciifolia.

4. Discussion

4.1. Impact of Drought Stress on Seed Germination and Physiological Characteristics of Sophora viciifolia

Drought represents a significant environmental challenge for plants, particularly in karst landscapes, affecting various aspects of plant life including seed germination, growth, and physiological processes [30]. Seed germination, a vital and sensitive phase in the life cycle of plants, is particularly susceptible to external factors, with drought stress being a major inhibitory factor [24]. The primary mechanism through which drought influences seed germination is osmotic stress, which results in decreased germination rates, delayed germination peak, and inhibited growth of both the embryo and radicle [24,25]. The findings of Liu et al. (2018) indicate that while low concentrations of PEG can enhance the germination of alfalfa (Medicago sativa) seeds, higher concentrations markedly suppress this process [30]. Correspondingly, our study demonstrated that a mild drought condition (5% PEG) significantly elevated the germination rate (GR), germination potential (GP), and germination index (GI) of seeds, whereas a severe drought condition (20% PEG) substantially decreased these germination metrics. This pattern suggests that S. viciifolia is capable of adapting to drought stress by modulating its germination strategies, aligning with observations reported in prior studies [11,14]. The ability of S. viciifolia to adjust its germination in response to varying levels of water availability highlights the plant’s resilience and potential for survival and propagation in drought-prone karst environments.

This finding underscores the dynamic adaptation mechanisms that S. viciifolia seedlings employ to counteract drought stress. The initial increase in peroxidase (POD) activity at a moderate drought stress level (5%) indicates an early defensive response, enhancing the scavenging of reactive oxygen species (ROS) generated due to water deficit conditions [31,32]. As drought stress intensifies to 20%, the significant uptick in catalase (CAT) activity and malondialdehyde (MDA) content reveals a heightened response to oxidative stress, indicating both enhanced antioxidant defense mechanisms and increased lipid peroxidation damage within cell membranes. This dual response mirrors the physiological adaptations observed in other plant species like Broussonetia papyrifera and Glycine max under similar environmental challenges [33,34].

Therefore, the ability of S. viciifolia seedlings to modulate the activities of crucial protective enzymes such as POD and CAT under varying degrees of water scarcity is a testament to their resilience against drought-induced oxidative stress. This adjustment helps in maintaining cellular homeostasis and mitigating the deleterious effects of drought stress, albeit with the trade-off of increased lipid peroxidation as evidenced by elevated MDA levels. The findings of this study contribute valuable insights into the complex biochemical and physiological strategies employed by S. viciifolia, a plant of ecological and medicinal importance, in navigating the challenges posed by drought conditions prevalent in karst environments.

4.2. Impact of Exogenous Abscisic Acid on Seed Germination and Physiological Traits of Sophora viciifolia under Drought Stress

Extensive research highlights the pivotal role of ABA in regulating seed germination and dormancy, showing that the application of exogenous ABA can significantly bolster plant resistance to various environmental stresses [34,35,36]. Studies have demonstrated that soaking seeds in low concentrations of ABA markedly enhances germination rates, germination potential, and the growth of the plumule and radicle under stress conditions, such as cold stress in Melissitus ruthenicus, while higher concentrations may have suppressive effects [34]. For instance, an ABA concentration of 8 mg·L⁻¹ has been shown to alleviate osmotic and ionic stress in Zea mays seeds caused by high soil salinity, thereby reducing damage [35]. Similarly, low-concentration ABA treatments have been reported to foster seed germination in Oryza sativa under high-temperature stress, thereby boosting the plant’s thermal tolerance [36].

Our study found that the germination indices (GR, GE, and GI) of S. viciifolia seeds exhibited an initial increase followed by a decrease with escalating concentrations of ABA, with the 50 mg·L⁻¹ ABA treatment significantly enhancing these indices. This pattern underscores the potential of an optimal level of exogenous ABA to counteract the suppressive effects of drought stress on seed germination, suggesting that ABA plays a crucial role in augmenting plant resilience to adverse conditions, particularly drought [37]. Furthermore, our findings revealed that different concentrations of exogenous ABA significantly elevated the activities of protective enzymes (POD, SOD, CAT) and the levels of malondialdehyde (MDA) and proline in S. viciifolia seedlings. This suggests that exogenous ABA contributes to improved osmoregulation and enzymatic activity under drought stress, thereby enhancing the seedlings’ capacity for ROS scavenging and peroxide detoxification, and safeguarding the integrity of cytoplasmic membrane structures and functions [38]. This study’s insights are congruent with research conducted by Nazar et al. (2017) and Poór et al. (2019), highlighting the beneficial role of exogenous ABA in enhancing plant physiological responses to drought stress [39,40]. The evidence points to exogenous ABA not only as a mitigator of drought stress effects on seed germination but also as a vital enhancer of drought resilience in plants, promoting a broader understanding of its utility in agricultural practices and ecological restoration efforts, especially in drought-prone environments like karst regions.

ABA plays a pivotal role in regulating plant growth and development, markedly enhancing stress resistance, particularly under conditions of drought. Through the application of a membership function comprehensive analysis, this study has determined that a concentration of 50 mg·L⁻¹ ABA is highly effective in promoting both the germination and subsequent growth of S. viciifolia seedlings, as well as in boosting plant enzyme activities. These findings underscore the potential benefits of ABA application in the cultivation of S. viciifolia, suggesting its viability as a strategy for enhancing plant resilience to drought. However, the application’s effectiveness can be influenced by various environmental factors, including soil type, the topographical landscape, and the composition of soil microbiota [41,42,43]. These elements may introduce certain constraints to the practical application of ABA, indicating the need for a thorough consideration of the specific cultivation conditions of S. viciifolia. This research contributes to the development of novel drought-resistant strategies and lays a theoretical foundation for the thriving cultivation of S. viciifolia in the challenging conditions of karst drought-affected soils.

5. Conclusions

The strategic incorporation of an optimal concentration of exogenous ABA further bolsters these adaptive mechanisms by increasing osmotic regulation substance levels and enhancing enzyme activities in seedlings, thus significantly boosting the species’ drought resilience. In this study, in response to drought stress simulated by PEG, S. viciifolia demonstrates remarkable adaptability to arid conditions through the modulation of seed germination strategies, augmentation of osmotic regulation substance content in seedlings, and amplification of protective enzyme activities. Furthermore, the comprehensive analysis conducted in this study, leveraging principal component analysis and membership functions, identifies the application of 50 mg·L⁻¹ exogenous ABA as the most effective treatment for enhancing S. viciifolia’s drought tolerance under experimental conditions. This finding offers a robust theoretical framework for fostering the successful cultivation and growth of S. viciifolia in the challenging drought-affected terrains of karst mountainous regions.