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Article

Effects of Drought Stress on the Growth and Physiological Characteristics of Idesia polycarpa Maxim

by
Xiaoyu Lu
1,2,
Yian Yin
1,2,
Maolin Yang
1,2,
Shucheng Zhang
1,2,
Zhangtai Niu
1,2,
Lingli Wu
1,2,* and
Chan Chen
3,*
1
State Key Laboratory of Utilization of Woody Oil Resource, Central South University of Forestry and Technology, Changsha 410004, China
2
Key Laboratory of Cultivation and Protection for Non-wood Forest Trees of Ministry of Education and the Key Laboratory of Non-Wood Forest Products of Forestry Ministry, Central South University of Forestry and Technology, Changsha 410004, China
3
Hunan Botanical Garden, Changsha 410116, China
*
Authors to whom correspondence should be addressed.
Horticulturae 2025, 11(7), 834; https://doi.org/10.3390/horticulturae11070834
Submission received: 11 June 2025 / Revised: 7 July 2025 / Accepted: 10 July 2025 / Published: 15 July 2025
(This article belongs to the Section Biotic and Abiotic Stress)
Browse Figures
Versions Notes

Abstract

Idesia polycarpa is a valuable woody oil plant with potential for horticultural and industrial applications. However, limited information is available regarding its drought tolerance during the seedling stage. In this study, one-year-old seedlings were subjected to five treatments based on soil relative water content (RWC): moderate drought (T1, 40 ± 5%), severe drought (T2, 20 ± 5%), control (CK, 70 ± 5%), and rewatering following moderate (T3) and severe drought stress (T4), with RWC restored to 70 ± 5%. Under drought stress, seedlings exhibited adaptive responses including reduced growth, enhanced antioxidant enzyme activity, osmotic regulation, and changes in endogenous hormone levels. Seedlings showed good tolerance and recovery under moderate drought, but severe drought caused substantial damage and limited post-rewatering recovery. Pearson correlation and principal component analyses revealed that betaine, APX, SA, IAA, ABA, chlorophyll (a + b) content, and crown growth were strongly associated with drought response and could serve as key indicators for drought resistance assessment in I. polycarpa. These findings provide insights into the physiological mechanisms of drought adaptation and support the development of a reliable evaluation system for drought tolerance in this promising species.

1. Introduction

Drought is regarded as one of the most destructive natural disasters worldwide, exerting a significant impact on agricultural systems, ecological balance, and human society. As the greenhouse effect intensifies and global temperatures continue to rise, the frequency and severity of droughts are predicted to increase, resulting in a broader range of regions being affected. Drought stress has been shown to cause plant growth inhibition, which can be promoted by timely rewatering to restore plant growth [1,2]. The ability of a plant to swiftly revert to its pre-drought state following rehydration is a crucial factor in determining its drought resilience [3]. Therefore, the assessment of a plant’s capacity to withstand drought stress should encompass its recovery potential. Drought tolerance in plants is defined as a comprehensive response to drought stress, which is manifested by numerous changes in external morphology, internal structure, growth, physiological characteristics, and the accumulation of bioactive components [4]. Regarding external morphology, drought stress severely inhibited the growth and development of Rhododendron ovatum, with obvious wilting and drying of the leaves [5]. In terms of physiological characteristics, drought stress resulted in the accumulation of osmotic substances such as soluble sugars and soluble proteins in the leaves of Lycium ruthenicum [6]. At the same time, antioxidant enzyme activities in plants change to adapt to the water-deficit environment due to the surge in reactive oxygen species content inside the plant under drought stress [7]. In addition, drought stress caused changes in hormone levels in plants, including growth hormones such as indole acetic acid (IAA) and gibberellin (GA) and the increased adversity-responsive hormone abscisic acid (ABA) [8]. In terms of leaf anatomical characteristics, due to the sudden decrease in water content in plants, the leaf morphology and structure changed to adapt to the environmental changes, which caused a significant reduction in the thickness of the leaf tissues [9].
Idesia polycarpa Maxim. (I. polycarpa), which belongs to the genus Idesia within the family Salicaceae, is a deciduous tree species. Its fruits and seeds are high in unsaturated fatty acids and thus receive the designation of the “oil reservoir in the air” [10]. I. polycarpa is notable for its height, adaptability, rapid growth rate, and robust vitality. Its natural distribution encompasses Sichuan, Guizhou, and Hubei provinces of China, with a recent focus on its introduction to regions such as Chongqing and Shaanxi provinces, to mitigate the effects of rocky desertification. As a new woody oilseed tree species, I. polycarpa is widely used in many fields such as landscaping, bioenergy, healthcare, and beauty care. It was listed as a national strategic reserve forest species in April 2020, which is of strategic importance to ensure national energy security, food and oil supply, ecological protection, and rural revitalization [11,12]. At present, domestic and international research on I. polycarpa mainly focuses on seedling breeding and fruit oil analysis, while little research has been reported on the drought tolerance of I. polycarpa seedlings. I. polycarpa is widely distributed, and there are many varieties; the known species are ‘Exuan 1’, ‘Yuji’, ‘Chuantong’, and so on. However, due to the lack of a comprehensive drought evaluation system, it is difficult to evaluate the drought tolerance of various varieties of I. polycarpa, which seriously restricts the promotion of I. polycarpa to the northwestern arid areas for afforestation. In this study, the one-year-old seedlings of I. polycarpa were taken as the test material, through the systematic study of the growth and physiological response of I. polycarpa seedlings to drought stress, to provide a scientific basis for I. polycarpa seedlings’ water management. It is proposed to establish an I. polycarpa drought tolerance evaluation system, to provide a reference for the screening of I. polycarpa drought-tolerant varieties, and to lay a practical basis for the popularization of I. polycarpa to the northwestern arid region.

2. Materials and Methods

2.1. Test Materials

Mature I. polycarpa fruits were collected from wild superior individual mother trees in Qishe Town, Xingyi, Guizhou Province (24°56′ N, 104°47′ E). After collecting the wild I. polycarpa fruits in November 2022, the pulp of the mature fruits was crushed and continuously kneaded to obtain the seeds. The floating impurities and poor-quality seeds were eliminated through clean water. Then, the plump seeds were air-dried in a cool, well-ventilated place for storage. In January 2023, a large number of high-quality tissue culture seedlings were obtained through the aseptic germination of the collected and stored I. polycarpa seeds. In March, the seedlings were transplanted into transparent plastic cups for hardening (Figure 1A). In early April, I. polycarpa seedlings were transplanted into 20 × 23 cm (height × diameter) polypropylene containers with a soil substrate composed of peat soil, perlite, and vermiculite (2:1:1) for continued growth (Figure 1B). In July, healthy one-year-old I. polycarpa seedlings with uniform height were selected and moved under a rain shelter that did not affect light or ventilation for acclimatization (Figure 1C). During this recovery phase, normal watering and maintenance practices were maintained.

2.2. Design of the Experiment

Three different water treatments were established based on soil relative water content (RWC) to simulate drought stress: control (CK, RWC = 70 ± 5%), moderate drought (T1, RWC = 40 ± 5%), and severe drought (T2, RWC = 20 ± 5%). The drought stress treatments lasted for 21 days. After the drought stress period, all drought-treated groups were rewatered to the control RWC level (70 ± 5%), forming the moderate drought rewatering group (T3) and the severe drought rewatering group (T4). Each treatment group, including the control, contained 20 plants, totaling 60 potted I. polycarpa seedlings.
Drought stress was simulated using a pot-based water control method. Irrigation was stopped on August 10, allowing soil moisture to decrease through natural evaporation until it reached the target RWC for each treatment group. This point was designated as day 0 of drought stress. The drought stress period lasted for 21 days. During this period, water lost through transpiration was replenished every 4 h between 8:00 and 19:00. A soil moisture meter (SANKU SK-100, Japan) was used to measure soil water content before and after each watering. Following the principle of frequent and small water additions, the probe of the moisture meter was inserted to a depth of 10 cm to ensure that the soil RWC remained within the specified range for each treatment.
On day 21 of the drought stress treatment, plants in all drought treatment groups were rewatered to the control RWC level (70 ± 5%), and the rewatering treatment lasted for 14 days. Throughout the experiment, seedlings were kept in a rainproof net house to prevent any external water input. To reduce water loss from soil surface evaporation, plastic film was used to cover the soil surface and maintain soil moisture.

2.3. Sample Collection

Measurements of growth, biomass, and photosynthetic physiological parameters were conducted on day 21 of drought stress and day 14 after rewatering. Sample leaves were also collected during each of these times, between 08:00 and 10:00 a.m. For each treatment, 8–10 healthy leaves were collected and labeled with the treatment name and sampling time. A portion of the samples was placed in resealable bags for the determination of leaf relative water content, relative electrical conductivity, and chlorophyll content. Another portion was wrapped in aluminum foil, immediately frozen in liquid nitrogen, and brought back to the laboratory for storage in an ultra-low temperature freezer (–80 °C). These samples were used for measuring the contents of osmotic adjustment substances, antioxidant enzyme activities, and endogenous hormone levels. Each treatment included three biological replicates for sampling.

2.4. Measurement of Growth Indexes

Before the initiation of the stress treatment, after the stress treatment, and following rewatering, plant height, ground diameter, and crown were measured to evaluate the changes in plant height growth (ΔH), diameter growth (ΔD), and crown growth under each treatment condition.
Biomass was determined using the whole-plant harvesting method, which was conducted at the end of the stress treatment and after the rewatering process. Upon completion of the treatments, the potting soil was loosened by rinsing with running water. The seedlings were then extracted, and any adhering soil was washed away. After removing residual moisture, the fresh weights of the aboveground parts (including stems, branches, and leaves) and the belowground parts (roots) of the seedlings were measured separately. The fresh weight per plant and the root–crown ratio (the ratio of belowground fresh weight to aboveground fresh weight) were subsequently calculated.

2.5. Photosynthetic Characteristics and Photosynthetic Pigment Indexes

Photosynthetic parameters including net photosynthetic rate (Pn), stomatal conductance (Gs), intercellular CO2 concentration (Ci), and transpiration rate (Tr) were recorded employing a portable photosynthesis measurement system (LI-6400, LI-COR, Lincoln, NE, USA) according to a set light level of 1200 μmol m−2 s−1. The level of chlorophyll was obtained utilizing the 95% ethanol–acetone extraction technique [13].

2.6. Physiological and Biochemical Indexes

The leaf relative water content (LRWC) was determined using the drying method [14]. Fresh leaves were collected, and their fresh weight was measured. The leaves were then immersed in distilled water for 24 h to achieve saturated fresh weight. Subsequently, the leaves were placed in an oven and dried at 80 °C for 8 h to obtain the dry weight. The relative water content of the leaves was calculated as (fresh weight-dry weight)/(saturated fresh weight–dry weight) × 100%.
The leaves were cut into strips of appropriate length (avoiding the midrib), and three fresh samples, each weighing 0.1 g, were quickly weighed and placed into graduated test tubes containing 10 mL of deionized water. The tubes were then stopped with glass caps and left to soak at room temperature for 12 h. The conductivity of the leachate (R1) was measured using a conductivity meter. Subsequently, the samples were heated in a boiling water bath for 30 min, cooled to room temperature, and thoroughly mixed before the conductivity of the leachate (R2) was measured again. Leaf relative electrical conductivity (REC) was calculated as R1/R2 × 100% [14].
Soluble Sugar (SS) was quantified using the anthrone–sulfuric acid method [15]. Fresh tissue was extracted in 80% ethanol, reacted with anthrone reagent at 100 °C for 10 min, and absorbance was measured at 620 nm against a glucose standard.
Soluble Protein (SP) was assessed via Coomassie Brilliant Blue G-250 dye-binding (Bradford assay) [16]. Tissue homogenate in phosphate buffer (pH 7.0–7.8) was centrifuged; supernatant reacted with Bradford reagent for 5–10 min, with absorbance at 595 nm quantified using the BSA standard.
Malondialdehyde (MDA) was determined by the thiobarbituric acid (TBA) reaction [17]. Tissue homogenate in 5% trichloroacetic acid (TCA) was reacted with 0.67% TBA at 95 °C for 30 min, and absorbance was read at 532 nm and 600 nm (turbidity correction), calculated using the extinction coefficient 155 mM−1 cm−1.
Proline content was quantified using an acidic ninhydrin kit (Ruixin Biotechnology Co., Ltd., Quanzhou, China), measured at 520 nm; betaine content via a Reinecke salt kit (Ruixin Biotechnology Co., Ltd., Quanzhou, China ) at 525 nm; POD activity by a guaiacol oxidation kit (Ruixin Biotechnology Co., Ltd., Quanzhou, China ) at 470 nm; SOD activity with an NBT photoreduction kit (Ruixin Biotechnology Co., Ltd., Quanzhou, China) at 560 nm; APX activity via an ascorbate oxidation kit (Ruixin Biotechnology Co., Ltd., Quanzhou, China) at 290 nm; CAT activity by an ultraviolet absorption kit (Ruixin Biotechnology Co., Ltd., Quanzhou, China) at 240 nm.
ELISA was used to quantify the endogenous levels of GA, IAA, ABA, and SA [18]. Fresh I. polycarpa leaves were homogenized in an extraction buffer and centrifuged to obtain the supernatant. Samples were added to antibody-coated ELISA plates, followed by incubation with specific antibodies and enzyme-labeled secondary antibodies. Absorbance was measured at 450 nm, and hormone concentrations were determined using a standard curve. The ELISA kits used were purchased from Ruixin Biotech Co., Ltd, Quanzhou, China.

2.7. Root System Indexes and Leaf Anatomy

After the drought stress test was finished and the plants were rewatered, the entire plant was gently uprooted, and the root systems were cleaned with flowing water to remove any dirt. The roots were then dried, and images of the root structure were captured using WinRHIZO 2013 (Regent Instruments Inc., Quebec City, QC, Canada). Root structural characteristics, including total root length, root surface area, root volume, and the number of root tips, were subsequently analyzed.
Leaf anatomy: Sections were prepared through the conventional paraffin sectioning steps, and the staining method was safranin fast green double staining. Fresh leaf tissues in each experimental group were collected and promptly fixed in FAA tissue fixative for over 24 h. The tissues were removed from the fixative, and the target tissues were trimmed flat with a scalpel in a fume hood. The sectioned specimens and corresponding labels were arranged in embedding frames. The dehydration cassettes were put into a dehydrator for gradient alcohol dehydration in sequence, followed by soaking in xylene until the tissues became transparent. The transparent tissues were then immersed in paraffin, and the wax-impregnated tissues were embedded in an embedding machine (JB-P5, Wuhan Junjie Electronics Co., Ltd., Wuhan, China). After trimming the wax blocks, 4-μm-thick sections were cut using a paraffin microtome. The sections were then stained and fixed with safranin and fast green. Slides were analyzed using a Nikon Eclipse E100 upright microscope (Nikon Corporation, Japan) and imaged with a DS-U3 camera (Nikon). The captured images were imported into CaseViewer to measure the thickness data of each tissue [19,20].

2.8. Statistical Analysis

Data collection and preliminary processing were all carried out utilizing Microsoft Excel 2019 (Microsoft Corporation, Redmond, WA, USA). The experimental data were analyzed using one-way ANOVA in SPSS 23.0 (IBM, New York, NY, USA), followed by Duncan’s test to assess the significance of observed differences. Graphs were generated according to Origin 2021 (OriginLab, Northampton, MA, USA), whereas plates were created with Adobe Photoshop 2021 (Adobe, San Jose, CA, USA).

3. Results

3.1. Effects of Drought Stress on Leaf Phenotypic Characteristics of I. polycarpa Seedlings

As shown in Figure 2, the leaf phenotypes of I. polycarpa seedlings varied under different treatments. Under drought stress, leaves exhibited varying degrees of damage. After 21 days of moderate drought stress (T1), the leaves turned yellow, developed yellow spots, showed raised veins, and some displayed surface wrinkling. After 21 days of severe drought stress (T2), more pronounced drought-induced damage was observed, including more extensive yellowing, increased wrinkling, and the appearance of brown spots in the mesophyll and along the leaf margins.
Following rewatering, leaf damage was alleviated to some extent in both treatment groups, but leaf morphology did not fully return to the state observed under normal water conditions. Fourteen days after rewatering in the T1 group, the leaves appeared greener than those at the end of the drought period, with fewer yellow edges and yellow spots. In the T2 group, although the leaves turned visibly greener 14 days after rewatering, yellow spots and yellowing along the edges were still present.

3.2. Effects of Drought Stress on Photosynthetic Pigment Content in Leaves of I. polycarpa Seedlings

Drought stress obviously reduced the chlorophyll content of I. polycarpa seedlings. T1 resulted in a reduction in chlorophyll a, chlorophyll b, and chlorophyll (a + b) contents by 49.62%, 51.67%, and 50.71%, respectively, in comparison to CK. Following rewatering (T3), however, chlorophyll a exhibited a reduction of only 7.49% compared to CK, while chlorophyll b and chlorophyll (a + b) demonstrated significantly lower levels, at 55.77% and 33.29%, respectively. In response to T2, there was a notable decline in chlorophyll a, chlorophyll b, and chlorophyll (a + b), with reductions of 51.82%, 75.64%, and 64.56%, accordingly, in comparison to CK. Following rewatering (T4), these indexes remained substantially below the control level, with decreases of 20.32%, 68.16%, and 45.88%, respectively, in comparison to CK (Table 1).

3.3. Effects of Drought Stress on Plant Growth of I. polycarpa Seedlings

The growth and biomass accumulation of I. polycarpa seedlings were impeded by drought stress. Table 2 indicates that under T1, ΔH, ΔG, crown growth, aboveground biomass, and belowground biomass were substantially less than CK by 78.63%, 55.69%, 113.45%, 35.32%, and 24.52%, respectively. After rehydration (T3), though, none of these indexes recovered to the untreated level, which was 62.39%, 71.09%, 86.5%, 33.55%, and 21.96%, respectively, lower compared to CK. I. polycarpa seedlings were particularly damaged by the severe drought treatment (T2); the ΔH, ΔG, crown growth, aboveground biomass, and belowground biomass were much lower—by 81.2%, 46.68%, 62.4%, 232.27%, and 59.00%, respectively—than those of CK. In contrast, the root–crown ratio increased by 39.72% and was significantly higher than that of CK. After rehydration, only the belowground biomass recovered to the level of the control group, while all other indicators remained significantly different from CK.

3.4. Effects of Drought Stress on Root Growth of I. polycarpa Seedlings

To enable them to adapt to the environment of water deprivation, I. polycarpa seedlings also experienced certain alterations in their root structure. In T1, as illustrated in Table 3 and Figure 3, root length and root tips increased greatly with CK, rising by 49.40% and 44.21%, respectively; on the other hand, root surface area and root volume decreased relative to CK, falling by 0.88% and 42.34%, respectively. Following watering again (T3), the root tips increased by 67.80% and the root surface area dropped by 13.36% in comparison to CK, but the root length and volume were still below the control level. During T2 treatment, root length increased by only 12.59% over CK, yet it did not vary considerably. Root surf area and volume were dramatically reduced by 32.40% and 61.85%, respectively. After watering continued, all indexes were restored to control level, except root volume, which was 18.49% below CK.

3.5. Effects of Drought Stress on Photosynthetic Characteristics of I. polycarpa Seedlings

During the prolonged drought, I. polycarpa seedlings’ leaves were similarly lacking in photosynthetic characteristics. While intercellular CO2 concentration (Ci) and transpiration rate (Tr) were not substantially different from CK, only 6.65% and 26.91% below CK, respectively, photosynthetic rate (Pn) and stomatal conductance (Gs) were notably lower than CK at moderate drought (42.22% and 63.22%, respectively). Gs fully recovered and exhibited minimal change from CK after rewatering, staying at 15.91%. When compared to CK, Pn, Gs, Ci, and Tr all showed notable declines in response to T2, reaching 63.22%, 81.32%, 21.47%, and 68.21%, respectively. I. polycarpa recovered well after the water supply was restored, with Gs, Ci, and Tr reaching levels similar to those at CK. Pn, however, was still 18.76% less than CK (Figure 4).

3.6. Effects of Drought Stress on LRWC, REC, and MDA Content of I. polycarpa Seedling Leaves

Drought stress harmed the leaf relative water content (LRWC) of I. polycarpa seedlings; at T1 and after water intake, values declined to those similar to CK. In particular, compared to CK, LRWC was 8.19% and 7.26% down, respectively. Under T2, the level was much lower, showing a loss of 18.62% compared to CK and a reduction of 14.04% even after rehydration. Leaf relative electrical conductivity (REC) and the MDA content of leaves increased during dry circumstances, but when the stress level increased, they showed a considerable rise in comparison to CK. The observed increases were 75.92% and 27.98% under T2 and 59.03% and 10.11% under T1, respectively. Additionally, the MDA content demonstrated a notable recovery following rewatering (Figure 5).

3.7. Effects of Drought Stress on Activities of Antioxidant Enzymes of I. polycarpa Seedling Leaves

The antioxidant enzyme activities in the leaves of I. polycarpa seedlings rose throughout stress caused by drought. POD, SOD, and CAT tended to rise and then minimize, whereas APX showed an effect of steadily increasing, as the drought stress intensified. POD, SOD, CAT, and APX were considerably above CK under T1, at 81.78%, 63.47%, 44.19%, and 37.96%, respectively. POD and APX recovered substantially after rewatering (T3), although SOD and CAT remained much higher—by 23.6% and 11.18%—than CK. At T2, POD, SOD, CAT, and APX were much higher than CK, at 71.75%, 31.2%, 33.15%, and 74.76%, respectively. After water retention (T4), all antioxidant enzyme activities started to fall, particularly POD, CAT, and APX, which stayed above CK by 19.33%, 13.82%, and 25.51%, respectively (Figure 6).

3.8. Effects of Drought Stress on the Osmoregulatory Substances of I. polycarpa Seedlings’ Leaves

Drought additionally harmed osmoregulatory substances in the leaves of I. polycarpa seedlings. Proline, betaine, and SS exhibited a propensity to steadily rise as drought stress intensified, while SP showed a tendency to rise and subsequently drop. Proline, betaine, SP, and SS were much lower than CK under T1 (14.27%, 39.58%, 34.2%, and 36.34%, respectively). Proline, betaine, and SS all recovered effectively after rewatering (T3), while SP was 22.71% above CK. While SP concentration stayed at the control level, the accumulation of osmoregulatory substances was considerably higher during periods of T2, with proline, betaine, and SS considerably higher than CK by 27.87%, 58.85%, and 55.52%, respectively. Rewatering (T4) allowed proline and SS to recover effectively, but betaine was still 21.02% over CK (Figure 7).

3.9. Effects of Drought Stress on Endogenous Hormones of I. polycarpa Seedling Leaves

I. polycarpa seedlings’ leaves respond to environmental changes, such as drought, by controlling endogenous hormone levels. In the leaves of I. polycarpa seedlings, drought caused a buildup of ABA and SA contents. As the stress level grew, the ABA content progressively rose while the SA content first climbed and subsequently declined. At T1, the concentrations of ABA and SA were 16.12% and 28.49%, respectively, higher than the level of CK. After rewatering (T3), ABA started to decline but was still 10.61% above CK; in contrast, SA grew and was 93.32% above CK after rewatering. At T2, ABA and SA were also above CK by 29.01% and 8.79%, and following rewatering (T4), by 13.04%. On the other hand, as the drought got worse, IAA and GA steadily dropped. IAA in T1 stayed at the control level, but GA was 12.61% less than CK. IAA and GA content increased with rewatering (T3); however, GA returned to the control level, at 29.19% and 8.13% higher than CK. IAA and GA in T2 were 27.25% and 13.98% higher than in CK, respectively, but both returned to control levels when the soil was rewatered (T4) (Figure 8).

3.10. Effects of Drought Stress on the Leaf Anatomy of I. polycarpa Seedling Leaves

The drought hurt the leaf anatomy of I. polycarpa seedlings, causing all leaf tissues to become thinner. As demonstrated in Table 4 and Figure 9, the thickness of upper epidermis (TU) and lower epidermis (TL) decreased and then increased with the deepening of stress, while the thickness of palisade mesophyll (TP), spongy mesophyll (TS), and the leaf thickness (LT) gradually decreased. At T1, upper epidermal thickness and leaf thickness altered dramatically; they were 39.51% and 5.46% below CK. Both went back to the control level after being rewatered (T3). Under T2, there was a significant and negative impact on upper epidermal thickness, palisade tissue thickness, spongy tissue thickness, and leaf thickness. These parameters were all reduced relative to CK by 34.97%, 14.10%, 12.78%, and 10.62%, respectively. Rewatering did not completely reverse the effects on the thickness of the palisade tissue, spongy tissue, or leaf; rather, the thickness of each was decreased by 14.54%, 11.78%, and 11.01% in comparison to CK, while the thickness of the upper epidermis was brought back to the control level.

3.11. Correlation Analysis

Based on the Pearson correlation analysis of growth, physiological, and leaf anatomical indicators in I. polycarpa seedlings under drought stress (Figure 10), it was demonstrated that chlorophyll a and chlorophyll (a + b) exhibited statistically significant negative correlations with betaine, proline, ascorbate peroxidase (APX), catalase (CAT), peroxidase (POD), malondialdehyde (MDA), and relative electrical conductivity (REC), while abscisic acid (ABA) and soluble sugars showed statistically significant positive correlations with relative conductivity, MDA, POD, CAT, APX, proline, and betaine; in addition, leaf structural indicators including thickness of palisade mesophyll, thickness of spongy mesophyll, and leaf thickness were significantly positively correlated with chlorophyll a, chlorophyll (a + b), aboveground biomass, and leaf relative water content (LRWC), but significantly negatively correlated with APX, proline, and betaine.

3.12. Principal Component Analysis

The principal component analysis (PCA) results demonstrated that PC1 and PC2 contributed 55.3% and 20.3% of the variance, respectively, cumulatively explaining 75.6% of the total variation—adequately capturing core data heterogeneity (Figure 11A, B). Along the PC1 axis, the positive loading direction was predominantly enriched with indicators, including Chl (a + b), Chl a, Chl b, aboveground biomass, crown growth, and leaf relative water content (LRWC), representing high photosynthetic efficiency and vigorous growth status. Conversely, the negative loading direction concentrated stress–response markers such as proline, betaine, malondialdehyde (MDA), and relative electrical conductivity (REC). The PC2 axis primarily correlated with belowground biomass, root-to-shoot ratio, and phytohormones (ABA, IAA, and SA), reflecting trade-offs in root–shoot allocation and hormonal regulation divergence. The cos2 value (cosine squared) is a key indicator that measures the projection strength of each variable (or observation) onto the principal components, reflecting its contribution. Figure 11C illustrates the cos2 values of different variables in the first two principal components, with higher contributions observed for betaine, APX, SA, IAA, ABA, Chl (a + b), Chl a, and crown growth.

4. Discussion

Drought stress has widespread and profound effects on plant growth and development, particularly under water-deficient conditions, where cell differentiation and expansion are inhibited, ultimately affecting overall plant growth and development [21]. The most visible effects of drought stress are changes in leaf morphology, such as wilting, yellowing, and necrosis at the leaf tips and margins. In this study, I. polycarpa seedlings exhibited leaf yellowing and necrosis under drought conditions, indicating significant growth impairment. Similar responses have been observed in Camellia oleifera [22] and Delphinium grandiflorum [23]. After rewatering, these stress symptoms were alleviated, but the leaf phenotype remained distinct from non-stressed plants.
Changes in leaf anatomical structure further reflected the impact of drought stress. The palisade tissue, which plays a central role in photosynthesis, directly affects photosynthetic efficiency and light tolerance [24]. Our results showed that drought stress significantly reduced the thickness of leaf tissues in I. polycarpa, indicating a restriction in growth due to water deficiency. Notably, the upper epidermal thickness of I. polycarpa leaves significantly decreased by 39.51% and 34.97% under moderate and severe drought conditions, respectively, compared to the control, demonstrating a remarkable reduction. After rewatering, leaf structure partially recovered under moderate drought conditions, whereas recovery was notably limited under severe drought, suggesting that rewatering has a constrained restorative effect on leaf structure.
Stomata serve as the primary channels for gas exchange in plants [25]. Under drought stress, Gs and Ci declined, directly limiting Pn. In this study, the decrease in Gs and Ci suggested that Pn limitation was mainly driven by stomatal factors. Additionally, Tr decreased, indicating that I. polycarpa adapted to drought by reducing water loss. After rewatering, gas exchange parameters partially recovered, consistent with findings in palm species following drought stress relief [26]. Drought stress also significantly inhibited chlorophyll synthesis and accelerated its degradation [27]. In this study, chlorophyll a and chlorophyll b contents in I. polycarpa seedlings declined significantly under drought conditions but increased after rewatering, suggesting that resuming normal water supply mitigates drought-induced damage to the photosynthetic system.
The root system, which directly interacts with the soil, is highly sensitive to water availability [28]. Under drought stress, I. polycarpa exhibited an increase in total root length and root tips, indicating an adaptive strategy to access deeper soil water and enhance drought tolerance. After rewatering, compensatory root growth occurred, suggesting that I. polycarpa possesses a certain degree of post-drought recovery capability.
Leaf relative water content (LRWC) is an indicator of plant drought tolerance [29]. In this study, LRWC decreased progressively with increasing drought severity, while relative electrical conductivity (REC) and MDA content increased significantly, indicating severe cellular membrane damage. After rewatering, LRWC recovered well under moderate drought conditions, demonstrating the resilience of I. polycarpa to mild drought stress.
Under drought stress, the contents of proline, betaine, and SP in I. polycarpa leaves significantly increased, while SP content peaked under moderate drought and then declined. After rewatering, these osmotic regulators partially recovered, indicating a certain degree of post-drought recovery ability. This response may be attributed to the plant’s regulation of intracellular solute concentrations to maintain cell turgor and water balance. Proline and betaine not only reduce cellular osmotic potential but also stabilize proteins and membrane structures, thereby mitigating drought-induced damage. Meanwhile, soluble sugars provide energy and carbon skeletons while undergoing osmotic adjustment [30].
Additionally, drought stress significantly enhanced the activity of antioxidant enzymes (POD, SOD, CAT, and APX) to mitigate oxidative damage caused by reactive oxygen species (ROS) [31,32]. After rewatering, enzyme activity declined, indicating a reduction in oxidative stress. SOD converts superoxide radicals into H2O2, which is further decomposed by POD, CAT, and APX, thereby limiting ROS accumulation [33]. This mechanism plays a crucial role in protecting cell membranes and biomolecules such as proteins and DNA, ultimately improving drought tolerance.
Endogenous hormones are vital in plant drought responses [34]. This study found that drought reduced GA and IAA levels, inhibiting growth to conserve water. Upon rehydration, both hormones recovered and exceeded control levels, indicating compensatory growth, likely through hormone balance regulation. ABA and SA increased significantly under drought stress. ABA induced stomatal closure to reduce transpiration, while SA likely enhanced stress tolerance and damage repair [35]. Rehydration led to ABA decline and stomatal reopening. Elevated SA levels suggest that I. polycarpa enhances drought resistance and recovery via SA-mediated signaling.
Plants respond to drought stress through various growth and physiological changes. Therefore, assessing drought tolerance requires a comprehensive evaluation of multiple growth and physiological parameters. In this study, correlation analysis revealed significant relationships (p < 0.05) among 18 growth and physiological indices. However, the high correlation between variables may lead to redundant information extraction. To address this issue, principal component analysis (PCA) was employed to reduce data dimensionality while retaining essential information, effectively mitigating redundancy [36]. In PCA, the cos2 value serves as a metric for assessing a variable’s contribution to the principal components. By analyzing cos2 values, variables with high explanatory power can be identified, aiding in data interpretation, key variable selection, and subsequent analysis. In drought tolerance studies, cos2 values help determine which growth and physiological indices are most representative of drought resistance. This study found that betaine, ascorbate peroxidase (APX), salicylic acid (SA), indole-3-acetic acid (IAA), abscisic acid (ABA), total chlorophyll (Chl (a + b)), chlorophyll a (Chl a), and crown growth exhibited high cos2 values in the first two principal components. These results suggest that these indices have strong explanatory power and can serve as key indicators for evaluating the drought tolerance of I. polycarpa.

5. Conclusions

Drought stress negatively impacted I. polycarpa seedlings by altering leaf morphology and anatomy, inhibiting photosynthesis, modifying root structure, disrupting membrane stability, and regulating endogenous hormones and antioxidant defenses. After rewatering, I. polycarpa exhibited partial recovery, though resilience declined significantly under severe drought conditions. Therefore, monitoring changes in leaf morphology and physiological indicators and implementing appropriate irrigation strategies are crucial for mitigating drought stress and promoting healthy seedling growth. This study identified betaine, APX, SA, IAA, ABA, chlorophyll a, total chlorophyll, and crown growth as key indicators for evaluating drought tolerance in I. polycarpa. These findings provide a scientific basis for assessing drought tolerance in I. polycarpa and lay the foundation for further exploration of its drought resistance mechanisms.

Author Contributions

Conceptualization, X.L. and L.W.; methodology, X.L.; software, X.L.; validation, X.L., Y.Y., and Z.N.; formal analysis, X.L.; investigation, L.W.; resources, X.L.; data curation, X.L.; writing—original draft preparation, X.L.; writing—review and editing, X.L.; visualization, S.Z.; supervision, M.Y.; project administration, C.C.; funding acquisition, L.W. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by Special Funds for Student Innovation and Entrepreneurship Programs in Hunan Province (grant number: [S202410538003]).

Data Availability Statement

Requests to access the datasets should be sent via email to luxy0115@163.com.

Acknowledgments

This study gratefully acknowledges the provision of laboratory facilities by the Central South University of Forestry and Technology and financial support from the Hunan Provincial College Students’ Innovation and Entrepreneurship Fund.

Conflicts of Interest

The authors declare no conflicts of interest.

Abbreviations

The following abbreviations are used in this manuscript:
RWCSoil relative water content
LRWCLeaf relative water content
RECLeaf relative electrical conductivity

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Figure 1. The experimental materials used in this study. (A) Tissue-cultured seedlings of I. polycarpa were transplanted into transparent plastic cups for acclimatization. (B) I. polycarpa seedlings were transplanted into pots for continued growth. (C) Healthy one-year-old I. polycarpa seedlings with uniform height were selected and moved under a rain shelter that did not affect light or ventilation for acclimatization.
Figure 1. The experimental materials used in this study. (A) Tissue-cultured seedlings of I. polycarpa were transplanted into transparent plastic cups for acclimatization. (B) I. polycarpa seedlings were transplanted into pots for continued growth. (C) Healthy one-year-old I. polycarpa seedlings with uniform height were selected and moved under a rain shelter that did not affect light or ventilation for acclimatization.
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Figure 2. Effects of drought stress on leaf phenotypic characteristics of I. polycarpa seedlings. CK: Control. T1: Moderate drought. T2: Severe drought.
Figure 2. Effects of drought stress on leaf phenotypic characteristics of I. polycarpa seedlings. CK: Control. T1: Moderate drought. T2: Severe drought.
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Figure 3. Scan of I. polycarpa seedlings root system.
Figure 3. Scan of I. polycarpa seedlings root system.
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Figure 4. Effects of drought stress on the net photosynthetic rate (A), stomatal conductance (B), intercellular CO2 concentration (C), and transpiration rate (D) of I. polycarpa. Different lowercase letters indicate significant differences between the treatments (p < 0.05).
Figure 4. Effects of drought stress on the net photosynthetic rate (A), stomatal conductance (B), intercellular CO2 concentration (C), and transpiration rate (D) of I. polycarpa. Different lowercase letters indicate significant differences between the treatments (p < 0.05).
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Figure 5. Effects of drought stress on leaf relative water content (A), leaf relative electrical conductivity (B), and MDA content (C) of leaves of I. polycarpa. Different lowercase letters indicate significant differences between treatments (p < 0.05).
Figure 5. Effects of drought stress on leaf relative water content (A), leaf relative electrical conductivity (B), and MDA content (C) of leaves of I. polycarpa. Different lowercase letters indicate significant differences between treatments (p < 0.05).
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Figure 6. Effects of drought stress on POD activity (A), SOD activity (B), APX activity (C), and CAT activity (D) of leaves of I. polycarpa. Different lowercase letters indicate significant differences between treatments (p < 0.05).
Figure 6. Effects of drought stress on POD activity (A), SOD activity (B), APX activity (C), and CAT activity (D) of leaves of I. polycarpa. Different lowercase letters indicate significant differences between treatments (p < 0.05).
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Figure 7. Effects of drought stress on proline content (A), betaine content (B), SP content (C), and SS content (D) of leaves of I. polycarpa. Different lowercase letters indicate significant differences between treatments (p < 0.05).
Figure 7. Effects of drought stress on proline content (A), betaine content (B), SP content (C), and SS content (D) of leaves of I. polycarpa. Different lowercase letters indicate significant differences between treatments (p < 0.05).
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Figure 8. Effects of drought stress on ABA content (A), SA content (B), IAA content (C), and GA content (D) of leaves of I. polycarpa. Different lowercase letters indicate significant differences between treatments (p < 0.05).
Figure 8. Effects of drought stress on ABA content (A), SA content (B), IAA content (C), and GA content (D) of leaves of I. polycarpa. Different lowercase letters indicate significant differences between treatments (p < 0.05).
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Figure 9. Leaf anatomy of I. polycarpa seedling leaves. LE: Lower epidermis; UE: Upper epidermis; SM: Spongy mesophyll; PM: Palisade mesophyll.
Figure 9. Leaf anatomy of I. polycarpa seedling leaves. LE: Lower epidermis; UE: Upper epidermis; SM: Spongy mesophyll; PM: Palisade mesophyll.
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Figure 10. Pearson’s correlation matrix analysis of growth, physiological, and leaf anatomical indicators of I. polycarpa seedlings. Chl a: Chlorophyll a; Chl b: Chlorophyll b; Chl (a + b): Chlorophyll (a + b); TU: Thickness of upper epidermis; TL: Thickness of lower epidermis; TP: Thickness of palisade mesophyll; TS: Thickness of spongy mesophyll; LT: Leaf thickness. * indicates significant correlation (p ≤ 0.05); ** indicates extremely significant correlation (p ≤ 0.01).
Figure 10. Pearson’s correlation matrix analysis of growth, physiological, and leaf anatomical indicators of I. polycarpa seedlings. Chl a: Chlorophyll a; Chl b: Chlorophyll b; Chl (a + b): Chlorophyll (a + b); TU: Thickness of upper epidermis; TL: Thickness of lower epidermis; TP: Thickness of palisade mesophyll; TS: Thickness of spongy mesophyll; LT: Leaf thickness. * indicates significant correlation (p ≤ 0.05); ** indicates extremely significant correlation (p ≤ 0.01).
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Figure 11. Principal component analysis of indicators of growth and physiology of I. polycarpa seedlings. (A) PCA 2D biplot; (B) scree plot; (C) biplot with cos2 values; high cos2 values are represented by red arrows, intermediate cos2 values are represented by yellow arrows, and low cos2 values are represented by blue arrows.
Figure 11. Principal component analysis of indicators of growth and physiology of I. polycarpa seedlings. (A) PCA 2D biplot; (B) scree plot; (C) biplot with cos2 values; high cos2 values are represented by red arrows, intermediate cos2 values are represented by yellow arrows, and low cos2 values are represented by blue arrows.
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Table 1. Effects of drought stress on chlorophyll a, chlorophyll b, and chlorophyll (a + b) contents of I. polycarpa. Different lowercase letters indicate significant differences between each treatment (p < 0.05).
Table 1. Effects of drought stress on chlorophyll a, chlorophyll b, and chlorophyll (a + b) contents of I. polycarpa. Different lowercase letters indicate significant differences between each treatment (p < 0.05).
TreatmentChlorophyll a Content
(mg·g−1 FW)		Chlorophyll b Content
(mg·g−1 FW)		Chlorophyll (a + b) Content
(mg·g−1 FW)
CK1.882 ± 0.001 a2.161 ± 0.082 a4.042 ± 0.082 a
T10.948 ± 0.004 c1.044 ± 0.094 b1.992 ± 0.091 c
T20.907 ± 0.017 c0.526 ± 0.038 d1.433 ± 0.055 d
T31.741 ± 0.086 a0.956 ± 0.139 bc2.696 ± 0.224 b
T41.499 ± 0.121 b0.688 ± 0.104 cd2.188 ± 0.225 bc
Table 2. Effects of drought stress on the growth of I. polycarpa seedlings. Different lowercase letters indicate significant differences between each treatment (p < 0.05).
Table 2. Effects of drought stress on the growth of I. polycarpa seedlings. Different lowercase letters indicate significant differences between each treatment (p < 0.05).
TreatmentΔH (cm)ΔG (mm)Crown Growth (cm)Aboveground Biomass (g)Belowground Biomass (g)Root–Crown Ratio
CK3.90 ± 0.15 a2.66 ± 0.29 a6.82 ± 1.00 a51.80 ± 5.72 a43.82 ± 1.66 a0.87 ± 0.10 c
T10.83 ± 0.41 b1.18 ± 0.26 bc−0.92 ± 0.16 bc33.51 ± 1.61 bc33.07 ± 1.44 b0.99 ± 0.03 bc
T20.73 ± 0.18 b1.42 ± 0.06 b−9.02 ± 0.27 d21.24 ± 1.98 c26.42 ± 2.32 b1.25 ± 0.06 b
T31.47 ± 0.44 b0.77 ± 0.06 cd0.92 ± 0.19 c34.42 ± 3.86 b34.19 ± 2.19 b1.01 ± 0.11 bc
T41.13 ± 0.22 b0.47 ± 0.05 d2.56 ± 0.80 b24.64 ± 4.16 bc42.69 ± 3.85 a1.79 ± 0.16 a
Table 3. Effects of drought stress on root growth of I. polycarpa seedlings. Different lowercase letters indicate significant differences between each treatment (p < 0.05).
Table 3. Effects of drought stress on root growth of I. polycarpa seedlings. Different lowercase letters indicate significant differences between each treatment (p < 0.05).
TreatmentRoot Length
(cm)		Root Surface Area
(cm 2)		Root Volume
(cm 3)		Root Tips
CK1923.44 ± 14.41 bc955.24 ± 20.94 a39.07 ± 2.11 a1119.0 ± 48.5 c
T12873.54 ± 228.60 a946.86 ± 26.38 a22.53 ± 0.85 c1613.7 ± 113.8 ab
T22165.50 ± 71.36 b645.79 ± 24.28 c14.91 ± 1.92 d1685.7 ± 29.8 ab
T31764.77 ± 123.61 bc827.66 ± 50.09 b33.45 ± 0.91 ab1877.7 ± 141.1 a
T41664.69 ± 81.47 c863.47 ± 15.04 ab31.85 ± 2.49 b1408.0 ± 47.1 bc
Table 4. Parameters of I. polycarpa leaf structure. TU: Thickness of upper epidermis; TL: Thickness of lower epidermis; TP: Thickness of palisade mesophyll; TS: Thickness of spongy mesophyll; LT: Leaf thickness. Different lowercase letters indicate significant differences between treatments (p < 0.05).
Table 4. Parameters of I. polycarpa leaf structure. TU: Thickness of upper epidermis; TL: Thickness of lower epidermis; TP: Thickness of palisade mesophyll; TS: Thickness of spongy mesophyll; LT: Leaf thickness. Different lowercase letters indicate significant differences between treatments (p < 0.05).
TreatmentTU (μm)TL (μm)TP (μm)TS (μm)LT (μm)
CK19.07 ± 0.90 b7.83 ± 0.38 b60.07 ± 1.34 a76.70 ± 2.16 a154.40 ± 3.99 a
T111.53 ± 0.99 a6.40 ± 0.56 b56.23 ± 2.09 abc72.87 ± 1.73 ab145.97 ± 0.75 b
T212.40 ± 0.72 a8.33 ± 1.28 ab51.60 ± 2.22 bc66.90 ± 0.69 b138.00 ± 2.79 bc
T319.50 ± 1.02 b11.47 ± 1.33 a57.10 ± 0.51 ab77.13 ± 3.59 a158.37 ± 1.03 a
T419.53 ± 1.55 b7.27 ± 0.73 b51.33 ± 1.44 c67.67 ± 2.72 b137.40 ± 2.08 c
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Lu, X.; Yin, Y.; Yang, M.; Zhang, S.; Niu, Z.; Wu, L.; Chen, C. Effects of Drought Stress on the Growth and Physiological Characteristics of Idesia polycarpa Maxim. Horticulturae 2025, 11, 834. https://doi.org/10.3390/horticulturae11070834

AMA Style

Lu X, Yin Y, Yang M, Zhang S, Niu Z, Wu L, Chen C. Effects of Drought Stress on the Growth and Physiological Characteristics of Idesia polycarpa Maxim. Horticulturae. 2025; 11(7):834. https://doi.org/10.3390/horticulturae11070834

Chicago/Turabian Style

Lu, Xiaoyu, Yian Yin, Maolin Yang, Shucheng Zhang, Zhangtai Niu, Lingli Wu, and Chan Chen. 2025. "Effects of Drought Stress on the Growth and Physiological Characteristics of Idesia polycarpa Maxim" Horticulturae 11, no. 7: 834. https://doi.org/10.3390/horticulturae11070834

APA Style

Lu, X., Yin, Y., Yang, M., Zhang, S., Niu, Z., Wu, L., & Chen, C. (2025). Effects of Drought Stress on the Growth and Physiological Characteristics of Idesia polycarpa Maxim. Horticulturae, 11(7), 834. https://doi.org/10.3390/horticulturae11070834

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