Next Article in Journal
Verbena officinalis L. Herb Extract, Its Amino Acid Preparations and 3D-Printed Dosage Forms: Phytochemical, Technological and Pharmacological Research
Previous Article in Journal
Biodiversity Conservation, a Crucial Step Towards Food and Nutritional Security, Food Justice and Climate Change Resilience in Africa
 
 
Search for Articles:
Title / Keyword
Author / Affiliation / Email
Journal
Article Type
 
 
Advanced Search
 
Section
Special Issue
Volume
Issue
Number
Page
 
Journals
Plants
Volume 14
Issue 17
10.3390/plants14172650
plants-logo
Submit to this Journal Review for this Journal Propose a Special Issue
► Article Menu

Article Menu

share Share announcement Help format_quote Cite question_answer Discuss in SciProfiles
first_page
settings
Order Article Reprints
Font Type:
Arial Georgia Verdana
Font Size:
Aa Aa Aa
Line Spacing:
Column Width:
Background:
Article

Exogenous Dopamine Alleviates Combined High Temperature and Drought Stress in Loquat [Eriobotrya japonica (Thunb.) Lindl.] Seedlings: Improvements in Photosynthetic Efficiency, Oxidative Damage and Osmotic Regulation

by
Xian Luo
1,
Ya Luo
1,
Xiao-Li Wang
1,
Xiao-Mei Kong
1,
Hui-Fen Zhang
1,
Li-Jin Lin
1,
Yu-Xing Li
1,
Ke-Wen Huang
2,
Qun-Xian Deng
1 and
Yong-Xia Jia
3,*
1
College of Horticulture, Sichuan Agricultural University, Chengdu 611130, China
2
Horticultural Research Institute, Chengdu Academy of Agriculture and Forestry Sciences, Chengdu 611130, China
3
College of Resources, Sichuan Agricultural University, Chengdu 611130, China
*
Author to whom correspondence should be addressed.
Plants 2025, 14(17), 2650; https://doi.org/10.3390/plants14172650
Submission received: 21 July 2025 / Revised: 15 August 2025 / Accepted: 23 August 2025 / Published: 26 August 2025
(This article belongs to the Special Issue Integrated Quality Regulation in Horticultural Crops)
Browse Figures
Versions Notes

Abstract

In recent years, high temperature and drought have severely impacted the growth and development of loquat [Eriobotrya japonica (Thunb.) Lindl.] plants. Although dopamine can improve the stress resistance of plants, its role in combined stress requires further exploration. This study investigated the alleviative effect and mechanism of exogenous dopamine on loquat seedlings subjected to the combined stress of high temperature and drought. The combined stress significantly reduced root viability, photosynthetic pigment content, and net photosynthetic rate (Pn) while markedly increasing reactive oxygen species (ROS) levels, thiobarbituric acid-reactive substances (TBARS) content, and electrolyte leakage (EL). The seedlings exhibited pronounced wilting symptoms, along with markedly reduced root surface area and volume. Dopamine treatment significantly alleviated combined stress-induced damage. This mitigation was manifested through substantially enhanced root viability, photosynthetic pigment content, Pn, antioxidant enzyme activities, and osmotic adjustment substances concomitantly with marked reductions in ROS, TBARS content, and EL. Dopamine significantly reduced seedling wilting severity and improved root morphological parameters. This study demonstrates that dopamine enhances loquat seedlings’ tolerance to combined stress through coordinated mechanisms: maintaining photosynthetic pigments and improving stomatal conductance to sustain photosynthetic efficiency, enhancing antioxidant enzyme activity and ROS scavenging capacity to mitigate oxidative damage, and promoting osmotic solute accumulation for osmotic potential regulation.

1. Introduction

In recent years, global climate warming has significantly increased the frequency, intensity, and duration of extreme dry-heat events [1,2]. As a synergistic abiotic stress factor, combined high temperature and drought stress poses an increasingly severe threat to agricultural ecosystems [3,4]. Notably, China’s high temperature and drought risk index reached its highest level since 1961 in 2022 [5], indicating that this compound stress poses a substantial risk to the country’s agricultural productivity and sustainable development.
Loquat [Eriobotrya japonica (Thunb.) Lindl.], a Rosaceae family member, ranks among the principal fruit trees in subtropical regions [6]. As both the origin and core distribution area of this species, China accounts for more than 66% of global cultivation area and yield, establishing it as the dominant worldwide producer [7]. Loquat thrives in warm and humid conditions, with optimal growth at 15–30 °C. Exposure to temperatures > 35 °C induces leaf wilting, fruit sunscald, and impaired root development [8]. Owing to its shallow root system, the species exhibits high sensitivity to water availability [9]. The seedling stage is critical for stress tolerance development, as combined high temperature and drought stress during this phase severely constrains mature tree growth and fruit quality formation.
High temperature or drought stress individually impairs photosynthesis, disrupts reactive oxygen species (ROS) homeostasis, and compromises osmotic adjustment in plants. Photosynthesis underpins plant growth, development, and energy acquisition, its efficiency provides a critical gauge of stress-induced impairment to primary productivity [10,11]. Concurrently, imbalanced ROS metabolism is a primary indicator of oxidative damage, effectively reflecting both membrane integrity impairment and oxidative stress severity [11,12]. The accumulation of osmoregulatory solutes constitutes an adaptive mechanism for turgor maintenance and osmotic stress counteraction; variations in solute levels directly signal a plant’s capacity to sustain water and osmotic homeostasis [13]. Mechanistically, high temperature predominantly induces thylakoid membrane disintegration and reduces rubisco activity, thereby decreasing both the net photosynthetic rate (Pn) and the maximum photochemical efficiency of PSII (Fv/Fm) [10,14]. In contrast, drought is the secondary stress induced by high temperature; it triggers stomatal closure, limiting CO2 diffusion, and similarly suppresses Pn [15,16]. Each stress independently induces excessive ROS accumulation, leading to membrane lipid peroxidation and inhibiting osmolyte synthesis (e.g., proline and betaine). This cascade weakens cellular water retention and ultimately curbs dry matter accumulation [9,17,18]. Critically, when high temperature and drought co-occur, they act synergistically, generating effects that substantially exceed the sum of their individual impacts [12].
Applying plant growth regulators represents a promising strategy for enhancing crop stress resilience. Notably, dopamine (DA)—a catecholamine present in animals and plants—mediates dual physiological functions: stress signaling and antioxidant defense [19,20]. Our prior work established that dopamine can improve the activity of antioxidant enzymes and enhance the ability to remove ROS, thereby improving the drought stress tolerance of loquat seedlings [21]. Similar studies have been confirmed in apples [22]. Under high temperature stress, dopamine preserves photosynthetic function by elevating chlorophyll content and Fv/Fm while concurrently upregulating antioxidant enzyme genes (e.g., SOD, CAT, POD, and APX) to enhance their activities. This dual action synergistically suppresses ROS accumulation, improving thermotolerance [23]. However, current research exclusively focuses on dopamine under single-stress conditions; its physiological roles and regulatory mechanisms under combined stress may differ. These mechanisms remain uncharacterized.
This study investigated photosynthetic characteristics, antioxidant systems, and osmoregulatory substances in loquat seedlings under combined high temperature and drought stress, aiming to explore the (1) mitigation effects of dopamine on combined stress in loquat seedlings and (2) the physiological mechanisms underlying dopamine-mediated tolerance to this combined stress.

2. Results

2.1. Plant Phenotypes and Root Morphological Parameters of Loquat Seedlings

Plant phenotype serves as a visual indicator of damage from combined high temperature and drought stress and dopamine-mediated alleviation. The dopamine (DA) treatment did not induce significant morphological changes in loquat seedlings under normal growth conditions (Figure 1). However, under combined high temperature and drought stress (HD), the seedlings exhibited pronounced wilting symptoms, including leaf drooping, withering, and curling. Notably, the DA application under stress conditions (HDDA) effectively alleviated the growth inhibition caused by HD stress.
As shown in Table 1, no significant differences in root morphological parameters were observed between the CK and DA treatments. Compared with CK, the HD treatment markedly reduced the root surface area, volume, average diameter, and number of tips by 36.30%, 29.69%, 40.04%, 30.21%, and 34.54%, respectively. Relative to HD alone, the HDDA treatment increased these parameters by 45.39%, 8.76%, 30.86%, 22.39%, and 16.11%, respectively.

2.2. TWC and RWC of Loquat Seedlings

As shown in Table 2, the DA treatment had no significant effect on the tissue water content (TWC) and relative water content (RWC) of leaves or roots under normal growth conditions. However, the HD treatment significantly reduced the TWC and RWC in both tissues. Compared with CK, the HD treatment decreased the leaf TWC and RWC to 38.02% and 30.79% below control levels and the root TWC and RWC by 19.15% and 25.62%, respectively. Furthermore, the HDDA treatment significantly increased the leaf TWC and RWC by 32.14% and 18.48% and the root TWC and RWC by 13.86% and 18.18% relative to those of the HD treatment alone.

2.3. Root Viability, TBARS Content and EL of Loquat Seedlings

Under normal conditions, the loquat seedlings showed no significant differences in root viability, thiobarbituric acid-reactive substances (TBARS) content, or electrolyte leakage (EL) between the DA treatment and the control (Figure 2). Compared with the control plants, the HD-treated plants exhibited a 54.94% decrease in root viability (Figure 2a), while the HDDA treatment resulted in a 32.64% increase compared with the HD treatment. The TBARS content and EL in the HD-treated plants were significantly higher than those in the control (Figure 2b and Figure 2c), with increases of 108.16% and 180.20%, respectively. In contrast, the HDDA treatment reduced both TBARS levels by 38.89% and EL by 29.65% compared with the HD-treated plants.

2.4. Photosynthetic Pigment Content and Photosynthetic Parameters of Loquat Seedlings

The carotenoid and total chlorophyll contents in the HD-treated loquat seedlings showed significant decreases of 12.5% and 24.24%, respectively, compared with the control. However, under the HDDA treatment, these pigment levels significantly increased by 42.86% and 22.00%, respectively, compared with those in HD-treated plants.
Compared with CK, the HD-treated loquat seedlings showed significant decreases in the net photosynthetic rate (Pn), transpiration rate (Tr), and stomatal conductance (Gs), whereas the intercellular CO2 concentration (Ci) was significantly increased. Under the HDDA treatment, the Pn, Tr, and Gs values were 2.21, 2.34, and 2.67 times higher than those in the HD-treated plants, respectively. Conversely, the Ci was significantly reduced by 8.25% compared with the HD-treated plants. (See Table 3).

2.5. Antioxidant System of Loquat Seedlings

2.5.1. Superoxide Anion Production Rate and H2O2 Accumulation

The DA treatment exerted no significant effects on the superoxide anion production rate (Figure 3a) or H2O2 content (Figure 3b) in the loquat seedlings under normal conditions. However, the HD treatment markedly induced ROS accumulation, increasing the superoxide anion production rate by 398.17% and H2O2 content by 209.47% compared with CK. In contrast, the HDDA treatment significantly attenuated these stress-induced ROS surges, reducing the superoxide anion production rate by 43.18% and H2O2 content by 47.62% compared with the HD-treated plants.

2.5.2. Antioxidant Enzyme Activities

Figure 4 demonstrates that under the HD treatment, the activities of superoxide dismutase (SOD), peroxidase (POD), catalase (CAT), and ascorbate peroxidase (APX) in the loquat seedlings were significantly higher than those in CK, with increases of 23.28%, 105.12%, 220.00%, and 392.37%, respectively. Compared with the HD treatment, the HDDA treatment further enhanced the activities of these antioxidant enzymes under high temperature and drought stress, showing increases of 7.02%, 48.13%, 40.63%, and 102.04%, respectively. No significant differences in SOD, POD, CAT or APX activities were observed between the DA-treated and CK-treated seedlings under normal conditions.

2.6. Osmotic Regulating Substance Accumulation of Loquat Seedlings

The DA treatment exerted no significant effects on osmotic regulating substance content under normal conditions (Figure 5). Compared with CK, the HD treatment significantly elevated betaine, proline (Pro), and soluble sugar (SS) levels by 37.66%, 37.88%, and 28.27%, respectively. Under combined stress, the HDDA treatment further enhanced the accumulation of osmotic regulating substances, increasing betaine, Pro, and SS levels by 223.30%, 87.91%, and 90.52%, respectively, compared with the HD-treated plants.

3. Discussion

3.1. Dopamine Alleviates Combined High Temperature and Drought Stress in Loquat Seedlings

Under the current background of global warming, high temperatures and drought have become the key factors constraining plant growth, development, yield, and quality. Root viability reflects the growth status and absorption function of plant roots [24]. Meanwhile, TBARS and EL are important indicators for assessing the extent of membrane lipid peroxidation and cell membrane integrity [25,26]. In this study, combined high temperature and drought stress impeded the physiological activities and growth of loquat seedlings, resulting in hampered root expansion, reduced root viability, declined cell membrane stability, leaf drooping, wilting, and decreased water content. These findings align with previous studies on tomatoes [27] and potatoes [28]. As a bioactive molecule with both signal transduction and antioxidant capabilities, dopamine can enhance a plant’s resistance to abiotic stress [19]. In this study, after dopamine treatment, the wilting severity of loquat seedlings was alleviated, and improvements were observed in the water content, root volume, surface area, viability and cell membrane stability of the seedlings. These findings are consistent with previous results in apples [22,29]. This demonstrates that dopamine can effectively enhance the tolerance of loquat seedlings to combined high temperature and drought stress and mitigate stress-induced damage. The mechanism might involve dopamine’s ability to enhance root oxidative capacity and increase the viscosity and elasticity of root tip cells, which stimulates root elongation [19,20], thereby improving root viability. Additionally, it may be related to dopamine-mediated photosynthesis, ROS homeostasis, and osmotic balance.

3.2. Physiological Mechanism of Dopamine’s Alleviation of Combined High Temperature and Drought Stress in Loquat Seedlings

Photosynthesis serves as the foundation of plant growth, providing the primary energy source and organic matter for plants. Its efficiency depends on the capture and conversion of light energy [30]. Chlorophyll and carotenoids are the key pigments responsible for this capture–conversion process [31]. In this study, combined high temperature and drought stress led to a significant decrease in photosynthetic pigment content, as well as reductions in Tr and Gs, ultimately causing a decline in Pn. This phenomenon occurred because high temperature inhibits key enzyme activity in chlorophyll synthesis, thereby impeding chlorophyll production [32]. Additionally, in this study, combined stress reduced the root vitality in loquat seedlings. This reduction weakened mineral nutrient absorption (e.g., nitrogen) and diminished substrate supply for chlorophyll synthesis [33]. Consequently, these factors synergistically exacerbated chlorophyll loss. This photosynthetic impairment resulted in photosynthetic reaction center dysfunction, impaired electron transport, and energy deficits that compromised CO2 utilization [34,35], thereby reducing Pn. Here, Pn decreased while Ci increased, indicating that a non-stomatal limitation was the cause of Pn decline [36]. After dopamine treatment, the loquat seedlings showed significant increases in photosynthetic pigment content, Tr, and Gs, along with stabilized Ci levels, thus resulting in a marked enhancement of Pn. This improvement can be attributed to three synergistic actions of dopamine: (1) It boosts root viability, facilitating the uptake of mineral nutrients (e.g., nitrogen) to provide sufficient substrates for chlorophyll synthesis [37]. (2) It upregulates the activity of key chlorophyll synthesis enzymes and directly scavenges ROS, thereby reducing oxidative damage to chlorophyll porphyrin rings [38,39]. (3) It modulates abscisic acid signaling to moderately open stomata, which elevates the Gs and CO2 supply, ultimately enhancing photosynthetic efficiency [40].
A brief and mild episode of stress triggers the production of ROS, which in turn induces the activities of antioxidant enzymes such as SOD, POD, CAT, and APX. In contrast, prolonged and severe stress causes a sharp and substantial accumulation of ROS that surpasses the capacity of these antioxidant enzymes to alleviate oxidative damage, resulting in a diminished ROS-scavenging ability and ultimately leading to membrane lipid peroxidation injury [11]. In this study, a significant accumulation of ROS, including superoxide anion and H2O2, was observed in loquat seedlings under combined high temperature and drought stress, consistent with prior findings in tomatoes [27] and potatoes [28]. These results indicate that the loquat seedlings suffered severe membrane lipid peroxidation damage under these stress conditions. As a stress response, ROS activated the activities of SOD, POD, CAT, and APX, yet the seedlings’ scavenging capacity was insufficient, leading to massive ROS accumulation [41]. Similar results have been confirmed in watermelons [42]. Under combined high temperature and drought stress, dopamine further enhanced the activities of antioxidant enzymes in the loquat seedlings while reducing ROS accumulation. These results align with previous studies on drought stress in apple trees [22]. This phenomenon may be attributed to dopamine’s dual mechanisms: It acts as a signaling molecule to directly trigger the expression of antioxidant enzyme genes [29] and to modulate stress-responsive transcription factors such as NAC, ERF, WRKY [43], and bHLH [44], thereby boosting antioxidant enzyme activity. Additionally, dopamine serves as a water-soluble antioxidant that directly scavenges ROS [22]. These combined effects effectively improve the stress resistance of loquat seedlings.
Plants accumulate osmoregulatory substances such as betaine, Pro, and SS to prevent excessive cellular water loss, maintain turgor pressure, reduce osmotic potential, and thereby ensure normal functioning of physiological metabolic processes under stress conditions [45,46]. In this study, combined high temperature and drought stress induced an increase in the contents of betaine, Pro, and SS in the loquat seedlings, which aligns with the reported results for winter wheat [47]. This represents another adaptive mechanism of loquat seedlings to combined high temperature and drought stress. However, osmoregulatory substances exhibit only marginal increases under stress conditions, resulting in severe cellular dehydration and consequential plant injury [48]. Following dopamine treatment, the loquat seedlings exhibited substantially elevated levels of betaine, Pro, and SS compared with those exposed to combined stress. This enhancement of osmoregulatory capacity aligns with previous findings of other hormones in grapes [49] and walnuts [50]. One possible explanation is that dopamine enhances osmotic stress-related gene expression and/or activates key biosynthetic enzymes, facilitating sucrose and malic acid accumulation via the upregulation of SPS1 (sucrose phosphate synthase) and MDH (malate dehydrogenase) [22,51]. These mechanisms collectively elevate the content of osmoregulatory substances, thereby maintaining cellular water potential and mitigating stress-induced damage, ultimately improving the thermotolerance and drought tolerance in loquat seedlings.

4. Materials and Methods

4.1. Materials

The experiment was conducted at Sichuan Agricultural University in Wenjiang (30°43′ N, 103°52′ E), China. The loquat plants used were one-year-old ‘Da Wu Xing’ seedlings cultivated in pots (25 cm in diameter and height; one plant per pot). The potting substrate was prepared by mixing field soil and universal nutrient soil at a 1:1 volumetric ratio, with a pH of 6.69, organic matter content of 35.37 g kg−1, alkali-hydrolyzable nitrogen of 143.23 mg kg−1, available phosphorus of 33.14 mg kg−1, and available potassium of 238.65 mg kg−1.

4.2. Experimental Design

First, loquat plants that were disease-free, pest-free, and exhibited uniform growth were selected. They were placed in a GZX-180F-type light incubator (Lichen Instrument Technology Co., Ltd., Shanghai, China) (day/night temperatures: 28 °C/18 °C; light/dark cycle: 12 h/12 h; light intensity: 120 μmol photons m−2 s−1; relative humidity (RH): 60%) for pre-cultivation. After one week, the plants were divided evenly into two groups for pretreatment: (1) foliar application of 100 μmol L−1 of exogenous dopamine (the optimal concentration determined through preliminary experiments, see Supplementary Materials); (2) control group: distilled water spray. The solutions were applied as a foliar spray, evenly coating the leaves until a thin water film (without droplet formation) covered the leaf surface. Applications were repeated every 2 days for a total of 3 applications. Following pretreatment, half of the plants from each group were randomly subjected to combined high temperature and drought stress in an RQH-350-type artificial climate chamber (Jinghong Laboratory Equipment Co., Ltd., Shanghai, China) (day/night temperatures: 38 °C/28 °C; light/dark cycle: 12 h/12 h; light intensity: 120 μmol photons m−2 s−1; RH: 60%). The remaining plants were kept under control conditions in the GZX-180F-type light incubator (day/night temperatures: 28 °C/18 °C; light/dark cycle: 12 h/12 h; light intensity: 120 μmol photons m−2 s−1; RH: 60%).
Four treatments were set up in the study. (1) Control (CK): foliar spray with distilled water under 28 °C/18 °C (day/night); (2) dopamine (DA): foliar spray with 100 μmol L−1 of dopamine solution under 28 °C/18 °C (day/night); (3) high temperature and drought stress (HD): foliar spray with distilled water under 38 °C/28 °C (day/night); (4) high temperature, drought and dopamine treatment (HDDA): foliar spray with 100 μmol L−1 of dopamine solution under 38 °C/28 °C (day/night). The experimental design consisted of three replicates per treatment, with fifteen pots per replicate.
Before pretreatment began, all pots were fully irrigated, and the soil relative water content (RWC) was measured using an SM150T portable moisture sensor (Delta-T Devices Ltd., London, UK). For the HD and HDDA treatments, soil moisture was regulated via the natural water depletion method [52]. When the soil RWC decreased to 40–45% of field capacity, this moisture level was maintained for 10 days using gravimetric measurements. The CK and DA treatments were kept at 70–80% field capacity. After 10 days, the 2nd–4th fully expanded leaves (from the apex) were harvested for physiological analysis.

4.3. Determination of Morphological Parameters

Representative seedlings were randomly selected from each treatment group for photographic documentation. Three randomly selected seedlings per treatment were used for root morphology analysis. The root systems were gently rinsed to remove adherent soil and scanned using a WinRHIZO LA-S plant image analysis system (WinRHIZO Pro LA-S; Wanshen Detection Technology Co., Ltd., Hangzhou, China). The scanned images were analyzed to determine key root morphological parameters, including root length, root surface area, root volume, average root diameter, and number of root tips.

4.4. Determination of Tissue Water Content and Relative Water Content

Root and leaf samples were collected, washed, and blotted dry before measuring fresh weight (FW). The samples were then immersed in water for several hours. After blotting dry, they were re-weighed. This immersion–blotting–weighing cycle was repeated at 1 h intervals until the saturated weight (Wt) stabilized (i.e., consecutive measurements showed negligible differences, indicating full saturation). Subsequently, the samples were placed in an oven at 105 °C for 30 min to deactivate enzymes, then dried at 60 °C until constant weight was achieved. Dry weight (DW) was recorded. The tissue water content (TWC) and relative water content (RWC) were calculated using the following formulas:
TWC (%) = [(FW − DW)/FW] × 100.
RWC (%) = [(FW − DW)/(Wt − DW)] × 100.

4.5. Determination of Photosynthetic Parameters, Photosynthetic Pigment Content, and Root Vitality

The net photosynthetic rate (Pn, unit: μmol m−2 s−1), transpiration rate (Tr, unit: mmol m−2 s−1), stomatal conductance (Gs, unit: mol m−2 s−1), and intercellular CO2 concentration (Ci, unit: μmol mol−1) were measured on the second fully expanded leaf below the apical meristem using an LI-6800 portable photosynthesis system (LI-COR, Lincoln, NE, USA). Measurements were conducted between 9:00 and 11:30 a.m. under controlled conditions: leaf chamber temperature (20–25 °C), CO2 concentration (380–420 μmol mol−1, regulated via an integrated CO2 cylinder), relative humidity (60%), and photosynthetically active radiation (800 μmol photons m−2 s−1).
Chlorophyll and total carotenoid contents were spectrophotometrically quantified in fresh leaf tissue according to Lichtenthaler [53] with modifications. Leaf samples were homogenized in 80% (v/v) aqueous acetone under subdued light conditions. The homogenate was centrifuged at 10,000× g for 10 min (4 °C), and the supernatant was collected for spectral analysis. Absorbance measurements were performed at 663 nm, 646 nm, and 470 nm using a double-beam spectrophotometer (Cary 3500-type, Agilent Technologies, Inc. Santa Clara, CA USA).
Pigment concentrations were calculated using the following equations:
Total Chlorophyll (mg L−1) = 7.15 × A663 + 18.71 × A646
Total carotenoids (mg L−1) = [1000 × A470 − 1.82 × (12.25 × A663 − 2.79 × A646) − 85.02 × (21.50 × A646 − 5.10 × A663)]/198
The final pigment content was expressed per unit fresh weight:
Pigment content (mg g−1 FW) = calculated concentration × extract volume/sample fresh weight
Root vitality was quantified via the triphenyl tetrazolium chloride (TTC) reduction method [54]. Fresh root apical segments (0.5 g, 5 mm length) were incubated in 6 mL of reaction solution containing 0.4% (w/v) TTC in a 0.1 M phosphate buffer (pH 7.4) at 37 °C in complete darkness for 2 h. The reaction was terminated with 2 mL of 1 M H2SO4. Formazan was extracted in ethyl acetate (final volume of 10 mL) and centrifuged (10,000× g, 4 °C, 10 min). The absorbance of the supernatant was measured at 485 nm using ethyl acetate as the blank.
A formazan standard curve was generated by dissolving 10 mg of TTF (1,3,5-triphenylformazan) in 100 mL of ethyl acetate (100 μg mL−1 stock solution). Serial dilutions (0–100 μg mL−1) were prepared, and A485 was measured for each standard.
The TTC reduction capacity was calculated as follows:
Root vitality (μg g−1 h−1 FW) = formazan concentration from standard curve ×
extraction volume/(fresh weight × incubation time)

4.6. Evaluation of Membrane Damage

Lipid peroxidation was assessed by quantifying the thiobarbituric acid-reactive substances (TBARS) content using the thiobarbituric acid (TBA) method [55]. Fresh leaf tissue (0.5 g) was homogenized in 5% (w/v) trichloroacetic acid (TCA) and centrifuged at 4000× g for 10 min. The supernatant was reacted with 0.67% (w/v) TBA in a boiling water bath for 30 min. After rapid cooling, absorbance was measured at 450 nm, 532 nm, and 600 nm. The TBARS content was calculated as follows:
TBARS (μmol g−1 FW) = [6.45 × (A532 − A600) − 0.56 × A450] × extract volume/fresh weight
Membrane permeability was assessed via electrolyte leakage (EL) using the method of Ifeduba [56]. Uniform leaf discs (5 mm diameter) were excised with a cork borer (major veins excluded), rinsed in distilled water to remove surface electrolytes, and immersed in 10 mL of distilled water. Samples were incubated at 25 °C for 1 h with gentle shaking. Initial conductivity (C1) was measured using a conductivity meter. Discs were then boiled in the same solution for 10 min and cooled to 25 °C, and then final conductivity (C2) was recorded. EL was calculated as follows:
EL (%) = C1/C2 ×100.

4.7. Determination of ROS

The superoxide anion production rate was quantified using the hydroxylamine oxidation method [57]. Fresh leaf tissue (0.5 g) was homogenized in ice-cold phosphate-buffered saline (PBS) and centrifuged at 12,000× g for 20 min (4 °C). The supernatant (1 mL) was reacted with 1 mL of a hydroxylamine hydrochloride solution for 1 h at 25 °C. Subsequently, 1 mL of sulfanilamide and 1 mL of α-naphthylamine were added, followed by incubation at 25 °C for 30 min. Absorbance was measured at 530 nm. A standard curve was generated by preparing NaNO2 solutions (0–50 μM in PBS) and processing them identically to the samples. The superoxide anion production rate was calculated as follows:
Superoxide anion production rate (μmol g−1 min−1 FW) = (nitrite concentration from standard curve −
endogenous nitrite control) × extraction volume/(fresh weight × hydroxylamine incubation time)
The hydrogen peroxide (H2O2) content was measured using a commercial assay kit (Jiancheng Biotech Inc., Nanjing, China). Fresh leaf tissue (0.1 g) was homogenized in 1 mL of an ice-cold extraction buffer (Reagent 1) and centrifuged at 8000× g for 10 min (4 °C). The supernatant was collected. To 0.5 mL of the supernatant, 100 μL of a reaction initiator (Reagent 2) and 200 μL of a chromogenic agent (Reagent 3) were added. The mixture was centrifuged at 4000× g for 10 min at 25 °C. After carefully discarding the supernatant, the pellet was resuspended in 1 mL of a developer solution (Reagent 4) and incubated at 25 °C for 5 min. Absorbance was measured at 415 nm against a reagent blank. The H2O2 standard was used instead of the sample in the standard preparation group.
Calculate net absorbance: ΔATest = ATest − ABlank ΔAStandard = AStandard − ABlank
H2O2 (μmol g−1 FW) = (ΔATest/ΔAStandard)/fresh weight

4.8. Determination of Antioxidant Enzyme Activities

Superoxide dismutase (SOD) activity was determined with the nitroblue tetrazolium (NBT) photochemical reduction method [58]. Fresh leaf tissue (0.5 g) was homogenized in 5 mL of an ice-cold phosphate buffer (50 mM, pH 7.8) and centrifuged at 12,000× g for 15 min (4 °C). The supernatant (crude enzyme extract) was kept on ice. The reaction mixtures contained 1.5 mL of a phosphate buffer (50 mM, pH 7.8), 0.3 mL of L-methionine (13 mM), 0.3 mL of NBT (75 μM), 0.3 mL of EDTA-Na2 (0.1 mM), 0.3 mL of riboflavin (2 μM), 50 μL of an enzyme extract, and 0.25 mL of distilled water. The controls replaced the enzyme extract with a phosphate buffer. Tubes were illuminated (75 μmol photons m−2 s−1, 30 min), with one dark control wrapped in foil. Absorbance at 560 nm was measured. One unit of SOD activity (U) was defined as the amount of enzyme required to cause a 50% inhibition of NBT photochemical reduction relative to the light-exposed control. SOD activity was calculated using the following equation:
SOD activity (U g−1 FW) = [(absorbance of light-exposed control − absorbance of sample) × reaction
volume]/[0.5 × absorbance of light-exposed control × fresh weight × volume of enzyme extract in reaction]
Peroxidase (POD) activity was determined by guaiacol oxidation [59]. Fresh leaf tissue (0.5 g) was homogenized in an ice-cold phosphate buffer (50 mM, pH 7.0) and centrifuged at 10,000 × g for 15 min (4 °C). The reaction mixtures contained 700 μL of a phosphate buffer (0.1 M, pH 7.0), 100 μL of guaiacol (20 mM), and 100 μL of an enzyme extract. The reactions were initiated with 100 μL of H2O2 (10 mM), and absorbance at 470 nm was recorded every 10 s for 1 min using a spectrophotometer. POD activity was calculated from the initial linear rate of change (ΔA470). One unit of POD activity (U) was defined as a change of 0.01 per minute in A470.
POD activity (U g−1 FW) = ΔA470 × reaction volume/(0.01 × fresh weight × reaction time × volume of
enzyme extract in reaction)
Catalase (CAT) activity was measured by H2O2 decomposition at 240 nm [58]. Fresh leaf tissue (0.5 g) was homogenized in an ice-cold phosphate buffer (50 mM, pH 7.0) containing 1 mM of EDTA and 1% (w/v) polyvinylpyrrolidone (PVP), followed by centrifugation at 12,000 × g for 15 min (4 °C). A supernatant (50 μL) was added to 1.95 mL of a phosphate buffer (50 mM, pH 7.0) in a quartz cuvette. The reactions were initiated with 1 mL of H2O2 (15 mM), and absorbance decrease at 240 nm (ΔA240) was recorded for 1 min. CAT activity was derived from the initial reaction rate (ΔA240). One unit of CAT activity (U) was defined as a change of 0.1 per minute in A240.
CAT activity (U g−1 FW) = ΔA240 × reaction volume/(0.1 × fresh weight ×
reaction time × volume of enzyme extract in reaction)
Ascorbate peroxidase (APX) activity was quantified via ascorbate oxidation at 290 nm [58]. Fresh leaf tissue (0.5 g) was homogenized in an ice-cold potassium phosphate buffer (50 mM, pH 7.0) containing 1 mM of ascorbate, 1 mM of EDTA, and 1% PVP, then centrifuged at 12,000× g for 15 min (4 °C). The reaction mixtures (1 mL final volume) contained 50 mM of a phosphate buffer (pH 7.0), 0.5 mM of ascorbate, and 0.1 mM of EDTA. After adding 50 μL of a supernatant, the reactions were initiated with 0.1 mL of H2O2 (1 mM), and absorbance decrease at 290 nm (ΔA290) was recorded for 1 min. APX activity was calculated from the initial reaction rate (ΔA290).
APX activity (μmol g−1 min−1 FW) = ΔA290 × reaction volume/(2.8 × 103 × fresh weight ×
cuvette path length × reaction time × volume of enzyme extract in reaction)

4.9. Analysis of Osmotic Regulating Substances

The proline (Pro) content was determined using the acidic ninhydrin method [60]. Fresh leaf tissue (0.5 g) was homogenized in 10 mL of 3% (w/v) sulfosalicylic acid and centrifuged at 10,000× g for 10 min. A 2 mL aliquot of the supernatant was reacted with 2 mL of an acid–ninhydrin reagent (1.25 g of ninhydrin dissolved in 30 mL of glacial acetic acid and 20 mL of 6 M H3PO4) and 2 mL of glacial acetic acid. The mixture was incubated at 100 °C for 1 h, cooled on ice, and extracted with 4 mL of toluene. After phase separation (10 min), the absorbance of the toluene layer was measured at 520 nm. A standard curve was generated concurrently using L-proline solutions (0–20 μg mL−1) processed identically to the samples. The absorbance values were plotted against proline concentration to derive a linear regression equation (y = ax + b). The proline content was calculated as follows:
Proline (μg g−1 FW) = (proline concentration determined from the standard curve ×
volume of toluene used for extraction)/fresh weight
The betaine content was analyzed using a commercial kit (Shanghai Yaji Biotechnology Co., Ltd., Shanghai, China). Dried leaf tissue (40 mg) was extracted in 1.6 mL of distilled water (60 °C, 30 min, shaking), followed by centrifugation at 10,000× g (25 °C, 10 min). A 300 μL aliquot of a supernatant (Test) and 300 μL of distilled water (Blank) were transferred to separate reaction tubes. To each tube, 500 μL of a precipitation reagent (Reagent I) was added. After vortexing, tubes were incubated at 4 °C for 2 h. Following centrifugation (12,000× g, 10 min, 25 °C), the supernatants were discarded. The pellets were washed with 500 μL of washing solution (Reagent II; 80% ethanol/ether mixture) and re-centrifuged under identical conditions, and then the supernatants were completely removed. The residual solvent was evaporated for 15 min in a fume hood. The pellets were dissolved in 300 μL color developer (Reagent III) by vigorous vortexing. Aliquots (200 μL) were transferred to a 96-well plate, and absorbance was measured at 525 nm using a microplate reader pre-equilibrated to 25 °C. A standard curve (0–200 μg mL−1 betaine) was processed identically to samples. The betaine content was calculated as follows:
Betaine (mg g−1 DW) = [betaine concentration from standard curve × extraction volume]/dry weight
Soluble sugars (SS) were assessed with the anthrone colorimetric method [61]. Fresh tissue (0.5 g) was extracted by boiling in 10 mL of distilled water for 30 min. After centrifugation (3000× g, 10 min), the pellet was re-extracted with 10 mL boiling water. Combined supernatants were brought to a final volume of 50 mL with distilled water. For the colorimetric reaction, 0.5 mL of extract was mixed with 1.5 mL of distilled water, 0.5 mL of ice-cold anthrone reagent (0.2% w/v in concentrated H2SO4), and 5 mL of concentrated H2SO4. The mixture was vortexed immediately and heated at 100 °C for exactly 1 min. After rapid cooling to room temperature, the absorbance measured at 630 nm. A standard curve was generated by preparing glucose solutions (0–200 µg mL−1) and processing them identically to the samples. Soluble sugar concentration was determined from the standard curve using the formula:
SS (mg g−1 FW) = (sugar concentration from standard curve × extract volume)/
(volume of extract assayed × fresh weight)

4.10. Statistical Analysis

All data are presented as the means ± SE of three independent replicates. The data were analyzed with a one-way ANOVA using SPSS 25.0 (IBM, Inc., Armonk, NY, USA). Tukey’s multiple comparison test was used to detect differences between treatments at p < 0.05. All figures were plotted using Origin 9.0 (OriginLab Corporation, Northampton, MA, USA).

5. Conclusions

Dopamine can enhance the tolerance of loquat seedlings to combined high temperature and drought stress. The primary mechanisms of action are as follows: (1) Dopamine improves the photosynthetic efficiency of loquat seedlings by maintaining photosynthetic pigment levels and increasing stomatal conductance. (2) It mitigates membrane lipid peroxidation damage by boosting the activity of antioxidant enzymes in the seedlings, thus effectively scavenging the ROS generated within the plants. (3) Dopamine promotes the accumulation of osmotic substances to regulate osmotic potential, thereby enhancing the seedlings’ high temperature and drought tolerance. In addition, dopamine may regulate this tolerance in loquat seedlings either through its own metabolites or by regulating other plant hormone levels, the specific mechanisms for these pathways requiring further investigation.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/plants14172650/s1, Figure S1: Root viability and SPAD in loquat seedlings; Figure S2: TBARS content and EL in loquat seedlings; Figure S3: Photosynthetic parameters in loquat seedlings.

Author Contributions

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

Funding

This research was funded by the SICHUAN PROVINCIAL DEPARTMENT OF SCIENCE AND TECHNOLOGY, grant number 2022ZHXC0097.

Data Availability Statement

The original contributions presented in this study are included in the article/Supplementary Materials. Further inquiries can be directed to the corresponding author.

Conflicts of Interest

The authors declare no conflicts of interest.

Abbreviations

The following abbreviations are used in this manuscript:
DADopamine
ROSReactive oxygen species
TBARSThiobarbituric acid-reactive substances
ELElectrolyte leakage
RHRelative humidity
SODSuperoxide dismutase
CATCatalase
PODPeroxidase
APXAscorbate peroxidase
HDHigh temperature and drought
HDDADopamine was applied under high temperature and drought stress
PnNet photosynthetic rate
TrTranspiration rate
GsStomatal conductance
CiIntercellular CO2 concentration
ProProline
SSSoluble sugar
SPSSucrose phosphate synthase
MDHMalate dehydrogenase
RWCRelative water content
PARPhotosynthetically active radiation

References

  1. Yang, M.X.; Mou, Y.L.; Meng, Y.R.; Liu, S.; Peng, C.H.; Zhou, X.L. Modeling the effects of precipitation and temperature patterns on agricultural drought in China from 1949 to 2015. Sci. Total Environ. 2020, 711, 135139. [Google Scholar] [CrossRef]
  2. Zandalinas, S.I.; Fritschi, F.B.; Mittler, R. Signal transduction networks during stress combination. J. Exp. Bot. 2020, 71, 1734–1741. [Google Scholar] [CrossRef]
  3. Habti, E.A.; Fleury, D.; Jewell, N.; Garnett, T.; Tricker, P.J. Tolerance of combined drought and heat stress is associated with transpiration maintenance and water soluble carbohydrates in wheat grains. Front. Plant Sci. 2020, 11, 568693. [Google Scholar] [CrossRef]
  4. Ostmeyer, T.; Parker, N.; Jaenisch, B.; Alkotami, L.; Bustamante, C.; Jagadish, S.V.K. Impacts of heat, drought, and their interaction with nutrients on physiology, grain yield, and quality in field crops. Plant Physiol. Rep. 2020, 25, 549–568. [Google Scholar] [CrossRef]
  5. China Meteorological Administration Climate Change Centre. Blue Book on Climate Change in China (2023); Science Press: Beijing, China, 2023; pp. 33–40. (In Chinese)
  6. Pawłowska, A.M.; Żurek, N.; Kapusta, I.; Leo, M.D.; Braca, A. Antioxidant and antiproliferative activities of phenolic extracts of Eriobotrya japonica (Thunb.) Lindl. fruits and leaves. Plants 2023, 12, 3221. [Google Scholar] [CrossRef]
  7. Wang, S.; Wang, Q.; Zhang, Y.; Qie, H.; Wang, H. Identification of two new S-RNases and molecular S -genotyping of twenty loquat cutivars [Eriobotrya japonica (Thunb.) Lindl.]. Sci. Hortic. 2017, 218, 48–55. [Google Scholar] [CrossRef]
  8. Wang, P. Practical Techniques for High-Quality and Efficient Cultivation of Loquat; China Agriculture Press: Beijing, China, 2008; pp. 26–30. (In Chinese) [Google Scholar]
  9. Wang, D.; Chen, Q.; Chen, W.; Guo, Q.; Xia, Y.; Wang, S.; Jing, D.; Liang, G. Physiological and transcription analyses reveal the regulatory mechanism of melatonin in inducing drought resistance in loquat (Eriobotrya japonica Lindl.) seedlings. Environ. Exp. Bot. 2021, 181, 104291. [Google Scholar] [CrossRef]
  10. Ji, W.; Luo, H.; Song, Y.; Hong, E.; Li, Z.; Lin, B.; Fan, C.; Wang, H.; Song, X.; Jin, S.; et al. Changes in Photosynthetic Characteristics of Paeonia suffruticosa under High Temperature Stress. Agronomy 2022, 12, 1203. [Google Scholar] [CrossRef]
  11. Ru, C.; Hu, X.; Chen, D.; Wang, W.; Song, T. Heat and drought priming induce tolerance to subsequent heat and drought stress by regulating leaf photosynthesis, root morphology, and antioxidant defense in maize seedlings. Environ. Exp. Bot. 2022, 202, 105010. [Google Scholar] [CrossRef]
  12. Hlahla, J.M.; Mafa, M.S.; Merwe, R.V.D.; Moloi, M.J. Tolerance to combined drought and heat stress in edamame is associated with enhanced antioxidative responses and cell wall modifications. Physiol. Plant. 2025, 177, e70187. [Google Scholar] [CrossRef]
  13. Berto, S.D.D.C.; Bezerra, F.S.; Morais, J.E.F.D.; Leal, L.Y.D.C.; Paulino, M.K.S.S.; Araújo, C.A.F.; Gonçalves, W.J.F.; Nascimento, C.W.A.D.; Souza, E.R.D. Silicon mitigates the effects of water deficit in ratoon sugarcane and enhances leaf water potential, osmotic adjustment, and biomass production. J. Plant Growth Regul. 2025, in press. [Google Scholar] [CrossRef]
  14. Zha, Q.; Xi, X.; He, Y.; Yin, X.; Jiang, A. Effect of short-time high-temperature treatment on the photosynthetic performance of different heat-tolerant grapevine cultivars. Photochem. Photobiol. 2021, 97, 763–769. [Google Scholar] [CrossRef] [PubMed]
  15. Liu, D.; Guo, H.; Yan, L.P.; Gao, L.; Zhai, S.; Xu, Y. Physiological, photosynthetic and stomatal ultrastructural responses of Quercus acutissima seedlings to drought stress and rewatering. Forests 2024, 15, 71. [Google Scholar] [CrossRef]
  16. Li, X.; Wang, S.; Zhu, L.; Zhang, P.; Qi, H.; Zhang, K.; Sun, H.; Zhang, Y.; Lei, X.; Li, A.; et al. Leaf hydraulic decline coordinates stomatal and photosynthetic limitations through anatomical adjustments under drought stress in cotton. Front. Plant Sci. 2025, 16, 1622308. [Google Scholar] [CrossRef] [PubMed]
  17. Gugliuzza, G.; Talluto, G.; Martinelli, F.; Farina, V.; Bianco, R.L. Water deficit affects the growth and leaf metabolite composition of young loquat plants. Plants 2020, 9, 274. [Google Scholar] [CrossRef]
  18. Chen, Y.; Deng, C.; Xu, Q.; Chen, X.; Jiang, F.; Zhang, Y.; Hu, W.; Zheng, S.; Su, W.; Jiang, J. Integrated analysis of the metabolome, transcriptome and miRNome reveals crucial roles of auxin and heat shock proteins in the heat stress response of loquat fruit. Sci. Hortic. 2022, 294, 110764. [Google Scholar] [CrossRef]
  19. Ahammed, G.J.; Li, X. Dopamine-induced abiotic stress tolerance in horticultural plants. Sci. Hortic. 2023, 307, 111506. [Google Scholar] [CrossRef]
  20. Jiao, X.; Li, Y.; Zhang, X.; Liu, C.; Liang, W.; Li, C.; Ma, F.; Li, C. Exogenous dopamine application promotes alkali tolerance of apple seedlings. Plants 2019, 8, 580. [Google Scholar] [CrossRef]
  21. Wang, X.; Xu, J.; Luo, X.; Luo, Y.; Lin, L.; Huang, K.; Zhang, H.; Deng, Q. Effects of exogenous dopamine on photosynthesis characteristics and antioxidant system of loquat seedlings under drought stress. J. Northwest AF Univ. (Nat. Sci. Ed.) 2025, 53, 109–116. (In Chinese) [Google Scholar]
  22. Gao, T.; Zhang, Z.; Liu, X.; Wu, Q.; Chen, Q.; Liu, Q.; Nocker, S.; Ma, F.; Li, C. Physiological and transcriptome analyses of the effects of exogenous dopamine on drought tolerance in apple. Plant Physiol. Bioch. 2020, 148, 260–272. [Google Scholar] [CrossRef]
  23. Raza, A.; Salehi, H.; Rahman, M.A.; Zahid, Z.; Haghjou, M.M.; Najafi-Kakavand, S.; Charagh, S.; Osman, H.S.; Albaqami, M.; Zhuang, Y.; et al. Plant hormones and neurotransmitter interactions mediate antioxidant defenses under induced oxidative stress in plants. Front. Plant Sci. 2022, 13, 961872. [Google Scholar] [CrossRef] [PubMed]
  24. Hodge, A.; Berta, G.; Doussan, C.; Merchan, F.; Crespi, M. Plant root growth, architecture and function. Plant Soil 2009, 321, 153–187. [Google Scholar] [CrossRef]
  25. Shang, Y.K.; Liu, S.Y.; Jiang, Y.Y.; Wang, L.; Liao, J.Q.; Yang, R.W.; Zhang, L. Effects on antioxidant enzymes, lipid peroxidation and photosynthetic of burdock (Arctium lappa L.) under water stress. Biol. Bull. 2022, 49, 114–123. [Google Scholar] [CrossRef]
  26. Liu, H.; Bao, G.; Dou, Z.; Liu, H.; Bai, J.; Chen, Y.; Yuan, Y.; Zhang, X.; Xi, J. Response characteristics of highland barley under freeze-thaw, drought and artemisinin stresses. BMC Plant Biol. 2022, 22, 126. [Google Scholar] [CrossRef]
  27. Zhou, R.; Kong, L.; Yu, X.; Ottosen, C.O.; Zhao, T.; Jiang, F.; Wu, Z. Oxidative damage and antioxidant mechanism in tomatoes responding to drought and heat stress. Acta Physiol. Plant. 2019, 41, 20. [Google Scholar] [CrossRef]
  28. Wang, X.; Shi, M.; Zhang, R.; Wang, Y.; Zhang, W.; Qin, S.; Kang, Y. Dynamics of physiological and biochemical effects of heat, drought and combined stress on potato seedlings. Chem. Biol. Technol. Agric. 2024, 11, 109. [Google Scholar] [CrossRef]
  29. Zhang, Z.; Tang, Z.; Jing, G.; Gao, S.; Liu, C.; Ai, S.; Liu, Y.; Liu, Q.; Li, C.; Ma, F. Dopamine confers cadmium tolerance in apples by improving growth, reducing reactive oxygen species, and changing secondary metabolite levels. Environ. Exp. Bot. 2023, 208, 105264. [Google Scholar] [CrossRef]
  30. Parkash, V.; Snider, J.L.; Awori, K.J.; Pilon, C.; Brown, N.; Almeida, I.B.; Tishchenko, V. Peanut (Arachis hypogaea L.) growth and photosynthetic response to high and low temperature extremes. Plant Physiol. Bioch. 2025, 220, 109479. [Google Scholar] [CrossRef] [PubMed]
  31. Fiutak, G.; Mohammadi, X.; Filipczak-Fiutak, M.; Jarzebski, M.; Sterczynska, M.; Pratap-Singh, A. Pre-harvest applications of pulsed light increases vitamin C, chlorophyll, carotenoids and proteins in alfalfa sprouts. Sci. Hortic. 2024, 332, 113200. [Google Scholar] [CrossRef]
  32. Chen, J.H.; Tang, M.T.; Jin, X.Q.; Li, H.; Chen, L.S.; Wang, Q.L.; Sun, A.Z.; Yi, Y. Regulation of Calvin–Benson cycle enzymes under high temperature stress. aBIOTECH 2022, 3, 65–77. [Google Scholar] [CrossRef]
  33. Zhao, C.; Wu, T.; Chang, A.; Fan, Z.; Li, X.; Li, C.; Zheng, M.; Sun, Y.; Wan, X.; Meng, J.; et al. Miscanthus sinensis ‘Gracillimus’ shows strong submergence tolerance implying its potential utilization in construction of ecological ditches. Agronomy 2025, 15, 109. [Google Scholar] [CrossRef]
  34. Lipiec, J.; Doussan, C.; Nosalewicz, A.; Kondracka, K. Effect of drought and heat stresses on plant growth and yield: A review. Int. Agrophys. 2013, 27, 463–477. [Google Scholar] [CrossRef]
  35. Ma, C.; Feng, Y.; Wang, J.; Zheng, B.; Wang, X.; Jiao, N. Integrative physiological, transcriptome, and proteome analyses provide insights into the photosynthetic changes in maize in a maize-peanut intercropping system. Plants 2024, 13, 65. [Google Scholar] [CrossRef]
  36. Lu, Z.; Ren, T.; Li, Y.; Cakmak, I.; Lu, J. Nutrient limitations on photosynthesis: From individual to combinational stresses. Trends Plant Sci. 2025, 25, 1360–1385. [Google Scholar] [CrossRef] [PubMed]
  37. Liu, X.M.; Gao, T.T.; Zhang, Z.J.; Tan, K.X.; Jin, Y.B.; Zhao, Y.J.; Ma, F.W.; Li, C. The mitigation effects of exogenous dopamine on low nitrogen stress in Malus hupehensis. J. Integr. Agric. 2020, 19, 2709–2724. [Google Scholar] [CrossRef]
  38. Ji, Z.; Liu, Z.; Han, Y.; Sun, Y. Exogenous dopamine promotes photosynthesis and carbohydrate metabolism of downy mildew-infected cucumber. Sci. Hortic. 2022, 295, 110842. [Google Scholar] [CrossRef]
  39. Kanazawa, K.; Sakakibara, H. High content of dopamine, a strong antioxidant, in Cavendish banana. J. Agric. Food Chem. 2000, 48, 844–848. [Google Scholar] [CrossRef] [PubMed]
  40. Cao, Y.; Du, P.; Zhang, J.; Ji, J.; Xu, J.; Liang, B. Dopamine alleviates cadmium stress in apple trees by recruiting beneficial microorganisms to enhance the physiological resilience revealed by high-throughput sequencing and soil metabolomics. Hortic. Res. 2023, 10, uhad112. [Google Scholar] [CrossRef]
  41. Khan, T.A.; Ahmad, A.; Saeed, T.; Yusuf, M.; Faisal, M.; Alatar, A.A. Investigating the influence of selenium and epibrassinolide on antioxidant activity, proline accumulation, and protein expression profiles in wheat plants experiencing heat and drought stress. Front. Plant Sci. 2024, 15, 1441483. [Google Scholar] [CrossRef] [PubMed]
  42. Ren, K.; Tang, T.; Kong, W.; Su, Y.; Wang, Y.; Cheng, H.; Yang, Y.; Zhao, X. Response of watermelon to drought stress and its drought-resistance evaluation. Plants 2025, 14, 1289. [Google Scholar] [CrossRef] [PubMed]
  43. Gao, T.; Wang, Y.; Liu, Y.; Ma, M.; Li, X.; Zhang, D.; Ding, K.; Li, C.; Zhou, Y.; Ma, F. Overexpression of tyrosine decarboxylase (MdTYDC) enhances drought tolerance in Malus domestica. Sci. Hortic. 2021, 289, 110425. [Google Scholar] [CrossRef]
  44. Liu, X.; Li, J.; Cao, H.; Tian, X.; Liu, S.; Liu, C.; Zhang, Z.; Mao, K.; Ma, F.; Li, C. Dopamine regulates its own synthesis via MdORG2 to improve low-nitrogen tolerance in apple plants. Plant Cell Environ. 2025, 48, 470–488. [Google Scholar] [CrossRef] [PubMed]
  45. Wang, M.; Gong, S.; Fu, L.; Hu, G.; Li, G.; Hu, S.; Yang, J. The involvement of antioxidant enzyme system, nitrogen metabolism and osmoregulatory substances in alleviating salt stress in inbred maize lines and hormone regulation mechanisms. Plants 2022, 11, 1547. [Google Scholar] [CrossRef]
  46. Wen, Y.; Zhou, L.J.; Xu, Y.J.; Hashem, A.; Abd_Allah, E.F.; Wu, Q.S. Growth performance and osmolyte regulation of drought-stressed walnut plants are improved by mycorrhiza. Agriculture 2024, 14, 367. [Google Scholar] [CrossRef]
  47. Chen, R.; Hu, X.; Chen, D.; Wang, W.; Zhen, J. Photosynthetic, antioxidant activities, and osmoregulatory responses in winter wheat differ during the stress and recovery periods under heat, drought, and combined stress. Plant Sci. 2023, 327, 111557. [Google Scholar] [CrossRef]
  48. Han, Z.; Zhang, Y.; Zhang, H.; Luo, W.; Chen, A. Effects of drought stress on chloroplast ultrastructure and physiological characteristics of Seriphidiumtran siliense seedlings. Pratac. Sci. 2025, 42, 1403–1413. (In Chinese) [Google Scholar]
  49. Niu, X.; Deqing, C.; Liang, D. Effects of exogenous melatonin and abscisic acid on osmotic adjustment substances of ‘Summer Black’ grape under drought stress. IOP Conf. Ser. Earth Environ. Sci. 2019, 295, 012012. [Google Scholar] [CrossRef]
  50. Zhong, Y.; Liu, C.; Wei, B.; Zhang, J.; An, Y.; Wang, L. Exogenous 5-aminolevulinic acid promotes osmotic stress tolerance of walnuts by modulating photosynthesis, osmotic adjustment and antioxidant systems. Forests 2023, 14, 1789. [Google Scholar] [CrossRef]
  51. Dai, Z.; Yang, J.; Li, D.; Chen, Y.; Shao, Z.; Wang, Y. The impact of dopamine on the photosynthetic performance and quality of Shine Muscat grape in the Hexi Corridor of Gansu province. J. Fruit Sci. 2025, 42, 94–111. (In Chinese) [Google Scholar]
  52. Ju, Y.L.; Yue, X.F.; Zhao, X.F.; Zhao, H.; Fang, Y.L. Physiological, micro-morphological and metabolomic analysis of grapevine (Vitis vinifera L.) leaf of plants under water stress. Plant Physiol. Bioch. 2018, 130, 501–510. [Google Scholar] [CrossRef]
  53. Lichtenthaler, H.K. Chlorophylls and carotenoids: Pigments of photosynthetic biomembranes. Methods Enzymol. 1987, 148, 350–382. [Google Scholar] [CrossRef]
  54. Clemensson-Lindell, A. Triphenyltetrazolium chloride as an indicator of fine-root vitality and environmental stress in coniferous forest stands: Applications and limitations. Plant Soil 1994, 159, 297–300. [Google Scholar] [CrossRef]
  55. Heath, R.L.; Packer, L. Photoperoxidation in isolated chloroplasts I. Kinetics and stoichiometry of fatty acid peroxidation. Arch. Biochem. Biophys. 1986, 125, 189–198. [Google Scholar] [CrossRef]
  56. Jia, Y.; Yue, P.; Li, K.; Xie, Y.; Li, T.; Pu, Y.; Xu, X.; Wang, G.; Zhang, S.; Li, Y.; et al. Mechanisms of cadmium tolerance and detoxification in two ornamental plants. Agronomy 2023, 13, 2039. [Google Scholar] [CrossRef]
  57. Yuan, Y.X. The production rate and malondialdehyde content of different tissue superoxide anions in wetland Phrgmintes australis. Sci. Technol. Eng. 2019, 19, 40–43. (In Chinese) [Google Scholar]
  58. Liu, T.; Jiao, X.; Yang, S.; Zhang, Z.; Ye, X.; Li, J.; Qi, H.; Hu, X. Crosstalk between GABA and ALA to improve antioxidation and cell expansion of tomato seedling under cold stress. Environ. Exp. Bot. 2020, 180, 104228. [Google Scholar] [CrossRef]
  59. Tian, Z.M.; He, J.D.; Wang, Z.Y.; Zhang, Z.; Quinet, M.; Meng, Y. Exogenous melatonin enhances drought tolerance and germination in common buckwheat seeds through the coordinated effects of antioxidant and osmotic regulation. BMC Plant Biol. 2025, 25, 613. [Google Scholar] [CrossRef]
  60. Lee, M.R.; Kim, C.S.; Park, T.; Choi, Y.S.; Lee, K.H. Optimization of the ninhydrin reaction and development of a multiwell plate-based high-throughput proline detection assay. Anal. Biochem. 2018, 556, 57–62. [Google Scholar] [CrossRef]
  61. Zhou, J.; Yang, S.; Ma, Y.; Liu, Z.; Tu, H.; Wang, H.; Zhang, J.; Chen, Q.; He, W.; Li, M.; et al. Soluble sugar and organic acid composition and flavor evaluation of Chinese cherry fruits. Food Chem. X 2023, 20, 100953. [Google Scholar] [CrossRef]
Plants 14 02650 g001
Figure 1. Plant phenotypes of loquat [Eriobotrya japonica (Thunb.) Lindl.] seedlings. CK: control, foliar spray with distilled water; DA: foliar spray with dopamine solution; HD: combined high temperature and drought stress, foliar spray with distilled water; HDDA: combined high temperature and drought stress, foliar spray with dopamine solution. Scale bar = 20 cm.
Figure 1. Plant phenotypes of loquat [Eriobotrya japonica (Thunb.) Lindl.] seedlings. CK: control, foliar spray with distilled water; DA: foliar spray with dopamine solution; HD: combined high temperature and drought stress, foliar spray with distilled water; HDDA: combined high temperature and drought stress, foliar spray with dopamine solution. Scale bar = 20 cm.
Plants 14 02650 g001
Plants 14 02650 g002
Figure 2. Root viability (a), TBARS content (b) and EL (c) of loquat [Eriobotrya japonica (Thunb.) Lindl.] seedlings. Data are presented as the means ± SE (n = 3). Different letters on the bars indicate significant differences (p < 0.05) among different treatments based on Tukey’s HSD test. CK: control, foliar spray with distilled water; DA: foliar spray with dopamine solution; HD: combined high temperature and drought stress, foliar spray with distilled water; HDDA: combined high temperature and drought stress, foliar spray with dopamine solution.
Figure 2. Root viability (a), TBARS content (b) and EL (c) of loquat [Eriobotrya japonica (Thunb.) Lindl.] seedlings. Data are presented as the means ± SE (n = 3). Different letters on the bars indicate significant differences (p < 0.05) among different treatments based on Tukey’s HSD test. CK: control, foliar spray with distilled water; DA: foliar spray with dopamine solution; HD: combined high temperature and drought stress, foliar spray with distilled water; HDDA: combined high temperature and drought stress, foliar spray with dopamine solution.
Plants 14 02650 g002
Plants 14 02650 g003
Figure 3. Superoxide anion production rate (a) and H2O2 content (b) of loquat [Eriobotrya japonica (Thunb.) Lindl.] seedlings. Data are presented as the means ± SE (n = 3). Different letters on the bars indicate significant differences (p < 0.05) among different treatments based on Tukey’s HSD test. CK: control, foliar spray with distilled water; DA: foliar spray with dopamine solution; HD: combined high temperature and drought stress, foliar spray with distilled water; HDDA: combined high temperature and drought stress, foliar spray with dopamine solution.
Figure 3. Superoxide anion production rate (a) and H2O2 content (b) of loquat [Eriobotrya japonica (Thunb.) Lindl.] seedlings. Data are presented as the means ± SE (n = 3). Different letters on the bars indicate significant differences (p < 0.05) among different treatments based on Tukey’s HSD test. CK: control, foliar spray with distilled water; DA: foliar spray with dopamine solution; HD: combined high temperature and drought stress, foliar spray with distilled water; HDDA: combined high temperature and drought stress, foliar spray with dopamine solution.
Plants 14 02650 g003
Plants 14 02650 g004
Figure 4. Activities of SOD (a), POD (b), CAT (c), and APX (d) of loquat [Eriobotrya japonica (Thunb.) Lindl.] seedlings. Data are presented as the means ± SE (n = 3). Different letters on the bars indicate significant differences (p < 0.05) among different treatments based on Tukey’s HSD test. CK: control, foliar spray with distilled water; DA: foliar spray with dopamine solution; HD: combined high temperature and drought stress, foliar spray with distilled water; HDDA: combined high temperature and drought stress, foliar spray with dopamine solution.
Figure 4. Activities of SOD (a), POD (b), CAT (c), and APX (d) of loquat [Eriobotrya japonica (Thunb.) Lindl.] seedlings. Data are presented as the means ± SE (n = 3). Different letters on the bars indicate significant differences (p < 0.05) among different treatments based on Tukey’s HSD test. CK: control, foliar spray with distilled water; DA: foliar spray with dopamine solution; HD: combined high temperature and drought stress, foliar spray with distilled water; HDDA: combined high temperature and drought stress, foliar spray with dopamine solution.
Plants 14 02650 g004
Plants 14 02650 g005
Figure 5. Contents of betaine (a), proline (b), and soluble sugar (c) of loquat [Eriobotrya japonica (Thunb.) Lindl.] seedlings. Data are presented as the means ± SE (n = 3). Different letters on the bars indicate significant differences (p < 0.05) among different treatments based on Tukey’s HSD test. CK: control, foliar spray with distilled water; DA: foliar spray with dopamine solution; HD: combined high temperature and drought stress, foliar spray with distilled water; HDDA: combined high temperature and drought stress, foliar spray with dopamine solution.
Figure 5. Contents of betaine (a), proline (b), and soluble sugar (c) of loquat [Eriobotrya japonica (Thunb.) Lindl.] seedlings. Data are presented as the means ± SE (n = 3). Different letters on the bars indicate significant differences (p < 0.05) among different treatments based on Tukey’s HSD test. CK: control, foliar spray with distilled water; DA: foliar spray with dopamine solution; HD: combined high temperature and drought stress, foliar spray with distilled water; HDDA: combined high temperature and drought stress, foliar spray with dopamine solution.
Plants 14 02650 g005
Table 1. Root morphological parameters of loquat [Eriobotrya japonica (Thunb.) Lindl.] seedlings.
Table 1. Root morphological parameters of loquat [Eriobotrya japonica (Thunb.) Lindl.] seedlings.
TreatmentLength
(cm)		Surface Area
(cm2)		Volume
(cm3)		Mean Diameter
(mm)		Number of Tips
CK1257.432 ± 84.211 ab712.033 ± 24.115 a70.036 ± 5.004 a1.925 ± 0.157 a214.775 ± 14.467 a
DA1311.926 ± 55.772 a728.285 ± 23.368 a72.069 ± 4.806 a2.084 ± 0.104 a225.817 ± 12.445 a
HD801.002 ± 67.605 c500.624 ± 26.007 b41.992 ± 8.427 c1.346 ± 0.112 c140.582 ± 6.404 c
HDDA1164.553 ± 57.623 b544.464 ± 45.018 b54.955 ± 4.720 b1.647 ± 0.153 b163.231 ± 4.940 b
Note: Data are presented as the means ± SE (n = 3). Different letters following the values in the same column indicate significant differences (p < 0.05) among different treatments based on Tukey’s HSD test. CK: control, foliar spray with distilled water; DA: foliar spray with dopamine solution; HD: combined high temperature and drought stress, foliar spray with distilled water; HDDA: combined high temperature and drought stress, foliar spray with dopamine solution.
Table 2. The tissue water content and relative water content in roots and leaves of loquat [Eriobotrya japonica (Thunb.) Lindl.] seedlings.
Table 2. The tissue water content and relative water content in roots and leaves of loquat [Eriobotrya japonica (Thunb.) Lindl.] seedlings.
TreatmentTWC (%)RWC (%)
LeafRoot LeafRoot
CK75.152 ± 1.193 a70.963 ± 3.795 a82.253 ± 3.613 a88.078 ± 2.668 a
DA79.910 ± 5.591 a73.195 ± 5.264 a84.482 ± 2.958 a86.825 ± 4.764 a
HD46.584 ± 4.701 c57.375 ± 3.073 c56.921 ± 6.430 c65.515 ± 2.173 c
HDDA61.558 ± 5.589 b65.320 ± 2.882 b67.440 ± 2.351 b77.424 ± 4.753 b
Note: Data are presented as the means ± SE (n = 3). Different letters following the values in the same column indicate significant differences (p < 0.05) among different treatments based on Tukey’s HSD test. CK: control, foliar spray with distilled water; DA: foliar spray with dopamine solution; HD: combined high temperature and drought stress, foliar spray with distilled water; HDDA: combined high temperature and drought stress, foliar spray with dopamine solution.
Table 3. Photosynthetic pigments and photosynthetic parameters of loquat [Eriobotrya japonica (Thunb.) Lindl.] seedlings.
Table 3. Photosynthetic pigments and photosynthetic parameters of loquat [Eriobotrya japonica (Thunb.) Lindl.] seedlings.
TreatmentCarotenoid
Content
(mg g−1 FW)		Chlorophyll
Content
(mg g−1 FW)		Pn
(μmol m−2 s−1)		Tr
(mmol m−2 s−1)		Gs
(mol m−2 s−1)		Ci (μmol mol−1)
CK0.083 ± 0.009 b0.662 ± 0.014 b14.681 ± 0.847 a3.900 ± 0.390 a0.134 ± 0.013 a204.858 ± 8.882 c
DA0.081 ± 0.003 b0.704 ± 0.005 a15.766 ± 0.725 a4.201 ± 0.333 a0.159 ± 0.009 a213.073 ± 1.123 c
HD0.067 ± 0.004 c0.499 ± 0.021 d3.770 ± 0.234 c1.343 ± 0.110 c0.030 ± 0.011 c251.606 ± 8.018 a
HDDA0.099 ± 0.007 a0.612 ± 0.024 c8.353 ± 0.240 b3.147 ± 0.376 b0.081 ± 0.019 b232.442 ± 1.375 b
Note: Data are presented as the means ± SE (n = 3). Different letters following the values in the same column indicate significant differences (p < 0.05) among different treatments based on Tukey’s HSD test. CK: control, foliar spray with distilled water; DA: foliar spray with dopamine solution; HD: combined high temperature and drought stress, foliar spray with distilled water; HDDA: combined high temperature and drought stress, foliar spray with dopamine solution.
Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.

Share and Cite

MDPI and ACS Style

Luo, X.; Luo, Y.; Wang, X.-L.; Kong, X.-M.; Zhang, H.-F.; Lin, L.-J.; Li, Y.-X.; Huang, K.-W.; Deng, Q.-X.; Jia, Y.-X. Exogenous Dopamine Alleviates Combined High Temperature and Drought Stress in Loquat [Eriobotrya japonica (Thunb.) Lindl.] Seedlings: Improvements in Photosynthetic Efficiency, Oxidative Damage and Osmotic Regulation. Plants 2025, 14, 2650. https://doi.org/10.3390/plants14172650

AMA Style

Luo X, Luo Y, Wang X-L, Kong X-M, Zhang H-F, Lin L-J, Li Y-X, Huang K-W, Deng Q-X, Jia Y-X. Exogenous Dopamine Alleviates Combined High Temperature and Drought Stress in Loquat [Eriobotrya japonica (Thunb.) Lindl.] Seedlings: Improvements in Photosynthetic Efficiency, Oxidative Damage and Osmotic Regulation. Plants. 2025; 14(17):2650. https://doi.org/10.3390/plants14172650

Chicago/Turabian Style

Luo, Xian, Ya Luo, Xiao-Li Wang, Xiao-Mei Kong, Hui-Fen Zhang, Li-Jin Lin, Yu-Xing Li, Ke-Wen Huang, Qun-Xian Deng, and Yong-Xia Jia. 2025. "Exogenous Dopamine Alleviates Combined High Temperature and Drought Stress in Loquat [Eriobotrya japonica (Thunb.) Lindl.] Seedlings: Improvements in Photosynthetic Efficiency, Oxidative Damage and Osmotic Regulation" Plants 14, no. 17: 2650. https://doi.org/10.3390/plants14172650

APA Style

Luo, X., Luo, Y., Wang, X.-L., Kong, X.-M., Zhang, H.-F., Lin, L.-J., Li, Y.-X., Huang, K.-W., Deng, Q.-X., & Jia, Y.-X. (2025). Exogenous Dopamine Alleviates Combined High Temperature and Drought Stress in Loquat [Eriobotrya japonica (Thunb.) Lindl.] Seedlings: Improvements in Photosynthetic Efficiency, Oxidative Damage and Osmotic Regulation. Plants, 14(17), 2650. https://doi.org/10.3390/plants14172650

Note that from the first issue of 2016, this journal uses article numbers instead of page numbers. See further details here.

Article Metrics

Article metric data becomes available approximately 24 hours after publication online.
Back to TopTop