Physiological and molecular responses of urban ornamental plants to heat stress

1 Introduction

In light of global warming and the increasing intensity and frequency of heat waves, the demand for heat-tolerant ornamental plants for urban green spaces has increased, contributing to environmental quality and resilience. According to IPCC projections, global temperatures are expected to rise between 1.5 °C and 2.4 °C by 2050, besides the frequency of severe heatwaves compared to present conditions may double or quadruple (IPCC, 2021). Climate change remains a major challenge, demanding research and sustainable practices, including resilient garden design and the efficient use of resources to mitigate its impacts and negative consequences (Paiva, 2023).

Beyond their aesthetic value, ornamental plants contribute significantly to enhancing environmental quality and the sustainability of urban areas. These plants may help improve air quality, mitigate urban heat island effects, reduce noise pollution, and provide important ecological services by supporting beneficial insects including pollinators, as well as other animals. Rapid city urbanization has increased the value of green spaces, and the use of ornamental plants in urban green infrastructure is particularly important as it can promote biodiversity, strengthen ecological resilience, and help regulate urban microclimates (Swetha et al., 2024). Evaluating and Identifying climate change resilient ornamental plant species and varieties continues to be a major challenge (Paiva et al., 2024).

Urban green infrastructure plays an important role in reducing urban temperatures by providing shading and improving thermal comfort. Its benefits can be further enhanced through management practices such as irrigation which helps plants survive extreme heat especially during heatwaves (Lam et al., 2020). However, although management practices can effectively contribute to reducing urban temperature, this is not enough. Great emphasis should be placed on the selection of climate-resilient plant species to ensure the long-term sustainability of parks and gardens (Du et al., 2019).

Plants are susceptible to different abiotic and biotic stresses. Among the abiotic, besides drought and salt, high temperature is an important stressor, mainly as a consequence of climate change (Lee et al., 2022). Heat stress represents a major challenge for agriculture causing irreversible damage to crop plants and ornamentals (Jagadish et al., 2021; Fernandes et al., 2023). Although plant responses to abiotic stresses have been investigated in significative number of species, most available studies have focus on drought and salinity, whereas comparatively fewer addressed heat stress. Moreover, only a limited number of these studies investigated ornamental species used in urban green spaces, despite their important role in maintaining urban ecosystem services and enhancing resilience to climate change.

While phenotypic observations are important, understanding plant responses at the physiological and molecular levels is essential to support species selection, breeding programs, and management practices in urban areas, as plant adaptation depends on integrated sensing and response mechanisms, activated according to the severity of the stress. Understanding ornamental plant used in urban landscapes performance, directly influences the ecological and functional benefits provided by green infrastructure. Despite extensive research on plant responses to heat stress across developmental stages, a comprehensive understanding of the molecular and physiological processes involved is still lacking.

Some of the negative effects of heat stress in addition to disruptions in physiological processes, inhibition of photosynthetic, destabilization of cell membranes and bursts of reactive oxygen species (ROS), include reduced shoot growth, leaf chlorosis, leaf etiolation, reduced pollen fertility, decreased yield, and even plant death (Du et al., 2019). For example, Viola×wittrockiana, an important ornamental plant widely used in garden flower beds, requires temperatures between 14 °C and 21 °C for optimal growth and flowering. This species does not tolerate temperatures above 30 °C, at which heat stress symptoms become evident (Du et al., 2019).

Physiological traits, gene expression profiles, and transcriptomic analyses are used to identify heat-sensitive and heat-tolerant species. In Clematis species, these approaches have revealed diverse heat response mechanisms with implications for the breeding of heat-resistant varieties (Zhang et al., 2021).

Climate change is increasing the frequency of extreme heat events and, consequently, plants may have diverse responses, including the activation of programmed cell death in specific cells or tissues, depending on exposure duration and temperature intensity (Distéfano et al., 2025). Heat stress is associated with osmotic and oxidative stresses. Tolerant plants respond by accumulating osmolytes and compatible solutes, activating ABA-mediated signaling pathways and enhancing antioxidant defenses that scavenge ROS (Masouleh and Sassine, 2020). Elevated temperatures induce heat stress and damage plants at morphological, physiological and molecular levels through disruptions in photosynthesis and respiration, the accumulation of misfolded proteins and excessive production of (ROS) (Sachdev et al., 2021). In response to moderate or extreme high temperatures, plants activate complex sensing and signaling pathways that drive physiological and morphological adaptations at cellular and molecular levels. These responses include changes in membrane fluidity, protein dynamics, gene expression, and the synthesis of heat shock proteins and hormones (Distéfano et al., 2025). Plant responses to elevated temperature can be assessed through physiological indicators, including stomatal regulation, cell membrane stability, canopy temperature, and chlorophyll content (Kumar et al., 2025).

Given the expansion of urban green spaces and their critical role in climate adaptation, a comprehensive synthesis of heat stress responses in ornamental plants used in these areas is timely and necessary. This review compiles and discusses current knowledge on their physiological and molecular responses and adaptations to heat stress, identifying mechanisms associated with thermotolerance, as well as highlighting existing research gaps. Ultimately, this work aims to advance the understanding of high-temperature effects and how these plants tolerate adverse conditions to better support urban climate change.

2 Methodology

This systematic literature review approach followed the PRISMA 2020 (Preferred Reporting Items for Systematic Reviews and Meta-Analyses) guidelines proposed by Page et al. (2022). The review methodology was conducted through four sequential stages: identification, screening, eligibility, and final inclusion.

During the identification stage, two databases Scopus and Web of Science (WofS) institutional reference libraries containing a recognized inventory of abstracts and citations from peer-reviewed literature, with a broad coverage across diverse scientific fields, were consulted. The search strategy was optimized using Boolean search strings, combining the terms “Urban Plant Resilience AND heat stress” OR “Heat stress AND ornamental plant” OR “Abiotic stress tolerance AND ornamental plant” OR “High-temperature tolerance AND ornamental plant” OR “Heat stress AND garden,” covering all documents published between 2016 and 2026, which resulted in 573 records, being Scopus = 196 and WofS = 377.

The search strategy targeted terms in the title, abstracts, and keywords. Filters regarding publication status (only published papers were considered), subject area, and language (English only) were used to select the studies. Only final-stage publications, belonging to the fields of Agricultural and Biological Sciences; Biochemistry, Genetics, and Molecular Biology; Environmental Sciences; were considered, reducing the number of records to 381 publications. The search was performed on July 17^th 2026.

Following this process, further screening was conducted to exclude duplicate articles (=55) and studies outside the scope of interest, based on title (n=182), abstracts (n=85). Subsequently, 59 full-text articles were considered eligible for full-text retrieval based on title and abstract screening criteria.

After full-text analysis, 28 articles that did not fulfill the main objective of this research (e.g., studies addressing abiotic stress without a specific focus on heat stress), three of them refer to ornamental plant in commercial production, one is a study with medicinal plant, plus one article in Chinese language and one review article (besides all filters applied, these remained on the list) were excluded. This resulted in n=25 studies included on the review, in accordance with strict systematic literature review (SLR) inclusion criteria. The overall reference list (n=65) also included additional studies used for concept definition and broader discussion (n=40). The complete selection process and numerical data across all stages are illustrated in the PRISMA 2020 flow diagram (Figure 1).

A summary of the studies, as well as quality evaluation, included in the systematical review is presented in Table 1.

Table 1. Summary of the characteristics of the studies included in the systematic review.

H=high; M=moderate.

2.1 Methodological quality assessment of included studies

The methodological quality of the included studies was independently assessed by two reviewers using a critical appraisal checklist adapted from the Joanna Briggs Institute (JBI) Critical Appraisal Tools and guidelines commonly applied to experimental studies in plant science (Barker et al., 2024). The original JBI criteria were adapted to evaluate aspects specifically relevant to experimental design, methodological rigor, reproducibility, and reporting quality.

The assessment considered the clarity of the experimental design, description and characterization of plant material, details of heat treatment and environmental conditions, use of control, replication, measurements, statistical analysis, reporting quality and reproducibility. Each criterion was rated as Yes (=1), No (=0), or Unclear (=0). Based on the number of criteria fulfilled, studies were classified as presenting low (L = 0-4), moderate (M = 5-7), or high (H = 8-10) methodological quality (Table 1). Two reviewers independently assessed the methodological quality of all included studies. Any disagreements were resolved through discussion until consensus was reached.

Of the 25 included studies, 21 (84.0%) were classified as having high methodological quality, and 4 (16.0%) as moderate methodological quality. No studies were classified as having low methodological quality. The most frequently identified methodological limitations were related to experimental design and incomplete descriptions of environmental conditions.

3 Heat stress and plant resilience mechanisms under climate change

Climate change has caused significant alterations in global weather patterns, including temperature fluctuations and the occurrence of unusually cold or hot conditions, as well as heat weaves across different seasons and regions (Distéfano et al., 2025; Kumar et al., 2025). Among these climatic changes, rising temperatures have become a major challenge for plants, negatively affecting physiological processes. An increase of 10–15 °C above the optimal temperature can induce a heat stress response in plants within a few hours and may become potentially lethal (Wahid et al., 2007). Heat stress, resulting from rising temperatures, is one of the major consequences of global warming, which is primarily driven by human activities and elevated CO₂ levels (Kumar et al., 2025).

3.1 General plant responses to heat stress

In response to heat stress, plants activate several regulatory mechanisms, including the maintenance of cell membrane stability, the upregulation of the reactive oxygen species (ROS) scavenging systems, and accumulation of heat shock proteins (Mittler et al., 2012). These mechanisms ensure metabolic continuity, limiting cellular damage under high-temperature conditions.

Although heat stress affects plant growth and development, survival depends on adaptive mechanisms associated with genetic traits that confer heat tolerance. These mechanisms involve adjustments in cellular and molecular processes as well as the regulation of diverse metabolic pathways and physiological processes (Kumar et al., 2025).

3.2 Plant diversity and thermotolerance strategies

Plants exhibit varying degrees of tolerance to heat stress depending on the species and can be classified into three groups: heat-sensitive species, relatively heat-resistant species, and heat-tolerant species (Hasanuzzaman et al., 2013). Thermotolerance in roses is a result of a coordinate role of molecular, biochemical, and physiological mechanisms (Wang et al., 2024). Similarly, studies on Rhododendron demonstrate that heat tolerance is a complex and genotype-dependent trait. Rhododendron cultivars exhibited different physiological, anatomical, chlorophyll synthesis, osmotic adjustment solutes, and cell membrane thermostability responses to heat stress, indicating cultivar-specific heat tolerance mechanisms (Shen et al., 2017). A more recent molecular study showed that heat tolerance in Rhododendron depends on the intensity of heat stress. Moderate temperatures activate protective metabolic pathways, whereas severe heat stress reduces root activity and alters root architecture. Furthermore, transcription factors play a key role in linking heat perception and metabolic adaptation, while oleanolic acid contributes to antioxidant defense under high-temperature conditions (Liu et al., 2025). The findings of these studies indicate that in Rhododendron, heat tolerance is not explained by a single physiological trait but through the integration of physiological, anatomical, and molecular mechanisms, that can vary according to genotype and stress severity.

3.3 Molecular and omics insights into heat stress responses

Transcriptomic and metabolomic analyses revealed that heat stress triggers genetic and biochemical changes in rose plants through the activation of several pathways, including flavonoid biosynthesis, Mitogen-Activated Protein Kinase (MAPK) signaling, and plant hormones. Under high-temperature conditions, physiological alterations such as reduced chlorophyll and protein content and increased proline accumulation are also observed. Flavonoids, heat shock proteins (particularly HSP70), and antioxidant enzymes such as peroxidase (POD) contribute to heat tolerance whereas malondialdehyde (MDA) serve as indicator of lipid peroxidation and oxidative damage. These insights into rose responses to high temperatures provide valuable information for breeding programs aimed at improving heat stress tolerance (Wang et al., 2024).

Similarly, exposure of tree peony (Paeonia suffruticosa) to 40 °C for 24 h was identified as a critical condition for inducing heat stress, resulting in marked phenotypic, physiological, and proteomic changes. A total of 100 heat-responsive proteins were identified, associated with signal transduction, protein synthesis and degradation, heat shock proteins, antioxidant defense, photosynthesis, energy conversion, and primary metabolism. These findings highlight the complex molecular network underlying thermotolerance (Ma et al., 2022).

At the gene expression level, heat stress in Petunia × hybrida ‘W115’ significantly altered the expression of genes involved in proline metabolism and heat stress signaling, particularly PhP5CR, PhHsf19, and PhHsf21, highlighting the coordinated regulation of osmotic adjustment and heat shock transcription factors underlying thermotolerance and identifying promising targets for molecular breeding (Yue et al., 2019).

Comparative transcriptomic analyses of the heat-tolerant Rhododendron ‘Gommer Waterer’ and the heat-sensitive R. ‘Fenjingling’ demonstrated extensive transcriptional reprogramming in response to prolonged heat stress, particularly in glutathione metabolism. The identification of GSTT1 as a hub gene highlights glutathione metabolism as an important molecular pathway underlying thermotolerance and provides a promising target for breeding heat-tolerant woody ornamentals (Zhou et al., 2026).

These studies demonstrated that thermotolerance is regulated by highly integrated molecular networks. Despite differences among ornamental species, omics approaches have consistently identified coordinated regulation of stress perception, signal transduction, antioxidant metabolism, osmotic adjustment, and protein homeostasis as central components of the heat stress response, reinforcing the potential of multi-omics strategies to identify consistent molecular markers for heat tolerance.

3.4 Applied perspectives and resilience strategies

To understand and develop strategies for improving plant thermotolerance it is important to understand the molecular mechanisms involved in heat perception and signaling. Advancing knowledge in this area is important for breeding approaches, the application of nanomaterials as stress-priming agents, and the development of sustainable strategies for biodiversity conservation, understanding species distribution, and predicting evolutionary responses to climate change (Calixto, 2025).

Among the potential strategies to enhance thermotolerance and mitigate the effects of heat stress, the application of exogenous compounds such as melatonin and spermidine represents a promising approach, as these compounds act by activating protective mechanisms. Exogenous application of melatonin alleviated the negative effects of heat stress in tall fescue (Festuca arundinacea) by promoting greater growth, protecting photosynthetic pigments, reducing oxidative damage, and enhancing antioxidant defenses (Alam et al., 2022). Melatonin application through foliar spraying of heat-stressed Rosa hybrida mother plants during the flowering period enhanced thermotolerance in seeds collected from these plants. The protective effects were associated with reduced oxidative stress and modulation of RhCOMT1 expression (Li et al., 2025). These findings suggest that melatonin may be a promising tool for improving heat tolerance in temperate grass species (Alam et al., 2022).

Similarly, spermidine application improves heat tolerance in tall fescue (Festuca arundinacea). These compounds act by stimulating antioxidant defenses, maintaining photosynthetic performance, and reducing oxidative damage under high-temperature conditions (Zhang et al., 2017).

Nevertheless, these results were obtained from plants maintained under controlled conditions, and the application of melatonin in ornamental plants exposed to urban heat stress remains poorly investigated. Further validation under field conditions, particularly in urban green spaces subjected to heat waves, is required to confirm its effectiveness as a strategy to enhance thermotolerance.

4 Morphological and structural responses of plants to heat stress

Plants respond to high temperatures by activating a range of adaptive strategies to reduce the heat-induced damages through two primary strategies: tolerance and avoidance. Avoidance mechanisms include leaf cooling as a result of high transpiration rates and morphological adjustments that reduce heat load, whereas tolerance mechanisms involve changes in leaf osmolality and thylakoid membrane lipid composition (Tarvainen et al., 2022). Plants respond to high temperatures through cooling strategies such as stomatal regulation during transpiration, which helps reduce leaf temperature (Kumar et al., 2025). Trees with large leaves and low transpiration rates tend to exhibit higher leaf temperature (Tleaf), increasing the risk of photosynthetic heat stress under a warming climate (Tarvainen et al., 2022). These two strategies are not exclusive, since they depend on species, genotype, developmental stage, and the intensity and duration of heat exposure.

Plants exposed to heat stress may exhibit symptoms such as leaf sunburn, reduced leaf size to minimize heat absorption, wilting, stem discoloration (Kumar et al., 2025) and changes in leaf shape and orientation (Hasanuzzaman et al., 2013). Some plants exhibit paraheliotropism, reorienting leaves vertically to reduce direct solar radiation during periods of intense heat. Although these adaptations help decrease thermal stress, a reduction in light interception also occurs, leading to reduced photosynthesis and biomass accumulation (Hussain et al., 2021).

Heat stress induces significant changes in cellular ultrastructure, including plasmolysis and degradation of thylakoid grana membranes, which lead to chlorophyll loss and reduced photosynthetic capacity. In Rhododendron under heat stress, the cultivar Liu Qiu Hong exhibited more pronounced ultrastructural damage, whereas cells of the cultivar Lan Yin remained relatively intact, indicating higher heat tolerance (Shen et al., 2017).

Extended heat stress reduced chlorophyll a, chlorophyll b, and total chlorophyll contents, indicating damage to cellular ultrastructures, especially chloroplasts. Comparative analyses of two Rhododendron cultivars demonstrated that these physiological changes were closely associated with anatomical adaptations. The heat-tolerant cultivar ‘Lan Yin’ exhibited reduced stomatal size, a lower percentage of open stomata, superior xylem vessel structure, and better preservation of leaf ultrastructure and chloroplast integrity, which contributed to limiting water loss and maintaining cellular stability under heat stress. In contrast, the heat-sensitive cultivar Liu Qiu Hong exhibited higher stomatal density, plasmolysis, chloroplast damage, and greater structural deterioration, indicating increased susceptibility to high temperatures (Shen et al., 2017). The comparison between the heat-tolerant and the heat-sensitive Rhododendron cultivars demonstrates that thermotolerance depends on not only a single mechanism, but also on the coordinated maintenance of anatomical integrity, stomatal regulation, and chloroplast stability.

Heat stress and the combination of drought and heat stresses negatively affected growth and morphological traits of purslane (Portulaca oleracea), causing reduced leaf area, decreased leaf succulence, reddish stem coloration, and severe wilting symptoms. Under high-temperature conditions, plants showed marked growth inhibition and loss of turgor, indicating the vulnerability of morphological traits to heat stress (Jin et al., 2016). Similarly, heat stress affected photosynthetic performance in tall fescue (Festuca arundinacea) leaves, with contrasting responses between the heat-sensitive genotype PI 234881 and the heat-tolerant genotype PI 578718. Combined heat and drought stress caused a more severe reduction in chlorophyll (Chl) content compared with heat stress alone, indicating that multiple conditions intensify damage to the photosynthetic apparatus (Bi et al., 2016). Besides improving antioxidant protection, exogenous spermidine applied in Festuca alleviated heat-induced damage to photosystem II (PSII) by increasing chlorophyll content, improving PSII efficiency, and enhancing the expression of psbA and psbB, core proteins of the PSII reaction center. These findings indicate that polyamines contribute to heat tolerance by preserving photosynthetic machinery under high-temperature conditions (Zhang et al., 2017).

Heat stress induced different physiological among Pelargonium genotypes under high temperatures. Heat-tolerant genotypes, including Pelargonium sidoides and Pelargonium ‘Chocolate Mint’, maintained higher leaf hue values, chlorophyll concentration indices (CCIs), chlorophyll a content, and the net assimilation rate (Pn) increased in P. sidoides. In contrast, heat-sensitive genotypes, such as P. stenopetalum and Pelargonium ‘Madame Thibault’, exhibited reduced whole-plant dry weight, lower chlorophyll content, leaf chlorosis, increased heat damage index, and decreased photosynthetic performance (Miao et al., 2026).

Analysis of these studies involving different ornamental plant species commonly used in urban green areas, indicates that heat-tolerant genotypes are characterized by better maintenance of growth, photosynthetic pigments, and carbon assimilation, whereas susceptible genotypes exhibit chlorosis, reduced biomass accumulation, and reduced photosynthetic performance.

Bauhinia brachycarpa (tree) exhibited greater photosynthetic thermotolerance, higher leaf mass per area (LMA), and higher Ca²⁺ concentration than B. comosa (liana). Under combined high-temperature and high-light conditions, the maximum quantum yield of photosystem II (Fv/Fm) declined in both species, although the reduction was significantly greater in B. comosa, indicating greater sensitivity of the photosynthetic apparatus in the liana species. These findings suggest that structural and physiological leaf traits contribute to heat tolerance and that lianas may be more vulnerable to future climate warming (Zhang et al., 2019).

A comparative study performed under natural field conditions involving 36 urban tree species revealed that evergreen species exhibited greater heat tolerance and lower leaf damage than deciduous species during natural heat events. A critical temperature (Tcrit) of up to 40 °C, corresponding to 15% initial damage to photosystem II was observed as a more sensitive indicator of leaf heat tolerance than other thermal indices. Other parameters, including leaf structural traits, such as leaf mass per area (LMA), leaf thickness (LT), and spongy tissue thickness (Ts), were positively correlated with heat tolerance, suggesting that greater leaf thickness and structural development may provide enhanced protection against high temperatures. These findings highlight the importance of integrating physiological and anatomical traits when selecting tree species for climate-resilience urban green spaces (Zhang et al., 2024).

Considering ploidy level as a potential factor influencing stress tolerance, studies in ferns (Dryopteris spp.) demonstrated that polyploids exhibit greater capacity to tolerate different environmental stresses compared with their diploid counterparts. These findings suggest that polyploidy may confer enhanced physiological flexibility and resilience under climate-related stress conditions (Blake-Mahmud et al., 2025).

In response to heat stress during the reproductive phase, plants may exhibit altered floral development and reproductive structures, with disruptions in inflorescence development, sporogenesis, gametogenesis, anthesis, pollination and fertilization (Resentini et al., 2023). Short-term heat stress did not cause significant wilting in rose leaves, due to the presence of increased wax deposition, although leaf softening occurred due to water loss (Wang et al., 2024).

Taken together, these studies demonstrate that morphological and anatomical responses to heat stress are highly species- and genotype-dependent. Morphological adjustments including reduction of leaf area, stomatal regulation, and altered leaf orientation contribute to reducing the effects of heat stress. Likewise, anatomical traits such as leaf thickness, chloroplast integrity, and xylem organization enhance tissue resistance to thermal damage, contributing to plant thermotolerance. Most available findings on ornamental plant heat responses result from studies under controlled conditions. Few of them consist of results under natural conditions with heat events, which remains a research gap and stimulates future studies to validate the results from controlled environment in field conditions.

5 Molecular signaling in heat stress response

At the molecular level, heat stress triggers changes in the expression of stress-related proteins involved in cellular sensing, signal transduction, and defense mechanisms allowing plants to perceive, respond to, and regulate heat stress through multiple interconnected signaling networks In addition, heat stress affects the transcription of genes encoding calcium sensors, receptor-like protein kinases (RLKs), transport proteins such as aquaporins, signal transducers, transcription factors, and chaperones (Priya et al., 2019). Heat stress induces extensive reprogramming of gene expression and protein synthesis with transcription factors playing a central role in regulating downstream stress-responsive genes, enabling plants to adjust their physiology and metabolism to elevated temperatures (Kumar et al., 2025).

In response to heat-stress in plants, an integrated regulatory network involving phytohormones, heat shock proteins (HSPs), transcription factors (TFs), and calcium-dependent signaling cascades is activated. Plant hormones regulate gene expression, maintain redox homeostasis, and trigger cellular defense mechanisms, often through interactions with protein kinases and molecular chaperones. These coordinated interactions contribute to enhanced thermotolerance and stabilization of cellular processes by increasing antioxidant activity and regulating stomatal function (Zubair et al., 2025). Within this regulatory network, heat stress activates complex transcription factors cascades that induce the de novo production of HSPs, highly conserved molecular chaperones that protects cellular structures and enhance stress tolerance (Kan et al., 2023). Among these regulators, heat shock transcription factors (HSFs) function as central regulators of the HSR, coordinating the expression of numerous genes responsive to stress conditions.

Osmoprotectants such as proline, glycine betaine, and trehalose, antioxidants, factors involved in signaling cascades, molecular chaperones and regulatory proteins accumulate in response to heat stress as constitute major tolerance mechanisms acting to protect proteins and membranes, while maintaining water relations balance (Zandalinas et al., 2017). In plants exposed to combined abiotic stresses, including heat stress, transcriptomic studies have revealed conserved patterns of gene regulation associated with stress tolerance, providing valuable insights and potential targets for the development of heat-resilient crops (Jia et al., 2017). Plants, under severe or combined stresses, often suppress multiple regulatory pathways related to growth, signaling, and stress responses, resulting in a broad downregulation of gene expression rather than the selective activation of defense mechanisms (Riaz et al., 2024).

In ornamental plants, these molecular mechanisms are supported by studies demosntrating species-specific regulatory responses. The heat shock transcription factor gene RhHsfA7 was identified as a positive regulator of thermotolerance in Rosa hybrida ‘Samantha’ highlighting the important role of HSF-mediated signaling in enhancing heat tolerance (Sun et al., 2025). Similarly, in rose (Rosa chinensis), exogenous trehalose application reduced the heat damage index, increased superoxide dismutase (SOD) activity, preserved photosystem II (PSII) efficiency, and mitigated oxidative damage. At the molecular level, trehalose upregulated the expression of RcTPS7b, which coordinated heat shock signaling and redox homeostasis, indicating that trehalose confers thermotolerance through a synergistic mechanism involving SOD-mediated ROS scavenging, membrane stabilization, and the activation of stress-responsive gene networks. Exogenous trehalose application not only provides immediate protection against heat stress but also enhances the transcriptional regulation of heat-responsive pathways, reinforcing plant thermotolerance (Ma et al., 2025).

Under extreme environmental conditions can lead not only to stress responses but also to programmed cell death. Exposure to heat stress can activate ferroptosis, a regulated cell death pathway, characterized by lipid peroxidation and specific cellular features (Distéfano et al., 2025).

Molecular insights into thermotolerance highlighted that plant stress responses extend beyond the activation of isolated genes or pathways, representing an integrated network that encompasses signal transduction, transcriptional regulation, antioxidant metabolism, and cellular protection.

5.1 Calcium signaling in plant heat stress responses

Calcium ions (Ca²⁺) act as key second messengers in plant signaling pathways. Heat stress induces a rapid calcium influx through membrane-associated channels, likely associated with changes in membrane fluidity and activation of heat-responsive signaling mechanisms. This increase in intracellular Ca²⁺ concentration activates downstream signaling cascades including interaction with calmodulins (CaMs), which are upregulated under heat stress in several species (Al-Quraan et al., 2010; Distéfano et al., 2025).

Cascades of calcium-dependent protein kinases (CDPKs) are enzymes found primarily in plants that function as sensors and transducers of calcium (Ca²⁺) signals in response to heat stress. Through the activation of downstream signaling pathways, CDPKs can indirectly modulate reactive oxygen species (ROS) homeostasis by regulating gene expression and act in regulation of ABA production. In turn, this hormone induces the synthesis of antioxidant enzymes, including ascorbate peroxidase (APX), glutathione peroxidase (GPX), glutathione reductase (GR), superoxide dismutase (SOD), and catalase (CAT) (Atif et al., 2019; Devireddy et al., 2021).

Flavonoids and calcium play important roles in plant adaptation to heat stress. High temperatures alter flavonoid metabolism in Rhododendron × pulchrum, while exogenous CaCl₂ enhances heat tolerance. Calcium promotes the accumulation of antioxidant flavonoids, improving reactive oxygen species scavenging and helping plants cope with heat-induced oxidative stress (Shen et al., 2024).

Exogenous calcium, jasmonates, and brassinosteroids can also regulate the effects of abiotic stressors such as high temperature on plant growth and development (Wang et al., 2022).

5.2 Reactive oxygen species and antioxidant defense in heat stress

Reactive oxygen species (ROS) play a central role in plant responses to heat stress acting both as signaling molecules and as mediators of oxidative damage. One of the major determinants of plant thermotolerance is the balance between ROS production and scavenging. Ascorbate peroxidase (APX) is one of the enzymes involved in mediated regulation of reactive oxygen species (ROS) removal in plant thermotolerance (Zubair et al., 2025).

Heat stress stimulates ROS as a response to oxidative stress but in tolerant plants, antioxidant enzymes and antioxidant metabolites are synthesized, such as ascorbate (AsA) and glutathione (GSH) while the activities of POD, CAT, and GR in susceptible cultivars decreased significantly. The major antioxidant enzymes include ascorbate peroxidase (APX), catalase (CAT), glutathione S-transferase (GST), superoxide dismutase (SOD), peroxidase (POD), and glutathione reductase (GR) (Du et al., 2019). Heat stress also activates antioxidant defense systems, including ROS-scavenging enzymes such as catalase, ascorbate peroxidase, and superoxide dismutase, as well as the accumulation of antioxidants such as ascorbic acid and flavonoids (Distéfano et al., 2025). These results highlight that thermotolerance involves restricting ROS accumulation and activation of an efficient antioxidant defense to protect cellular homeostasis.

Exposure to heat stress led to changes in physiological indicators such as proline, soluble sugar, and ascorbate peroxidase (APX) activity in petunia. Furthermore, the expression of most genes involved in the proline metabolism pathway was altered under heat stress. In particular, the expression patterns of the proline synthetase gene PhP5CR, indicate that this gene together with PhHsf19 and PhHsf21 may represent promising targets for molecular breeding aimed at improving heat tolerance in petunia (Yue et al., 2019).

The heat-tolerant Rhododendron cultivar Lan Yin showed greater capacity for physiological adjustment under heat stress compared with the sensitive cultivar Liu Qiu Hong. Under this condition, ‘Lan Yin’ maintained higher chlorophyll and carotenoid contents, reducing damage associated with malondialdehyde (MDA) and H₂O₂ accumulation, and stronger antioxidant and osmotic adjustment responses, including an increase on SOD and POD activities, as well as greater proline, soluble protein, and sugar contents while reducing total soluble protein levels. As important response, a reduction on oxidative damage and improvement in cell membrane stability occurred (Shen et al., 2017). A comparative study involving Rhododendron cultivars, showed that the heat-tolerant genotype ‘Gommer Waterer’ maintained membrane integrity under prolonged heat stress despite elevated H₂O₂ levels. This response was associated with increased glutathione peroxidase (GPX) and glutathione S-transferase (GST) activities, promoting efficient detoxification of lipid peroxidation products. In contrast, the heat-sensitive cultivar Fenjingling exhibited weaker antioxidant responses and reduced glutathione recycling, emphasizing the importance of glutathione-mediated antioxidant defense for maintaining cellular integrity under heat stress (Zhou et al., 2026). These studies reinforce the importance of antioxidant efficiency, as a critical determinant of thermotolerance.

Conditions of high temperatures (35/28 °C, day/night) negatively affect physiological parameters such as MDA content, relative electrolyte leakage (REL), and SOD activity, as well as reduce chlorophyll content and net photosynthetic rate in roses, that is sensitive to heat and associated with excess humidity stress conditions, favor diseases occurrence (Xie et al., 2019). Heat stress increased the activities of superoxide dismutase (SOD) and peroxidase (POD), whereas catalase (CAT) activity remained unchanged in purslane (Portulaca oleracea) submitted to stress conditions (Jin et al., 2016). Several exogenous compounds enhance antioxidant metabolism under heat stress. Exogenous application of spermidine (Spd) improved heat tolerance in tall fescue (Festuca arundinacea) by reducing ROS accumulation, enhancing antioxidant enzyme activities (SOD, POD, and APX), and maintaining cell membrane stability under heat stress. These effects were associated with lower H₂O₂ and superoxide accumulation, indicating that polyamines contribute to thermotolerance by strengthening antioxidant defenses and protecting cellular structures from oxidative damage (Zhang et al., 2017). Melatonin plays an important role as a reactive oxygen species (ROS)-scavenging molecule, protecting the photosynthetic machinery from oxidative damage induced by heat stress, maintaining photosynthetic efficiency, and enhancing plant thermotolerance (Ahmad et al., 2021). Exogenous melatonin application stimulates heat tolerance in Festuca arundinacea under heat stress by modulating antioxidant metabolism, reducing H₂O₂ and malondialdehyde (MDA) accumulation, reducing oxidative damage and promoting thermotolerance (Alam et al., 2022).

Analysis of these studies indicates that exogenous compounds promote plant thermotolerance primarily by enhancing endogenous antioxidant defenses, highlightening ROS homeostasis as a key factor determinant of heat tolerance.

5.3 Hormonal regulation of plant responses to heat stress

Heat stress alters phytohormone levels, including ethylene (ET), salicylic acid (SA), jasmonate (JA), abscisic acid (ABA), brassinosteroids (BRs), cytokinins (CKs), auxins, and gibberellins (GA). Their modulation is often associated with enhanced thermotolerance, although the roles of these hormones vary according to stress intensity, plant species, and the affected tissues. Among them, ABA rapidly accumulates under heat stress and promotes stomatal closure, helping to maintain water balance (Distéfano et al., 2025). Plant hormones play important roles in heat stress tolerance. ABA, salicylic acid (SA), and ethylene increase in response to high temperatures, whereas auxins, cytokinins, and gibberellins tend to decline (Kumar et al., 2025).

Among these hormones, ABA is a key regulator for plant resilience to heat stress, as it promotes stomatal closure to reduce water loss, modulates reactive oxygen species (ROS) levels and activates the expression of heat-responsive genes under high-temperature conditions (Sharma et al., 2020). The heat stress response (HSR) proteins function in signaling cascades that attenuate heat-induced damage, integrating responses to environmental and hormonal signals, including light and hormones such as brassinosteroids (BRs) and abscisic acid (ABA). In particular, ABA activates heat shock factors (HSFs), which regulate heat stress-responsive transcriptomes and induce the expression of protective genes (Dündar et al., 2025). Short-term heat stress can activate heat shock proteins (HSPs) as well as calcium and phytohormone signaling pathways, in Festuca arundinacea, contributing to thermotolerance. However, prolonged exposure to heat stress can result in leaf senescence in consequence of chlorophyll degradation and stress-related pathways, including NAC transcription factors, while suppressing photosynthesis-related genes and catalase (CAT) expression (Qian et al., 2020). These findings highlight that phytohormones responses under exposure to heat conditions are dynamic, shifting from initial protective actions to senescence pathways.

The application of exogenous salicylic acid (SA), 6-benzyladenine (6-BA), and ABA enhanced heat tolerance in roses by regulating photosynthetic properties, such as chlorophyll content, net photosynthetic rate, transpiration rate, water utilization, cell membrane stability in leaves, and antioxidant enzyme activity. Under high temperatures, 6-BA shows the strongest protective effect (Wang et al., 2022). The effect of plant growth regulators (PGRs) on plant function are dose-dependent, and excessively high concentrations may reduce stress tolerance (Wang et al., 2022).

5.4 Role of heat shock proteins in regulation of plant heat stress response

Heat shock factors (HSFs) and heat shock proteins (HSPs) are central components of the heat shock response (HSR). This response is modulated by signaling molecules such as calcium ions (Ca²⁺), nitric oxide (NO) and phytohormones, which regulate HSF activity and activate downstream HSR pathways (Kang et al., 2024). Their expression is regulated by heat shock factors (HSFs), which bind to heat shock elements (HSEs) in the promoters of HSP genes. Transcriptome analysis of a heat-tolerant pansy cultivar under heat stress revealed 5,708 differentially expressed genes in Viola×wittrockiana, highlighting HSF, HSP and ascorbate peroxidase as important regulators of thermotolerance (Du et al., 2019). HSF–HSP regulation represents one of the most important molecular mechanisms associated with plant thermotolerance.

Heat shock proteins (HSPs) are key molecular chaperones involved in plant thermotolerance, and are rapidly activated in response to heat stress (Lee et al., 2022; Kumar et al., 2025). The primary functions of HSPs include stabilization and refolding denatured proteins, preventing protein aggregation, and facilitating the degradation of irreversibly damaged proteins, enhancing plant survival under heat stress conditions (Kumar et al., 2025).

HSPs are classified according to molecular weight into five major families such as HSP100 (~100 kDa), HSP90 (~90 kDa), HSP70 (~70 kDa), HSP60 (~60 kDa), and small HSPs (~15–30 kDa). Under heat stress, they are mainly regulated by heat shock transcription factors (HSFs), being the HSP70 and HSP90 expression typically increases about tenfold, whereas sHSPs can be induced up to 200-fold, highlighting their role in enhancing thermotolerance for and maintenance of cellular homeostasis during thermal stress by stabilizing, refolding and preventing aggregation of denatured proteins (Du et al., 2019; Kumar et al., 2025).

The Hsp90 acts as a molecular chaperone and, under abiotic stress conditions, interacts with proteasomes systems to maintain protein homeostasis. It is also involved in chloroplast protection, light stress resistance, hormone signaling, signaling pathways and activating immune defenses, and autophagy (Lee et al., 2022; Wu et al., 2026). Plants adapted to semi-arid and arid environments often exhibit enhanced HSP synthesis, which contributes to improved tolerance to elevate leaf temperatures and supports cellular metabolic stability under heat stress (Ahmad et al., 2021).

Heat shock proteins are involved not only in cellular protection during heat stress but also in the regulation of reproductive development and floral organ function. HSP gene HSP70–16 analysis in Arabidopsis thaliana, a model plant, demonstrated an important role played in flower opening and mild heat stress responses. In this study, under elevated temperatures, mutants lacking HSP70–16 exhibited sepal fusion, abnormal floral organ development, reduced fertilization, and lower seed setting (Chen et al., 2019).

In ornamental species, comparative analyses of heat-tolerant and heat-sensitive Oncidium cultivars concluded that the genotype ‘GR’, a thermotolerant, employs a multi-layered defense strategy involving activation of small heat shock proteins (HSP20). In contrast, the heat-sensitive cultivar ‘HC’ exhibited reduction on protein homeostasis and increased oxidative damage (Luo et al., 2025).

The results of these studies reinforce that thermotolerance is associated with regulation of protein protection and stress signaling, rather than isolated metabolic reactions.

5.5 Epigenetic modifications regulating plant responses to heat stress

Epigenetic modifications play a key role in plant heat stress responses by regulating gene expression and long-term stress memory, enhancing tolerance to recurrent heat events, acting in close coordination with morphological, structural, physiological, and molecular mechanisms. Exposure to elevated temperatures can induce DNA demethylation of heat-responsive genes, promoting the activation of protective mechanisms (Talarico et al., 2024; Kumar et al., 2025).

Epigenetic modifications including histone modifications and noncoding RNAs play a critical role in regulating gene expression under heat stress (Distéfano et al., 2025; Kumar et al., 2025). These mechanisms can also contribute to transgenerational tolerance, enabling progeny to exhibit enhanced resilience to ancestral stress developing an epigenetic memory (Kumar et al., 2025).

In this context, stress memory is a functional outcome of epigenetic regulation, whereas plants maintain stress-induced changes after recovery from heat exposure. Under extreme temperature conditions, plants can continuously activate stress-tolerance mechanisms, although prolonged responses can lead to growth inhibition. Plants present adaptive traits that persist over time, which are actively regulated through the establishment and resetting of stress memory mechanisms (Nishio et al., 2024).

Understanding gene expression patterns and their functional roles during plant development is vital for designing targeted strategies to improve heat-stress tolerance (Kumar et al., 2025). However, beyond the temperature range that induces thermomorphogenesis, cells perceive elevated temperatures and activate specific molecular and epigenetic responses depending on the severity and the duration of heat exposure (Distéfano et al., 2025).

One of the best-characterized manifestations of epigenetic regulation is transcriptional memory mechanism, enhanced in plants as response to stress and is referred to as Thermomemory. In Festuca arundinacea, transcriptional memory of FaHSP17.8-CII was associated with reduction on ROS accumulation and high PSII activity in chloroplasts after heat stress, enhancing thermotolerance (Bi et al., 2021). These molecular responses are closely associated with physiological changes, including oxidative stress, membrane stability, and photosynthetic performance. In Festuca, heat stress resulted in increased in malondialdehyde (MDA) content and electrolyte leakage whereas chlorophyll content and the activities of superoxide dismutase (SOD) and peroxidase (POD) decreased. Comparative analyses among genotypes showed that the heat-tolerant genotype (‘TF71’) maintained membrane stability by increasing the proportion of unsaturated fatty acids. The membrane fluidity is preserved, contributing to greater heat tolerance under high-temperature conditions (Hu et al., 2018). Among the cellular processes affected by heat stress, the photosynthetic apparatus is one of the primary targets of oxidative damage. High temperatures impair photosystem II (PSII), Rubisco, cytochrome b559, and plastoquinone, disrupting the oxygen-evolving complex and interrupting electron transport. As a consequence, reactive oxygen species (ROS) accumulate, leading to protein degradation, membrane destabilization, and, under severe stress, programmed cell death (PCD). Chloroplast small heat shock proteins, such as HSP21, contribute to thermotolerance by stabilizing the PSII and preserving photosynthetic function under heat stress. In Festuca arundinacea, heat stress inhibits photochemical activity by reducing the maximum quantum yield of PSII (φPo). Exogenous L-ascorbate (Asc), which accumulates in chloroplast, alleviates these effects by reducing ROS accumulation, protecting the PSII D1 protein, and maintaining photosynthetic efficiency during heat stress (Chen et al., 2017).

Overall, epigenetic regulation is not limited to the modulation of individual genes, but involves a coordinated transcriptional memory associated with physiological and biochemical adjustments that enhance plant thermotolerance. This complex regulatory network integrates stress perception, gene expression, membrane stability, and photosynthetic protection, contributing to both heat stress responses and long-term adaptive capacity (Figure 2).

6 Conclusion

Although substantial research has been conducted on plant stress responses, relatively few studies have specifically addressed heat stress, leaving several important knowledge gaps. Furthermore, limited attention has been given to ornamental plants used in urban green areas, despite their crucial role in climate regulation and the provision of ecosystem services that improve urban environmental quality.

High temperatures induce a wide range of structural, physiological, biochemical, and molecular changes in plant cells. The nature and extent of these responses, as well as the severity of the resulting damage, depend on the intensity, duration, and frequency of heat exposure. Understanding how plants respond, adapt, and maintain vigor under extreme temperature conditions is essential for advancing our knowledge of thermotolerance mechanisms.

Overall, thermotolerance is not the result of isolated pathways, but rather results from the coordinated integration of multiple mechanisms, including morphological and anatomical adjustments, molecular signaling, hormonal regulation, antioxidant defenses, osmotic adjustment, membrane stability, chloroplast protection, protein homeostasis, maintenance of photosynthetic function, and epigenetic regulation. These adaptive strategies are closely associated with species- and genotype-specific responses, resulting in heat-sensitive and heat-tolerant phenotypes. These phenotypes represent valuable indicators for the selection of ornamental plants and for breeding programs aimed at improving climate resilience and adaptive potential in urban green areas. The development of studies on ornamental species and cultivars under realistic urban conditions may provide important insights, as these approaches allow the assessment of the combined effects of multiple abiotic stress such as heat, water deficit, salinity, and nutrition limitations. Such studies can provide valuable support for the identification and selection of resilient genotypes better adapted to urban environments.

Moreover, identifying physiological, biochemical, and molecular markers associated with thermotolerance will facilitate the development of more efficient selection strategies. This information is critical for the design and management of resilient and sustainable urban green spaces that can continue to provide efficient ecosystem services.

However, current knowledge is still concentrated in a relatively small number of ornamental species, limiting our understanding of plant adaptive mechanisms in response to heat stress. Furthermore, a considerable proportion of the available studies has focused on Festuca arundinacea, a turfgrass species frequently used in heat stress research. Expanding comparative studies involving a wider range of ornamental species will be essential to identify physiological and molecular markers of thermotolerance, supporting ornamental breeding and the efficient selection of genotypes adapted and resilient to climate change for use in urban landscaping. Ultimately, integrating this information and these advances into the selection of species and cultivars for urban landscape planning will contribute to the development of more climate-resilient green areas, while reducing maintenance requirements, and enhancing the sustainability and functionality of urban green infrastructure.

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Footnotes

Edited by: Daniela Romano, University of Catania, Italy

Reviewed by: Justin Quetone Moss, Oklahoma State University, United States

Arezoo Paravar, University of Hormozgan, Iran