Urban green spaces play a crucial role in mitigating the urban heat island effect, providing shade, improving air quality, and supporting pollinators and other wildlife. But their effectiveness depends heavily on the plants themselves. When temperatures soar, heat-sensitive species can suffer leaf chlorosis, reduced photosynthesis, membrane destabilization, and even death. The review, led by Patricia Duarte de Oliveira Paiva and colleagues at the Federal University of Lavras in Brazil, systematically searched Scopus and Web of Science for studies published between 2016 and 2026, ultimately identifying 25 studies that met strict eligibility criteria. Of these, 21 were rated as high methodological quality, providing a robust evidence base for understanding how ornamental species respond to elevated temperatures.

The review found that heat tolerance in ornamental plants is not the product of a single mechanism but rather the coordinated integration of multiple strategies operating across scales—from visible morphological adjustments to invisible molecular cascades. Heat-tolerant genotypes consistently maintain photosynthetic pigments, membrane stability, and carbon assimilation, while susceptible ones exhibit chlorosis, reduced biomass, and impaired photosynthetic performance. The authors emphasize that thermotolerance depends on the intensity, duration, and frequency of heat exposure, as well as on species- and genotype-specific traits.

At the physiological level, plants deploy a range of cooling and protective strategies. Stomatal regulation helps reduce leaf temperature through transpiration, while some species exhibit paraheliotropism—reorienting leaves vertically to minimize direct solar radiation. Anatomical traits also matter: studies of urban tree species found that evergreens maintained superior water status and lower leaf temperatures during natural heatwaves compared with deciduous species, and that leaf thickness and spongy tissue thickness correlated positively with heat tolerance. In Rhododendron, the heat-tolerant cultivar ‘Lan Yin’ preserved chloroplast integrity and xylem structure, while the sensitive cultivar ‘Liu Qiu Hong’ suffered plasmolysis and chloroplast damage.

Underlying these visible responses is a sophisticated molecular machinery. When temperatures rise, plants activate heat shock transcription factors (HSFs) that bind to heat shock elements in the promoters of heat shock protein (HSP) genes. These molecular chaperones—classified into families such as HSP100, HSP90, HSP70, HSP60, and small HSPs—stabilize and refold denatured proteins, preventing aggregation and facilitating the degradation of irreversibly damaged proteins. In tall fescue (Festuca arundinacea), the small heat shock protein FaHSP17.8-CII was shown to enhance thermotolerance by preserving photosystem II stability and reinforcing heat stress memory. In roses, the transcription factor RhHsfA7 positively regulates heat tolerance by activating heat-responsive genes and enhancing antioxidant capacity.

Calcium signaling plays a central role in heat perception. Heat stress induces a rapid influx of calcium ions through membrane-associated channels, activating calmodulins and calcium-dependent protein kinases (CDPKs) that modulate downstream responses, including the regulation of reactive oxygen species (ROS) homeostasis and abscisic acid (ABA) production. ABA, in turn, promotes stomatal closure to conserve water and activates the expression of heat-responsive genes. Other hormones, including salicylic acid, jasmonates, and brassinosteroids, also participate in the heat stress response, though their roles vary depending on species, tissue, and stress intensity.

Reactive oxygen species occupy a dual role in heat stress biology: they act as signaling molecules that trigger protective responses, but excessive accumulation leads to oxidative damage to lipids, proteins, and DNA. The balance between ROS production and scavenging is a major determinant of thermotolerance. Tolerant plants synthesize antioxidant enzymes—including superoxide dismutase, catalase, ascorbate peroxidase, and glutathione reductase—as well as non-enzymatic antioxidants such as ascorbate and glutathione. In Rhododendron, the heat-tolerant cultivar ‘Gommer Waterer’ maintained membrane integrity under prolonged heat stress by increasing glutathione peroxidase and glutathione S-transferase activities, while the sensitive cultivar exhibited weaker antioxidant responses and reduced glutathione recycling.

Exogenous compounds offer promising tools for enhancing thermotolerance. Melatonin application alleviated heat stress in tall fescue by promoting growth, protecting photosynthetic pigments, and enhancing antioxidant defenses, while in roses it boosted heat tolerance through modulation of the RhCOMT1 gene. Spermidine, a polyamine, improved heat tolerance in tall fescue by reducing ROS accumulation and maintaining cell membrane stability. Trehalose application and overexpression of the trehalose-6-phosphate synthase gene enhanced heat tolerance in roses by preserving photosynthetic stability and mitigating oxidative damage. However, the authors caution that most of these results were obtained under controlled conditions, and validation in real urban environments remains limited.

Epigenetic mechanisms add another layer of complexity. Heat stress can induce DNA demethylation of heat-responsive genes, histone modifications, and the action of noncoding RNAs, enabling plants to establish a form of stress memory. This ‘thermomemory’ allows plants to respond more rapidly and robustly to recurrent heat events, and in some cases can be transmitted across generations, providing progeny with enhanced resilience to ancestral stress. In tall fescue, transcriptional memory of FaHSP17.8-CII was associated with reduced ROS accumulation and sustained photosystem II activity after heat stress, illustrating how epigenetic regulation translates into physiological protection.

The review highlights significant gaps in current knowledge. Most available studies have focused on a small number of species—particularly tall fescue, a turfgrass frequently used in heat stress research—leaving the responses of many ornamental species used in urban landscapes poorly characterized. Moreover, few studies have examined plant responses under natural urban conditions, where plants face combined stresses including heat, drought, and poor soils. Expanding comparative studies across a wider range of species will be essential for identifying reliable thermotolerance markers that can support breeding programs and the selection of climate-resilient genotypes. Ultimately, the authors argue, integrating these advances into urban landscape planning will contribute to greener, cooler, and more sustainable cities capable of withstanding the heat challenges of the coming decades.

Subject of Research: Physiological and molecular mechanisms of heat stress tolerance in urban ornamental plants

Article Title: Physiological and molecular responses of urban ornamental plants to heat stress

Article References: Paiva, P. D. D. O., Magalhães, H. S., & Paiva, R. (2026). Physiological and molecular responses of urban ornamental plants to heat stress. Frontiers in Horticulture, 5, Article 1923332. https://doi.org/10.3389/fhort.2026.1923332

Image Credits: AI Generated

DOI: 10.3389/fhort.2026.1923332

Keywords: heat stress, ornamental plants, urban green infrastructure, thermotolerance, heat shock proteins, reactive oxygen species, antioxidant defense, epigenetics, climate change, plant hormones, photosystem II, urban heat island