Physiological and Molecular Biology of Ornamental Plants—2nd Edition

1. Introduction

Ornamental plants, celebrated for their rich floral variation, vivid colors, and pleasant fragrances, hold enormous cultural, ecological, and economic value. Their traits are determined by intricate physiological processes and molecular mechanisms that govern pigmentation, secondary metabolism, reproductive success, and stress adaptation. However, many of these mechanisms remain only partially understood. This Special Issue brings together 14 contributions that collectively advance our understanding of ornamental plant biology. These papers cover three thematic areas, floral traits, floral scent and secondary metabolism, and adaptive responses to environmental stresses, providing a comprehensive overview of the latest progress in ornamental plant physiology and molecular biology. Collectively, these studies clarify the regulatory networks involved in floral pigmentation, secondary metabolite pathways, and stress-responsive processes, while showing how integrative approaches are advancing knowledge of ornamental trait formation and resilience.

2. Floral Traits: Color and Development

Flower color is one of the most visible and economically important traits in ornamentals [1]. Osmanthus fragrans is a heritage evergreen ornamental widely used in gardens and urban landscapes, valued for its compact crown and intensely fragrant blossoms across diverse cultivar groups [2]. In O. fragrans, two complementary studies investigated carotenoid metabolism in detail. One identified a mechanism in which delayed starch degradation leads to chromoplast structural abnormalities, preventing carotenoid cleavage and ultimately deepening floral pigmentation (contribution 1). The other focused on the red-flowered cultivar ‘Yanzhi Hong’ and showed that lycopene accumulation, together with reduced α-carotene synthesis due to downregulation of LYCE and LYCB genes, accounts for its distinctive rouge-red petals (contribution 2).

Anthocyanin-based pigmentation represents another dimension of floral coloration. Transcriptomic profiling of four flower colors in Impatiens uliginosa revealed differential expression of structural genes such as C4H, FLS, PAL, and ANS, in addition to transcription factors including MYB and bHLH, indicating that both biosynthetic genes and regulators shape anthocyanin accumulation patterns (contribution 3). Another study in I. uliginosa demonstrated that copper stress disrupts pigment biosynthesis pathways, leading to visible petal color changes (contribution 4).

Floral development, including organ growth and pollen fertility, is fundamental to reproductive success in ornamentals. Resource allocation plays a central role in floral growth and function. Citrus flowers combine ornamental and edible value: their highly fragrant blossoms support both landscaping use and traditional culinary applications [3]. In Citrus maxima, an analysis of C:N:P stoichiometry and non-structural carbohydrates across floral organs and phenophases revealed stage- and organ-specific partitioning, showing how coordination between carbon reserves and nutrient demand underpins organ development during anthesis (contribution 5). A genome-wide survey of the DIR gene family in Rosa chinensis identified 33 RcDIRs, including a tandem cluster, with RcDIR12 exhibiting pollen-specific promoter activity; functional assays confirmed expression in mature pollen grains, providing evidence for pollen-specific DIR function in roses (contribution 6).

Together, these findings illustrate how pigment biosynthetic pathways, nutrient metabolism, and reproductive gene networks jointly shape floral coloration, organ development, and pollen function in ornamentals. Integrative analyses across these processes are providing a more coherent view of the molecular and physiological basis of floral traits.

3. Floral Scent and Secondary Metabolites

Floral fragrance is a defining trait of many ornamentals, contributing both to ecological interactions and commercial appeal. Orchids, with nearly 30,000 species worldwide, are especially noted for their diverse floral morphologies and complex fragrance profiles [4]. They also provide important model systems for understanding the molecular basis of floral scent, particularly volatile terpene biosynthesis. In Dendrobium chrysotoxum, genome-wide identification of terpene synthase (TPS) genes revealed 37 family members, among which three TPS-b genes encode enzymes that catalyze the production of linalool, a dominant monoterpene shaping the floral scent profile (contribution 7). Similarly, in Cymbidium ensifolium, 30 TPS genes were characterized and classified into three subfamilies, with CeTPS1 and CeTPS18 highly expressed in flowers and localized to chloroplasts, indicating specialized roles in terpenoid metabolism during floral development (contribution 8). Together, these studies demonstrate that orchids have evolved a diverse TPS repertoire, with distinct subfamilies differentially contributing to the unique fragrance profiles that underpin their ornamental and ecological value.

Beyond orchids, pyrethrum (Tanacetum cinerariifolium) exemplifies the dual roles of secondary metabolites in ornamentals. It produces two classes of bioactive compounds with distinct ecological functions: the volatile (E)-β-farnesene, which mediates plant–insect interactions, and the non-volatile pyrethrins, which act as natural insecticides [5]. Transcriptomic analysis combined with GC–MS-based metabolite profiling revealed asynchronous regulation for pyrethrins, where gene expression did not correspond with metabolite accumulation. In contrast, both the expression of (E)-β-farnesene biosynthetic genes and the accumulation of the metabolite were concentrated in peduncles. Jasmonate signaling, particularly through MYC2, emerged as a central hub coordinating these processes (contribution 9). This finding highlights a complex regulatory divergence between volatiles and non-volatiles within the same plant, providing valuable insights into how ornamental plants balance ecological interactions with defense-oriented chemistry.

Collectively, recent work indicates that floral fragrance and defensive chemistry are coordinated through context-dependent partitioning of biosynthetic flux between volatile and non-volatile branches. Within this architecture, the diversification of terpene synthases structures volatile terpene profiles in orchids, while non-volatile defenses are regulated in parallel, linking bouquet composition to ecological function and applied aims in ornamentals.

4. Adaptive Strategies Under Environmental Stresses

Ornamental plants are frequently exposed to a wide range of abiotic stresses, and understanding their adaptive strategies is essential for both horticultural practice and basic research. Among them, Rhododendron, one of the largest woody plant genera, is highly valued for its remarkable diversity of flower colors and ornamental appeal [6]. It also serves as a representative system for studying how root–microbe interactions contribute to stress responses. In Rhododendron delavayi, metabolomic and microbiome analyses revealed that waterlogging stress triggered the secretion of root metabolites capable of recruiting beneficial microbes, which persisted into the recovery phase and contributed to stress alleviation (contribution 10). A complementary study in R. decorum showed that cadmium exposure not only reshaped rhizosphere microbial communities but also suppressed the accumulation of key secondary metabolites such as flavonoids, thereby weakening both chemical defenses and symbiotic stability (contribution 11).

Beyond Rhododendrons, additional contributions broaden the perspective on stress adaptation in ornamentals. Hydrangea macrophylla represents a unique case where abiotic stress tolerance directly underpins ornamental quality. Sepal bluing depends on aluminum accumulation, making this species an important model for linking stress adaptation with visible floral traits. Transcriptome analyses of contrasting cultivars under Al treatment identified 43 differentially expressed genes, including 7 associated with Al accumulation. Among them, HmALS3.1 emerged as a key candidate involved in Al transport (contribution 12). Cinnamomum camphora is a widely used evergreen ornamental and street tree throughout subtropical East Asia, prized for its dense, shade-casting crown and fragrant blossoms [7]. In C. camphora, transcriptomic analyses under cold stress uncovered calcium- and ethylene-mediated signaling pathways, with ERF transcription factors acting as pivotal regulators, thereby providing a molecular framework for understanding how woody ornamentals perceive and respond to low temperature (contribution 13). At a broader scale, a field survey in northern Italy documented the emergence of new ornamental plant diseases under climate change. Beyond reporting new pathogens, this study highlighted the role of “living-lab” gardens as early warning systems and emphasized integrated management approaches, ranging from resistant cultivars and climate-aware cultural practices to biological control and molecular diagnostics, as essential tools to address shifting pathogen pressures (contribution 14).

Collectively, these studies support a systems-level view of stress adaptation in ornamentals, where molecular regulation, secondary metabolism, and plant–microbe interactions operate together to confer resilience. The contributions further delineate practical measures, including resistant cultivars, climate-adaptive cultivation, biological control, and molecular diagnostics, for sustaining ornamental plant performance under environmental change.

5. Conclusions

This Special Issue highlights major advances in ornamental plant physiology and molecular biology. Studies on floral traits have clarified the metabolic and transcriptional networks underlying pigmentation and development, while research on floral scent and secondary metabolism has identified key gene families shaping fragrance and defense. Work on stress adaptation has revealed the complex physiological adjustments and ecological interactions that sustain ornamental performance under adverse conditions.

Across these themes, the contributions delineate coordinated regulatory networks that connect pigment and floral development with the biosynthesis and allocation of volatile and non-volatile metabolites. They also show how stress-responsive processes, together with plant–microbe interactions, add an important layer of resilience to ornamental species. In doing so, the papers exemplify established integrative approaches that combine gene family analyses, expression profiling, metabolite measurements, and targeted functional assays to resolve mechanisms of trait formation and ecological performance.

Looking ahead, priorities include expanding research beyond a narrow set of model ornamentals to under-represented taxa and traits, such as fragrance complexity, floral longevity, and architectural features. Strengthening field and multi-environment validation of molecular findings and advancing comparative and translational studies that connect mechanism with breeding and cultivation, will be equally important. Continued attention to robust phenotyping and data standardization will further enhance reproducibility and cross-study synthesis. Taken together, the fourteen contributions deepen the scientific foundation of ornamental biology and outline practical directions for innovation in research, breeding, and sustainable cultivation.