Melatonin was found in plants in the late 1990s, but its function, signaling, and interaction with other phytohormones still unclear. Melatonin research in plants has increased substantially in recent years, including reports on the impact of this putative plant hormone under biotic and abiotic stress situations. Temperature extremes, salt, drought, hypoxia or anoxia, nutrient deficiency, herbicides, UV radiation stress, and heavy metal toxicity are all important obstacles to horticulture crop production worldwide. To deal with these environmental challenges, plants have evolved complex signaling networks. Phytohormones are essential for controlling plant growth, development, and stress responses. Melatonin, a pleiotropic chemical present in a variety of species, has recently emerged as a powerful regulator of plant abiotic stress tolerance. The purpose of this review is to investigate the interplay between melatonin and phytohormones in the control of abiotic stress responses in horticultural crops. We explore the interactions of melatonin with several phytohormones under various abiotic stresses.

Silicon plays a crucial role in enhancing plant tolerance to various abiotic and biotic stresses, including drought, salinity, heavy metals, and pathogen/pest attacks. Its application has shown promising results in improving stress tolerance and productivity in medicinal plants. This review synthesizes findings from numerous studies investigating the mechanisms by which silicon confers stress tolerance, including the regulation of antioxidant systems, water relations, nutrient homeostasis, phytohormone signaling, and stress-responsive gene expression. Additionally, it examines the effects of silicon supplementation on the production of valuable secondary metabolites and essential oils in medicinal plants. Silicon application can significantly mitigate stress-induced damage in plants, including medicinally important species such as borage, honeysuckle, licorice, Damask rose, savory, basil, and eucalyptus. The deposition of silicon in cell walls provides physical reinforcement and acts as a barrier against pathogen invasion and insect herbivory. Furthermore, silicon fertilization can enhance the production of valuable secondary metabolites in medicinal crops under stress conditions. The findings underscore the potential of silicon fertilization as a sustainable strategy for improving the productivity and quality of medicinal crops under changing environmental conditions, highlighting the need for further research to elucidate the molecular mechanisms underlying silicon-mediated stress tolerance and practical applications in medicinal plant cultivation.

Plant growth regulators can affect the primary and secondary metabolites of various plant species. However, the effect of paclobutrazol (PBZ) on the composition of lavender oil, especially related to the terpenoid pathway, is still unclear in literatures. In this study, the effect of PBZ as a foliar spray (0.200, 400 and 600 ppm) on the vegetative growth, phytochemical content, and both antioxidant and antimicrobial properties of lavender oil were investigated. The results indicated that all examined PBZ treatments led to a significant (p ≤ 0.05) decrease in growth parameters compared to the untreated plants. Meanwhile, the yield of essential oil was significantly decreased by the treatment of PBZ at 200 ppm compared to the control. In contrast, applied-PBZ significantly enhanced the chlorophyll content and displayed a marked change in the composition of the essential oil. This change included an obvious and significant increase in 3-carene, eucalyptol, γ–terpinene, α-pinocarvone, caryophyllene, β-vetivenene, β-santalol, ledol, geranyl isovalerate, farnesol, caryophyllene oxide, and phytol percentage. Generally, the highest significant values were achieved by the treatment of 400 ppm compared to the other treatments. Furthermore, this treatment showed the highest free radical scavenging activity against DPPH (1,1-diphenyl-2-picrylhydrazyl) by 13% over the control. Additionally, to determine the antimicrobial activities of the extracted oil, each treatment was examined against two strains of Gram positive bacteria (S. aureus and B. cereus), two strains of Gram negative bacteria (S. enteritidis and E. coli), and two fungal species (C. albicans and A. niger) represent the yeast modal and filamentous fungus, respectively. The findings demonstrated that all examined species were more sensitive to the oil that was extracted from lavender plants, treated with 400 ppm PBZ, compared to the other concentrations.

Catharanthus roseus is a medicinal plant that produces indole alkaloids, which are utilized in anticancer therapy. Vinblastine and vincristine, two commercially important antineoplastic alkaloids, are mostly found in the leaves of Catharanthus roseus. ĸ-carrageenan has been proven as plant growth promoting substance for a number of medicinal and agricultural plants. Considering the importance of ĸ-carrageenan as a promoter of plant growth and phytochemical constituents, especially alkaloids production in Catharanthus roseus, an experiment was carried out to explore the effect of ĸ-carrageenan on the plant growth, phytochemicals content, pigments content, and production of antitumor alkaloids in Catharanthus roseus after planting. Foliar application of ĸ-carrageenan (at 0, 400, 600 and 800 ppm) significantly improved the performance of Catharanthus roseus. Phytochemical analysis involved determining the amount of total phenolics (TP), flavonoids (F), free amino acids (FAA), alkaloids (TAC) and pigments contents by spectrophotometer, minerals by ICP, amino acids, phenolic compounds and alkaloids (Vincamine, Catharanthine, Vincracine (Vincristine), and vinblastine) analysis uses HPLC. The results indicated that all examined ĸ-carrageenan treatments led to a significant (p ≤ 0.05) increase in growth parameters compared to the untreated plants. Phytochemical examination indicates that the spray of ĸ-carrageenan at 800 mg L⁻¹ increased the yield of alkaloids (Vincamine, Catharanthine and Vincracine (Vincristine)) by 41.85 μg/g DW, total phenolic compounds by 3948.6 μg gallic/g FW, the content of flavonoids 951.3 μg quercetin /g FW and carotenoids content 32.97 mg/g FW as compared to the control. An amount of 400 ppm ĸ-carrageenan treatment gave the best contents of FAA, Chl a, Chl b and anthocyanin. The element content of K, Ca, Cu, Zn and Se increased by treatments. Amino acids constituents and phenolics compounds contents were altered by ĸ-carrageenan.

Pedilanthus tithymaloids is a popular temperate indoor succulent ornamental plant and is highly marketable. However, the appropriate propagation techniques should be adopted to solve the rooting issues of cuttings. Therefore, the present research was designed to specify the optimal indole − 3−butyric acid (IBA) concentration and rooting medium type for obtaining good rooting and healthy plants. Three IBA levels (0, 1.0, 2.0, and 3.0 g L^− 1) and five growing mediums (sand, sand: clay, sand: peat moss, sand: perlite, and sand: vermiculite as 1:1, v/v) were used. The experiment was performed in a randomized complete block design in three replications. The obtained data illustrated that roots number per cutting and rooting percentage reached the maximum values by treating P. tithymaloids cuttings with 3.0 g L^− 1 IBA x sand: perlite or sand: vermiculite medium. Cultivating the treated-IBA 3.0 g L^− 1 cuttings of P. tithymaloids in sand: perlite showed the most enhancements in plant height, branches number and leaves number, leaf area and herb dry weight. However, the heaviest root dry weight was noticed with IBA 3.0 g L^− 1 x sand: peat moss medium. Except sand medium, all other tested media in combination with IBA at 1.0, 2.0–3.0 g L^− 1 showed remarkable stimulation effect on chlorophyll content. Sand: vermiculite mixture x IBA at 2.0 g L^− 1 was the effective practice for improving carbohydrates percentage. By studying the anatomical changes of the adventitious root of P. tithymaloids plant, it was found that there was an increase in the root diameter, phloem and xylem tissues and xylem vessels diameter due to growing in sand: perlite x 3.0 g L^− 1 IBA. In conclusion, dipping the cuttings bases of P. tithymaloids in 3.0 g L^− 1 IBA and cultivation in sand: perlite considered the most effective combination practice for better rooting ability and growth and could be advisable for commercial production.

Melatonin can mitigate the adverse effects of salt stress in plants, but the combined effects of it and bacterial bio-stimulants on enhancing plant resilience to salinity remain poorly understood. To fill in such a knowledge gap, we investigated the interaction between melatonin and bacterial bio-stimulants on the growth, productivity, and physiological characteristics of Calendula officinalis seedlings under salt stress. Using a pot-sand culture method, we tested various concentrations of exogenous melatonin (distilled water, 50, 100 µM) combined with a bacterial mixture (5 ml/pot at 10⁸ CFU/ml) of Azotobacter chroococcum, Bacillus megaterium, and Pseudomonas fluorescens. The seedlings were subjected to different saline irrigation levels with NaCl (tap water, 42.8, 85.6, and 128.3 mM) over two growing seasons in 2023 and 2024. The experiment was designed as a factorial study using a randomized complete block design with three replicates. NaCl concentrations of 85.6 mM and 128.3 mM negatively affected seedling growth and yield. Foliar application of 100 µM melatonin combined with bacterial inoculation significantly enhanced leaf area and shoot dry weight. Inflorescence parameters improved notably, with increases in total carotenoids and flavonoids in petals. Total chlorophyll content, mineral percentages (N, P, K), and proline increased significantly, while sodium and chloride levels decreased. The activities of polyphenol oxidase and peroxidase increased significantly and also improved relative water content and reduced electrolyte leakage. Our findings suggest that the combination of exogenous melatonin and bacterial inoculation can effectively mitigate the adverse effects of salt stress on Calendula seedlings.