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Solar ultraviolet (UV) radiation mainly includes UVA (320–400 nm). UVA intensity varies depending on the season and geographic location, while it is projected to rise owing to climate change. Since it elicits secondary metabolism, additional knowledge on the UVA dependence of phytochemical production is required for both farmers and processors, particularly under natural settings. In this field study, the pheno-morphological traits and essential oil composition responses to UVA intensity were addressed in three Thymus species [T. daenensis (endemic to Iran), T. fedtschenkoi (semi-endemic), T. vulgaris (common thyme)]. During growth, three UVA levels (ambient, enriched, excluded) were realized in combination with spraying protectants [water (control), melatonin, glutathione, iron-zinc nanofertilizer]. In T. daenensis, enriched UVA caused early flowering. The height of T. daenensis was the longest under enriched UVA, and the shortest under excluded UVA. In control plants, enriched and excluded UVA stimulated the accumulation of oxygenated metabolites in T. daenensis and T. fedtschenkoi. Altogether, under enriched UVA some phenolic compounds (e.g., thymol, carvacrol, γ-terpinene) increased in the essential oil of all three species, but others decreased. In all taxa, glutathione caused a significant essential oil content reduction. Iron-zinc nanofertilizer increased essential oil accumulation in T. daenensis and T. vulgaris. Treatments also induced an alteration of the essential oil composition. In conclusion, cultivation regime effects on the essential oil quality (composition) and quantity were strongly species dependent. T. deanensis underwent the most consistent enhancement under UVA, making the species more adaptable to climate change, whereas T. fedtschenkoi the least.

In this study, screening of Artemisia dracunculus accessions was investigated under water deficit based on physiological and phytochemical traits. The results clearly indicated that water deficit significantly reduced the relative water content, chlorophyll, and carotenoid contents and increased malondialdehyde, electrolyte leakage, and antioxidant activities. The responses of tarragon accessions to water deficit, however, were inconsistent. The HPLC analysis revealed the presence of chlorogenic, syringic, ferulic, vanillic, chicoric, and p-coumaric acids as major phenolic acids, while quercetin and herniarin were detected as the predominant flavonoid and coumarin compounds in the extracts. Our findings revealed that the water deficit not only increased the amounts of herniarin, luteolin, apigenin, caffeic acid, and syringic acid, but also introduced quercetin that was not present under normal conditions in Estahbanat. Nevertheless, these results were highly impacted by the accession type. The results indicated that Hamadan, Varamin and Estahbanat accessions could be introduced as tolerant accessions. Given the very different responses of tarragon accessions to water deficit and the diversity between these accessions, the findings of the present study could be an effective step in identifying and achieving homogeneous, drought-tolerant and high-yield potential accessions, and may help tarragon breeding programs as well as development of cultivation.