Introduction
Rosemary (Rosmarinus officinalis L.) is one of the most economically important medicinal and aromatic plants in Lamiaceae family, owing to its high-value essential oil and diverse bioactive secondary metabolites. The yield and chemical composition of these metabolites are strongly influenced by environmental conditions and harvest stage. Although the individual effects of environmental factors and harvest timing have been widely investigated, studies simultaneously evaluating their combined influence under contrasting climatic conditions remain limited. Therefore, the present study aimed to investigate the interactive effects of growing region and harvest time on vegetative growth, photosynthetic pigments, phytochemical traits, essential oil yield, and volatile composition of rosemary cultivated under the diverse climatic conditions of Khuzestan Province, southwestern Iran.
Materials and Methods
The study was conducted from 2022 to 2024 in four commercial rosemary fields located in Dezful, Baghmalek, Dezpart, and Bidrobeh, Khuzestan Province, Iran. Plants were harvested in November (autumn), May (spring), and August (summer). The experiment followed a nested design, with harvest time nested within growing region. Plant height, fresh and dry biomass, chlorophyll a, chlorophyll b, total chlorophyll, carotenoids, total phenolic content, total flavonoid content, antioxidant activity (DPPH assay), and essential oil percentage were determined using established analytical methods. Essential oil was extracted by hydrodistillation using a Clevenger-type apparatus, and its chemical composition was characterized by GC–MS based on comparisons of mass spectra with the NIST library and retention indices. Data was analyzed using SAS software, and mean comparisons were performed using Duncan's multiple range test at P ≤ 0.05.
Results and Discussion
Harvest time exerted a substantially greater influence than growing region on almost all measured traits. While growing region had no significant effect on vegetative growth, photosynthetic pigments, phytochemical traits, or essential oil percentage, harvest time nested within growing region significantly affected nearly all evaluated characteristics. The highest fresh biomass was recorded during the August harvest in Baghmalek (761.00 g plant⁻¹), followed by Dezful (699.25 g plant⁻¹) and Bidrobeh (670.25 g plant⁻¹). The maximum dry biomass was also obtained in August, reaching 277.25 g plant⁻¹ in Bidrobeh, indicating that summer conditions favored vegetative biomass accumulation. Photosynthetic pigments exhibited pronounced seasonal variation. The highest chlorophyll a (0.80 mg g⁻¹ FW), chlorophyll b (0.44 mg g⁻¹ FW), total chlorophyll (1.24 mg g⁻¹ FW), and carotenoid content (0.39 mg g⁻¹ FW) were recorded during the May harvest in Dezful, whereas the lowest pigment concentrations occurred in Bidrobeh during August. These results suggest that moderate spring conditions promote pigment biosynthesis, whereas high summer temperatures reduce pigment stability. The highest total flavonoid content (8.60 mg QE g⁻¹ DW) was observed during the May harvest in Dezful, while the maximum total phenolic content (62.5 mg GAE g⁻¹ DW) was recorded during the August harvest in Dezpart. Antioxidant activity remained relatively stable across treatments but reached its highest value (80.92%) during the November harvest in Dezful. Essential oil percentage was highest during the May harvest in Dezful (3.73%) and lowest during the August harvest in Baghmalek (1.39%), indicating that spring conditions were more favorable for essential oil accumulation than summer conditions. GC–MS analysis identified approximately 35 volatile constituents, predominantly oxygenated monoterpenes, monoterpene hydrocarbons, and sesquiterpenes. The major compounds were 1,8-cineole, α-pinene, β-pinene, camphor, camphene, limonene, borneol, isoborneol, bornyl acetate, verbenone, caryophyllene, and α-caryophyllene. Harvest time had a markedly greater effect on essential oil composition than growing region. The highest concentration of 1,8-cineole (21.11%) was detected during the May harvest in Bidrobeh, whereas camphor (5.68%), borneol (4.52%), and bornyl acetate (3.53%) reached their highest levels during the November harvest in Baghmalek. These seasonal shifts indicate dynamic regulation of terpene biosynthesis throughout plant development in response to seasonal environmental conditions.
Conclusion
Harvest time was the primary factor influencing biomass production, phytochemical characteristics, essential oil yield, and volatile composition of rosemary under the climatic conditions of Khuzestan Province, whereas growing region exerted comparatively minor effects. Spring harvest optimized essential oil yield, photosynthetic pigment content, flavonoid accumulation, and 1,8-cineole concentration, whereas summer harvest maximized vegetative biomass production. Moreover, seasonal variation substantially altered the chemical profile of the essential oil, highlighting the importance of harvest timing as a practical strategy for tailoring rosemary essential oil to specific pharmaceutical, nutraceutical, food, and cosmetic applications.