**5. Conclusions**

This study provides useful information on the seasonal variations of the quality and quantity of secondary metabolites as well as the antioxidant activity of the Arabian lilac. The results showed that weather conditions in the early spring and autumn were favorable to produce essential oil and increase the amounts of β-caryophyllene, sabinene, and manool. Although EO content decreased during the summer and winter seasons, the plants collected in these seasons contained higher caryophyllene oxide, phytol, (6Z,10Z)-Pseudo phytol, manool oxide and 7 α-hydroxy-manool. The optimum time of harvesting plants to achieve the highest quantity of flavones and flavanols was summer, while the samples collected in the winter are rich in anthocyanins. It is recommended to harvest the plant in summer and winter, when it exhibits the highest TPC, TFC, and has the most powerful antioxidant activity. Therefore, the knowledge of the impact of environmental factors, such as seasonal changes, can help producers to choose the best period for plant harvesting and production of plant products richer in the desired compounds for exploitation in pharmaceutical and food industries.

**Author Contributions:** M.M.S. designed the experiments, harvested the materials and analyzed the essential oils. A.B.D. and P.B.D. performed the bioactivities. M.M.S. and A.B.D. wrote, and S.V., M.I., and J.M. revised the manuscript.

**Funding:** This project was financially supported by the Shahid Chamran University of Ahvaz.

**Acknowledgments:** The authors are thankful to Mehrangiz Chehrazi (Department of Horticultural Science, Faculty of Agriculture, Shahid Chamran University of Ahvaz, Ahvaz, Iran) for her excellent assist in taxonomical identification of plant material.

**Conflicts of Interest:** The authors declare no conflict of interest.
