The effect of UV radiation, time, and storage conditions on essential oil compounds and some physiological traits of Satureja mutica Fisch. & C. A. Mey.

Satureja mutica Fisch & C.A. Mey is a perennial plant that can be cultivated under irrigation conditions and in rainfed drylands. It has anti-flatulent, appetizing, sexual enhancement, anti-fungal and anti-bacterial effects. Drying increases product shelf life by stopping enzymes, pathogens, and microorganisms. The factors that affect essential oils and their content are temperature, drying time, and the type of plant. Microwaves for drying are one of the newer methods. Their advantages include shortening drying time, maintaining quality, reducing energy consumption, preserving dried plants' color, and improving effective ingredients.

The flowering shoots were collected from the Research Institute of Forests and Rangelands research farm in 2019. Plants in the insectarium were exposed to UV-B light treatment through UV-B lamps. The plants were turned over every half hour. Irradiation agents were selected based on the preliminary test, including durations of 0, 2.5, 5, and 10 hours. After irradiation, the samples were stored in sealed bags under two conditions: refrigerator at 4°C and room temperature. The storage time was 0, 120, and 240 hours. Therefore, the sources of variation include irradiation duration in four levels (0, 2.5, 5, and 10 hours), storage conditions in two conditions (room temperature 25-27°C and fridge with a temperature of 4°C), and storage duration factor in three levels (0, 120 and 240 hours).202 grams of fresh samples (equivalent to 80 grams of dry matter) and 80 grams of dried samples in other treatments were extracted by water distillation in 3 repetitions for 2 hours. The obtained essential oils were stored in sealed bottles inside the refrigerator at 4 °C until injected into GC and GC/MS devices for quantitative and qualitative analysis. Carbohydrate, phenol, and free radical inhibition (by the DPPH method) were measured. Data were analyzed using SPSS version 18, and the means were compared with the LSD test at P ≥ 0.05.

Variance analysis of essential oil percentage showed that UV-B factor, storage condition (SC), storage time (ST) at the 1% level, and SC ST interaction at the 5% level had a significant effect. Due to the increase in irradiation duration, the percentage of essential oil extracted from the plant increased. This percentage increased to 0.42% in the control and 0.71% in the 10-hour treatment. Based on a mean comparison, the amount of essential oil in the shade increased from 0.48% to 0.54%. Mean comparison of the effect of storage conditions (SC) on storage time (ST) revealed that the highest percentage was related to the shade storage treatment for 240 hours with 0.61%. The mean comparison of the interaction effect of the three treatments showed that the percentage of thymol was the highest at 45% in the conditions without irradiation refrigerator×without storage. The results of the simple correlation of the traits demonstrated that thymol had a significant negative relationship with α-thujone, α-terpenine, and carvacrol. Carvacrol showed a significant negative correlation with major compounds, paracymene and thymol. It was observed that the highest antioxidant activity was obtained from 10 hours of UV-B radiation. A mean comparison of the effect of UV-B rays on the amount of phenol indicated that its amount increased in the 2.5-hour treatment compared to the other treatments. The highest antioxidant percentage could be achieved after 10 hours of irradiation in the refrigerator for 240 hours.

The conversion of compounds occurs by irradiation of UV-B rays in the drying stages, and the amount of paracymene and thymol is reduced, and the amount of carvacrol is increased. Therefore, in S. mutica, which has thymol and carvacrol compounds, drying treatments aiming to increase the specific composition can be very effective. Also, the amount of thymol and carvacrol in dried plants may decrease compared to fresh plants. The increase in antioxidant activity in the treatment of 10 hours of UV-B radiation confirms the viability of plant cells and their efforts to prevent environmental stress through the increase of antioxidant enzymes.