Background and objectives: Ultraviolet (UV) radiation, as a part of the electromagnetic spectrum, has significant consequences for plant physiology. The increased depletion of the stratospheric ozone layer due to industrial activities has led to an increase in UV-B radiation reaching the Earth's surface. This radiation acts as an oxidative stressor and, by generating reactive oxygen species (ROS), damages cellular components such as DNA, proteins, and membrane lipids. In response, plants employ complex defense mechanisms, including enzymatic (e.g., catalase and peroxidase) and non-enzymatic (e.g., phenols and carotenoids) antioxidant systems. Among these, medicinal plants are of particular importance due to their inherent ability to produce valuable secondary metabolites. Periwinkle (Vinca minor L.), from the Apocynaceae family, is a medicinal and ornamental plant containing valuable indole alkaloids such as vincamine, known for its modulating effects on cerebral blood circulation and neuroprotective properties. However, the physiological responses of this plant to UV radiation, especially considering the variable of radiation pattern (intermittent vs. continuous), have been less studied. This research was designed and conducted to investigate the effect of different types of UV radiation (UV-A and UV-B) under two patterns of intermittent and continuous irradiation on the physiological characteristics, antioxidant defense system, and secondary metabolite content of periwinkle.
Materials and Methods: This research was conducted as a completely randomized design with 7 treatments and 3 replications in the greenhouse of Malayer University (Hamedan, Iran). Periwinkle plantlets, after in vitro culture in MS medium and rooting, were transferred to pots containing Hoagland's solution. After one week of establishment, they were exposed to UV radiation treatments for 9 days. The treatments included: control (only fluorescent light), intermittent UV-A, continuous UV-A, intermittent UV-B, continuous UV-B, intermittent UV-AB, and continuous UV-AB. UV radiation was applied using 40-watt broadband lamps manufactured by Q-Lab (USA), providing the closest match to the natural solar spectrum, for 45 minutes in the middle of the 16-hour photoperiod. The intermittent pattern consisted of three 15-minute periods with 15-minute intervals, and the continuous pattern involved 45 minutes of uninterrupted radiation. At the end of the treatment period, leaf samples were harvested and immediately frozen in liquid nitrogen. The measured traits included the activity of antioxidant enzymes (catalase, peroxidase, and ascorbate peroxidase), malondialdehyde (MDA) content, photosynthetic pigments (chlorophyll a, b, and carotenoids), total phenol, proline, soluble sugars, and fresh and dry root weight.
Results: The results showed that UV radiation had a significant effect on most of the measured physiological traits. In the enzymatic defense sector, the highest catalase enzyme activity was observed in the intermittent UV-A treatment (0.44), and the highest ascorbate peroxidase activity was recorded in the intermittent UV-AB treatment (0.55), both showing a significant difference from the control. However, guaiacol peroxidase activity was not affected by the treatments. Regarding photosynthetic pigments, the intermittent UV-B treatment exhibited a powerful photoprotection strategy, marked by a significant peak in carotenoids (0.93 mg/g) and a sharp decline in chlorophyll b (0.1 mg/g). The continuous UV-B treatment also showed the highest chlorophyll a (0.85 mg/g) but failed to increase carotenoids significantly. Total phenol content increased significantly compared to the control only in the intermittent UV-B treatment (28.49 mg GAE/g), identifying it as the most effective elicitor of the phenylpropanoid pathway. A key and unexpected finding was the significant decrease in malondialdehyde in all UV treatments compared to the control, indicating the remarkable efficiency of the plant's antioxidant defense system. Soluble sugar content decreased significantly in the intermittent UV-B treatment, reflecting the consumption of carbon reserves to supply the energy and carbon skeletons required for the biosynthesis of defensive metabolites. Proline changes were significant only in the continuous UV-B treatment, showing a decrease compared to the control. Fresh and dry root weight showed no significant change under any treatment.
Conclusion: This study demonstrated, for the first time, that the physiological response of periwinkle to UV radiation depends not only on the type of spectrum but also on the irradiation pattern. Intermittent UV-B, as the most efficient elicitor, achieved a level of cellular protection by coordinated induction of enzymatic (increased catalase) and non-enzymatic (significant increase in total phenol and carotenoids) antioxidant defenses, reducing lipid peroxidation even below the control level. This defensive success was accompanied by the consumption of sugar reserves, highlighting the metabolic cost of these responses. In contrast, continuous UV-B, by decreasing proline and failing to increase phenols and carotenoids, likely exceeded the plant's tolerance threshold. From a practical perspective, the results of this study demonstrate the potential of intermittent UV-B radiation as a non-chemical elicitor for enhancing secondary metabolites in periwinkle; however, confirming its commercial application requires further complementary studies on a larger scale, over longer growth periods, and with an evaluation of final plant yield.