Physiological and Biochemical Changes Induced by UV-B Radiation in Rosemary Plants Grown Under Salinity Stress

Plants are exposed to solar ultraviolet radiation due to use of sunlight for photosynthesis. Additionally, salinity in soil or water influences the plant productivity and quality considerably. Moreover, when plants are simultaneously exposed to multiple stresses, one form of stress can affect the response to other stress. Particularly, it has been shown that they can benefit from dual tolerance as salinity and UV-B radiation are applied together. In order to understand the effects of UV-B radiation and salinity stress on some physiological and biochemical parameters, one-year-old cuttings of rosemary plants were grown under different levels of ultraviolet B radiation (0, 4.32 and 6.05 kJ m^-2 d^-1) and salinity stress (control, 50, 100 and 150 mM NaCl). The results showed that 4.32 kJ m^-2 d^-1 UV-B treatment significantly increased plant biomass up to 17.9% as compared to control. However, by increasing salinity to 150 mM, plant biomass significantly decreased up to 18.1%, as compared to control. Regardless of UV-B treatments, plants, grown under 100 mM salinity stress, had produced 2.8 times higher total phenolic compounds (TPC) and also have greater antioxidant activity (33.1%) in comparison to control. Both treatments, enhanced UV-B radiation and salinity stress, significantly increased the concentration of proline, hydrogen peroxide (H₂O₂) and malondealdehyde (MDA). In relation of total soluble sugar (TSS) and ion content, both treatments acted in an opposing manner. In turn, the enhanced UV-B radiation decreased concentration of TSS and the Na⁺ content in leaves, salinity stress increased the concentration of TSS, as well as the Na⁺ content in leaves and root. The plants grown under 150 mM salinity level accumulated 5.32 and 2.83 times higher Na⁺ ions in leaves and roots, respectively, than control. In addition, salinity significantly decreased the relative water content (RWC), photosynthetic pigments and K⁺ content in leaves and roots. The interaction between UV-B irradiation and salinity showed that the UV-B radiation improved the K⁺ content in leaves, RWC and membrane stability and consequently resulted in a better tolerance of rosemary to salinity.