Methyl Jasmonate Counteract UV-B Stress in Barley Seedlings

The role of exogenously applied phytohormone methyl jasmonate (MeJA) in counteracting the ultraviolet B (UV-B) stress in barley seedlings was investigated. Barley seedlings (Hordeum vulgare L., cv. Alfa) 4 days old were supplied with 5 x 10(-5) m MeJA through the roots for 3 days and then exposed for 2 days for 5 h per day to UV-B (312 nm, biological effectiveness of UV-B radiation 28.8 kJ m(-2)day(-1)). The rate of (CO2)-C-14 fixation, PSI and PSII activities and chlorophyll content decreased, but flavonoids, H2O2, malondialdehyde, proline and UV-B induced compounds increased after UV-B treatment. The rate of photosynthetic oxygen evolution was more strongly inhibited by UV-B-irradiation than PSI and PSII efficiency. MeJA itself increased the content of free proline, which acts as a stress protector due to its radical scavenging ability. Increased superoxide dismutase, catalase and peroxidase (POX) activities in the leaves and in the roots and the POX isoforms induction revealed the MeJA involvement in plant tolerance to oxidative stress caused by UV-B irradiation. It was shown that pre-treatment with MeJA counteracted UV-B stress. Therefore, it was suggested that MeJA could acts as a mediator in plant defense responses to UV-B irradiation by enhancing the activity of antioxidant system and free radical scavenging capability of plant cells.