Ultraviolet-B (UV-B) radiation is increasing in the result of stratospheric ozone depletion and other unknown factors. It has been supposed that increase in UV-B radiation will cause the various detrimental effects on plants. A variety of experiments have been conducted to examine the effect of UV-B and enhanced UV-B radiation. UV-B irradiation causes the growth reduction and the visible injury on the leaves, probably due to the DNA damage produced by UV-B radiation. Most of the DNA damage products caused by UV-B are cyclobutane pyrimidine dimmer, which is restored by photolyase using energy of UV-A to blue light region. Photolyase gene in cucumber seedlings began to be transcripted about 3 hr after sunrise, and showed the highest activity of photolyase about at noon. In addition, the enzyme activity was highest at 25 to 30℃. Cucumber seedlings showed high resistance to UV-B in the experimental conditions with high photolyase activity, suggesting that plant damages, caused by UV-B, is mediated by DNA damage. Plants have two mechanisms related to protection from UV-B radiation. One is the restoration of DNA damages mainly catalyzed by photolyase as mentioned above, and the other is UV-absorbing substances, phenolic substances such as flavonoids, which are accumulated in vacuoles of epidermal cells. The former, photolyase activity, is high in the young stage of leaves and rapidly decreases with aging. In contrast, the later, phenolic substances, is low at young stage and increases with progress of age. The enzyme activity and amount of the substances related to the two protection mechanisms seem to have complemental relation each other.Recently, it has been shown that photolyase is induced by light with wavelength of around 310 nm. DNA damage is induced by light with wavelength below 300 nm. Activation of photolyase is induced by light with wavelength of 375-425 nm. Intensity of sunlight is stronger with longer wavelength. Thus, the DNA damage caused by UV-B in plants is effectively restored in the natural light condition, resulting in plant survival under severe sunlight
