Research on biosynthesis of UV absorbing substances in epidermis of plant leaves and their function to protect the plants from injury by UV radiation.

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Radiation of ultraviolet-B (UV-B) region (280-320nm) causes growth retardation of plants and visible injury such as chlorosis on leaf surface. These damages seem to be under the control of the environmental conditions during plant growth. The extent of damage is thought to depend on the differences of content of ultraviolet (UV) absorbing substances in the epidermis of plant leaves. In the present studies, the content of UV absorbing substances in leaf epidermis of pea mutant and broad bean was compared with resistance of these plants to ultraviolet-C (UV-C). In the next experiments, effects of UV-B radiation on first leaves of cucumber plants and effects of environmental conditions on the sensitivity of plant to UV-B were examined. Effects of UV-B radiation on enzymic properties of glutathione reductase (GR), rate-limiting enzyme in metabolic pathway to detoxify hydrogen peroxide, were also examined. Outline of the results obtained are as follows.(1)Epidermis of pea leaves has flavono … More id-like substances, which have absorbance peaks at 260 and 320 ns and little absorbance in visible region. The content of these substances showed a parallelism with resistance to UV-B radiation.(2)UV-B irradiation caused the growth retardation of cotyledons and first leaves of cucumber seedlings and chlorosis on first leaves at 20ﾟC. But, the growth retardation could not be found on the irradiation at 25ﾟC, though chlorosis was observed.(3)high intensity of ultraviolet-A (UV-A) restored the retardation of growth of first leaves when it was given together with UV-B. But, chlorosis caused by UV-B was not restored by UV-A radiation.(4)UV-B radiation lowered the affinity of GR in cotyledons to substrates. It was clarified in the present work that the sensitivity of cucumber plants to UV-B was dependent on temperature and light condition at the time of UV-B radiation. Mechanism of plant damage caused by UV-B radiation, defense mechanism to UV-B and, in addition, effects of environmental conditions on biosynthesis of UV absorbing substances must be more investigated. Less