| Enhanced UV-B Radiation resulting from ozone depletion is one of the global environmental problems. Up to now, the effects of UV-B Radiation on organisms have been studied extensively. However, the investigated materials in these studies were mainly herbaceous plants, very few woody plants were explored, Furthermore, most of these experiments were conducted indoors with strickly controled conditions, the conclusions from these studies have many inconsistencies. In this experiment, 1 used the woody plant, ginkgo biloba L., as my experimental material. The ginkgo biloba plant were treated with UV-B Radiation with the intensities of 0 u W.cm-2(CK), 10.0 W.cm-2(L) and 22 u W.cm~-2(H), the duration of UV-B treatment was eight hours per day(from AM9:00 to PM17:00), all plants were treated with thirty-two days.The effects of UV-B Radiation on the photosynthesis, metabolisms of nitrogen, nucleic acids, secondary substance and active oxygen and cell membrane system were studied, the results showed as follows:(1) Effects of enhanced UV-B Radiation on photosynthesis of Ginkgo bilobaIncreased UV-B Radiation promoted the photosynthesis of Ginkgo biloba in a short-term, the photosynthetic pigments(including chla, chlb and CAR), Hill reaction activity and net photosynthetic rate of ginkgo biloba leaves increased in the first sixteen days. However, with the passage of time enhanced UV-B Radiation tended to inhibit the photosynthesis of ginkgo biloba, and the content of photosynthetic pigments, Hill reaction activity and net photosynthetic rate of ginkgo biloba leaves tended to decrease. Enhanced UV-B Radiation reduced the ratio of chla/chlb, it indicated that the chla/chlb-binding proteins were destroyed. In addition, the stomatal resistance of ginkgo biloba leaves exposed to high UV-B radiation increased, but that of leaves exposed to low UV-B radiation did not. The analysis showed that the degradation of photosynthetic pigments and the damage to the structure and function of PS II were the main reasons that led to the decrease in photosynthetic rate of ginkgo biloba leaves.(2) Effects of enhanced UV-B Radiation on the nitrogenous metabolism in ginkgo biloba leavesThe activities of NR and GPT in ginkgo biloba leaves exposed to enhanced UV-B Radiation increased firstly, and then decreased. The GDH activities of treated plants were lower than those of control plants during the whole experiment. Furthermore, the GDH activities of plants in H group were lower than those of plants in L group. Enhanced UV-B Radiation reduced the contents of NO3"-N in ginkgo biloba leaves. Above mentioned results showed that enhanced UV-B Radiation influenced the absorption of nitrate, assimilation of nitrogen and synthesis of amino acids, and might further affect the synthesis of proteins. As for the metabolism of proteins, the activities of caseolytics increased obviously in ginkgo biloba leaves exposed to enhanced UV-B Radiation, it also increased with the increase of UV-B Radiation and the exposure time. Compared with the control plants, the contents of soluble proteins in leaves of treated plants increased firstly, and then decreased. The contents of soluble proteins in leaves of treated plants were lower than those of control plants at the later stage. The contents of total free amino acids increased in ginkgo biloba leaves exposed to UV-B Radiation, content of total free amino acids in leaves of plants in H group was higher than that of plants in L group, the foliage contents of soluble proteins in treated plants increased at first was probably due to the the synthesis of some resistance proteins induced by UV-B Radiation. Because of the increase in proteolytic enzyme activities, the soluble proteins contents of treated plants decreased, and the free amino acids as the hydrolytic products of proteins increased. In aword, under UV-B Radiation, the nitrogenous metabolism in ginkgo biloba leaves was in disorder, which decreased the activities of synthetases, but increased the activities of lysis enzymes.(3) Effects of enhan...
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