Effects Of Enhanced UV-B Radiation On Growth And Ecophysiological Of Acer Davidii And Betula Albo-sinensis Seedlings

(Chengdu Institute of Biology, Chinese Academy of Sciences, Chengdu 610041) Abstract: At Maoxian Ecological Station of Chinese Academy of Sciences located in northwest Sichuan province, 2-year-old seedlings of two native maples (Acer davidii and Betula albo-sinensis) were potted and grown in the open. The plants received an enhanced UV-B radiation (280-320 nm) of 0.27μw·cm-2·s-1 (7.7 KJ·m-2·d-1) was applied with an artificial lamp system, which approximated the predicted enhanced UV-B reaching the earth surface when stratosphere ozone was depleted by 15% in the local area, and the control plant received the ambient UV-B. During the experimentation, the height and diameter of every seedling was measured, and the gas exchange was made to examine the capacity of photosynthesis. Chlorophyll fluorescence were used to determine changes in the efficiency of light utilization for electron transport, the occurrence of photoinhibition of photosystem II photochemistry and the possibility of stomatal patchiness across leaves. Additionally, leaves were sampled to determine the chlorophylls, carotenoid and UV-absorbing compounds content. After four months' treatments, the seedlings were sampled to determine the total biomass, leaf area, N and P content and morphological of transverse section .The results show as follows: 1 The total biomass, total leaf area, leaf number and single leaf expanding of both species were reduced by enhanced UV-B treatment.The enhanced UV-B radiation slowered the growth of Acer davidii and accelerate the growth of Betula albo-sinensis. 2 Enhanced UV-B had no singnificant effects on N and P contnts of both species. 3 Results showed that chlorophyll content of both species were marked reduced. The effects of enhanced UV-B radiation on the photosynthesis of Acer davidii were that the maximal net photosynthetic rate was significantly lowered, the respiration rate and light compensation point were raised, but the quantum yield was not significantly affected. Simultaneity, the results showed that the enhanced UV-B had no effect on the maximal net photosynthetic rate and light compensation point of Betula albo-sinensis, lowered its respiration rate and quantum yield. The diurnal changes in net photosynthetic rates, water use efficiency, quantum efficiency of photosystem II centers (Fv/Fm) and quantum yield of photosystem II photochemistry (ΦPSП) of Acer davidii seedlings were reduced under enhanced UV-B radiation, while diurnal changes in net photosynthetic rates, water use efficiency of Betula albo-sinensis was unaffected and its quantum efficiency of photosystem II centers (Fv/Fm) and quantum yield of photosystem II photochemistry (ΦPSП) were increased. 4 The enhanced UV-B radiation reduced the epidermis thickness of Acer davidii and palisade tissue thickness of Betula albo-sinensis. Contrary to what was expected, enhanced UV-B radiation decreased the carotenoid UV-absorbing compound content in leaves of both species suggesting a reduction in protection provided.