Effect of Rare Earths on Plant under Supplementary Ultraviolet-B Radiation: Ⅰ Effect of Cerium on Growth and Photosynthesis in Rape Seedlings Exposed to Supplementary Ultraviolet-B Radiation

Effect of cerium （Ce 3+） on growth and photosynthesis in rape seedlings exposed to two levels of ultraviolet-B radiation （UV-B, 280～320 nm） was studied with hydroponics under laboratory conditions. The growth of rape seedlings exposed to two levels of UV-B irradiation （0.15 and 0.35 W·m -2/T2） was both heavily restrained. The aboveground growth indices including stem （plant） height, leaf number, leaf area, leaf fresh/dry weight and stem fresh/dry weight were obviously decreased by 13.2%～44.1% （T1） and 21.4%～49.3% （T2）. Compared to CK, and except active absorption area of roots, the belowground indices main root length, root volume and fresh/dry weight by 14.1%～35.6% （T1） and 20.3%～42.6% （T2）, respectively. For Ce+UV-B treatments, the aboveground and belowground growth indices were decreased by 4.1%～23.6%, 5.2%～23.3% （Ce+T1） and 10.8%～28.4%, 7.0%～27.8% （Ce+T2）, lower than those of UV-B treatments mentioned above. These results show that Ce has protective effect on plant against injury of UV-B radiation. Furthermore, the protective effect of Ce on seedlings exposed to T1 level of UV-B radiation is superior to T2 level. Chlorophyll content, net photosynthesis rate, transpiration rate, stomatal conductance and water use efficiency in UV-B treatments decrease dramatically, whereas intercellular CO2 concentration increases. Although these indices in Ce+UV-B treatments decrease compared with those of CK, the decrease in Ce+UV-B treatments are lower than those in UV-B treatment. This phenomenon indicates that the ecophysiological protective effect of Ce is based on improving photosynthesis in plants. The dynamic curves of photosynthesis indices show that the course of light-repair is shortened and the injury to rape seedlings by UV-B radiation stress is alleviated by Ce.