Abstract: Taking Medicago sativa as an experimental material, the experiment was implemented in the plastic greenhouse in Shenyang, April-October, 2015. Lanthanum (La) was sprayed in the early flowering stage, conducted nine treatments, which were CK (no UV-B radiation and La treatment), T1 (only subjecting to 5% UV-B radiation), T13 (subjecting 5% UV-B radiation and spraying 30mg·L-1 La), T16 (subjecting 5% UV-B radiation and spraying 60mg·L-1 La ), T19 (subjecting 5% UV-B radiation and spraying 90mg·L-1 La), T2 (only subjecting to 10% UV-B radiation), T23 (subjecting 10% UV-B radiation and spraying 30mg·L-1 La), T26 (subjecting 10% UV-B radiation and spraying 60mg·L-1 La ), and T29 (subjecting 10% UV-B radiation and spraying 90mg·L-1 La), to reveal the effects of spraying La in early flowering stages on physiological characteristics of Medicago sativa under enhanced UV-B radiation. The results showed that, with the increased of UV-B radiation, Chla, Chlb, A, E, gs, Ci, Fv/Fm, qP and NPQ declined compared with those of CK, so it was concluded that UV-B radiation can reduce the photosynthetic characters of Medicago sativa leaves; On the contrary, with sprayed the concentrations of La (sprayed the 30mg·L-1 of La under UV-B radiation enhanced by 5%, sprayed the 60mg·L-1 of La under UV-B radiation enhanced by 10%), the above indicators significantly enhanced by 12.91%, 11.93%, 22.22%, 22.25%, 96.26%, 7.10%, 12.85%, 11.55%, 123.66% and 140.81%, 63.12%, 37.90%, 22.80%, 108.28%, 6.08%, 3.81%, 37.95%, 199.68%, respectively. Results showed that sprayed La can alleviate the damage of photosynthetic characters of Medicago sativa leaves under UV-B radiation. In addition, the content of soluble sugar and soluble protein was also increased in the growth period, it demonstrated that UV-B radiation and La could promote the synthesis of sugar and protein of Medicago sativa leaves.

Key words: Lanthanum(III), UV-B radiation, Medicago sativa, Photosynthetic characteristics, Chlorophyll fluorescence parameters