Physiological Adaptation Of Alfalfa (Medicago Sativa) To Enhanced Ultraviolet Radiation And Soil Water Deficit

Alfalfa is one of the most important legume forages all over the world. It is wildly planted in China and plays important economic, ecological and social roles in animal production, grassland establishment, and grassland protection. However, China is a country facing severe water shortage. which limits the distribution of alfalfa in arid areas. Meanwhile, the increase of Chlorofluorocarbons induced from human activities reduced the concentration of O3 in atmosphere. resulted in enhanced ultraviolet radiation, thus severely influenced growth and development of alfalfa.In the current study, two alfalfa (Medicago sativa) cultivars with different drought resistances, Aohan (high resistance) and Sanditi (low resistance), were used to analyze the responses of leaf cuticular wax content and crystal structure, leaf water potential (LWP), contents of malondialdehyde (MDA) and chlorophyll, and ultraviolet absorbing compounds under soil water deficit and enhanced ultraviolet radiation (UVR) stresses. The ultraviolet radiation includes three doses,0 w/m2(Control),0.05 w/m2 and 0.10 w/m2. The radiation was applied from 12:00 to 13:00 using ultraviolet lamp. distance of which was adjusted everyday to keep constant radiation dose. Soil water deficit includes two levels, normal water treatment (maintaining 75% of the saturated water capacity), and drought treatment (maintaining 40% water capacity). The radiation treatment was applied for 10 consecutive days with 3 replicates after soil water condition matched treatments. The main results were:1. Alfalfa cultivar. ultraviolet radiation and soil water deficit significantly influenced the contents of leaf cuticular wax and chlorophyll, and leaf water potential. Cultivar and water treatment existed significant interaction effects on content of wax.2. The leaf surface of alfalfa was covered by thick wax platelets, vertically distributing over leaf surface. Solely water deficit had no significant effect on wax crystalloid.3. Under the conditions of 0.05 w/m2 UVR dose or combined treatments of UVR and soil water deficit. the platelet wax crystalloid on leaf surface of both alfalfa cultivars melted, which increased covering areas and UVR reflection rate, and thus would reduce cuticular water loss.4. Under 0.1 w/m2 UVR dose, some crystal platelets paralleling leaf surface while vertically distributing above other crystalloid, appeared on leaf surface of Aohan. which efficiently reduced the effect of UVR on plant physiology, while the wax crystalloids of Sanditi further melted.5. The content of leaf wax content in Aohan was significantly higher than that in Sanditi. Under combined treatments of UVR and soil water deficit, high-drought resistant cultivar maintain higher level of LWP to control water loss by increasing wax deposition on leaf surface.6. The contents of ultraviolet absorbing compounds had no significant response to enhanced UVR, indicating that the wax crystalloid structure on alfalfa leaf surface had direct defense mechanism to UVR, which partly might delay or replace the defense mechanisms from secondary metabolic compound such as ultraviolet absorbing compounds.