Physiological Response Of Old World Bulustem Under Enhanced UV-B And Drought Stresses

In this paper, old world bluestems grown in the pots were subjected to UV-B and drought stresses. There were three UV-B radiation treatments, some without UV-B supplementation (T0), ambient UV- B radiation plus supplemental 1.94 kJ m^-2 day-1 levels of UV-B radiation (T1), ambient UV-B radiation plus 3.0 kJ m^-2 day-1supplemental levels of UV-B radiation (T2). The drought treatments are as below: well-watered treatment (W), medium drought treatment (D1) and serious drought treatment (D2). We studied the effects of enhanced UV-B radiation and drought stress singly and there combination on the physiological characteristics of old world bluestem.The results from the experiments were shown as follows:1. Changes of photosynthetic pigmentIn all of the treatment, the contents of Chla,Chlb and Car decreased, but the ratio of Chla/b and Chl/Car almost unchanged. The result indicated that both UV–B radiation and drought stresses can inhibit the synthesis of photosynthesis pigment or damage the decomposition of photosynthetic pigments. UV-B radiation can aggravate the damage of drought stress to plant.2. Changes of The chlorophyll fluorescence parameters Under single UV-B or drought stress, the minimal fluorescence (Fo) and maximum fluorescence (Fm) decreased noticeably. However, under the combination of UV-B and drought stresses, Fo,Fm had slight variation though lower than the control. It indicated that there was some interaction between the two kinds of adverse factors. In all of the treatment, the ratio of the variable to maximum fluorescence (Fv/Fm) and the ratio of the variable to minimal fluorescence (Fv/Fo) had slight change. It may be that the stress had not damage the PSⅡ.3. Changes of the gas exchange indexes Pn,Gs and Tr increased with the enhancement of UV-B radiation, but Ci only changed a little Pn,Gs,Tr. The change of Gs, Ci, Tr and Ls indicated that the stoma activity was not the mostly effect factor of Photosynthesis of old world bulustem. The combination of UV-B and drought stresses can alleviate the damage of their single effect.4. Change of antioxidant enzymes activity Both the single sress and combination of UV-B and drought stress had no significant to SOD activity. POD activity decreased first and then increased with the enhancement of UV-B stress. POD activity increased significantly under drought stress, which indicated that the ability of scavenge H2O2 was enhanced. Both the UV-B radiation and drought stress could increase the CAT activity to resist the antioxidant ability. However, CAT activity declined significant under the combined treatment of UV-B radiation and drought stress.5. Changes of lipid PeroxidationMalondialdehyde (MDA) content was higher than control under the enhanced UV-B radiation or drought stress. It indicated that UV-B radiation and drought treatment induced lipid peroxidation. There existed correlation between relative conductivity and MDA content. They increased with the enhancement of drought degree and UV-B radiation. However, under the treatment of their combination, MDA content had a different trend with the relative conductivity.6. Changes of osmotic adjusting materialsThe contents of soluble protein and free proline declined obviously under the enhanced UV-B radiation (p<0.05). Drought stress increase soluble protein content slightly, but the free proline content decreased significantly compared with CK. When exposed to both the drought and UV-stresses, contents of soluble protein and free proline had a rising trend.7. Changes of flavonoid contents and PAL activityThe contents of flavonoid was increased considerably under enhancd UV-B radiation. PAL, which is the major enzyme of secondary metabolites synthetic, its activity obviously rose and cuased increasing of andflavonoids. Drought stress had no distinct effect on flavonoid content and PAL activity. However, Drought could relive the decrease of flavonoid resulted by UV-B radiation. When the old world bluestem were treated under UV-B stress, extra drought stress could increase the PAL activity distinctly(P<0.05).The old world bluestem started a series of characteristic respond to enhanced UV-B and drought stress to relieve the damage of adverse environment. The conclusion of this paper will help us understanding the combined effects of UV-B and drought stress on plants in global climate change,which would on happen in our country.