| In recent years, there have been a large number of anthropogenic pollutants such as chlorofluorocarbons (CFCs) and other ozone antagonists were released into the stratosphere. The depletion of the stratosphere ozone is leading to increasing levels of solar ultraviolet-B radiation reaching the Earth's surface. In addition, a lot of CO2 and other greenhouse gases were released into the atmosphere since the burning of fossil, which is leading to the growing global climate warming. Therefore, UV-B radiation, the greenhouse effect and acid rain have become the three major environmental problems affected human survival. Currently, there have been a lot of researches about the effect of single-factor such as the UV-B radiation and high temperature on plants, but the researches about interactive effects of UV-B radiation and high temperature on plants were less, and these researches were only confined to a few plants and a lower temperature range. From the information available shows that the negative effect of UV-B radiation could be reduced by appropriate temperature, which mainly as photosynthetic rate were increased of plants by the appropriate temperature. However, the way of the interaction between the UV-B radiation and high temperature is universal? When the temperature is further raised, while the impact way of the interaction between the UV-B radiation and high temperature might change, these problems need to be further studied. In the present study, the wheat seedlings were used as plant materials, the growth and the antioxidant system of wheat seedlings under three temperature (26℃,31℃and 36℃)combined with UV-B radiation (4.3KJm-2·d-1) were been studied. The purpose of the study is to reveal the similarities and differences impact ways between the UV-B radiation and different temperatures. Followings are the main results and conclusions:1. The growth of wheat seedlings could be promoted by appropriate temperature, but the growth was inhibited by too high temperature significantly:the activity of SOD, CAT and APX and the content of Flavonoids and Car were increased under 31℃treatment, but its ability to eliminate reactive oxygen species may be limited, the reactive oxygen species such as O2·- and H2O2 were maintained at a high level, the lipid peroxidation (the content of MDA) and leaf relative conductance were still higher significantly, these showing that the temperature caused a certain degree of oxidative stress on the plant. However, the content of chlorophyll was increased by this temperature, indicating the oxidative damage at this time was not enough to affect the growth and development of plant. So, the reason of the promotion on the growth of plant by appropriate temperature is that the promotion of photosynthesis, meanwhile, the ability of plant resistance to oxidative stress is also improved. Although the activity of SOD, CAT and APX was still higher significantly under 36℃treatment, other antioxidant enzymes (POD and GR) and non-enzyme antioxidants (Flavonoids, Car and GSH) were inhibited by high temperature significantly, and thus the total active oxygen scavenging capacity of plant decreased significantly, showing the levels of reactive oxygen species such as O2·- and H2O2 were further increased, the degree of lipid peroxidation (reflected by the MDA content) deeper and membrane integrity (could be seen by the relative conductivity) were damaged significantly, and the content of chlorophyll lower, the photosynthetic capacity decreased significantly, all these inhibited the growth of plant.2. UV-B radiation inhibited the growth of wheat seedlings under room temperature (26℃). Because, on the one hand, UV-B radiation could cause the reactive oxygen species such as O2·- and H2O2 in a high level, the lipid peroxidation increased and the membrane integrity impaired; on the other hand, UV-B radiation deceased the content of chlorophyll, and inhibited the photosynthesis. Under this temperature, the activity of antioxidant enzymes such as SOD, APX, CAT and POD were higher, the content of non-enzyme antioxidant such as Flavonoids and Car were increased, these all show that, on the one hand, the plant can resist the damage caused by UV-B radiation by improving the antioxidant capacity, on the other hand, these also show the capacity of the defense system is limited.3. The UV-B radiation had no distinct inhibition on the growth of wheat seedlings under 31℃, it may be related to the UV-B radiation could further promote the activity of the antioxidant enzymes such as SOD, APX and CAT increased, and the content of non-enzymes antioxidant such as Flavonoid and Car increased, and thus the ability of scavenging reactive oxygen species of plant was enhanced further, so that the level of reactive oxygen species such O2·- and H2O2 were not change, and there were not affect on the level of membrane lipid peroxidation, membrane integrity and the content of chlorophyll apparently. All these show that the appropriate temperature will help improve the resistance ability of plant on the UV-B radiation.4. Though the UV-B radiation could increase the activity of SOD, APX and CAT further under 36℃, the activity of POD and GR and the content of Flavonoids, Car and GSH were lower further at the same time, Thus the overall active oxygen scavenging ability of plant reduced further, it caused the level of reactive oxygen species such O2·- and H2O2 increased, membrane lipid peroxidation intensified further and the membrane integrity damaged serious; the content of chlorophyll declined significantly, photosynthetic capacity decreased at the same time, which inhibited the growth of plant. It can be seen that when the temperature is too high, the high temperature and UV-B radiation exacerbate the damage to the plant in a synergistic manner. In short, the interactive effect between UV-B radiation and high temperature vary duo to temperature. Appropriate temperature (such as 31℃) is help for enhancing the resistance to UV-B radiation of plant; but when the temperature is too high (such as 36℃), the interactive effect of high temperature and UV-B radiation exacerbates the damage to the plant. The different effect of UV-B radiation on the growth of plant under different temperatures is the overall performance of the different effects on active oxygen metabolism system and photosynthesis of UV-B radiation under different temperatures. |
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