| Summary: | 碩士 === 國立中興大學 === 生命科學系 === 92 === Photosynthesis is energy source for plant growth and development. And the efficiency of photosynthesis has a closely relationship with environmental conditions. Low temperature results in a net decrease in photosynthetic efficiency. In high light environment plants absorb energy that exceeds the consumed by photosynthetic processes. Too much energy can lead to increased production of reactive oxygen species (ROS). Then ROS damage the photosynthetic apparatus and decrease photosynthetic efficiency. We would like to understand how low temperature and high light damage photosynthesis. So we study green alga Chlorella which has a chloroplast structure comparable to that of higher plants. In addition, we want to clarify the relationship between the growth and the activity of superoxide dismutase (SOD) which is the first line of cellular defense against oxidative stress. Chlorella DT and 8b are almost alike in their physiology, but DT strain is a desiccation tolerance strain. With optimal illumination of 120 ■mol m-2s-1, the cell growth rate and chlorophyll fluorescence Fv/Fm value were nearly the same between DT and 8b at various temperature. When temperature was below 15oC, DT and 8b could not survive. The SOD activity of DT was higher than that of 8b at low temperature. With higher illumination of 240 ■mol m-2s-1, Fv/Fm rations of DT decreased more greatly than those of 8b in the early period of low temperature acclimation. As for SOD activity, we found that DT had higher SOD activity and more induced isoenzymes than 8b at 15oC. In our study, we approve that a combination of high light and low temperature greatly increases the photoinhibition process. DT uses SOD more efficiently than 8b to decrease the damage of photoinhibition under the stress of low temperature.
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