UV-B誘導芥藍切離葉片老化過程中之抗氧化反應

Abstract

UV-B是波長為280~320 nm之輻射線，由於現代工業釋放出空氣污染物氟氯碳化物（chloroflurocarbons, CFCs），使得大地平流層臭氧含量減少，導致地球表面接受的UV-B輻射量增加。本研究之目的為了解UV-B所促進的芥藍切離葉片老化與氧化逆境的關係，藉由分析抗氧化酵素之活性及內生抗氧化物之變化，以明瞭葉片受到UV-B傷害時抗氧化系統之反應。本試驗以芥藍‘蕙津’ (Brassica oleracea L. Albograbra Group cv. ‘Veg Gin’) 苗齡21天之第二片本葉為試驗材料。切離葉片插於裝有去離子水的三角瓶中，給予短時間過量UV-B照射處理(0、0.5 kJ m-2、1.5 kJ m-2)，再將切離葉片移至25℃黑暗觀察4 天。結果顯示隨著UV-B處理劑量提高，葉綠素計(SPAD) 讀值下降幅度顯著，說明UV-B處理確實可以加速芥藍切離葉片之老化。葉片經UV-B照射後第1天會促使內生H2O2 、MDA與逆境乙烯含量的開始增加，且明顯高於對照組，表示有氧化逆境之產生。本試驗結果顯示葉片經UV-B照射後，葉片中SOD的活性會減少，APX、GR、POX 與CAT 等抗氧化酵素比活性分別在1至3天後增加，抗氧化物 (GSH、Ascorbate) 等含量也被誘導增加，表示葉片中清除H2O2之機制會被誘導增加。在本試驗中，切離葉片的抗氧化酵素比活性與抗氧化物質雖因氧化逆境發生而增加，由於無法繼續維持細胞內氧化逆境的平衡，因而導致葉片之死亡。

Abstract Ultraviolet-B (UV-B) is the ultraviolet radiation at wavelengths between 280 and 320 nm. The concern about the increasing exposure of living matter on earth’s surface to UV-B radiation has been rising in the last two decades, owing to the depletion of stratospheric ozone by the accumulation of chlorofluorocarbons and other air pollutants. It is important to know the effects of UV-B radiation on the physiological activities of organisms living on earth. The purpose of this research was to study the changes in the antioxidant system in excised leaves of Chinese kale in response to UV-B radiation, and to find out the relationship between oxidative stress and UV-B induced leaf senescence. Fully developed second leaves of 3-week old seedlings of ‘Veg-Gin’ Chinese kale (Brassica oleracea L. Albograbra Group) were excised and held in Erlenmeyer flasks containing distill water. These leaves were irradiated with UV-B light at 0.5 and 1.5 kJ m-2 dosage, and were kept in darkness at 25℃ for 4 days to induce leaf senescence. UV-B irradiation cause a rapid decrease in the chlorophyll content, which is measured by the Minolta SPAD chlorophyll meter, of the excised leaf; and the rate of chlorophyll loss paralleled with the dosage applied. This indicates that UV-B will accelerate the senescence of excised Chinese kale leaves. One day after UV-B irradiation, there were significant increase in hydrogen peroxide, malondialdehyde contents and stress ethylene production in the leaves, indicating the presence of oxidative stress. With respect to the changes in oxidative enzymes, UV-B irradiation effectively lowered the superoxide dismutase(SOD) activity. The specific activities of ascorbate peroxidase(APX), glutathione reductase(GR), peroxidase(POX) and catalase(CAT), increased after 1 to 3 days of irradiation. The concentrations of glutathione and ascorbate, both are endogenous antioxidants, also increased after UV-B irradiation. These results suggest that the antioxidant system responsible for the eradication of hydrogen peroxide were activated in the UV-B irradiated leaves. However, due to the inability of the antioxidant system to overcome the excessive reactive oxygen species induced by the high dosage of UV-B irradiation, the excised leaves eventually died from oxidative stress.