Studies on the Relationship between Calcium and Cadmium Stress of Rice Seedlings

本論文係以水稻品種台中在來一號 (Oryza sativa L. cv. Taichung Native 1,TN1) 為材料，探討 (一) 過氧化氫在水稻黃化幼苗根鎘逆境下所扮演之角色；(二) 鈣對水稻黃化幼苗根鎘逆境之影響；(三) 缺鈣對水稻幼苗鎘逆境之影響。 黃化幼苗根之鎘毒害為抑制根長與冠根數目、減少乾重累積及造成細胞死亡。鎘處理3小時，catalase (CAT) 活性與glutathione (GSH) 含量明顯下降，過氧化氫於6小時顯著累積。前處理GSH提升GSH含量可減緩鎘毒害。前處理DPI (diphenylene iodonium) 抑制NADPH oxidase活性使過氧化氫不累積，同時減緩根長與乾重抑制及細胞死亡增加之鎘毒害，但不影響冠根數目。過氧化氫處理亦可觀察到黃化幼苗根根長與乾重之抑制及細胞死亡增加等鎘毒害，顯示水稻黃化幼苗根之鎘毒害係因過氧化氫累積所致。上述結果說明，鎘會抑制水稻黃化幼苗根之CAT活性造成過氧化氫累積，過氧化氫累積會抑制根長與乾重及增加細胞死亡造成鎘毒害。 鈣添加可減緩水稻黃化幼苗根之鎘毒害，同時減少水稻根之鎘吸收與鎘所誘導累積之過氧化氫含量。鈣添加減緩之鎘毒害可經過氧化氫處理而加劇。以上結果顯示，鈣可降低水稻黃化幼苗根之鎘含量，進而減緩鎘誘導之過氧化氫累積而降低過氧化氫抑制之根長與根乾重與細胞死亡之增加。 缺鈣處理顯著降低水稻幼苗鈣含量，同時提升第二片葉片superoxide dismutase (SOD) 活性與降低CAT活性及GSH含量，削弱水稻幼苗之抗氧化系統。因此，缺鈣幼苗經後續鎘處理可加劇水稻幼苗鎘毒害 (第二片葉葉綠素與蛋白質之降低、malondialdehyde (MDA) 含量之增加，同時增加鎘之吸收。缺鈣幼苗經後續鎘處理會加劇鎘誘導之SOD、CAT、ascorbate peroxidase (APX) 與glutathione reductase (GR) 活性之增加及ascorbate含量降低。缺鈣會導致水稻幼苗之熱休克相關基因Oshsp17.3表現下降，熱休克處理保護水稻鎘逆境之效果消失，顯示水稻熱休克處理誘導增加之Oshsp17.3表現需要鈣離子參與。Rice (Oryza sativa L. cv. Taichung Native 1,TN1) seedlings were used to investigate (a) the role of hydrohen peroxide (H2O2) in cadmium (Cd) toxicity in rice seedling roots, (b) the effect of calcium (Ca) on Cd toxicity in rice seedling roots, and (c) the effect of calcium (Ca) deficiency on Cd toxicity of rice seedlings. Measurements of root length, crown root number, root dry weight and cell death in roots were selected as Cd toxicity index in rice seedling roots. Catalase (CAT) activity and GSH content decreased in 3 h after Cd treatment. Cd-induced H¬2O2 accumulation occurred 6 h after Cd treatment. Pretreatment of GSH, which increased GSH content, resulted in alleviation of Cd toxicity. Pretreatment of diphenylene iodonium (DPI) NADHP oxidase inhibitor, not only decreased H2O2 level but also alleviated Cd-inhibited root length, root dry weight and Cd-increased cell death. Cd decreased crown root number is not related to H2O2. The results obtained indicated that Cd-decreased CAT activity resulted in H2O2 accumulation. The accumulation of H2O2 in turn decreased root length and dry weight, and increased cell death. Exogenous addition of Ca was observed to reduce Cd toxicity, Cd uptake and Cd-induced H2O2 accumulation. Ca-reduced Cd toxicity can be revered by H2O2 appilication. These results suggest that Ca- reduced Cd toxicity is mediated through inhibition of Cd uptake, which in turn increased CAT activity and reduced H2O2 accumulation. The Ca deficiency treatment caused reduction of Ca concnetration in rice seedling. Ca deficiency resuled in decrease in CAT activity and GSH content and increase in SOD activity. Ca deficiency increased the toxicity of leaves of rice seedlings to subsequent Cd treatment. Cd concentration was higher in Ca-deficient shoot amd roots than their respective control shoot and roots. Cd-induced activities of SOD, CAT, APX, and GR and Cd-decreased AsA content in Ca-deficient seedlings were pronounced than Ca-sufficient seedlings. HS-induced Oshsp17.3 expression and HS protection of Cd stress were odserved in Ca-sufficient but not in Ca-deficient rice seedlings. Thus, it can be concluded that Ca is involved in the HS-induced expression of Oshsp17.3