摘要
以天然抗碱盐生植物虎尾草(Chloris virgata)为材料,对其进行不同强度的碱胁迫(molar ratio of NaHCO_3 : Na_2CO_3 = 1 : 9, 40,80 mM, pH 9.93–9.97)和盐胁迫(molar ratio of NaCl : Na_2SO_4 = 1 : 9, 40,80 mM,pH 6.48–6.65)短期处理。首先对两种胁迫下茎叶中有机酸组分及含量进行分析,结果表明:在碱胁迫下,不仅茎叶中有机酸总量显著上升,而且不同组分的变化差异明显,其中柠檬酸增幅最大,苹果酸次之。而盐胁迫下有机酸组分及含量均无显著性变化。然后,根据有机酸积累特点,选定与此代谢变化有关的酶,并对其活性进行测定,以便进一步在酶学水平上探讨其抗碱生理机制。结果表明:在碱胁迫处理下,虎尾草茎叶中的柠檬酸合成酶、苹果酸合成酶及异柠檬裂解酶的活性均显著上升,而顺乌头酸酶及磷酸烯醇式丙酮酸羧化酶的活性没有显著性变化;在较强的碱胁迫下(80mM),苹果酸脱氢酶活性明显下降。盐胁迫下,除了苹果酸合成酶活性上升以外,其它酶活性均无显著性变化。实验结果有力地证明了:发生在酶水平上的有机酸代谢调节可能就是虎尾草适应碱胁迫的生理生化基础。其中柠檬酸合成酶、苹果酸合成酶和异柠檬裂解酶可能是决定其抗碱性的关键酶。
此外,用不同强度的碱胁迫和盐胁迫(molar ratio of NaHCO_3 : Na_2CO_3 = 1 : 1, 50,100,150, pH 9.93–9.98; molar ratio of NaCl : Na_2SO_4 = 1 : 1, 50,100,150, pH 6.32–6.64)对虎尾草幼苗进行短期胁迫处理,通过测定叶片光合、叶绿素荧光、光合色素、K~+、Na~+含量、磷酸烯醇式丙酮酸羧化酶活性等指标,分析了虎尾草幼苗在面对两种短期胁迫时的不同响应,从而在此角度再次证明碱胁迫是不同于盐胁迫的一种非生物胁迫。实验结果表明:在短期碱胁迫下,随着盐分浓度的增加,光合指标、叶绿素荧光参数、光合色素含量及磷酸烯醇式丙酮酸羧化酶活性均有明显的相应变化。与之相反,这些参数在盐胁迫下有很小或并没有显著的变化。同时,这两种短期性的胁迫均造成了虎尾草叶片Na~+离子的积累及K~+离子含量的减少,从而使Na~+ / K~+比值升高。与盐胁迫相比,碱胁迫下Na~+离子及K~+离子变化更加显著,Na~+ / K~+比值也得到急剧的升高。在盐碱两种短期胁迫下,虎尾草幼苗不同的光合、荧光响应及其它指标的变化表明,碱胁迫是一种不同于盐胁迫的生物胁迫,其对植物具有更大的破坏性和伤害性。与短期的盐胁迫相比,在碱胁迫下植物除了受到渗透胁迫及离子胁迫的威胁,其还要遇到碱胁迫所特有的高pH胁迫。植物根部周围的这种高pH胁迫造成了植物体内离子平衡的严重失调,使Na~+在叶片大量积累,而K~+含量急剧减少,产生更加严重的离子毒害,其可能导致虎尾草幼苗光合作用降低,叶绿素荧光等参数的改变。
Chloris virgata, a naturally alkali-resistant halophyte, was chosen as the test material for our research. The seedlings of C. virgata were treated with varying short salt stress (1:9 molar ratio of NaCl to Na_2SO_4; pH 6.70-6.72; 40, 80 mM) and alkali stress (1:9 molar ratio of NaHCO_3 to Na_2CO_3; pH 10.46-10.62; 40, 80 mM). Firstly, the composition and content of organic acids in shoots was analyzed, the results indicated that there was not only a significant increase in total organic acids, but also obvious changes in different components of organic acids under the alkali stress. The increment of citrate was the largest and malate followed. However, none of the organic acids showed significant alteration in the content and component under salt stress. According to the characteristics of accumulation in organic acids, some enzymes in the related metabolism were selected, and determined in order to further explore the physiological mechanism of the alkali tolerance in the level of enzyme. The results showed that the activities of citrate synthase (CS, EC 4.1.3.7), malate synthase (MS, EC 4.1.3.2), NADP-isocitrate dehydrogenase (NADP-ICDH, EC 1.1.1.42) and isocitrate lyase (ICL, EC 4.1.3.1) increased significantly, but the activities of aconitase (EC 4.2.1.3) and phosphoenolpyruvate carboxylase (PEPCase, EC 4.1.1.31) had little change under alkali stress. Malate dehydrogenase (MDH, EC 1.1.1.37) activity decreased obviously under a stronger alkali stress (80 mM). However, under salt stress, besides increased MS activity, there was no significant change for other enzymes. These results strongly demonstrated that the enzymatic regulation of organic acid metabolism may be the biochemical basis of the alkali tolerance for C. virgata. CS, MS and ICL might be the key enzymes which decide the alkali tolerance of C. virgata.
In addition, seedings of C. virgata were subjected to different intensity short-term (3 days) alkali-stress (1:1 molar ratio of NaHCO_3 to Na_2CO_3; pH 9.93–9.98; 50, 100,150 mM) and salt stress (1:1 molar ratio of NaCl to Na_2SO_4; pH 6.32–6.64; 50,100,150 mM). The data of Photosynthesis parameters, chlorophyll fluorescence, photosynthesis chlorophyll content, activity of PEPCase and content of k~+, Na~+ indicated that different response were existed under alkali stress and salt stress. These results indicated that salt stress and alkali stress are two different stresses. Photosynthesis parameters, chlorophyll fluorescence, photosynthesis chlorophyll content, activity of PEPCase and content of k~+ , Na~+ indicated had visible change under short alkali stress. In contrast, there were a little or no significant effects in C. virgata under short term salt stress. The different results in C. virgata indicated that alkali stress with greater destructive effects, different from salt stress, is an important abiotic stress, and worthy of further study. Plant are imposed high pH under alkali stress, the key feature, different from salt stress, may disturbs root structure, induces increase in Na~+ and decrease in K~+ in leaves.The High leaf Na~+ concentration probably reduces PN due to ion toxicity which disorderd photosynthetic apparatus, changed fluorescence and other parameters of C. virgata.
引文
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