【作者】 孙立；

【导师】 吴良欢； 丁悌平；

【作者基本信息】 浙江大学 ， 环境修复与资源再生， 2008， 博士

【摘要】 硅是地壳和土壤中含量仅次于氧的大量元素，从单细胞的藻类到维管植物，几乎所有的生命体中都可以发现含硅组织的存在。硅是硅藻生长的必需元素，虽然还没有被列为高等植物生长的必需元素，但它的有益作用已被广泛关注。硅不仅是生物体的重要的组成部分，而且还可以缓冲土壤pH值、调节大气CO₂浓度。因此，植物硅生理和硅生物地球化学一直是人们研究的热点。研究表明，在岩石风化和生物吸收硅的过程中会发生硅同位素分馏，在不同的物质中留下独特的硅同位素特征值。近年来，随着硅同位素测试精度的提高和快速测量技术的出现，利用硅同位素信号来研究硅生物地球化学循环开始逐渐引起人们的关注。目前关于硅同位素的研究主要集中海洋和河流系统，已报道的生物数据主要是硅藻、海绵体等海洋生物。陆生植物对全球硅循环和硅同位素平衡有重要的影响，但是国际上关于陆生植物的硅同位素组成只有零星的报道，还未开展系统研究。本课题以水稻为对象，研究了陆生高等植物硅同位素组成的分市规律和分馏机理，得到以下主要结论：（1）试验中观察到两种类型的硅同位素分馏：1、在水稻内部不同组织器官之间存在硅同位素分馏。2、在水稻与其生长环境之间存在硅同位素分馏。（2）水稻不同器官间存在明显的硅同位素组成的差异。常规水培条件下水稻不同器官中δ³⁰Si值变化范围是-2．7‰～2．1‰，各器官δ³⁰Si值变化顺序为：茎＜根＜叶＜稻壳＜糙米。限定硅源水培条件下水稻不同器官中δ³⁰Si值变化范围是-2．3‰～2．6‰，各器官δ³⁰Si值变化顺序为：茎＜根＜叶＜稻壳＜糙米。可以看出，水稻各器官δ³⁰Si值的变化顺序显示出一个明显的趋势。除根外，从下部器官到上部器官δ³⁰Si值逐渐增加。水稻单独器官—稻叶不同部位同样存在明显的硅同位素组成的差异。δ³⁰Si值的变化范围分别为：常规水培，-2．5‰～2．3‰；限定硅源水培，-2．6‰～1．7‰，两种培养方式稻叶各部位δ³⁰Si值变化顺序相同，叶鞘＜叶片基段＜叶片中段＜叶片顶段，也显示出从下部到上部逐渐增加的趋势。（3）植物体内硅同位素分馏的成因是硅随蒸腾流运输到各器官的过程中，相对质量较轻的²⁸Si优先沉淀在蒸腾流先到达的器官，使得蒸腾流中剩余的溶解性硅酸富含³⁰Si，随着蒸腾流的运输，硅沉淀的过程继续进行，溶解硅酸的δ³⁰Si值逐渐升高，因而在硅后续沉淀的器官较先沉淀的器官有更高的δ³⁰Si值，最终在蒸腾流末端沉淀的SiO₂获得最高的δ³⁰Si值，整个过程是一个类似于同位素动力学分馏的瑞利行为（Rayleigh-like behavior）。在水稻样品中观察到根部的δ³⁰Si值比茎高。按照瑞利模型来解释植物体内的硅同位素分馏，根部δ³⁰Si值应该最低，因为根是硅进入植株后最先沉淀的器官。结合硅生理研究数据分析显示，取样时在植物根部存有大量的溶解态硅，且这部分溶解硅比根部的沉淀硅有更高的δ³⁰Si值，而所测量的根δ³⁰Si值是根部溶解硅和沉淀硅的混合值，因此显示根部δ³⁰Si值比茎高。（4）水稻与外界环境中溶解硅之间存在硅同位素分馏(植株³⁰Si的亏缺)。这个现象指出在植物根部吸收硅时存在轻硅同位素优先进入植物的生物分馏作用，其成因可能是硅随质流进入植物时，轻硅同位素有更高的扩散系数。本次试验观察到水稻生物硅与外界溶解硅之间的硅同位素分馏系数(α_Pl-Sol)为0．9986～0．9996。（5）对植物内部及植物与环境之间硅同位素变化特征研究表明，在本试验条件下水稻对硅的吸收和体内运输都主要受蒸腾流的影响，硅吸收为随质流扩散的被动形式占主要成分。试验中观察到植物吸收硅时的生物分馏效应可以用来解释长江、黄河河水中δ³⁰Si值较高的成因，也为研究全球硅同位素平衡提供了重要的基础数据。更多还原

【Abstract】 Silicon （Si） is the second most mass-abundant element after oxygen in theEarth’s crust. From unicellular algae to vascular plants, numerous organisms arefound to produce siliceous structures. Si is essential for diatoms and is ’quasi-essential’for higher plant growth. Although Si is traditionally not considered as an essentialelement for plants, the beneficial effects of Si on the growth, development, yield anddisease resistance have been observed in a wide variety of plant species. Dissolved Siis absorbed in large amounts by terrestrial vegetation and weathering of silicatesremoves CO₂ from the atmosphere. Thus, there is a steadily growing scientific interestin the plant physiology and biogeochemistry of Si.It has been demonstrated that Si isotope is fractionated during weathering andbiological activity, which could provide unique information about numerous physicaland biological processes. Since recent developments in Si isotope measurementtechniques have provided a high degree of precise and rapid sample analysis, usingstable isotope of Si as proxies for understanding Si biogeochemical cycle has attractedsignificant scientific interest. The most work have particularly focused on ocean orriver systems. Some biogenic Si isotope data have been reported and the most wasfocused on marine materials （e.g. diatoms, sponges and radiolarian）. It has alreadybeen evident that the processes of Si uptake and releasing by terrestrial plants alsoplay an important role on Si biogeochemical cycle and global Si isotope balance.However, to date there have only been very limited records of Si isotope compositionon terrestrial plants. The general objective of this study is to investigate Si isotopefractionation in rice to lay the groundwork for understanding biogeochemical Si cycleand mechanism of plant Si acquisition and allocation. The main results aresummarized as follows:（1） The results obtained from this study suggest: two types of kinetic Si isotopefractionations occur during the plant development: one when Si is taken up by plantroots and the other when silica precipitates in plant tissues and organs.（2） The silicon isotope compositions of rice exhibit significant variations. Theδ³⁰Si values varied from -2.7‰to 2.1‰among different organs in normal hydroponicrice, by the order stem＜root＜leaf＜husk＜grain （means brown grain in thisdissertation）. Theδ³⁰Si values varied from -2.3‰to 2.6‰among different organs in Si-definite hydroponic rice, by the order stem＜root＜leaf＜husk＜grain. Theδ³⁰Sivalues exhibited a significantly gradient with a progressive increase from lower toupper organ except root.The silicon isotope compositions of rice leaf also exhibit significant variations.Theδ³⁰Si values varied from -2.5‰to 2.3‰among different parts in normalhydroponic rice leaf. Theδ³⁰Si values varied from -2.6‰to 1.7‰among differentparts in Si-definite hydroponic rice leaf. There was a consistent increasing trend ofδ³⁰Si values from lower to upper tissues （leaf sheath＜leaf blade base＜leaf blademiddle＜leaf blade top）.（3） We suggested that the variation pattern of Si isotope compositions could beexplained by the principle of kinetic isotope fractionation, according to whichdissolved H₄²⁸SiO₄ tends to precipitate preferentially, leaving the residual solutionenriched in H₄³⁰SiO₄. Preferential precipitation of light Si isotope contributed to aprogressive isotope fractionation with the transpiration stream moving from theuptake sites to the transpiration termini. Thus, Si isotope composition of the plantorgans was consistent with their position along the trajectory of the transpirationstream. We proposed that Si isotope fractionation in plant was a Rayleigh-likebehavior.Theδ³⁰Si value of stem was lower than that of root in rice. As discussed above,theδ³⁰Si of the root should be more negative than that of the stem, considering rootwas the foremost organ of dissolved Si precipitation within plant. A possibleexplanation was that the SiO₂ extracted from the rice root for Si isotope analyses wascomposed of depositional Si （SiO₂-nH₂O, opal, or phytoliths） and dissolved Si（monomeric silicic acid） in the root and the dissolved Si, isotopically heavier than thedepositional Si in the root, might play an more important role.（4） The ³⁰Si-depletion of whole rice plant relative to external nutrient solutiondisplayed that light Si isotope entered plants more readily than heavy Si isotope. Thisphenomenon indicated that biologically mediated Si isotope fractionation occurredduring uptake by the root. Si uptake might be dominated by mass-flow, which willfavour the light isotope because of its greater diffusion coefficient. In this experiment,the Si isotope fractionation factor (α_Pl-Sol) was estimated to be 0.9986～0.9996 in rice.（5） The Si isotope fractionation among different organs and between whole plantand source solution indicated that Si uptake and transport might be dominated bymass-flow, ion channels or via electrogenic pumps rather than by carrier-mediated transport. The contribution of passive component （mass-flow driven） played animportant role in Si accumulating plants in this experiment.In light of the Si isotope fractionation between plants and source solutions, wespeculate that biologically-mediated fractionation has greater influence on the Siisotope composition in the Yangtze River and Yellow River than other rivers. Theresults obtained in this study lay the groundwork for understanding Si biogeochemicalcycle and global Si isotope balance.更多还原