Stable Silicon Isotope Composition And Fractionation In Rice

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.