植物中硅矿化作用的硅同位素示踪研究

Study on Silicon Isotope Tracing for Silica Mineralization in Plants

项目首次对田地生长的水稻与竹子和室内栽培水稻中氧化硅的含量、形态、分布及硅同位素组成进行了系统研究。研究发现水稻中的氧化硅含量有由根到茎、叶、稻壳逐渐增高的趋势,但在米粒中含量急剧降低。竹子中的氧化硅含量也显由杆到枝、叶逐渐增高的趋势。在竹子和水稻的根部,氧化硅都集中在内皮层;而在其地上部分(竿、枝、叶、壳),氧化硅主要出现于外皮层。在单株水稻和竹子中都发现不同器官间存在显著的、系统的硅同位素分馏。水稻的δ30Si显示有由根到茎降低,而后向叶、壳和米逐渐增高的趋势。竹子的δ30Si也显由根到竿降低,而后向枝、叶增高的趋势。这种硅同位素变化可能是由植株内体液中的溶解硅在竿、枝、叶、壳相继沉淀出氧化硅时,产生瑞利过程的硅同位素分馏的结果。研究得出竹子和水稻中溶解硅与沉淀硅间的硅同位素分馏系数分别为0.9981和0.9996。研究发现水稻根和竹根从土壤溶液中吸取硅时,也存在硅同位素动力分馏。竹子与水稻吸收硅与土壤可溶硅之间的硅同位素分馏系数分别为0.9988和0.9989。研究得出:1)水稻与竹子由外界吸收的含硅化合物主要为正硅酸;2)被动吸收是其吸收硅的重要形式;3)蒸发作用是硅在这些植物中迁移和沉淀的主要机制。研究结果为理解植物中硅吸收、搬运和沉淀硅的方式与机制和探讨植物在硅、碳生物地球化学循环方面的作用提供了可靠的证据。

A systematic investigation on contents, morphology, distribution and silicon isotope compositions of silica was undertaken the first time on field grown rice plants and bamboos, as well as on laboratory grown rice plants. It was found that the SiO2 contents in rice plant increased generally from roots, through stem and leaves, to husks, but decreased drastically from husks to grains. Similarly the silica contents in bamboo plant increased also from stem, through branch, to leaves, although decreased slightly from root to stem. The silica was found to occur exclusively in the endodermis cells in the roots of rice and bamboo, but mainly in epidermal cells in their aerial parts （stem, branch, leaves and husks）. Large and systematic silicon isotope fractionation was observed among different organs of individual plant of rice and bamboo. The δ^30Si value of rice organs decreased from roots to stem, and then increased through leaves, husks and grains. Similarly the δ^30Si values of bamboo organs decreased from roots to stem, but then increased from stem, through branch, to leaves. This kind of silicon isotope variation might be caused by isotope fractionation in a Rayleigh process when SiO2 precipitated in stem, branches, leaves and husks gradually from plant fluid. The Si isotope fractionation factor between dissolved Si and precipitated Si is inferred to be 0.9981 in bamboo plant and 0.9996 in rice plant. Another biochemical silicon isotope fractionation was also observed in process of silicon uptake by roots of rice and bamboo from soil solution. The silicon isotope fractionation factor between plant and growth solution is inferred to be 0.9988 for bamboo and 0,9989 for rice. Based on above observations, it can be inferred that： a） Monosilicic acid is the major silicon-bearing component taken up by roots of rice and bamboo; b） Passive uptake of silicon is important for these plants; c） Evapo- rate-transpiration may be the major mechanism for the transportation and precipitation of silicon in these plants. The quantitative data obtained in this study provide us a solid base for understanding the mechanisms of silicon absorption, transportation and precipitation in plants and the role of plant growth in the continental Si cycle.