『Abstract
　The silicon isotope fractionation between rice plant and nutrient solution was studied experimentally. Rice plants were grown to maturity with the hydroponic culture in a naturally lit glasshouse. The nutrient solution was sampled for 14 times during the whole rice growth period. The rice plants were collected at various growth stages and different parts of the plants were sampled separately. The silica contents of the samples were determined by the gravimetric method and the silicon isotope compositions were measured using the SiF4 method.
　In the growth process, the silicon content in the nutrient solution decreased gradually from 16 mM at starting stage to 0.1-0.2 mM at harvest and the amount of silica in single rice plant increased gradually from 0.00013 g at start to 4.329 g at harvest. Within rice plant the SiO2 fraction in roots reduced continuously from 0.23 at the seedling stage, through 0.12 at the tiller stage, 0.05 at the jointing stage, 0.023 at the heading stage, to 0.009 at the maturity stage. Accordingly, the fraction of SiO2 in aerial parts increased from 0.77, through 0.88, 0.95, 0.977, to 0.991 for the same stages. The silicon content in roots decreased from the jointing stage, through the heading stage, to the maturity stage, parallel to the decrease of silicon content in the nutrient solution. At the maturity stage, the silicon content increased from roots, through stem and leaves, to husks, but decreased drastically from husks to grains. These observations show that transpiration and evaporation may play an important role in silica transportation and precipitation within rice plants
　It was observed that the δ³⁰Si of the nutrient solution increased gradually from -0.1‰ at start to 1.5‰ at harvest, and the δ³⁰Si of silicon absorbed by bulk rice plant increased gradually from -1.72‰ at start to -0.08‰ at harvest, reflecting the effect of the kinetic silicon isotope fractionation during silicon absorption by rice plants from nutrient solutions. The calculated silicon isotope fractionation factor between the silicon instantaneously absorbed by rice roots and the silicon in nutrient solution vary from 0.9983 at start to 0.9995 at harvest, similar to those reported for bamboo, banana and diatoms in direction and extent. In the maturity stage, the δ³⁰Si values of rice organs decreased from -1.33‰ in roots to -1.98‰ in stem, and then increased through -0.16‰ in leaves and 1.24‰ in husks, to 2.21‰ in grains. This trend is similar to those observed in the field grown rice and bamboo.
　These quantitative data provide us a solid base for understanding the mechanisms of silicon absorption, transportation and precipitation in rice plants and the role of rice growth in the continental Si cycle.』

1. Introduction
2. Rice growth experiment and analytic methods
　2.1. Rice growth experiment
　2.2. Sample collection and preparation
　2.3. SiO2 extraction and SiO2 content determination
　2.4. Silicon isotope analysis
3. Results and discussion
　3.1. The silicon content of the nutrient solution and the rice plant
　　3.1.1. The silicon content of the nutrient solution
　　3.1.2. The silica contents and distribution in different organs of the rice plants
　3.2. The silicon isotope composition of the nutrient solution and the rice plant
　　3.2.1. The silicon isotope composition of the nutrient solution
　　3.2.2. The silicon isotope composition of the rice plants
4. Silicon isotope fractionation between rice plants and nutrient solution
　4.1. The variation of δ³⁰Si values of bulk silicon, dissolved silicon and gel like silica in the nutrient solution
　4.2. The variation of δ³⁰Si values of silicon absorbed by the rice plants and silicon isotope fractionation factor between rice plant and dissolved silicon in nutrient solution
5. Implications
　5.1. Implications for the mechanism of Si uptake by rice roots
　5.2. Implications to the study of the global Si cycle
6. Conclusions
Acknowledgements
References