Silicon isotope fractionation in rice plants, an experimental study on rice growth under hydroponic conditions

Silicon (Si) isotope composition and Si distribution among different rice plant organs and different parts of rice leaf at maturity were studied, which may provide new insights into the mechanism of Si accumulation in plants and biogeochemical Si cycle. An isotope ratio mass spectrometer (IRMS) was used to examine Si isotope fractionation by rice plant grown in a hydroponic system. The observed Si-30-depletion (about 0.3 parts per thousand) of whole plant relative to external nutrient solutions suggested biologically mediated Si isotope fractionation occurred during uptake. However, it was not possible to judge the Si uptake mechanism with the data. For delta Si-30 variation within plant, there was a consistent increasing trend from lower to upper tissues (stem < leaf < husk < grain; leaf sheath < leaf blade base < leaf blade middle < leaf blade top). The phenomenon, reflecting kinetic isotope effects, could be explained that isotope fractionation during Si deposition in rice plant was a Rayleigh-like behavior. The range (-2.7 parts per thousand to 2.3 parts per thousand) of delta Si-30 variation among rice plant tissues in present experiment exceeded that (-1.7 parts per thousand to 2.5 parts per thousand) of phytoliths observed previously in continents, which would enhance understanding the role of phytoliths on globe Si isotope balance.