Silicon isotopic fractionation by banana (Musa spp.) grown in a continuous nutrient flow device

The determination of the plant-induced Si-isotopic fractionation is a promising tool to better quantify their role in the continental Si cycle. Si-isotopic signatures of the different banana plant parts and Si source were measured, providing the isotopic fractionation factor between plant and source. Banana plantlets (Musa acuminata Colla, cv Grande Naine) were grown in hydroponics at variable Si supplies (0.08, 0.42, 0.83 and 1.66 mM Si). Si-isotopic compositions were determined on a multicollector plasma source mass spectrometer (MC-ICP-MS) operating in dry plasma mode. Results are expressed as delta Si-29 relative to the NBS28 standard, with an average precision of +/- 0.08 parts per thousand (+/- 2 sigma(D)). The fractionation factor (29)epsilon between bulk banana plantlets and source solution is -0.40 +/- 0.11 parts per thousand. This confirms that plants fractionate Si isotopes by depleting the source solution in Si-28. The intra-plant fractionation Delta Si-29 between roots and shoots amounts to -0.21 +/- 0.08 parts per thousand. Si-isotopi c compositions of the various plant parts indicate that heavy isotopes discrimination occurs at three levels in the plant (at the root epidermis, for xylem loading and for xylem unloading). At each step, preferential crossing of light isotopes leaves a heavier solution, and produces a lighter solution. Si-isotopic fractionation processes are further discussed in relation with Si uptake and transport in plants. These findings have important implications on the study of continental Si cycle.