Sulfur Stable Isotope Discrimination in Rice: A Sulfur Isotope Mass Balance Study.

The use of sulfur (S) stable isotopes to study S metabolism in plants is still limited by the relatively small number of studies. It is generally accepted that less S stable isotope discrimination occurs during sulfate (SO ) uptake. However, S metabolism and allocation are expected to produce separations of S stable isotopes among the different plant S pools and organs. In this study, we measured the S isotope composition of the main S pools of rice plants grown under different SO availabilities in appropriate closed and open hydroponic-plant systems. The main results indicate that fractionation against S occurred during SO uptake. Fractionation was dependent on the amount of residual SO in the solution, showing a biphasic behavior related to the relative expression of two SO transporter genes ( and ) in the roots. S isotope separations among S pools and organs were also observed as the result of substantial S isotope fractionations and mixing effects occurring during SO assimilation and plant S partitioning. Since the S stable isotope separations conserve the memory of the physiological and metabolic activities that determined them, we here underline the potential of the S/S analysis for the detailed characterization of the metabolic and molecular processes involved in plant S nutrition and homeostasis.