Importance of plant species and external silicon concentration to active silicon uptake and transport.

Here, we characterized silicon (Si) uptake and xylem loading in Oryza sativa, Zea mays, Helianthus annuus and Benincase hispida in a series of hydroponic experiments. Both active and passive Si-uptake components co-exist in all the plants tested. The active component is the major mechanism responsible for Si uptake in O. sativa and Z. mays. By contrast, passive uptake prevails in H. annuus and B. hispida at a higher external Si concentration (0.85 mM), while the active component constantly exists and contributes to the total Si uptake, especially at a lower external Si concentration (0.085 mM). Short experiments showed that Si uptake was significantly suppressed in O. sativa and Z. mays by metabolic inhibitors or low temperature, regardless of external Si concentrations. By contrast, Si uptake in H. annuus and B. hispida was inhibited more significantly by metabolic inhibitors or low temperature at lower (for example, 0.085 mM) than at higher (for example, 1.70 mM) external Si concentrations. It can be concluded that both active and passive Si-uptake components co-exist in O. sativa, Z. mays, H. annuus and B. hispida, with their relative contribution being dependent much upon both plant species and external Si concentrations.