Silicon isotope fractionation and uptake dynamics of three crop plants: laboratory studies with transient silicon concentrations

Abstract. Silicon has been recognized an important element in global biogeochemical cycles for a long time. Recently, its relevance for global crop production gains increasing attention. Silicon is beneficial for plant growth and is taken up in considerable amounts by crops, likewise rice or wheat. The incorporation of silicic acid from the soil solution into the plants is accomplished by a variety of strategies (rejective, passive and active) that are subject to an intense debate. To forge a new perspective on the underlying processes, we investigated how the silicon stable isotope fractionation during plant growth depends on uptake strategy, transpiration, water use, and Si transfer efficiency. Crop plants with a rejective (tomato, Solanum lycopersicum and mustard, Sinapis alba) and active (spring wheat, Triticum aestivum) uptake were hydroponically grown for 6 weeks. Using inductively coupled plasma mass spectrometry, the silicon amounts and the isotopic composition of the nutrient solution, the roots, and the shoots were determined. Wheat revealed the highest Si transfer efficiency from root to shoot followed by tomato and mustard. All three species preferentially incorporated light 28Si, with a fractionation factor 1000∙ln(α) of −0.33 ‰ (tomato), −0.55 ‰ (mustard) and −0.43 ‰ (wheat). Even though the rates of active and passive Si root uptake differ, the physico-chemical processes governing Si uptake and stable isotope fractionation do not, they are governed by a diffusion process. In contrast, the transport of silicic acid from the roots to the shoots depends on the preceding precipitation of silicic acid in the roots and the presence of active transporters at the root endodermis. Plants with a significant biogenic silica precipitation in roots (mustard, and wheat), preferentially transport silicon enriched in 30Si into their shoots, whereas the transport in tomato is governed by a diffusion process and hence preferentially transports light silicon 28Si into the shoots.