Abstract. That silicon is an important element in global
biogeochemical cycles is widely recognised. Recently, its relevance for
global crop production has gained increasing attention in light of possible
deficits in plant-available Si in soil. Silicon is beneficial for plant
growth and is taken up in considerable amounts by crops like rice or wheat.
However, plants differ in the way they take up silicic acid from soil
solution, with some species rejecting silicic acid while others actively
incorporate it. Yet because the processes governing Si uptake and regulation
are not fully understood, these classifications are subject to intense
debate. To gain a new perspective on the processes involved, we investigated
the dependence of silicon stable isotope fractionation on silicon uptake
strategy, transpiration, water use, and Si transfer efficiency. Crop plants
with rejective (tomato, Solanum lycopersicum, and mustard, Sinapis alba) and active (spring wheat, Triticum aestivum) Si
uptake were hydroponically grown for 6 weeks. Using inductively coupled
plasma mass spectrometry, the silicon concentration and isotopic composition
of the nutrient solution, the roots, and the shoots were determined. We
found that measured Si uptake does not correlate with the amount of
transpired water and is thus distinct from Si incorporation expected for
unspecific passive uptake. We interpret this lack of correlation to indicate
a highly selective Si uptake mechanism. 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)
between growth medium and bulk plant. Thus, 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. We suggest that isotope
fractionation during root uptake is governed by a diffusion process. In
contrast, the transport of silicic acid from the roots to the shoots depends
on the amount of silicon previously precipitated in the roots and the
presence of active transporters in the root endodermis, facilitating Si
transport into the shoots. Plants with significant biogenic silica
precipitation in roots (mustard and wheat) preferentially transport
silicon depleted in 28Si into their shoots. If biogenic silica is not
precipitated in the roots, Si transport is dominated by a diffusion process,
and hence light silicon 28Si is preferentially transported into the
tomato shoots. This stable Si isotope fingerprinting of the processes that
transfer biogenic silica between the roots and shoots has the potential to
track Si availability and recycling in soils and to provide a monitor for
efficient use of plant-available Si in agricultural production.
Silicon uptake and isotope fractionation dynamics by crop species