Silicon uptake and isotope fractionation dynamics by crop species

Abstract

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 isotope fractionation with a low‐silica rice mutant reflects plant uptake strategy in response to different silicon supply levels

“…Regardless of Si supply levels, we found 28 Si enrichment in the entirety of the rice plants relative to the external solution. Similar phenomena were also found by Frick et al (2020) in wheat (Triticum aestivum L.), tomato (Solanum lycopersicum L.), and mustard (Sinapis alba L.) cultured in nutrient solution. This isotope fractionation was assumed to be caused by the difference between the diffusion coefficients of 28 Si(OH) 4 and 30 Si(OH) 4 .…”

Silicon Isotope Analyses of Soil and Plant Reference Materials: An Inter‐Comparison of Seven Laboratories

“…The vertical envelope between the dotted lines shows the array defined by the expanded uncertainty (U, k = 2) around the arithmetic mean of the full procedure replicates for each reference material. n = 3, Frick et al 2020). These published analyses were performed at GFZ using the same protocol in the same laboratory as reported for GFZ in this study.…”

“…In addition to the data acquired during this study, another published result for ERM‐CD281 is available ( δ 30 Si = −0.28 ± 0.08‰, 2 s , n = 3, Frick et al . 2020). These published analyses were performed at GFZ using the same protocol in the same laboratory as reported for GFZ in this study.…”

Silicon Cycling in Soils Revisited

“…Phytogenic silica can be found in living plants within cells (i.e., in the cell wall and the cell lumen), forming relatively stable, recognizable phytoliths, or in intercellular spaces and extracellular (cuticular) layers forming relatively fragile silica structures [108,109]. Phytogenic Si can be found in almost any plant organ, e.g., in leaves and stems as well as in roots [110,111]. Phytoliths can be frequently found in most soils and show a specific morphology that can be used for the taxonomic identification of plants [108,109,112].…”

“…Si absorption by plants is controlled by specific influx (called Lsi1 and Lsi6) and efflux (called Lsi2) channels, which have been found especially in crops such as rice (Oryza sativa), wheat (Triticum aestivum), or sorghum (Sorghum bicolor) (see Ma and Yamaji [119] for a detailed review). However, it should be kept in mind that the mechanisms behind the uptake, transport, and accumulation of Si in plants (active vs. passive Si transport) as well as Si-induced plant resistance (mode of action of Si in plants) are still not fully understood, and thus are under controversial discussion (see, e.g., Frick et al [110], Coskun et al [120], Exley et al [121], and Exley [122]). For example, it was found that plant functional groups strongly affect Si stocks in aboveground biomass, with grasses increasing and legumes decreasing the aboveground biomass Si stocks [123].…”