Summary by Daniel Puppe, a researcher at the Leibniz Centre for Agricultural Landscape Research (ZALF), Müncheberg, Germany.
Since centuries chemical extractions have been in use in different cultures to answer various purposes. Ancient alchemists used cold and hot extraction techniques, for example, to extract mercury from cinnabar (Marchini et al., 2022). In the Middle Ages the search for the so-called “philosopher`s stone”, a mythic alchemical substance that was believed to convert base metals into gold, had highest priority using laboratory techniques like heating, distillation, or extraction (Kauffman, 1985) (Fig. 1). And nowadays alkaline extractions, for example, are used in silicon (Si) research to determine Si contents in plant materials. In fact, alkaline chemicals are widely used to extract Si from plant samples, because this method is comparably simple, fast, and cheap.
Fig. 1 | The Alchemist in Search of the Philosopher’s Stone, painting by Joseph Wright of Derby (1771), public domain.
Generally, the quantification of Si pools (e.g., Si in the standing crop) and fluxes in agricultural plant-soil systems is crucial from an agro-ecological point of view. This is because humans largely affect global Si cycling caused by huge Si losses from intensively used land with negative impacts on yields and plant performance (see, e.g., Katz et al., 2021). Moreover, crops’ need for Si supply and Si fertilization efficacy can be evaluated by analyzing Si contents in plant materials. However, although alkaline extractions are commonly used to determine Si contents in plant materials, studies on their efficacy are surprisingly rare. Actually, there are only very few studies published that analyzed the comparability of results derived from different Si determination methods (Guntzer et al., 2010; Meunier et al., 2014; Nakamura et al., 2020), although an accurate quantification of Si contents in plant materials is crucial for assessing the Si status of an agricultural plant-soil system. Besides, an understanding of the relationships between Si contents of different plant materials (e.g., husk or straw) quantified with those methods, plant phytolith contents, and soil Si availability was missing until now.
In our study we performed two different alkaline (i.e., sodium carbonate (Na2CO3) and Tiron) extractions to determine Si contents in husk and straw samples of winter wheat (Triticum aestivum). Results obtained from hydrofluoric (HF) acid digestion were used as reference as this method is assumed to completely dissolve Si in a plant material. We evaluated the efficacy of simple, single-step Na2CO3 and Tiron extractions in relation to complex, multistep HF extractions and analyzed the relationships between Si and phytolith contents in plant samples and plant available Si concentrations in corresponding soil samples.
Our experiments showed that, irrespective of the plant material used (husk or straw), Tiron was more efficient in Si extraction compared to Na2CO3. While Na2CO3 extractions systematically underestimated Si contents in the tested plant materials, the results of Tiron extractions were comparable to the results of HF digestions, at least for husk samples (Fig. 2). Thus, our results clearly corroborated previous studies that reported the suitability of single-step alkaline extractions for the determination of Si contents in plants. Phytolith contents in plant samples were strongly positively correlated to Si contents obtained from extractions, especially in husk samples, which generally showed the highest Si contents. Plant available Si in soil samples was also reflected best in husk Si contents. From our results we concluded that alkaline extractions represent promising alternatives for replacing the highly corrosive and toxic multistep digestion based methods for a reliable determination of Si contents in plants.
Fig. 2 | Alkaline-extracted Si contents in plant materials (husk (A), straw (B), and husk + straw (C) of winter wheat) plotted against corresponding total Si contents (SiHF). Linear trendlines are given in light blue and corresponding equations and coefficients of determination are stated in blue (Si_Tiron) and orange (Si_Na2CO3) boxes. Dotted lines represent the lines of equality (y = x). Figure taken from Puppe et al. (2023).
Enjoyed the blogpost? Read the full paper at:
Puppe, D., Kaczorek, D., Buhtz, C., & Schaller, J. (2023). The potential of sodium carbonate and Tiron extractions for the determination of silicon contents in plant samples — A method comparison using hydrofluoric acid digestion as reference. Frontiers in Environmental Science, 11, 1145604. https://doi.org/10.3389/fenvs.2023.1145604
Siliceous Plants 