Silica uptake from nanoparticles and silica condensation state in different tissues of Phragmites australis

https://doi.org/10.1016/j.scitotenv.2012.10.016Get rights and content

Abstract

Silicon is described as beneficial for grasses by enhancing yield and fitness via a considerable contribution to pathogen, drought, and pest resistance. Silicic acid is the predominant form for uptake and transport within the plant and will precipitate in leaves. But it is unknown whether polymeric nanosilicon compounds in its synthetic form, with an increasing concentration in aquatic environments, can be suitable for plant nutrition. Therefore, we investigated the uptake, transport, and deposition of silicic acid/silica within plants using synthetic nanosilica. Our results show a significant difference in silicon (Si) content within the different tissues of Phragmites australis. The nanosilica had been dissolved prior to the uptake by plants. The chemical form of Si during uptake was not traceable. A significant enhancement in the condensation state of the silica was found from root to leaves especially from culm to leaf tips visible by the increasing content of Q4-groups in the NMR spectra. We conclude that synthetic nanosilica has the same quality as source for the beneficial element Si like natural silica. Since the condensation state is described to control silica solubility, we suggest that different condensation states within the plant may result in different remobilization of silicon during decomposition of the plant material.

Highlights

► Silicon precipitation occurs in tissues with transpiration function. ► Silica condensation state in grass depends on dominant function of the tissue. ► Nanosilica is mainly dissolved prior to uptake.

Introduction

Silicon as the second most abundant element on the Earth's crust is known to be beneficial or even essential for many organisms. Many terrestrial (e.g. grasses) and aquatic (emergent and submerged macrophytes) plants accumulate high amounts of amorphous Si in their tissues (Schoelynck et al., 2010, Struyf et al., 2007). In ecosystems dominated by Si-accumulating species like grasses, high amounts of Si are cycled each year via plant root uptake and litter fall (Cornelis et al., 2010, Melzer et al., 2010). A plant species likely to be important in Si cycling is common reed (Phragmites australis TRIN. ex Steudt.), which is one of the most abundant wetland plants worldwide. Amorphous silicon is beneficial with positive effects on biomass production of grasses (Eneji et al., 2008, Schaller et al., 2012), as well on pathogen and herbivore defense (Fawe et al., 1998) and amelioration of abiotic stress (Ma, 2004).

Silicon is immobilized by biomineralization as plant opals (phytoliths) in cell walls (Epstein, 1999, Sangster, 1970) and as amorphous silica forming a Si double layer immediately beneath the cuticle (Currie and Perry, 2007, Schaller et al., 2012). The uptake of Si mainly in the form of amorphous silica is either passive (unspecific) or active by ATP mediated transporters via (special groups of) aquaporins with subsequent genetic control (Ma et al., 2004, Ma and Yamaji, 2006).

Current findings have shown that P. australis cannot avoid the surplus uptake of Si into aboveground biomass (Schaller et al., 2012). Hereby Si seems to be transported within plants by transpiration driven water transport and is precipitated mainly in tissues with photosynthesis (transpiration) function. It was shown that the speciation of Si in grasses differs between uptake/transportation tissues and tissues with transpiration function. In roots and culm, Si exists mainly as silicic acid [Si(OH)4], an uncharged monomeric molecule (at pH values below 9) whereas in leaf blades and leaf sheath, Si(OH)4 mainly polymerizes forming silica gel [SiO2·nH2O] (higher condensation state) (Ma and Yamaji, 2006). A more precise determination of the change in Si speciation within different plant tissues (fine roots, coarse roots, rhizomes, culms, leaf sheath, leaf blades and leaf tips) has not been done so far. Furthermore, nothing is known about the uptake and transport of pyrogenic synthetic nanoparticles of amorphous silica within the different tissues, whereas an uptake via aquaporins can be neglected considering the particle size (Liu and Zhu, 2010). The production and application of synthetic silica nanoparticles increased during the last years (Som et al., 2011). It is known that silica nanoparticles occurring in the air can easily enter the lungs of mammals by air inhalation and will be distributed within their body by lymph and blood (Silva et al., 2012). While air-born silica nanoparticles washed out by rain result in higher concentrations of silica nanoparticles in the aquatic environment. Silica nanoparticles may also enter the aquatic environments directly from textiles by washing (Som et al., 2011). These particles are in turn difficult to remove from the water pathway (Chang et al., 2007, Chin et al., 2006), which points to the higher availability of silica nanoparticles for plants growing in aquatic environments.

Consequently, the uptake of Si from pyrogenic artificial amorphous nanosilica and at the same time the condensation state of Si within the different plant tissues were investigated in a pot experiment using P. australis.

Section snippets

Plant material and experimental conditions

Rhizomes of P. australis were collected at the beginning of March 2009 from a permanently submerged littoral stand in Lake Großteich 10 km northeast of Dresden, Germany (51°08′N, 13°43′E). The rhizomes were carefully washed with tap water to remove attached sediment before choosing pieces of about 15 cm length with a diameter of about 5 cm and the same morphology. Three pieces of rhizome were planted at 10 cm depth in each of thirty-six 15-L polyethylene buckets containing 1 kg of peat (Borena,

Distribution of silica within the plant

Silica uptake in the root system is reported to be either unspecific or mediated by special groups of aquaporins with subsequent genetic control (Ma et al., 2004, Ma and Yamaji, 2006). Once taken up, silica is transported within the plant precipitating mainly in tissues with high photosynthetic activity and related transpiration (Ma and Yamaji, 2008). Our results reveal significant differences (p < 0.001, Kruskal–Wallis-test) in Si content between different tissues at the end of the experiment (

Conclusion

Our results show a significant gradient of the silicon content within P. australis. The silica nanoparticles have been dissolved prior to uptake by plants, whereby the uptake in particulate form is not traceable. Furthermore, it was shown that synthetic nanosilica has the same quality in regard as source for the beneficial element silicon compared with natural silicon sources. A significant change in the condensation state of the silica was found within the plant (gradient from culm to leaf

Acknowledgments

The authors are grateful to Mrs. R. Schulze (Bioanalytical Chemistry, TU Dresden) for ICP-OES measurements.

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