ABSTRACT

Horsetail (Equisetum arvense L.) is a perennial herb which creates during the life cycle spring and summer stems. The selected species and populations were monitored in the years 2009-2011 in three different natural locations in Laborecká vrchovina (Slovakia). Samples were collected by destructive methods in all three locations. Silicon content was determined in dry biomass by AAS. Silicon content in plants ranged from 21.11 ± 3.24 g·kg⁻¹ to 32.80 ± 8.03 g·kg⁻¹. The highest content of silicon exhibited samples of the September collection. We found that the location and the year in terms of silicon content were not statistically significant. The main sources for statistical variability in the accumulation of silicon were during the collections.

1. Introduction

Horsetail (Equisetum arvense L.) is a herbaceous perennial plant. It has separate sterile non-reproductive and fertile spore-bearing stems, growing from a perennial underground rhizomatous stem system.

The plant contains several substances which can be used medicinally. It is rich at minerals silicon (10%), potassium, and calcium. Silicon is generally classified as an element, which is important for plants at their structure, physiology and for protection [1]. Silicon is taken up by the roots in the form of silicic acid [Si(OH)₄], an uncharged monomeric molecule, when the solution pH is below 9 [2]. Silicic acid is a soluble form of silicon and one of the basic form, which is absorbed and used by plants. Polymerized silicates belong to the group of the hardest materials in plant tissues. Silicon helps to raise the plant health by the creating of strongest and more resistant structures. Plants which are attacked by the herbivores tend to accumulate more oxides [3].

It is one of the oldest plants on earth and what remain today from tree-sized fossils are the field horsetails. They were used in historical times for scouring pots and polishing pewter and were commonly called “scouring rushes”. Horsetails have found extensive application in medicine as a source of silica, as it can amount to 25% of the dry weight of the plant [4]. Silicon from horsetail promotes the growth and stability of the skeletal structure. An invention describing a pharmaceutical composition based on Equisetum arvense for the treatment of bone diseases, particularly osteoporosis [5]. A few European clinical studies have determined that fractured bones heal much more quickly when horsetail is taken. The incidence of osteoporosis is, likewise, more greatly reduced when some horsetail is added to the diet. Horsetail extract is also added in a composition used against psoriasis [6].

Researchers [7,8] believe that the medicinal property of horsetail is due to its high silica content. Horsetail has been used as a folklore medicine for treatment of various conditions such as tuberculosis, as a catarrh in the kidney and bladder regions, as a hematostatic for profuse menstruation, nasal, pulmonary and gastric hemorrhages, for brittle fingernails and loss of hair, for rheumatic diseases, gout, poorly healing wounds and ulcers, swelling and fractures and for frostbite [9]. 

Silicon increases the resistance against mold. Recent researches noted that first reaction after fungi attack is higher at silicic acid presence [10]. Growing of the plants is supported by the silicates, which allows better flexibility and extensibility of cells walls [11]. The content of silicon had negative correlation with lignin and cellulose at wetland macrophytes. It substitutes the mechanical role of these polymers [12].

The aim of this research was to determine the content of the silicic acid, its relation to spatial structure and environmental conditions at the natural habitat of selected plant species.

2. Matrial and Methods

2.1. Plant Material and Description of Collection Locality

Samples of horsetail were collected by the destructive methods in the years 2009, 2010 and 2011, each year from May to September (June—first collection, July— second collection, August—third collection, September— fourth collection). The plant material reduced to its biomass after drying to a constant weight (minimum 24 hours) in 85˚C - 105˚C in a dryer with ventilation.

When we chose the locality for collecting the plant samples we encountered regular dispersion of horsetail in Laborecka vrchovina (Slovakia). The three localities are at different altitudes, different distances from rivers and are orientated depending on sun radiation and slope of the terrain. Locality 1 (L1) is on level ground and 30 m from the river, 196.4 m altitude, 49˚03'44.47''N, 21˚57'45.22''E. Locality 2 (L2) has high groundwater and is oriented to the south, 225.5 m altitude, 49˚03'57.71''N, 21˚58'00.89''E. Locality 3 (L3) is shaded and oriented to the north, 205.5 m altitude, 49˚03'43.20''N, 21˚58'40.42''E.

2.2. Analytic Methods and Equipment

The mineralization technique for the sample analysis required nitric acid HNO₃ (65%) and hydrofluoric acid HF (46%) which were bought from the Sigma Aldrich Co. The silicon standard (Si) was from company Ultra scientific. We used a 2% solution of HNO₃ and deionized water with the conductivity of <0.1 µS for extract stabilization. We used a Speedwave 2 Berghof for mineralization, with a voltage of ~230 V, frequency of 50/60 Hz, power consumption of 1610 W, and a magnetron frequency of 2450 MHz. Pressure vessels DAP-60K, volume 60 ml, maximum pressure 40 bar, maximum temperature 230˚C, maximum weight < 300 mg, minimum volume of acids > 5 ml. Atomic Absorption Spectrophotometers (AAS 7000) from Shimadzu company. Fully automatic dual—beam instrument with 3D-optic system, automatic 6-lamp holder, correction of background by D-lamp with SR-correction of spectral interferences. Software shares preset operating parameters for all elements and standard samples.

2.3. Sample Preparation by Microwave Mineralization

Dried samples of horsetail were weighted at 0.1 g with the precision of 0.0001 g. Thereafter, they were stored in pressure vessels, DAP-60K, and then 8 ml HNO₃ (65%) and 2 ml HF (46%) were added. The thermal program was 0˚C - 145˚C/5min, 220˚C/40min, 200˚C/15min. Mineralized samples were cooled until room temperature. After that, we prepared samples for AAS analysis to determine the silicon at wave length 251.61 nm. The aim of this research was to determine the content of the silicic acid, its relation to spatial structure and environmental conditions in the natural habitat of selected plant species.

3. Results

We consider the effect of three factors for the silicon content in the dry matter of horsetail (Table 1), which are from three locations (L1, L2, L3), three years (2009, 2010, 2011) and four collection times (1, 2, 3, 4). Since we do not have values for each combination of factors (we did not collect samples 3 and 4 at L3 locality) and to assess the significance of factors influencing the content of silicon in the dry matter of horsetail, we used an analytical model of variance to calculate the main effects.

Our calculations show the highest average content of silicon was measured at the sample from locality L2 (27.23 ± 7.59 g·kg⁻¹) and the lowest content was from the locality L3 (23.79 ± 2.73 g·kg⁻¹). Relative to the year, the highest average silicon content was from the year 2011 (26.96 ± 3.41 g·kg⁻¹), but the amounts reached

similar percentages in the years 2009 and 2010. The collection factor showed the highest disproportion in the measured values. The highest accumulation of silicon was from the fourth collection (32.80 ± 8.03 g·kg⁻¹) and the lowest content was measured in the first collection (21.11 ± 3.24 g·kg⁻¹).

3.1. Analysis of Variance of the Silicon Content at Dry Mater

The results from the analysis of the variance of the silicon accumulation in the dry mater of horsetail (Eqisetum arvense L.) are shown in Table 2. It describes the insignificance of the locality factor (F = 1.37, p = 0.275). Analysis of the year factor showed the same—the content of silicon in three different years (2009-2011) in horsetail was not statistically significant (F = 0.92, p = 0.413).

The only statistically significant factor was the collection (F = 7.79, p = 0.001) (Figure 1). It means that the content of silicon in the dry matter of horse tail was different in each collection.

Figure 1 shows the average silicon concentration in the dry matter from each collection. The highest average values were recorded in the fourth collection from 26.44 ± 1.32 g·kg⁻¹ to 32.80 ± 8.03 g·kg⁻¹. The lowest contents were measured in the first collection in each locality and