The results obtained by the application of selenium as micro fertilizer, in order to increase its content in several medicinal plant species (chamomile, peppermint, nettle and marigold) are presented in this paper. The research has been conducted through the field experiment on humogley soil, with the application of selenium in the form of Na-selenate, dissolved in water (0.001-0.02%), which had been used for soil and plants sprinkling in the beginning of the vegetation period. The following Se-doses were applied: 0, 50, 500 and 1000 g Se/ha. Humogley soil, where the experiment has been conducted, as well as the most soils in SR Yugoslavia, contains very low total selenium (about 0.2 ppm). The investigated medicinal plants contained 0.04-0.07 ppm of selenium in the control variants, which is below its optimal level (0.1-1 ppm). Yet, the application of the lowest dose (50 g Se/ha) increased its content in the chamomile, peppermint and marigold up to optimal values (0.2-0.5 ppm), while somewhat higher values were obtained for nettle (1.3 ppm). Each one of the four investigated medicinal plant reacted on the increasing of the Se dose. The greatest increase of Se content was obtained for nettle (to 95 ppm) and the lowest for marigold (to 4.5 ppm). The absorbed selenium in the medicinal plants was incorporated into the organic compounds and its significant portion (11-30%) could be extracted by hot water (nettle, peppermint and chamomile) and in such way (teas) used to satisfy a great part of daily human requirement in this element.

Key words: selenium, micro fertilizer, chamomile, peppermint, nettle, marigold.

Selenium (Se) is one of the most interesting microelements. It seems that it is not essential for plant growth, although it replaces sulfur in the amino acids. On the other hand, it is important element in human and animal nutrition. In the case of its pronounced deficiency, as well as in the case of its higher concentrations, various diseases in humans and animals often occur (Adriano, 1986).

There is a significant deficiency of Se in soils (less then 0.2 ppm) in the many parts of the world (China, Finland, New Zealand, etc.), which is connected to its deficiency for plants and various health problems in humans and animals (Adriano, 1986). In FR Yugoslavia, according to previous research (Maksimović et al., 1989, 1992, Jakovljević et al., 1995) high deficit of selenium in the arable soils, growing plants and animal food products has been found. Human intake of selenium in our country by regular meals amounts to 25-30 mg (Djujić et al., 1995), which satisfies only 25-30% of the optimal daily requirements in this element.

The solving of this problem for the first time started in Finland by application of Se in the NPK-fertilizers, which then have been widely used. The result was significant increase of Se level in the plants and improvement of the health situation of the entire population in the following years (Shorrocks, 1986).

Our intention was to significantly increase selenium content in different medicinal plants by fertilization, which might have positive effects in the following: increment of human daily supply with this element through teas, prevention of certain diseases by the consumption of phytopreparations based on plants enriched with Se and usage of some of the plants (nettle) for the animal food mixtures.

Designed research program was carried out through the field experiment on the humogley soil at the Experimental field station of the Institute for Medicinal Plants Research "Dr Josif Pančić" in Pančevo. The following medicinal plant species have been included in the research: chamomile [Chamomilla recutita (L.) Rausch.], peppermint (Mentha piperita L.), nettle (Urtica dioica L.) and marigold (Callendula officinalis L.). The plants were cultivated during the year 1995, on the 3 x 3 m specified plots. Na-selenate (Na₂SeO₄), with 42% of pure selenium, was used as Se-fertilizer. Corresponding amounts of this salt were dissolved in water and then diluted for uniform spreading on the soil surface, immediately after planting. The concentrations of these solutions were from 0.001 to 0.02%. The treatments of the experiment were: 1. Ć; 2. Se-1 (50 g Se/ha); Se-2 (500 g Se/ha); 4. Se-3 (1000 g Se/ha). There were several harvests during the spring and summer of the same year, i.e. chamomile - 3, peppermint - 2, nettle - 3 and marigold - 4. During 1996 the residual effect of the applied selenium on its content in the peppermint and nettle has been monitored.

After the harvest, plant samples were at first air dried, then dried at 70°C and ground. The following plant parts were analyzed: flowers - chamomile and marigold, leaf - peppermint and nettle and stem - nettle.

Total selenium content in dried and ground plant material was determined by AAS method, with the application of hydride vapor, after the digestion with acids (HNO₃ and HClO₄, with the addition of H₂O₂) and reduction of Se⁺⁶ to Se⁺⁴ with conc. HCl. The similar method was applied for the determination of total selenium in the soil, before and after the field experiment. Beside that, available selenium content in the soil was also determined after the extraction with hot water (soil: water ratio - 1:2).

The ethanolic extracts (ratio 1:1) and teas (4 g on 250 ml of hot water) were made from the dried plant samples. Such extracts were also subjected to the total selenium analysis, according to the described method.

As was mentioned, the field experiment was conducted on the humogley soil, which is characterized as rather fertile one, considering its chemical properties. It is slightly calcareous, with quite high humus (about 4.0 %) and nitrogen (0.22 %) content and well supplied with available potassium and essential microelements.

However, the total selenium content is rather low (0.200 ppm). Only 5% of the total selenium in the soil is presented in the available forms (Se soluble in water). As a consequence of this, Se content in the control variants of all investigated medicinal plants was also low (0.04-0.7 ppm), as could be seen from Tab.1.

Significant increase of Se content (Tab. 1.) in all investigated species, especially in nettle, chamomile and peppermint, has been achieved by its application in the form of micro fertilizer. It is noticeable that the increase of applied Se doses influenced its higher accumulation in the plants. But, it is important to emphasize that, as a response to the application of even the lowest Se dose (50 g Se/ha), its content in chamomile, peppermint and marigold plants increased up to optimal levels (0.1-1 ppm).

Further, pronounced influence of the harvest time on the selenium content in the plants was also noticed. Se content decreased rapidly in the second and the third cut, especially in nettle and peppermint. This is, certainly, influenced by the high mobility of selenium in the form of selenate, when its greater portion is being leached in deeper soil layers during the course of vegetation.

The residual effects of applied Se-fertilizer on its content in the peppermint and nettle plants, which has been studied in the second experimental year, was obtained for the higher doses, but the Se content in the plants was much lower than in the first experimental year.

The average distribution of selenium from added fertilizer is shown in Tab.2. It can be seen that the plants had taken the lowest portion of applied Se. For chamomile and peppermint it amounts to 0.5-2.0%, and for nettle 3-13%. Between 20 and 30 % of added Se remained bound in the plow soil layer, while the highest portion had been lost, partly by leaching in the deeper soil layers and partly by volatilization. It has to be emphasized that the residual Se in the soil has been strongly bounded, since the content of available Se remained within initial amounts (5% from total Se).

From the analysis of the plant's extracts it can be seen (Tab. 3.) that much higher portion of selenium, which had been taken by the investigated medicinal plants (chamomile, peppermint and nettle), was being extracted by hot water (teas), than by ethanol. It is understandable, since the absorbed Se was incorporated into the amino acids and proteins. The greatest percentage of the extraction was obtained from chamomile (30%), somewhat lower from peppermint and much lower from nettle. Chemical analysis proved that Se, extracted with hot water, is organically bound. According to the obtained results, the use of such teas could satisfy a great part of daily human requirement in selenium. This, of course, needs to be proved through the medical researches.

Much lower portion of selenium is extracted by ethanol, than by hot water, but its concentration per unit of volume is ten times higher, due to much narrow dry matter : extractant ratio (1:1). Accordingly, ethanolic extracts, as well, could be used for production of phytopreparations enriched with selenium.

High selenium content in the nettle's leaves and stem (Tab. 3.), as well as its organic nature, indicates a possibility that this raw material, as the source of Se, could be used for preparation of the animal food mixtures, especially for the animals low in weight.

The obtained results have shown the possibility of significant increase of selenium content in the medicinal plants: chamomile, peppermint, nettle and marigold, with Na-selenate applied in the soil in the form of micro fertilizer.

The optimal selenium level for plants (0.1-1 ppm) has already been achieved with lower dose of 50 g Se/ha.

Application of the high selenium doses (500 and 1000 g/ha) had as a result with its high content in the following medicinal plants: nettle, chamomile and peppermint. This Se was incorporated into the organic compounds.

Since organic selenium can partly be extracted by hot water (teas) and ethanol, such extracts could be used for human daily supply with selenium (teas) and for production of the phytopreparations (ethanolic extracts).

Ground nettle tissue could be used for enrichment of animal food mixtures with selenium.

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