Foliar application of selenium, in the form of sodium selenate, had a significant effect on the increase of the Se content in tomato fruits. Other antioxidants such a vitamin C and carotenoids will not be negatively affected. The concentration of total carotenoids and polyphenols is variety dependant.

An experiment was founded in 2016 in Botanical Garden of Slovak University of Agriculture (below BG SUA) in field conditions. Area is situated in very warm agro-climatic region, very dry sub-region. The mean annual temperature was 10 °C. Meteorological measurements were carried out by the help of meteorological station in the area of botanical garden, SUA in Nitra. The mean monthly air temperature and average rainfall for the year 2016 were evaluated by the climate normal 1961 – 1990 (Table 1 and Table 2). Sowing was carried out on March 14^th, 2016 in a heated greenhouse BZ SUA. There were included 7 tomato determinate varieties (Table 3). According to the techniques of tomato cultivation it was done preparing of the land. According to agrochemical analysis of the soil (Table 4) in case of all three variants there was applied nitrogen fertilizer DASA^®26/13 – Ammonium nitrate with sulfur (26% N and 13% of S) in dosage 254 kg.ha^-1 (60% of recommended normative).

The field experiment was established by block method in three replications, plants were planted in uniform plant spacing 0.7 x 0.3 m. Planting of pre-growing planting was carried out on May 15^th, 2016. According to necessity the supplementary irrigation was applied, as well as loosening and weeding of the crop. The application of selenium was done on June 26^th, 2016 at the growth stage of fruits creation on the first inflorescences and second inflorescence establishment. Selenium was foliar applied in solution Na₂SeO₄. During the growing season the crop is in the process of setting up third-fourth inflorescence fertilization with nitrogen fertilizers DASA^®26/13 (169 kg.ha^-1, it means 40% of the recommended normative). In the second half of vegetation protection of the plants against fungal diseases was carried out. Collection was realized in August 17^th, 26^th and in September 9^h, 14^th 2016 in full botanical ripeness of the fruits. In the small plot experiment there were observed the following options:

1^st variant (K) – without applying of selenium (control),

For laboratory analysis were used only fresh fruits in full botanical maturity of all observed varieties of tomato after first harvesting of the fruits. Both control as well as the variant fortified with sodium selenite were used. An average sample of 2000 g was prepared from the harvested fruit yield. We homogenized the opposite two quarters of each sliced fruit within average sample and used it for the determination of total carotenoids and vitamin C content. Fresh fruit analysis was performed within 24 hours after harvest.

The qualitative characteristics were estimated in laboratory of Department of vegetable growing, SUA, in Nitra involving:

Total carotenoids estimation – Carotenoids were estimated by spectrophotometric measurement of substances absorbance in petroleum ether extract on spectrophotometer PHARO 100 Spectroquant^® at 450 nm wavelengths.

Ascorbic acid estimation (AA) – HPLC method of vitamin C content estimation Stan, Soran and Marutoiu (2014) was used by the help of liquid chromatograph with UV detector, for separation was used RP C18 column, mobile phase was methanol:water (5:95, v/v), UV detection was adjusted to 258 nm (HPLC fy. VARIAN).

Total polyphenols content estimation (TPC) – total polyphenols were determined by the method of Lachman (2011) and expressed as mg of gallic acid equivalent per kg fresh mater. Gallic acid is usually used as a standard unit for phenolic content determination because a wide spectrum of phenolic compounds. The total polyphenol content was estimated using Folin-Ciocalteau assay.

Selenium content – mineralization of the plant material took place in the microwave mineralizer type CEM Mars X–press (microwave digestion oven). Quantitative determination of Se was done by using of ET-AAS method with Zeeman background correction. Atomic absorption spectrometer SpectrAA240FS (Varian, Mulgrave Virginia, Australia) was used to measure the total selenium content (Hegedűs et al., 2008). Conditions for selenium measurement were set in the equipment according to the recommendations of the manufacturer (Rothery, 1988) for ET-AAS technique. In the research, chemicals with analytical purity were used.

The analysis of variance (ANOVA), the multifactor analysis of variance and the multiple Range test were done using the Statgraphic Centurion XVII (StatPoint, USA).

The total carotenoid content in the fresh fruits of our observed hybridized tomato varieties ranged from 8.17 to 10.28 mg.100g^-1 of fresh matter for control variant and from 8.03 to 11.45 mg.100g^-1 after selenium application (Table 5). The highest carotenoid content obtained UG-Red F1 variety. The average values in our tested varieties were comparable to those of Mendelová et al. (2012) showing an average content of total carotenoids in tomato varieties (Báb, Žiara PK, Champion, Roti) ranging from 4.8 to 7.0 mg.100g^-1.

Due to the foliar application of selenium, we observed a slight decrease in total carotenoid content in three hybrid varieties: Townsville F1, Triple Red F1 and Brixol F1. For other varieties of tomatoes, foliar application of selenium has resulted in increased carotenoid content.

Based on statistical evaluation of the data obtained by the multifactor analysis of variance, we can state that the variety had a proven impact on the total carotenoids content in fresh fruits and tomato juice. The effect of the variant on the total carotenoid content in fruits wasn´t statistically significant (Figure 1).

Vitamin C (ascorbic acid) is one of the most important antioxidants in tomato. The content of antioxidants in tomato fruits depends on a number of factors such as ripeness, cultivation conditions and the variety itself (Hart and Scott, 1995; Kotíková et al., 2011), while the skin of fruits contains more vitamin C than the flesh (George et al., 2004).(Figure 2)

Carli et al. (2011) tested 7 parental lines and 8 tomato hybrids and found that the vitamin C content in the fruits ranged from 9.1 to 16.1 mg.100g^-1 of fresh matter. Our results show that similar levels of vitamin C content have also been found in our experiment. The vitamin C content measured in fresh fruits of 7 hybrid tomato varieties was ranging from 11.90 to 19.44 mg.100 g^-1 for control variant and from 11.46 to 20.14 mg.100g^-1 of fresh mass for variant with foliar application of selenium (Figure 3).

The highest vitamin C content was in the Triple Red F1 variety in both experimental variants. Foliar application of selenium has a positive effect on the vitamin C increase only in Troquay F1, Townsville F1 and Triple Red F1 and Brixol F1 varieties.

By statistical analysis of the obtained data based on vitamin C content we can conclude, that the effect of the variety on the vitamin C content in the fruits of evaluated Lycopersicon esculentum varieties has been proven (Figure 3). Among the observed variants, we didn´t find any significant differences in the vitamin C content in fresh tomato fruits (Figure 2).

During the evaluation of total polyphenol content in tomato fruit, we found that within the selenium control variant, polyphenols content ranged from 3588.30 mg GAE.kg^-1 of dry matter in Durpeel F1 variety to 4944.78 mg GAE.kg^-1 dry matter in Torquay F1 variety. Similar results are also reported by Luthria, Mukhopadhyay and Krizek (2006), who determined the total polyphenols in tomatoes ranging from 3400 to 4400 mg GAE.kg^-1.

By foliar application of selenium at a dose of 150 g.ha^-1, all of the tomato varieties, except the Brixol F1 variety, increased the content of polyphenols in its fruits. For this variant, the content of total polyphenols ranged from 3603.40 mg GAE.kg^-1 to 5350.70 mg GAE.kg^-1 of dry matter (Table 6).

Kavalcová et al. (2014), based on the onion field experiments reported that the sodium selenate applied in different doses didn´t have a significant effect on the total polyphenols content in the consuming parts of evaluated onion varieties.

On the contrary, authors Hegedűsová et al. (2017), on the basis of two-year field experiment, confirmed the significant influence of foliar application of selenium on increasing the polyphenols content in peas within two varieties of Pisum sativum.

By statistical evaluation of our results, we can conclude that the effect of the variety on the total polyphenols in the fruits was statistically significant. Although the effect of foliar application of selenium had a slight increase in the total polyphenols in the fruits, the effect of the variant on the total polyphenols content wasn´t confirmed (Figure 4).

To ensure the necessary income of selenium by human population, different ways of supplementing are used in the world. One of them is the addition of selenium into fertilizer preparations, which according to the authors Ducsay, Ložek and Varga (2007) can be considered as the most effective method of selenium intake, whereby selenium gets into individual food chain links. During the evaluation of the effect of selenium foliar application on selenium content in tomato fruit we found, that in experimental year 2016 selenium content in the fruits of studied varieties ranged in the control variant from 0.028 mg.kg^-1 to 0.060 mg.kg^-1 of dry matter. Foliar application of selenium caused an increase in selenium content in the fruits of observed tomato varieties and its content was in the range of 0.378 mg.kg^-1 in a variety Durpeel F1 to 0.990 mg.kg^-1 dry matter in a variety Uno Rosso F1 (Figure 6). Hegedűsová et al. (2017) also wrote about the positive effect of foliar application of selenium in the form of sodium selenate in the blooming phase of the pea,on the increase of selenium content in immature seeds (varieties: Ambrosador and Premium). They point to the fact that the total content of selenium in the consuming part is affected not only by the dose of applied selenium but also by the climatic conditions of the given growing year.

Using the statistical analysis of variance, we didn´t detect the difference in selenium content between the evaluated varieties. We recorded a statistically significant effect of the variant on the selenium content in tomato fruits (Figure 5).