Fruit and vegetables are an important part of human nutrition and their adequate daily consumption can help prevent serious diseases (WHO, 2003). The WHO (2004) recommended at least 400 g of fruit and vegetables daily to prevent chronic diseases such as heart and cancer, type 2 diabetes mellitus and obesity, as well as to prevent and reduce micronutrient deficiencies, particularly in less developed countries (WHO, 2003).

Vitamin C is the most important vitamin in fruits and vegetables. Except for humans and other primates, most phylogenetically lower animals can synthesize this vitamin (Rekha et al., 2012). More than 90% of vitamin C in the human diet comes from fruit and vegetables. The highest amounts of this vitamin occur in blackcurrants, citrus fruits, spinach, tomatoes, red peppers (Lewin, 1976; Lee and Kader, 2000; Saxholt et al., 2008; Rekha et al., 2012).

Vitamin C improves immune system function, acts as an antioxidant, and reacts with oxygen and other molecules (Rekha et al., 2012; Soni et al., 2017). According to EFSA (2017), the reference intake of vitamin C for a healthy adult should be 90 mg per day. Vitamin C content in vegetables and fruits depends on several factors, such as variety, habitat, growing year, storage and processing (Mařáková, 2008; Matějková and Petříková, 2010; Oyetade et al., 2012; Combs, 2017).

Vitamin C is a very unstable compound. It is rapidly oxidized with oxidizing agents, especially iron, copper, various enzymes (e.g. ascorbase, peroxidase, cytochrome oxidase) and air/oxygen (Jeney-Nagymate and Fodor, 2008). Moreover, the stability of vitamin C decreases with increasing temperature, pH (Jeney-Nagymate and Fodor, 2008) and light access (Nováková, Solich and Solichová, 2008). During processing, the stability of vitamin C is higher in fruit than in vegetables due to their lower pH (Mařáková, 2008).

Nitrates are created in fruits and vegetables mainly via the nitrification of ammonia nitrogen. They play an important role in plant nutrition, growth and development. Higher nitrates concentrations accumulate in leaves, while lower concentrations are present in roots, tubers, seeds and fruits. For this reason, leafy vegetables (spinach, lettuce, etc.) are the most important source of nitrates and may contain more than 2000 mg.kg^-1 (Maynard et al., 1976; Santamaria, 2006).

Nitrates themselves are relatively nontoxic, but their metabolites (nitroso compounds), which could be formed in the human body, can cause several health problems (Santamaria, 2006). Consumption of vegetables with high nitrate content may increase the risk of methemoglobinemia or gastrointestinal cancer (Du, Zhang and Lin, 2007). Bevc et al. (2012) reported an association between nitrates and cancer of the bladder, ovary, stomach and liver. According to EFSA (2008), the acceptable daily intake for nitrates (ADI) is 3.7 mg per kg of body weight per day. Temme et al. (2011) reported a real average daily nitrate intake of 1.38 mg per kg of body weight per day. The main sources of nitrates are vegetables (mainly lettuce) and drinking water. Due to the undesirable effect of nitrates on human health, their content in certain kinds of vegetables is regulated by Commission Regulation (EC) No 1881/2006.

On the other hand, intake of small amounts of nitrates can also have a positive effect on the gastrointestinal tract (Duncan et al., 1999) and the cardiovascular system (Webb et al., 2008; Sobko et al., 2010; Lundberg et al., 2011). The negative effect of nitrates on human health can be significantly limited by the simultaneous presence of vitamin C (Kopec, 2010; Shehata, 2010).

The accumulation of nitrates in fruit and vegetables depends on many factors - mainly their type or genotype, variety, maturity, climatic conditions (light intensity, air temperature and carbon dioxide concentration), agronomic factors (e.g. timing and form of nitrogen application), soil type and harvest time (Sorensen, Johansen, and Poulsen, 1994; Colla et al., 2018) and also by heat treatment (i.e., frying, baking, cooking), storage or preservation (Prasad and Chetty, 2008; Temme et al., 2011).

Farmers' markets are a form of selling agricultural and food goods to the general public, with the aim of a) supporting small and medium-sized agricultural growers, breeders and food producers; b) supplying citizens with fresh agricultural crops and food of mainly national and regional origin; c) creating new social space which, in addition to the sale of agricultural goods, serves to get people together and to bring the urban population closer to the agricultural season and natural cycles; d) revitalizing selected urban areas and improving their ambience. Farmers' markets usually take place in the open air. Only seasonal and local goods should be sold at farmers' markets (Bohatec, 2011; Sedláček, 2015).

The most common products at farmers' markets include fruit and vegetables (Brůčková, 2012). Unlike organic food, there is no legal or generally accepted definition for local food. Many authors describe it as “local food” such foods that were produced within 100 km of the consumer. Other authors argue that local foods are those produced in a certain region or country.

Another positive factor for buying local food is its freshness. At farmer's markets, we find fresh fruits and vegetables that were picked just before the sale, while in supermarkets we can come across food that has traveled to the counters for several days and matured on the way. This also relates to the carbon footprint; the production of local food is significantly more energy-efficient and thus more environmentally friendly (Brůčková, 2012; Kukla 2012).

For most consumers, fruits and vegetables that come from farmers' markets are considered healthier than usual “conventional” supermarket products. However, it is not clear whether these farm products contain more nutrients and/or less harmful substances (Wunderlich et al., 2009). Therefore, this work aimed to compare the content of vitamin C and nitrate in selected fruit and vegetables purchased at farmers' markets and in supermarkets.

Vitamin C and nitrates levels were monitored in three different kinds of fruit (apples, plums and strawberries) and three different kinds of vegetables (spinach, red peppers and tomatoes). The Jonagold apple variety was used for analysis. For other analyzed fruit and vegetables the variety was not declared by supermarkets and farmer´s markets. Fruit and vegetables (except spinach) were purchased in the period September-November (spinach in the period October-November) in four different supermarkets and four different farmers' markets in the same locality. The same kind of fruit or vegetables were always bought at the same time in the supermarket and the farmers' market.

Many factors are affecting the level of vitamin C in fruit and vegetables and, therefore, its content can vary a lot. From this point of view, our experimentally measured values are generally in line with literature data. Of the assayed samples, the highest content of vitamin C was in red peppers. Its average contents were 78.9 mg.100g^-1 (from farmers' markets) and 105.5 mg.100g^-1 (from supermarkets). This is less than observed in other studies. McCance and Widdowson (2014) reported an average content of 120 mg.100g^-1 of vitamin C in red peppers in their study; Lee and Kader (2000) and Saxholt et al. (2008) reported values up to 151 mg.100g^-1. Strawberries had the second-highest average vitamin C content of 52 mg.100g^-1 (supermarket) and 66 mg.100g^-1 (farmers' markets). Similar results were obtained by Lewin (1976), Lee and Kader (2000), and Saxholt et al. (2008), who reported vitamin C levels in strawberries ranging from 35 to 60 mg.100g^-1. Vitamin C levels in strawberries are comparable to those in citrus fruits (40 – 50 mg.100g^-1) (Lee and Kader, 2000).

Spinach is a vegetable with a high content of vitamins and minerals. Our value for vitamin C content was similar to values observed by Bureau et al. (2015) (23.7 mg.100g^-1). The vitamin C content we found in tomatoes is following literature values, 30 – 35 mg.100g^-1 (Lewin, 1976). According to George, Kaur, Khurdiya, et al. (2004), however, the vitamin C content of tomato pulp may be 84 to 324 mg.100g^-1. The deviations of some of our results for the content of vitamin C from the data in the literature may have been affected by analyzing samples purchased in the autumn and the way of their storage (Matějková and Petříková, 2010).

Our results indicated that only in the case of strawberries, tomatoes, and peppers from farmers' markets and supermarkets were significant differences in vitamin C content found. The biggest difference was in strawberry samples. Strawberries from farmers' markets contained 27.4% more vitamin C than strawberries from supermarkets. In contrast, the content of vitamin C in plums from farmers' markets and supermarkets was almost identical. There is not much information in the literature comparing products from farmers and supermarkets. Studies are usually focused on the differences between organic and conventional fruit and vegetables and their findings are often controversial (Silva et al., 2018; Andrade et al., 2017). Andrade et al. (2017) tested the quality of strawberries grown in organic and conventional systems. The content of vitamin C in organic strawberries was (49.07 mg.100g^-1) and in conventional strawberries (52.32 mg.100g^-1). According to these authors, the content of vitamin C in strawberries does not differ significantly between the conventional and organic systems. The authors state that vitamin C content depends on many factors, including variety, ripeness, growing conditions and harvest time. These factors could then lead to significant variations in results, both between studies and within studies.

Esch et al. (2010) looked for differences in vitamin C content between organic and conventional fruits.

The tested fruits were oranges, mangoes, kiwi, lemons, gala apples and red apples. Of these six fruits, the only lemon showed a significant difference between organically (higher content) and conventionally grown samples. This study confirmed that the content of vitamin C depends on several factors, not just the way of cultivation. Wunderlich et al. (2009) reported that pre- and post-harvest conditions have a major effect on vitamin C levels in vegetables.

Based on our results, the hypothesis that fruit and vegetables from farmers' markets have a higher vitamin C content can, therefore, neither be accepted.

Fruit and vegetables also contain substances that could have a negative impact on human health. For this reason, the second half of this work was focused on nitrate content in fruits and vegetables from farmers' markets and supermarkets. It can be stated that there were significant differences in the nitrates content among the assayed kinds of fruits and vegetables.

According to Prugar (2008) and Colla et al. (2018), fruit and vegetables can be divided into four categories according to nitrates content. The highest nitrate content is in leafy vegetables such as spinach. Maynard et al. (1976) and Santamaria (2006) reported that nitrates accumulate primarily in leaves, while lower nitrate concentrations are present in roots, tubers, seeds and fruits. For this reason, leafy vegetables (spinach, lettuce, parsley, etc.) are considered the most important source of nitrates. Tomatoes and peppers belong to the group with nitrates content of less than 250 mg.kg^-1 (Colla et al., 2018).

Maximum levels for nitrates content are given by Commission Regulation (EC) No 1881/2006 only for salad and spinach. The maximum limit for spinach is set in two categories, i.e. 3500 mg NO₃.kg^-1 for the harvest from 1 October to 31 March and 2500 mg NO₃.kg^-1 for the harvest from 1 April to 30 September. The highest measured nitrates content in the spinach sample in our case was 2969 mg.kg^-1. As the purchase of the samples took place in October, it can be assumed that the maximum limit for nitrate content was not exceeded.

Prugar (2008) and Colla et al. (2018) classified spinach into the “very high nitrates category”, i.e. more than 2000 mg.kg^-1. In this study, the results of spinach from farmers' markets with average nitrates content of 1391 mg.kg^-1, would fit to the “high nitrates category” (i.e. 1000 – 2000 mg.kg^-1). Supermarket spinach contained 47.5% more nitrates than spinach from farmers' markets. This difference was most likely due to different growing conditions. Fruit and vegetables from wholesalers come very often from greenhouses.

The main factors contributing to the increased nitrate concentration are temperature, lack of light and strong fertilizer concentration. In contrast, products from farmers' markets are usually grown in the open air in fields with enough light. Moreover, it is best to buy vegetables from the afternoon harvest. In the morning, the nitrate content in fruits and vegetables is higher than in the evening. It is therefore recommended to harvest agricultural products in the afternoon (Sorensen, Johansen and Poulsen, 1994; Colla et al., 2018).

Muramoto (1999) conducted a similar study with iceberg lettuce, Roman lettuce and spinach from supermarkets (conventional cultivation) and farmers' markets (organic cultivation). This author concluded that samples of spinach from the supermarket (average nitrates content 2540 mg.kg^-1) had a significantly higher nitrates concentration than samples from farmers' markets (average nitrates content 1810 mg.kg^-1). Muramoto (1999) also suggested that the use of nitrogen fertilizers may be the main cause of this difference. In other studies, Barker (1975) and Stopes et al. (1989) reported a positive correlation between the number of nitrogen fertilizers and nitrates accumulation in spinach.

Compared to vegetables, the fruit samples had much lower nitrates content. Bahadoran et al. (2016) reported an average of 27 mg nitrates per 100 g plums in their study. The highest nitrates content of our fruit samples was found in strawberries, which belong (together with bananas) to a “higher nitrates category”. Walker (1990) reported up to 150 mg of nitrates in 1 kg of strawberries in his study. We observed average nitrates content of strawberries from 116.63 mg.kg^-1 (from farmers' markets) to 122.39 mg.kg^-1 (from supermarkets). The explanation for the high nitrate content could be the presence of a thin, porous layer on the surface of strawberries, their frequent fertilization, or the way of growing in greenhouses where the lack of light causes the accumulation of nitrates.

In the case of our samples, no significant difference was found between strawberries from supermarkets and farmers' markets. This observation is consistent with the study of Bordeleau et al. (2002), who also examined the difference in nitrates content of strawberries from farmers' markets and supermarkets. Other samples – plums, red pepper, tomatoes, and apples contained less than 250 mg of nitrates per kg, thus belonging to the low nitrate fruit and vegetable category according to the nitrates content classification (Prugar, 2008; Colla et al., 2018).

In three cases out of six, the fruit and vegetables tested by us from farmers' markets contained statistically lower nitrates concentrations than those purchased in supermarkets. In these cases, the hypothesis that fruits and vegetables from farmers' markets have lower nitrates content than fruits and vegetables from the supermarket can, therefore, be accepted. On the other hand, there was no significant difference between the nitrates contents of strawberries and red peppers from farmers' markets or supermarkets, whereas tomatoes had significantly higher nitrates content when bought from farmers' markets.

Studies that have found significantly lower levels of nitrates in products from farmers' markets generally explain this by a lower fertilization rate. Bordeleau et al. (2002) stated that mineral fertilizers have a greater effect on nitrates levels than livestock fertilizers or humus. Other important factors influencing nitrates content are weather and light conditions.

The results of this work showed that the content of ascorbic acid and nitrates differs significantly depending on the type of fruit and vegetables. The most important source of vitamin C from the assayed fruits and vegetables was red pepper. One hundred grams of it would sufficiently cover the recommended daily dose of 90 mg. Comparing the different kinds of fruit and vegetables, significant differences in vitamin C content between samples from farmers' markets and samples from supermarkets were found in strawberries, red peppers and tomatoes.

Analyses of nitrates content showed significantly lower nitrates content in spinach, apples and plums from farmers' markets than from supermarkets. The most important source of nitrates was spinach, but the content did not exceed the limit set by legislation. Compared to vegetables, the fruit samples had much lower nitrates contents. Strawberries had the highest nitrates content of the fruit samples examined.

Overall, there were significant differences in nitrate content in fruit and vegetables from supermarkets and farmer´s markets. On the other hand, differences in vitamin C content were not found.

Since the majority of studies compare the quality of organic and conventional products rather than comparing the nitrates and vitamin C contents of fruit and vegetables from farmers' and supermarkets, it would be better to focus on this area in the future. Our findings suggest that farm products may not always be a better choice in terms of vitamin C and nitrates content than fruit and vegetables purchased in supermarkets.