## 1 Introduction  
Flowering Chinese cabbage, commonly known as caixin in Mandarin Chinese or choy sum in the Cantonese dialect, originated from China [1]. The vegetable is still little known by Europeans. The leaves and flowering stem are the most valuable parts of flowering Chinese cabbage. The plant is a rich source of glucosinolates, polyphenols, vitamin C, amino acids and other chemical compounds [2]. Based on previous studies [3; 4], factors such as season (planting date), cultivation technology and nitrogen nutrition are important and significantly modify the yield of flowering Chinese cabbage. It was also reported that foliar spraying affected plant responses, such as macro and microelement contents, pigment and phenolic levels as well as antioxidant properties. It was previously reported that the optimal level of nitrogen nutrition is in the range of up to 90 mg\(\cdot\)dm\({}^{-3}\) (mod. 180 kg per ha), while increased nutrition does not usually significantly improve the yield [4]. However, others indicated the N dose level from 440 to 490 kg per ha (i.e., 220-245 mg\(\cdot\)dm\({}^{-3}\) in a 20 cm layer) as the optimal nutrition, which is a balance between the biological value for plants and the economic costs of cultivation [5]. Although scientists compared the influence of several different fertilisers on the growth of flowering Chinese cabbage, including the effects of controlled release of urea nitrogen [6], ammonium nitrogen [7] and the _Trichoderma_ biofertiliser [8], there is still a lack of literature data on the quality of flowering Chinese cabbage grown with different N sources in the same experimental conditions. Nitrogen in various forms is used in plant production. One of the most popular is ammonium nitrate (NH4 NO3, 34% N). At an optimal concentration, ammonium nitrate promotes the growth and improves the nutritional quality of flowering Chinese cabbage in both pot cultivation and hydroponics [3; 4]. Application of 100 kg N per ha\({}^{-1}\) of ammonium nitrate increased the sugar content in sugarcane (_Saccharum_ spp.) by 7-25% [9]. [PERSON] et al. [10] pointed out that appropriate N--NH4/N--NO3 ratios increased N absorption and assimilation and thus promoted the growth of flowering Chinese cabbage. Urea (46% N) is the most concentrated solid nitrogen fertiliser. In soil, it changes to ammonium carbonate, which may temporarily cause a harmful local high pH. An experiment conducted by [PERSON] et al. [5] found that the urea fertiliser tended to increase the yield of flowering Chinese cabbage in the first stage of growth, but then, with a further increase in urea, the growth was inhibited. Thus, the optimal dose of urea for flowering Chinese cabbage was estimated to be 440-490 kg per ha\({}^{-1}\). When the application of urea exceeded the optimal dose, high levels of nitrate accumulation occurred [11; 12]. As sources of N, calcium (15.5% N), magnesium (10.5% N) and sodium (16% N, 26% Na) nitrates were also used as fertilisers. These metal ions (Ca2+, Mg2+ and Na+) play crucial roles in photosynthesis, glucose transport, nucleic acid synthesis and protein synthesis in plants, as well as in other processes that affect the quality of plants. They are necessary for stimulating plant growth [13; 14; 15] and affect amino acid content [16], pigment content and colour of plants [15]. _Brassica_ plants, such as cabbage, are often considered to be calcicoles, i.e., plants which accumulate free calcium ions (Ca2+) [17]. According to the authors' knowledge, there are no data on the effects of various nitrate sources on the yield and quality of flowering Chinese cabbage grown in the same experimental conditions. Thus, the aim of our study was to investigate the influence of N sources and N doses on the growth and qualitative characteristics of flowering Chinese cabbage. The following scientific hypothesis was formulated: N sources and N nutrition levels modify the yield and quality of flowering Chinese cabbage. The effect depends on the system of cultivation.