*2.5. Brassica Leafy Vegetables*

The group of brassicaceous leafy vegetables, formerly referred to as cruciferous vegetables, includes a wide range of species with potential health-promoting properties such as kale (*Brassica oleracea* var. *sabellica*), pack choi (*Brassica rapa* var. *chinensis*), mizuna (*Brassica rapa* var. *japonica*), watercress (*Nasturtium o*ffi*cinale* R.Br.), wild and salad rocket (*Diplotaxis tenuifolia* [L.] DC and *Eruca cesicaria* [L.] Cav., respectively). According to FAOSTAT [27] database, roughly 12% of the vegetables grown worldwide are members of the Brassicaceae family. Recent reports [96,97] have suggested that brassicaceous leafy vegetables constitute valuable sources of phytochemicals. They contain high levels of vitamins (C, E [as α- and γ-tocopherols] and K [phylloquinone]), carotenoids, and phenolic compounds. In addition to the latter phytochemicals, *Brassica* leafy vegetables are characterized by sulfur-containing glucosinolates and methylcysteinsulfoxide compounds. The genetic factor is the most important and influential one in terms of modulating the biosynthesis and accumulation of phytochemicals in *Brassica* leafy vegetables [97]. In a comparative study of antioxidant molecules in four *Brassica* leafy vegetables (mizuna, salad rocket, watercress, and wild rocket), the authors observed a large variability in phytochemical concentrations [98]. For instance, watercress showed the highest polyphenol and vitamin C content, while salad and wild rocket were characterized by high concentrations of kaempferol and quercetin derivatives and finally mizuna exhibited significant concentrations of isorhamnetin and sinapic acid [98]. The authors highlighted the potential value of salad *Brassica* leafy vegetables as dietary sources of antioxidants conferring a wide range of positive health effects against type 2 diabetes and cardiovascular diseases.

Kale is a leafy *Brassica* species considered a potent source of glucosinolates and isothiocyanates, their main breakdown products. A study from the CDC (center of disease control and prevention) reported kale being ranked 15th among "powerhouse" vegetables and fruits [99]. The most abundant glucosinolates in kale were: 3-(methylsulphinyl)propyl, 2-propenyl and also 4-(methylsulphinyl)butyl glucosinolates. Genotypic variation within eight cultivars of kale ("Starbor", "Beira, "Scarlet", "Premier", "Olympic Red", "Toscano", "Dwarf Siberian", and "Red Russian") revealed that "Beira" and "Olympic Red" were characterized by the highest total concentration of glucosinolates and were proposed as functional foods [100]. In another experiment carried out by Hahn et al. [101] on 25 kale cultivars, the authors observed a great variation in glucosinolate profiles. Similarly, Ferioli et al. [102] reported higher variation in the aliphatic compared to the indole glucosinolates (9- and 5-fold, respectively) across 25 kale cultivars harvested from different European countries (Italy, Portugal, and Turkey). The presence of isothiocyanates in *Brassica* leafy vegetables has been reported to confer anti-diabetic, anti-inflammatory and anti-cancer properties [103–108]. Although less popular, even "ornamental cabbage" or kale (*Brassica oleracea* L. var. *acephala* DC.) have genotypes characterized by the accumulation of different pigments [109–111]. *Brassica* leafy vegetables, in particular kale, are considered additionally as a rich source of carotenoids (lutein and β-carotene), as well as chlorophylls (a and b). Carotenoid concentrations of 33 kale cultivars were analyzed and quantified [112]. Zeaxanthin was the most abundant carotenoid in 21 cultivars. Moreover, American and hybrid cultivars and accessions were characterized by high concentrations of zeaxanthin, whereas, German landraces, German commercial varieties, Italian, and red-colored kale varieties exhibited high concentrations of chlorophyll a and b [112].

Emerging market trends catering to shifting consumer perceptions of quality [32], have resulted in colored *Brassica* leafy vegetables (e.g., violet kale or pack choi) containing anthocyanins garnering the attention of nutritionists and horticultural scientists. Recently, Mageney et al. [112] proposed that the anthocyanin content could be used as a marker to differentiate between varieties/cultivars. Testing green and red-pigmented pack choi, Zheng et al. [113] observed that red pack choi produced higher concentrations of carotenoids, total phenolic compounds, total flavonoids, glucosinolates, and anthocyanins compared to its green counterpart. Importantly, the regular intake of anthocyanins from such colored leafy vegetables has been positively correlated with the prevention of various liver diseases, and also with the reduction of colon cancer, hepatic inflammation and oxidative stress [114].

The main pigments isolated in the studied leafy vegetables are presented in Table 1.


