## 4 Factors Influencing the Phytochemical Compounds of _Brassica_ Vegetables
### Environmental and Agricultural Factors
#### 4.2.4 Fertilization Practices  
A correct fertilization plan is fundamental for obtaining high quality, healthy and safe vegetables. The nutritional and sensorial profile of _Brassica_ is conditioned by the availability of fertilizers and nutrients as they determine the biosynthesis of secondary metabolites. Countless studies have been conducted on the effect of sulphur fertilization on phytochemical concentration, mainly on GLS production, considering their sulphuru nature [136; 137]. There is a correlation between the increase in sulphur supply and higher levels of total GLS [138], in turnip [136], kale [137] and broccoli, mainly when associated with a reduction in water, at the expense of yield [139]. [PERSON] and co-authors [119] suggested that the effect of sulphur application on GLS varies with the development stage of broccoli plants and differs for each kind of GLS; in fact, they found an increase in total GLS content at the start of the inflorescence development, followed by a rapid decrease thereafter. Increasing sulphur fertilization brought about a positive impact in the synthesis of polyphenols, such as flavonoids and phenolic acids, increasing the total antioxidant capacity in turnip top (_B. rapa ssp. Sylvestis_) [113], and broccoli [110]. Sulphur fertilization in pre-harvest (from 2.6 mmol/L to 6.5 mmol/L) increases the lipophilic and hydrophilic antioxidant capacity but does not affect the nitrate and chlorophyll contents in ready-to-eat \"friariello\" product [140]. Sulphur deficiency induced an increased vulnerability of _Brassica_ crops to diseases and fungal pathogens [141]. Sulphur fertilization, besides improving the antioxidant activity, it is also associated with a genotype-dependent significant reduction in leaf nitrate content, since it enhances the incorporation of nitrogen into organic compounds and consequently reducing the leaf nitrate concentration [113]. Nitrogen is the main constituent of chlorophyll structure: for this reason, its availability influences the content of carotenoids such as lutein and \(\beta\)-carotene, indeed high NO\({}_{3}\)-N:NH\({}_{4}\)-N ratio led to a higher content of both [142]. Consequently, the colour and pigmentation of leafy vegetables are also improved [132]. Nitrogen fertilisation led to a decrease in the total GLS content [136]; nonetheless, it acts differently according to the type of GLS; in fact, abundant nitrogen applications increase progoitrin and decrease sinigirin concentration in _Brassica napus_[143]. A reduced nitrogen fertilisation generated an increase in the bioactive compound content, mainly phenolics, as nitrogen stress triggers the gene expression of flavonoid pathway enzymes [128]. Combined fertilisation with NO\({}_{3}\)-:NH\({}_{4}\)+ is the optimal solution to maintain plant growth and increase the total GLS content [144]. An optimal balance between nitrogen and sulphur fertilisation influences the biosynthesis of secondary metabolites [21]. GLS, for example, can be enhanced by the presence of low nitrogen and high sulphur fertilizers: this balance influences the quantity and the quality of GLS produced, according to the corresponding amino acids synthetized. Some authors reported the effect of different nitrogen/sulphur combinations on GLS content in _Brassica_, with an increasing amount of nitrogen (80-320 kg/ha) applications. When enough sulphur was available (60 kg/ha), there were no effects on total GLS content, but their production moved to indolics; when the combination was with a low concentration of sulphur supply (10-20 kg/ha), the arylaliphatic and aliphatic GLS decreased [138]. Increased nitrogen/sulphur ratio pushes the plants towards the vegetative growth, at the expense of GLS production [136]. [PERSON] et al. [145] showed that the type of fertilisation may influence mineral composition in plants: nitrogen fertilisation was negatively associated with potassium (K) and calcium (Ca) content in broccoli, while sulphur fertilisation increased manganese (Mn) and zinc (Zn), and decreased copper (Cu).