Melatonin treatment in broccoli florets shows interesting results for the postharvest conservation of this vegetable frequently consumed worldwide. Visual and quality characteristics are clearly improved by treatments around 100 μM melatonin. Short treatments by immersion or spray induce a lower loss of chlorophylls and carotenoids, and therefore, an improvement in the chromatic spectrum of broccoli florets, as can be seen by a greater degree of hue angle (greener, bluer, less yellow). This anti-senescence effect was first described in 2009 by the authors in barley leaves [23], and has subsequently been verified in a multitude of plant species, both edible and wildtype, where an inhibitory effect of melatonin on activating senescence transcription factors was demonstrated [18,27,32,39,67,68]. In addition, broccoli florets preserve freshness better, with less hydric and texture loss. In general, the maintenance and cold transport of broccoli florets is essential for an acceptable shelf life. Temperatures around 4 °C are ideal for reaching a maximum duration of about 20–22 days. Melatonin treatments clearly enhance visual and quality characteristics, improving color, texture and shine, and are able to extend its shelf life between 5–8 days with maximum quality. Other parameters such as soluble solids, total acid content, astringency, bitterness and vitamin C content are improved with melatonin treatments.

Regarding the compounds of nutraceutical interest in broccoli florets, melatonin treatments increase the contents of total phenols and flavonoids, and some specific ones such as rutin and quercetin. Melatonin induces the expression of diverse transcripts of phenolic metabolism. In white and red cabbage seedlings, melatonin upregulates phenolic-related genes such as PAL, C4H, CHS, CHI, F3H, DFR and UFGT, among others [7]. Moreover, in fruits such as berries and kiwifruit, melatonin induces several flavonoid biosynthesis genes [28,29]. Melatonin has also been found to cause a general activation of the primary and secondary metabolism that leads to high levels of carbohydrates, lipids and amino acids, and a better energy level and redox status [38,39].

A characteristic of Brassicaceae, though not exclusive to it, is its relevant level in glucosinolates, which are a group of secondary metabolites. Evolutionarily, glucosinolates originated twice, so that they are found in two unrelated lines of plants: in the Brassicales order (mainly Brassicaceae, Capparaceae and Caricaceae families) and in the Putranjivaceae family [69]. These compounds contain N- and/or S- in their chemical structure, formulated as β-thioglucoside-N-hydroxysulfates. More than 120 glucosinolate compounds have been identified, and they can be classified as aliphatic glucosinolates (originating from methionine, valine, leucine or isoleucine), indole glucosinolates (originating from tryptophan) and aromatic glucosinolates (from phenylalanine or tyrosine) [70]. These secondary metabolites are relevant in the defense system of plants due their fungicide, bactericide, nematicide and allelopathic properties [71]. Moreover, Brassicaceae’s powerful odor and taste (“mustard oil bomb”) seems to repel herbivores, defending the plant from excessive consumption [72]. Likewise, the potent cancer chemoprotective and/or anti-oncogenic activity of glucosinolates and isothiocyanates (their hydrolysis products) has been described, promoting the cultivation and human consumption of glucosinolate-containing plant species as healthy foods [70,73,74].

In an experiment on broccoli florets without cooling, the contents of total aliphatic and indolic glucosinolates declined by more than 50% after 3 days of storage. However, the decrease was alleviated by melatonin treatment [63]. In broccoli florets stored at 20 °C [63] and also at 4 °C [65], melatonin induced total glucosinolate content, including the biosynthesis of several aliphatic glucosinolates such as GER and GRA, and decreased others such as GNA and PRO; moreover, indolic glucosinolates such as GBS, NGBS and 4MGBS were increased. Sulforaphane (SFR), an isothiocyanate product, was additionally increased (Figure 1). Melatonin upregulated the expression of several genes related to glucosinolate biosynthesis such as the transcription factors MYB28 and MYB34, and several transcripts of glucosinolate biosynthesis enzymes such as GS-Elong, UGT74B1, ST5b, FMOGS-OX1 and TGG1, and CYP83A1, CYP79F1 and CYP79B2, but AOP2 and ESP were downregulated by melatonin. The upregulation by melatonin of MYO/TGGs induced glucosinolate hydrolysis, reflecting the accumulated levels of sulforaphane in stored broccoli (Figure 1) [63,65]. In addition, in Chinese cabbage, a similar glucosinolate biosynthesis activation by melatonin was observed. In this case, an interesting study on the fungicide activity of glucosinolate-rich extracts against Sclerotinia sclerotiorum (stem rot disease) and its relationship with the melatonin-inducing capacity of glucosinolate biosynthesis was demonstrated [53]. An overall view is provided in the proposed model of Figure 1, but although we know that during the storage of broccoli glucosinolates are synthesized [75], and that melatonin is able to over-activate the biosynthesis of these compounds, it is important to point out that the conditions of temperature and light/dark determine the balance between glucosinolate biosynthesis and hydrolysis up to isothiocyanates, which considerably alter the flavors of the product by the time it reaches the consumer.