**4. Discussion**

The two species provided very different yields that, according to the literature, place borage among the most productive species for microgreens (grown in a peat-based substrate) and purslane among the less productive [32,33]. Specifically, the fresh yield of borage was higher compared to plant species that were sown at a similar density (e.g., purple and green basil, tatsoi, mibuna) or that were harvested after a similar time (e.g., jute, kohlrabi, basil) [32,34]. If fresh yield is standardized considering the plant density (the number of seeds per unit of surface), borage still ranked among the most productive species, following radish and Swiss chard [32,34]. The fresh weight and hypocotyl length of borage was also higher than in another study on the same species, most likely due to an almost double growing cycle (23 vs. 12 days) [35]. On the other hand, purslane's yield was low, only higher than chicory [33], slightly below jute, basil, and rocket [32,34,36,37]. NUS are often disregarded due to their modest yield, but this is also related to the cultivation in marginal and poor soils, and to low or absent agricultural input [38]. The good performance of purslane, which deserves further consideration, is likely to be related to the succulent nature of the plant, as also implied by the high percentage of dry matter of the edible product [31,39].

Our evaluation also explored the mineral composition of the microgreen since nutritional parameters are considered one of the most interesting assets of this class of horticultural products, thus essential for promoting their commercial value [40]. There were significant differences between the two species, with the low-yield purslane providing the highest accumulation for the mineral elements of major dietary importance, such as K, Ca, Mg, Mn, Fe, and Zn. In absolute terms, both species presented a predominant presence of K and Ca, as also previously seen in purslane [41] and other microgreens [39]. It is significant that P was present in amounts similar to K, to an extent higher than previously observed in other species [32,39]. In addition, both species presented a low Na/K ratio, an index of dietary importance [42]. The observed ratio is close to the lowest level recorded in microgreens [32,39]. Purslane demonstrated a higher accumulation efficiency (on dry matter basis) also of the non-essential plant elements. This was shown for some (i.e., Ba and Cd) of the non-essential elements for humans (i.e., Al, Ba, Cd, and Pb). Consistent with the literature on other species, Mn, Zn, and Fe were among the most abundant micro-elements [39,43].

The high antioxidant activity is thought to be one the potential dietary advantages provided by microgreens [44]. Purslane resulted as the most valuable species also having a higher ascorbate content. Ascorbates are usually the most abundant water-soluble antioxidant molecules in plant cells [45] and it has been previously noted that faster growing microgreens have a higher quantity of ascorbic acid than sprouts [32]. The total ascorbate concentration of borage and purslane was within the range of a study on 12 species of microgreens (between 25.1 and 147.0 mg 100 g<sup>−</sup><sup>1</sup> fw) [46]. Differences between borage and purslane were not present in the main pigments of the plants, but purslane had a higher β-carotene/lutein ratio. This represents a useful feature since it is important that vitamins are present in plants in a form that can be easily absorbed. Specifically, it has been reported that lutein has an inhibitory effect on β-carotene absorption [47].

The antioxidant activity measured with the three methods consistently indicated the better performance of purslane compared to borage, coherent with the higher content of total polyphenols. The analysis of the phenolic compounds revealed the large diversity that exists between the two analyzed species. Regarding the absolute content, while the amount of total phenols in borage was comparable to that of several microgreens, purslane can be ranked among the top-producing microgreen species of phenolic compounds [32,39]. The polyphenols in borage and purslane were higher than in rocket, lettuce, mustard, and tatsoi [37,43], and lower than in basil and coriander [43]. Purslane extracts were characterized by a predominant amount of rutin [48]. This flavonol glucoside is common in the plant kingdom, especially in the Polygonaceae and Fabaceae families [49]. Rutin was not detected in several Brassica species, while it was highly concentrated in coriander

microgreens [32]. A major source of rutin in the Mediterranean diet is represented by capers, olives, and asparagus. The amount detected in purslane microgreens is higher than in most vegetables and fruits, except for some berries, such as red raspberry and black currant [50,51]. Rutin has several beneficial properties, and is used in pharmacology for its vasoprotective and capillary stabilizing activities [52]. On the other hand, borage had a more composite phenolics' profile evident for both the phenolic acids and flavonoids. Among phenolic acids, ferulic acid for borage and caffeoyl quinic acid for purslane were confirmed as being major compounds in these species also at the microgreen stage [53]. Considering the literature relative to the adult plants, borage had a significant amount of salicylic acid glucoside. Salicilates (e.g., salts or esters of salicylic acids) are known to be involved in the plant response to biotic stress. Specifically, inactive forms of the plant hormone salicylic acid are made through conjugation with glucose (or other small organic molecules), to be stored in plant vacuoles [54]. Moreover, the borage flavonoid profile was characterized by a more balanced amount of kaempferols and luteolins, among the most widely distributed flavones in the plant kingdom [55].
