Antioxidant activity and fatty acids quantification in Sicilian purslane germplasm

Abstract Portulaca oleracea is an annual succulent herb in the family Portulacaceae. It is a nutritious vegetable with high antioxidant properties and, it is among the richest plant source of ω-3 fatty acids, as well as a rich source of ω-6 fatty acids, ascorbic acid, tocopherols and beta-carotene. In the present study, three purslane populations under different Mediterranean environmental conditions for two years, for future valorization as novel food sources of omega-3 fatty acids, were evaluated. In particular, biomorphological characteristics, total phenols and fatty acids content were determined. The antioxidant activities were evaluated using 2,2-diphenyl-1-picrylhydrazyl assay. The population “Cas” appears to have higher antioxidant activity than the other two populations (“Cal” and “S. Ven”).The saturated fatty acid content is influenced only by the year of collection, while the polyunsaturated fatty acid by the populations. The most abundant unsatured fatty acids are linoleic and linolenic acids and “Cas” attained the highest contents. Graphical Abstract


Introduction
Portulaca oleracea L. (purslane) is an annual herbaceous plant with reddish stems and alternate leaves from family Portulacaceae. Purslane is distributed in many parts of the world and specifically the tropical and subtropical areas (Zhou et al. 2015). Portulaca oleracea is highly adaptable to many different environments. Danin et al. (2014) showed that P. oleracea flourishes in numerous biogeographical locations worldwide and is highly adaptable to drought, saline and nutrient-deficient conditions. These characteristics give purslane competitive advantage over many other cultivated crops. Ezekwe et al. (1999) showed that accession by planting date interaction influence levels of crude protein, total lipids, and carbohydrate contents, while studies (Petropoulos et al. 2016;Montoya-Garc ıa et al. 2018) evaluating crop management practices linked to breeding selection have been confirmed to allow for managing the content of the main bioactive compounds and consequently increase the added value of the final product. In the Mediterranean regions the aerial parts of the plant are consumed as vegetable and worldwide researchers and nutritionists have studied this plant as a potential vegetable crop for humans as well as animals (Zhou et al. 2015). It is a well-known plant in the European Traditional Medicine as a diuretic, febrifuge, antiseptic, antispasmodic, vermifuge (Xiang et al. 2005) and anticholinesterase activities (Xiu et al. 2018). Catap et al., (2018) showed the leaf extracts produce immune-protective properties and antispasmodic effects in vivo in mice. Purslane possess renowned antioxidant properties, mainly acting as free radical scavenger, metal quencher and lipid peroxidation inhibitor. These effects are mainly attributed to its phenolic constituents (Erkan 2012;Yang et al. 2018) and several fatty acids, which promote the optimum cardiovascular function, mainly by acting as anti-inflammatories and anti-nociceptive, and furthermore they could reduce the risk of human cancer (Oliveira et al. 2009;Moreau & Savage 2009;Erkan 2012;Amirul et al. 2014;Zhou et al. 2015;Corsaro et al. 2016;Sadeghi et al. 2016;Iranshahy et al. 2017). In addition, purslane can act as an immunomodulatory substance, whereas according to Silva and Carvalho (2014), phenolic extracts from leaves stems and flowers have a significant DNA protective effect against hydroxyl radicals. As above mentioned, purslane is abundant in x-3 fatty acids, particularly in a-linolenic acid for which it is considered one of the richest plant sources (Petropoulos et al. 2016). Apart from a-linolenic acid, which represents nearly up to 30% of purslane oil, other essential fatty acids have also been detected in plant tissues, such as palmitoleic, palmitic, linoleic, oleic and stearic acids, as well as trace amounts of 20:5x-3 and 22:6x-3, namely eicosapentaenoic acid and docosahexaenoic acid, respectively. Several authors have shown that the lipid and phenolic fractions of many plant foods have health and nutritional values (Lo Turco et al. 2016;Salvo et al. 2017;Aghraz et al. 2018;Cicero et al. 2018). The main aim of the research was to establish the fatty acid composition of three purslane populations in an attempt to correlate such composition with the environmental conditions. The secondary aim was to assess the health and nutritional values of lipid fraction of investigated samples for future valorization as omega-3 fatty acids food sources.

Biomorphological characterization
Results of the analysis of variance for most characteristics showed significant differences among populations in both the years of collections (Table S2).
Significant interaction "P" X "Y" has been observed for stems and leaves incidences on total biomass. Highly significant sums of squares for populations differences were found in the combined analysis of variance for biomass productions. Plant dry weight (g plant À1 ) and its partitioning (% of total plant weight) are reported in Table 1. On average of populations, the years of collection did not influence the plant dry weight, but its partitioning. In 2017 plants produced more stems than leaves.
Averaged for year of collection, the population "Cassibile" had the lowest plant biomass, mainly characterized by the presence of leaves (29.9% of total biomass). Differences in color of leaves were observed among genotypes and years of collection ( Figure 1).
In particular, the population influenced for over the 60% of total variation all the indices of color. The brightness (L Ã ) resulted highest in Cassibile for both years, while Figure 1. L Ã , a Ã and b Ã variation in relation to population and year of collection. The different letters between years within the same population indicate significant differences at P < 0.05 (Ducan's Test). Table 1. Biomass (g plant À1 dry weight) and its partitioning (% of total plant weight), on average of the two years of collection in purslane in relation to site of collection. the population S.Ven resulted more dark with lowest a Ã and highest b Ã values, confirming the color is a genotype parameter dependent. Table 2 shows the concentration of TPC crude extracts and the antioxidant activities (IC 50 ) of P. oleracea populations. In this study, TPC were determined compared with standard gallic acid. The Year of collection, did not affect these parameters, the amount of TPC resulted, averaged for the two years 241 mg 100 g À1 D.W., with an IC 50 of 1.28 mg/mL. Out of year of collection, the population "Cas" appears to have the lowest IC 50 and the highest TPC values among the three populations, indicating its higher antioxidant activity.

Total phenol content, antioxidant activity and fatty acids content of purslane
As regards fatty acids, our results refer to whole plants since whole aerial part of the plant is edible. Purslane is considered a rich source of fatty acids, and its content and composition could be considered a key quality factor for genotype evaluation. ANOVA showed that saturated fatty acid content resulted influenced only by the year of collection, while the polyunsaturated FA resulted influenced by the populations. Only the amount of oleic acid was influenced by both the factors under study.
Among FAMEs, on averaged for populations and years of collection, the most abundant fatty acids were linoleic and linolenic acids, with 0.76 ± 0.26 and 0.83 ± 0.27 mg/g, respectively. Among the populations, on average of the two years, "Cas" has the highest contents, with 1.01 ± 0.22 mg/g (linoleic acid) and 1.07 ± 0.23 mg/g (linolenic acid). Palmitic acid was detected at significant amounts in all populations (Table 3).
Moreover, linoleic acid was the prevailing fatty acid for all the genotypes with a ratio of linolenic: linoleic ratio higher than 1. The data reported by Petropoulos et al. (2015) on chemical composition of six genotypes of common purslane grown central Greece showed that PUFA/SFA ratio was higher than 0.45 and ranged from 1.31 to 1.92, with great differences among the studied genotypes, and agree with our results. Moreover Uddin et al. (Uddin et al. 2012) have also reported that mature leaves of wild purslane were rich in PUFAs and palmitic acid. Oliveira et al. (2009) identified twentyseven fatty acids in the leaves samples of purslane germplasm collected in Portugal. Linolenic acid resulted the most abundant, ranging from 27.7 to 39.1%,   followed by palmitic (19.3-24.3%) and oleic acids (11.6-19.5%); even if x-6:x-3 ratio resulted lower respect that reported by Uddin et al. (2014). These differences are probably due to the time of the germplasm harvest, which occurs in autumn in Portugal, while during summer period in Sicily. The correlation analysis of the studied chemical characteristics showed that similar parameter has a highly significant correlation, while among other parameters the correlation is either nonsignificant or less significant or has moderate relation (Table 4). In particular, it is worthwhile to note that the antioxidant activity, expressed as IC 50 , resulted significantly correlated not only with TPC content, but also with unsaturated FAMEs content.

Conclusion
The obtained results indicate that different purslane populations had different chemical composition and nutritional value; in particular, population "Cas" appears to have higher content of total phenols and higher antioxidant activity than the other two populations ("Cal" and "S.Ven"). Also the polyunsaturated FA, that are important for their health beneficial effects, resulted influenced by genotypes. The most abundant unsatured fatty acids are linoleic and linolenic acids and among the populations, also in this case "Cas" population has the highest contents of these fatty acids. The years of collection seem to influence only the partitioning of the plants and saturated fatty acid content. Thus, purslane is a potentially valuable healthy compounds food source, with a significant impact on human health and therefore its daily consumption should be highly recommended. Considering the high abundance of purslane in phytochemicals and nutrients, confirmed by recent studies (Santini et al. 2017), it could be suggested that interesting pharmacological effects and health benefits (antioxidant, antiinflammatory, hypo-cholesterolemic and hypo-glycemic effects) could be achieved through the use of pursulane in functional foods.

Disclosure statement
No potential conflict of interest was reported by the authors.