Amaranthus species are widely used as grain and leaf vegetables around the world and are potential dietary sources of antioxidants and phenolic compounds. In this study, we examined the variation in total flavonoid contents, total polyphenol contents, and antioxidant activities among 120 accessions of nine Amaranthus species. The antioxidant activity of DPPH (2,2-diphenyl-1-picrylhydrazyl) of the 120 amaranth accessions ranged from 1.1 (A. tricolor) to 75.2 (A. tricolor mg ascorbic acid equivalents (AAE)/g in 2018, and 8.5 (A. tricolor) to 68.8 (A. dubius) mg AAE/g in 2019. ABTS (2,2′-azinobis (3-ethylbenzothiazoline 6-sulfonate)) antioxidant activity ranged from 16.7 (A. tricolor) to 78.3 (A. hypochondriacus) mg AAE/g in 2018, and 36.6 (A. tricolor) to 79.2 (A. dubius) mg AAE/g in 2019. Total flavonoid content (TFC) of 2018 and 2019 ranged from 21.7 (A. caudatus) to 52.7 (A. hybridus) and from 22.3 (A. viridis) to 54.7 (A. tricolor), respectively Antioxidant activities were compared using two methods and all components were measured in plants grown both in 2018 and 2019. We identified wide variation among the accessions and between plants grown in the two years. Antioxidant activities and phytochemical contents were consistently negatively correlated. The nine species and 120 accessions clustered into three groups according to their antioxidant activities, total flavonoid contents, and total polyphenol contents in each year. These results provide information about the nutritional profiles of different Amaranthus species.

We used ultraperformance liquid chromatography coupled with a photodiode-array detector and electrospray ionization quadrupole time-of-flight mass spectrometry (UPLC-PDA/ESI-Q-TOF/MS) to rapidly and accurately quantify 17 phenolic compounds. Then, we applied this method to the seed and leaf extracts of two Amaranthus species to identify and quantify phenolic compounds other than the 17 compounds mentioned above. Compounds were eluted within 30 min on a C18 column using a mobile phase (water and acetonitrile) containing 0.1% formic acid, and the specific wavelength and ion information of the compounds obtained by PDA and ESI-Q-TOF/MS were confirmed. The proposed method showed good linearity (r² > 0.990). Limits of detection and quantification were less than 0.1 and 0.1 μg/mL, respectively. Intra- and interday precision were less than 2.4% and 1.8%, respectively. Analysis of amaranth seed and leaf extracts using the established method showed that the seeds contained high amounts of 2,4-dihydroxybenzoic acid and kaempferol, and leaves contained diverse phenolic compounds. In addition, six tentatively new phenolic compounds were identified. Moreover, seeds potentially contained 2,3-dihydroxybenzaldehyde, a beneficial bioactive compound. Thus, our method was an efficient approach for the qualitative and quantitative analysis of phenolic compounds, and could be used to investigate phenolic compounds in plants.

Amaranth is a nutritionally valuable crop, as it contains phenolic acids and flavonoids, yielding diverse plant secondary metabolites (PSMs) like phytosterol, tocopherols, and carotenoids. This study explored the variations in the contents of seventeen polyphenolic compounds within the leaves of one hundred twenty Amaranthus accessions representing nine Amaranthus species. The investigation entailed the analysis of phenolic content across nine Amaranthus species, specifically A. hypochondriacus, A. cruentus, A. caudatus, A. tricolor, A. dubius, A. blitum, A. crispus, A. hybridus, and A. viridis, utilizing ultra performance liquid chromatography with photodiode array detection (UPLC-PDA). The results revealed significant differences in polyphenolic compounds among accessions in which rutin content was predominant in all Amaranthus species in both 2018 and 2019. Among the nine Amaranthus species, the rutin content ranged from 95.72 ± 199.17 μg g⁻¹ (A. dubius) to 1485.09 ± 679.51 μg g⁻¹ (A. viridis) in 2018 and from 821.59 ± 709.95 μg g⁻¹ (A. tricolor) to 3166.52 ± 1317.38 μg g⁻¹ (A. hypochondriacus) in 2019. Correlation analysis revealed, significant positive correlations between rutin and kaempferol-3-O-β-rutinoside (r = 0.93), benzoic acid and ferulic acid (r = 0.76), and benzoic acid and kaempferol-3-O-β-rutinoside (r = 0.76), whereas gallic acid showed consistently negative correlations with each of the 16 phenolic compounds. Wide variations were identified among accessions and between plants grown in the two years. The nine species and one hundred twenty Amaranthus accessions were clustered into six groups based on their seventeen phenolic compounds in each year. These findings contribute to expanding our understanding of the phytochemical traits of accessions within nine Amaranthus species, which serve as valuable resources for Amaranthus component breeding and functional material development.