**1. Introduction**

Amaranth (*Amaranthus* spp.) is a gluten-free pseudocereal that is cultivated primarily in Mexico and South America, but also thrives in all temperate-tropical areas of the world [1]. Moreover, in certain regions of the world, such as eastern Africa, amaranth leaves are consumed as a vegetable because it is a fast-growing plant available most of the year. There has been a renewed interest in this ancient and highly nutritious food crop due to the excellent nutritional value of seed and leaves [2–4]. Both seeds and leaves are rich sources of proteins, which constitute up to 15–43% and 14–30% of fresh matter (FM), respectively. Amaranth proteins have a well-balanced amino acid composition [4], high bioavailability [3], and good functional properties [5]. Dietary fiber, vitamins and precursors of vitamins (ascorbic acid, riboflavin, tocols, carotenoids), as well as minerals (Ca, Fe, Mg, K, Cu, Zn, and Mn) are other important nutrients present in seeds and leaves of amaranth. Their contents are high compared to these in some cereals and green leafy vegetables [1,2,4]. Nutrient composition causes an increasing interest in amaranth as a food ingredient, especially in the production of gluten-free products [6,7].

In addition to macro- and micronutrients, amaranth contains secondary plant metabolites, which may play a significant role in the human diet due to their potential health beneficial effects [8]. Considerable research has been conducted over the past years on phenolic compound profile of amaranth seeds [8–11] and their functional and bioactive properties [12–15]. However, recent findings confirmed that leaves and other aerial parts of *Amaranthus* also are important sources of phenolic compounds [16–18]. Among phenolic compounds, those belonging to hydroxycinnamic acids, benzoic acids, and flavonols and their glycosides were identified in amaranth leaves, flowers, and stalks [18,19]. Steffensen et al. [20] found additional hydroxycinnamyl amides in aerial parts of young plants. Phenolic compounds are well-known as antioxidants. Conforti et al. [21] showed that amaranth leaf extracts contained phenolics, inhibited nitric oxide production, and scavenged free radicals. The reducing potential and antioxidant activity in lipid systems of various parts of amaranth shoot system were determined as well [16,19,22]. Other phytochemicals with antioxidant activity which occur in amaranth are betalains, especially betacyanins [23]. The contents of these pigments vary among *Amaranthus* species and genotypes [24]. If they are present in the plant, they are accumulated mainly in seedlings, leaves, and inflorescences.

Although the phenolic compound and betacyanin profiles of individual botanical parts of amaranth as well as their antioxidant potential are well-known, information about effects of the growth cycle on these phytochemicals and their activity is still scarce. It is important to identify the changes in antioxidant activity and phenolic compound composition by the growth stage of the amaranth.

In a previous paper [25], we reported the nutritive characteristics of *A. caudatus*, and in particular, we evaluated the effects of plant aging on the chemical composition, gross energy, *in vitro* true digestibility, neutral detergent fiber digestibility and fatty acid profile. However, to the best of our knowledge, no research has been conducted to investigate the change of antioxidant activities and phenolic composition of extracts of *A. caudatus* during plant growth. Barba de la Rosa et al. [26] stated that the knowledge of *Amaranthus* spp. as a source of phytochemicals will increase their importance as a potential source of antioxidant compounds in the human diet.

The aim of this study was to compare the antioxidant activity, TPC, and individual phenolic compounds of *A. caudatus* during plant growth, to identify the ideal growing stage to achieve the maximum antioxidant properties, and to optimize its use as a source of antioxidants for use in food production.
