**4. Conclusions**

Comparison of antioxidant activity and phenolic composition of the aerial parts of *A. caudatus* showed growth cycle dependent effects. The amaranth in shooting and budding stages had the lowest antioxidant activity and the lowest total phenolic content. The earlier (vegetative) and later (early flowering and grain fill) stages formed two groups, both with high antioxidant activity, though with differing dominant classes of phenolic compounds. The content of flavonols (mainly rutin) increased with growth cycle, and these compounds could be primarily responsible for the antioxidant activity of amaranth in early flowering and grain fill stages. In plants in vegetative stages, especially the early vegetative stage, the participation of hydroxycinnamic acid derivatives in the pool of phenolic compounds was high. The contribution of these compounds to high antioxidant activity of amaranth in vegetative stages seemed significant. It is worth noting the predominance of rutin quantity among individual phenolic compounds in all *A. caudatus* growth states, as well as its increasing content during the growth cycle.

The extracts obtained from amaranth in vegetative, early flowering, and grain fill stages, due to their high content of hydroxycinnamic acid derivatives and rutin, can be a valuable source of antioxidants that can be exploited for the production of nutraceuticals or used as a functional food ingredient.

**Author Contributions:** Conceptualization, M.K., F.G., R.A. and P.G.P.; methodology, M.K., G.M. and M.A.J.; formal analysis, M.K. and M.A.J.; investigation, M.K., E.L. and M.A.J.; resources, E.L. and G.M.; writing—original draft, M.K., F.G. and P.G.P.; writing—review and editing, M.K., F.G., R.A. and P.G.P.; visualization, M.A.J.; supervision, R.A.

**Funding:** This research received no external funding.

**Acknowledgments:** The Italian National Research Council provides a visiting grant to Ryszard Amarowicz in the framework of a free exchange programme between the Polish Academy of Sciences and the Italian National Research Council. The authors thank Wiesław Wiczkowski for assistance in the HPLC-MS/MS analysis.

**Conflicts of Interest:** The authors declare no conflict of interest.
