Health-promoting phytochemicals of stinging nettle (Urtica dioica L.) grown under organic farming in Italian environments

Urtica dioica L. (stinging nettle) is a neglected underutilized species, with potential as a future multi-purpose crop. To address the requisite for information on the influence of agronomical factors on health promoting bioactive components in leaf material under organic management, commercial stinging nettle was cultivated in three locations (Emilia Romagna, Italy) and analyzed over four harvests (September 2020 - September 2021). Significantly higher levels of polyphenol and flavonoid components as well as antioxidant activity (ferric reducing antioxidant potential [FRAP] and 1,1-diphenyl-2-picrylhydrazyl [DPPH]) were obtainable in May and July in the second cultivation year. Only the location at Lizzano yielded significantly higher biomass in combination with higher polyphenol content and FRAP in May, DPPH in May and July and ascorbate in July and September. Throughout 2021, limiting water supply was the major determinant for the significantly lower yield and higher polyphenol (flavonoid) content, respectively, at the rainfed location of Tresigallo. Temperature and growing degree days were inversely correlated to polyphenol content and antioxidant activity, implicating temperature as the major determinant of the latter under non-limiting water supply in the mountainous region of Lizzano. Provided agronomic practices are implemented, including effective weed removal, uniform water supply (supplementary irrigation) and organic amendments, satisfactory yields are obtainable combined with high levels of bioactive components. This shows promise for stinging nettle cultivation towards increased use in the medicinal/herbalist sector.