Concentration and composition of flavonol glycosides and phenolic acids in aerial parts of stinging nettle ( Urtica dioica L.) are affected by nitrogen fertilization and by harvest time

Summary Plants of Urtica dioica were cultivated in three years with four different nitrogen (N) levels (0, 100, 200 and 400 kg N ha -1 ) and the aerial parts were harvested three times per year to investigate if the content of flavonoids and phenolic acids were affected. The flavonol glycosides quercetin-3- O -glucoside, quercetin-3- O -rutinoside, isorhamnetin-3- O -rutinoside, and kaempferol-3- O -rutinoside and the phenolic acids 5- O -caffeoylquinic acid, 3- O -caffeoylquinic acid, 5- O -feruloylquinic acid and 2- O -caffeoylmalic acid were identified and quantified in extracts of the aerial parts of U. dioica by LC-DAD-MS and RP-HPLC-DAD, respectively. High N-levels reduced the concentration of total flavonoids significantly from an average of 10 mg g -1 dry matter (DM) at 0 kg N ha -1 to 5 mg g -1 at 400 kg N. The effect of N levels on total phenolic acids was only significant in the second harvest each year with a reduction from an average of 30 mg g -1 DM at 0 kg N ha -1 to 23 mg g -1 at 400 kg N ha -1 . The composition of flavonoids changed significantly among the major flavonoids with later harvest time resulting in an increase in the content of quercetin-3- O -rutinoside and a decrease of the quercetin-3- O -glucoside content. From the present study it appears that cultivation of U. dioica herba for medicinal purposes with a high yield of bioactive compounds is a compromise between a high yield of plant material and the content of flavonol glycosides and phenolic acids in the harvested product.