Changes in Total Polyphenol Content and Antioxidant Capacity of Stinging Nettle (Urtica dioica L.) from Spring to Autumn

Total polyphenol content and antioxidant/reducing capacity of stinging nettle (Urtica dioica L.) leaves and roots collected from wildgrown plants were investigated during the vegetation period. From both fresh and dried samples of leaves and roots, water extracts were prepared by brewing at 60, 80 and 100 degrees C for 3 hours, and ethanolic extracts of 20 % (v/v) and 70 % (v/v) by extracting at room temperature for 72 hours. The total polyphenol content was determined spectrophotometrically with Folin-Cioceltau reagent and the antioxidant capacity was measured by ferric reducing ability of plasma (FRAP) assay. Our results showed that the optimal harvest time is in the spring (April). Water extracts had the highest total polyphenol content and antioxidant capacity in this period. The amount of valuable compounds released increased by higher extraction temperature in both plant parts. In water extracts of nettle leaves, two times higher polyphenol content was obtained than in that of roots. Both kind of ethanolic extractions resulted in a higher polyphenol content in the leaves harvested in the spring period. For the roots, it was higher for samples collected in the autumn, which is also reflected in the values of antioxidant capacity. Water extracts of fresh leaves harvested in April had more than twice higher total polyphenol content than in October. For dried samples, raising the temperature did not cause a significant change in the total polyphenol content, however, it has resulted in increased antioxidant capacity both for the dried leaf and root samples.