Effect of nitrogen and potassium application on growth, total phenolic, flavonoid contents, and antioxidant activity of Eleutherine palmifolia

Eleutherine palmifolia is one of the medicinal plants widely used by the Dayak and Kutai tribes in Borneo Island, Indonesia as traditional medicines that can treat various diseases. Identification of the phytochemical content of E. palmifolia is very important to determine its potential as a medicinal plant. A significant factor in the success of the growth of E. palmifolia is the amount of fertilizer applied as a source of nutrients. In this study, the influence of various amounts of nitrogen (N) and potassium (K) fertilizers on the growth, total phenolic and flavonoid content (TPC and TFC), and antioxidant activity was investigated in different extracts of E. palmifolia. The treatments included 0, 46 or 92 kg/ha of N combined with 12.5 kg/ha of K, and 0, 25 or 50 kg/ha of K combined with 46 kg/ha of N. Using water, ethanol, or n-hexane as solvents, each sample was extracted with sonication method. TPC and TFC of the bulb extracts were quantified using Folin-Ciocalteu and aluminum chloride assays, respectively. Antioxidant activity was analyzed using 2, 2′-diphenyl-1-picrylhydrazyl (DPPH) and ferric reducing antioxidant power (FRAP) assays. Results showed that combining N and K had no significant effects on plant height, the number of tillers, leaves, or bulbs, and bulb diameter, while the chlorophyll reading value, total plant weight, and fresh bulb weight were significantly increased. The results indicated that the availability of N and K has a substantial effect on TPC, TFC, and antioxidant activity. The aqueous extract had the highest TPC and DPPH scavenging activity, whereas the ethanol extract exhibited the highest TFC and antioxidant FRAP activity. Therefore, to improve fresh bulb weight, phenol and flavonoid content, and antioxidant properties of E. palmifolia aqueous and ethanol extracts, supplementation of 46 kg/ha N and 12.5 kg/ha K is recommended.