Effects of potassium sulfate [K2SO4] on the element contents, polyphenol content, antioxidant and antimicrobial activities of milk thistle [Silybum marianum]

Background: Silybum marianum L. (Milk thistle) is native to the Mediterranean basin and is now widespread throughout the world. It's sprout is used as a herbal medicine for the treatment of liver disease for centuries. The seeds of milk thistle contain silymarin, an isomeric mixture of flavonolignans [silybin, silychristin, and silydianin. Silymarin acts as a strong anti-hepatotoxic. Objectives: The objective of this study was to evaluate the influences of potassium sulfate [K₂SO₄] fertilizer doses on polyphenol content, some nutrient elements, antioxidant and antimicrobial activities of milk thistle at experimental fields of Ordu University in Turkey. Methods: The antimicrobial activities of seed ethanol extracts and seed oil were tested in vitro against Pseudomonas aeruginosa (P. aeruginosa), Escherichia coli, (E. coli) Staphylococcus aureus (S. aureus), Aspergillus niger (A. niger) and Candida albicans (C. albicans) using the disc diffusion method. Free radical scavenging activity of the ethanolic extracts of milk thistle was determined spectrophotometrically by monitoring the disappearance of 2, 2-diphenyl-1-picrylhydrazil (DPPH·) at 517 nm according to the method described by Brand-Williams et al.^[17] The phenolic contents in the ethanolic extracts of milk thistle were determined according to the procedure described by Slinkard and Singleton^[19] with a slight modification of using a Folin-Ciocalteu phenolic reagent. The amount of total flavonoid in the ethanolic extracts was measured by aluminum chloride [AlCl₃] colorimetric assay. The ions in aerosol samples were determined by using Dionex ICS 1100 Series ion chromatography. Results: Seed and seed oils obtained from obvious doses of potassium sulfate [0, 30, 60, 90 and 120 kg ha ^-1 fertilizer applications showed antimicrobial activities against E. coli, A. niger and P. aeruginosa. The application of 90 kg ha^-1 of K₂SO₄ on seed oil resulted in the highest antimicrobial activities. At 100 µg mL^-1and 200 µg mL^-1, except the highest potassium application [120 kg ha ^-1 extract, all extracts showed high and similar DPPH scavenging activity. The highest phenolic compounds were obtained with 30 kg ha^-1 of K₂SO₄, whereas the use of 60 kg ha^-1 caused the highest total flavonoid content. This plant is a good source of K⁺, Ca⁺², PO4^-3, and Cl^-1. Conclusion: In this study, increasing doses of potassium sulfate had significant effect on element, polyphenol content, antioxidant and antimicrobial activities of the milk thistle.