Effect of Harpin protein as an elicitor on the content of phenolic compounds and antioxidant capacity in two hydroponically grown lettuce (Lactuca sativa L.) varieties.

Abstract Harpin protein was used as an elicitor in green (butterhead) and red (oak leaf type) varieties of hydroponically grown lettuce (Lactuca sativa L.). Its impact on polyphenolic content and antioxidant capacity was evaluated. Harpin protein was sprayed three days before harvesting in concentrations of 45, 60, and 120 mg/L. Green lettuce samples showed a positive response to Harpin by significantly increasing its phenolic content from 12 (control) to 16.43 mg gallic acid equivalents (GAE)/100 g dry weight (DW) (120 mg/L Harpin treatment) which means a 36.9% increase. Chlorogenic acid was the only phenolic compound that increased its concentration after lettuce induction with Harpin treatment. Antioxidant capacity, evaluated by 2,2-diphenyl-1-picrylhydrazyl (DPPH˙) and ferric reducing antioxidant power (FRAP) methods, also showed a significant increase (46.5 and 64.3%, respectively) in Harpin-treated green lettuce. The red lettuce variety showed none or a negative response to Harpin treatment for all parameters evaluated. These results show that elicitation may be an effective strategy to enhance phenolic content in lettuce varieties.