Seasonal variation of the main individual phenolics and juglone in walnut (Juglans regia) leaves

Context: Walnut [Juglans regia L. (Juglandaceae)] is a rich source of phenolic compounds, including phenolic acids, naphtoquinones and flavonoids. The increasing interest in the powerful biological activities of plant phenolics has outlined the necessity of determining their content in leaves of different walnut cultivars. Objective: In this study, walnut leaves from walnut cultivars, originating from the same orchard and from the same year of production, were analyzed for their content in ellagic acid, rutin, myricetin and juglone. In addition, the seasonal variation of these major individual phenolics from June to August was determined. Materials and methods: An HPLC method was used for identification and quantification of ellagic acid, rutin, myricetin and juglone contained in the methanol extract of walnut leaves in nine different cultivars grown under the same agricultural, geographical and climatic conditions. Results: Cultivars and sampling date had statistically significant influence on the phenolics contents in walnut leaves. The results showed that ellagic acid, rutin, myricetin and juglone were more abundant in July 15th samples (average content is 84.62 mg/100 g FW, 98.9 mg/100 g FW, 178.09 mg/100 g FW and 73.81 mg/100 g FW, respectively). Their contents increases similarly in all the cultivars; therefore, the walnut leaves should preferentially be collected until early August, when phenolics content is higher. Discussion and conclusion: The results reported here show that genotype and its interaction with the environment could make significant differences in leaf polyphenols. Walnut leaves may become a noticeable source of compounds with health protective potential.