Seasonal variations of naphthoquinone contents (juglone and hydrojuglone glycosides) in Juglans regia L.

The aim of the present study was to determine the content of naphthoquinones (juglone and hydrojuglone glycosides) in different tissues of walnut plants (Juglans regia L.) through the growing season, for the French cultivar ’Franquette’ and the Slovenian cultivar ’Rubina’. Husk, leaves, bark, catkin and pistillates were studied. Naphthoquinones contents in the catkins (male flowers) and pistillates (female flowers) of J. regia, have to the best of our knowledge not been defined in any metabolic studies to date. The naphthoquinones were identified and quantified by HPLC/MS. The content of all individual and total naphthoquinones and the total phenolics content in the catkins were consistent with those in the bark, which suggests that the phenolic compounds are redistributed from the catkins to the bark after flowering to prevent the loss of these valuable plant compounds. The lowest content of naphthoquinones was in the husks and bark for both cultivars, while the highest content was observed in the leaves. The highest content of hydrojuglone glucoside was observed in the pistillate followed by catkin and bark, while the highest content of hydrojuglone rhamnoside and hydrojuglone pentoside was observed in leaves. The highest content of juglone was observed in the husk followed by the catkin. Quantification of the naphthoquinones in the different tissues of J. regia during different growth periods provides valuable data on when the extraction of individual naphthoquinones might be useful for further isolation or for pharmaceutical studies