Identification and quantification of major phenolic constituents in Juglans regia L. leaves: healthy vs. infected leaves with Xanthomonas campestris pv. juglandis using HPLC-MS/MS

Abstract

The present study was designed to characterise and quantify the major phenolic constituents in healthy leaves and leaves infected with Xanthomonas campestris pv. juglandis. A comparison among six different cultivars: 'Fernor', 'Fernette', 'Franquette', 'Rubina', 'Sava' and 'Krka', with the same agricultural, geographical and climatic conditions, was made. Liquid chromatography coupled with a mass spectrometer (HPLC-MS/MS) was used to identify and quantify the compounds. A total of 52 compounds were identified based on mass spectra and literature. Among them, 15 hydroxycinnamic acids, 6 flavanols, 2 flavones, 22 flavonols and 7 naphthoquinones were identified. Two flavones and three naphthoquinones were reported for the first time in J. regia leaves. In addition, two naphthoquinones, which are reported to play an active role in the process of juglone formation, were confirmed in all six cultivars. In the process of MS fragmentation, compounds were fragmented up to MS6 fragments and in some cases both MS2 fragments were further fragmented, providing comprehensive data. Total analysed phenolic content (TAPC) and total phenolic content (TPC) concentrations were higher in infected leaves, suggesting that phenols play a major role in plant defence. In the case of walnut bacterial blight, the contents of flavanols and total hydroxycinnamic acids were higher in infected leaves in all cultivars, suggesting that they could play a key role in a plant’s response to this economically important disease.

Keywords

• Hydrojuglone
• Juglone
• Naphthoquinones
• Phenolic compounds
• Xanthomonas campestris pv. Juglandis
• 1,4-Naphthoquinone