Phenolic response in green walnut husk after the infection with bacteria Xanthomonas arboricola pv. juglandis M. Mikulic-Petkovsek * , A. Slatnar, R. Veberic, F. Stampar, A. Solar University of Ljubljana, Biotechnical Faculty, Agronomy Department, Chair for Fruit, Viticulture and Vegetable Growing, Jamnikarjeva 101, SI-1000 Ljubljana, Slovenia article info Article history: Accepted 28 September 2011 Keywords: Juglans regia Bacterial blight Hydroxycinnamic acids Flavonoids Hydroxybenzoic acids Resistance abstract The research was performed on healthy green walnut fruits and on fruits infected with Xanthomonas arboricola pv. juglandis (Xaj). Fruits of cultivars ‘Cisco’, ‘Sampion’, ‘Fernette’, ‘Seiferdorfer’ and genotypes ‘Zdole’ and ‘Erjavec’ were sampled at phenophases Gf, Gf þ 30 and Gf þ 45. In the green husk tissue the content level of gallic acid, three hydroxycinnamic acids, catechin and five quercetin glycosides was determined with the high-performance liquid chromatograph coupled with mass spectrometer (HPLC- MS). During the growing season, the content of phenolic compounds decreased and was related to the physiological stage of the fruits and cultivar analyzed. The cumulative content of ten determined poly- phenols in healthy walnuts was cultivar dependent, and weakly correlated to the blight susceptibility observed in the orchard. In comparison to healthy husk tissue, the infected husks contained up to 5 fold more hydroxycinnamic acids, up to 3 fold more gallic acid, up to 4.3 fold more quercetins and up to 23 fold more catechin. The cultivars ‘Cisco’ and ‘Zdole’ showed the strongest post-infectional accumulation of the phenolic compounds. An essential influence of quercetin-3-O-rhamnoside, as well as 4-O-p- coumaroylquinic, 3-O-caffeoylquinic and 3-O-p-coumaroylquinic acid on the walnut blight severity was confirmed and points out to the role of these phenolic compounds in the walnut resistance against bacterial blight. Ó 2011 Elsevier Ltd. All rights reserved. 1. Introduction Persian walnut (Juglans regia L.) is cultivated commercially throughout southern Europe, northern Africa, eastern Asia, the USA, western South America and Australia. The tree is not affected by many pests and diseases, but walnut blight (Xanthomonas arboricola pv. juglandis (Pierce) Vauterin et al.) (Xaj) may cause economically important reductions in fruit yield and tree vigor in all production areas, especially those with high spring and summer rainfall [1]. The disease causes severe damage to all current-season green tissue, such as leaves, twigs, buds, petioles, rachides, male and female catkins, nutlets and kernels [2,3]. Host susceptibility depends on the cultivar, plant organ, weather conditions, leafing date, and previous history of the disease in the orchard [2,4e7]. Preventive copper bactericides are applied in order to control the disease, although they are not always effective, in spite of frequent treatments and complete spray tree-coverage. Moreover, copper resistant populations of the bacterial pathogen are often evolved; copper accumulates in the soil, and consequently disturbs the metabolism and performance of walnut trees [2,8]. Insight into the physiological response of walnut tree to the infection with Xaj may explain an incidence and severity of bacte- rial blight in different cultivars during their ontogenetic develop- ment. Resistance to blight may be related to phenolic compounds, as it is reported for some economically important pests and diseases of plants in general and also in several fruit species [9e11]. Phenolic compounds are toxic to pathogens and many of them, such as flavanols and hydroxycinnamic acids can act as passive or inducible barriers against herbivores or microbial pathogens. In response to the pathogen attack, the content and composition of polyphenols can change, playing an active role in induced resis- tance to the pathogens [9,12]. Phenolic compounds may also clarify the genetically deter- mined resistance of different cultivars to a certain pest and/or disease. As assumed Michalek et al. [10], the resistance genes of a resistant cultivar act as regulatory genes of phenol synthesis. The accumulated defense phenols in the resistant cultivars may account for a more efficient response against the pathogen [9]. Up to date, such constitutive resistance has most frequently been confirmed in apple fruits and Venturia inaequalis [9,11,13], and also in walnut shoots and X. arboricola pv. juglandis [7]. * Corresponding author. Tel.: þ386 1 320 31 41; fax: þ386 1 423 10 88. E-mail address: maja.mikulic-petkovsek@bf.uni-lj.si (M. Mikulic-Petkovsek). Contents lists available at SciVerse ScienceDirect Physiological and Molecular Plant Pathology journal homepage: www.elsevier.com/locate/pmpp 0885-5765/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.pmpp.2011.09.006 Physiological and Molecular Plant Pathology 76 (2011) 159e165