Xylella fastidiosa (Xf) is a slow growing, vector-transmitted, xylem-limited and gram-negative bacterium first described by Wells in sod. in 1987. The pathogen can infect more than 80 plant species and genera. Large economical damage has been reported in the Americas from where the bacterium originates, namely in grapevine, citrus, and almond. Since its first detection in Europe, in southern Italy in 2013, it has destroyed millions of olive trees and negatively affected almonds in Spain. Due to the xylem-limiting nature of Xf growth, infected (Xf+) leaves show effects in their mineral composition compared to healthy leaves (Xf-). In grapevine higher concentration of calcium and magnesium, but lower concentrations of potassium and phosphor have been reported for Xf+ leaves compared to Xf- leaves. There is no information on tissue-specific distribution of elements due to Xf infection, thus the aim of our study was to evaluate distribution of elements in Xf+ and Xf- leaves of two almond cultivars (Glorieta and Masbovera), and in grapevine (Cabernet Sauvignon) grown on calcareous or red soil. Leaf cross-sections were prepared, or necrotic spots were excised from almond leaves, and tissue-specific element analysis was performed using micro-particle induced X-ray emission (micro-PIXE) at “Jožef Stefan” Institute, Ljubljana. Spectra were fitted, and distribution maps were generated in GeoPIXE II software. There were cultivar-dependent differences in concentration of sulphur, chlorine, calcium, and potassium in specific tissues in Xf- and Xf+ leaves. Necrotic spots on the almond leaves were encircled with silicon and iron, centrally there was an enrichment of phosphorus and sulphur and exclusion of chlorine and potassium. In grapevine, results depended on the soil and significant differences in phosphorus and sulphur concentrations in different leaf tissues when comparing Xf- and Xf+ leaves were observed. This is the first report on the use of micro-PIXE in Xf infected crop leaves.
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