Vessels occlusion frequency as cultivar-associated xylem trait in Olea europaea

Xylem vessel occlusions emerge in response to abiotic and biotic stresses, contributing both to hydraulic protection and to defense against vascular pathogens. However, their constitutive occurrence and inter-cultivar variability have rarely been quantified as a phenotypic trait in woody crops. In this study, we evaluate constitutive vessel occlusion frequency as a stable anatomical trait in Olea europaea and assess its potential as a predictive varietal marker for biotic stress tolerance. Using a field collection of 97 olive cultivars grown under uniform conditions at the CREA-OFA Mirto Crosia site, we quantified the number of occluded vessels in one-to two-year-old branches by combining optical and confocal microscopy on Safranin/AstraBlue-stained transverse sections. Occlusion values were used to classify cultivars from low to high occlusion frequency, and a subset of 40 representative cultivars was selected for detailed analyses. To explore the metabolic background associated with this trait, 1H-NMR profiling was performed on leaf extracts from the same plants. Our results show that readiness to vessel occlusion formation is a constant, cultivar-specific anatomical feature, with consistent differences among genotypes despite interannual variability and environmental susceptibility. We propose that occlusion frequency integrates both xylem morphology and the plant's physiological capacity to activate occlusion processes, thereby complementing mean vessel diameter as a determinant of vascular resistance. This conceptual framework is particularly relevant in the context of Xylella fastidiosa outbreaks in Mediterranean olive groves, where constitutive xylem traits may predispose cultivars to differential levels of tolerance. By treating vessel occlusion frequency as a measurable phenotypic trait, our study provides a basis for integrating xylem anatomy into selection strategies aimed at enhancing resilience to vascular pathogens in olive.