The Impact of Xylem Geometry on Olive Cultivar Resistance to Xylella fastidiosa: An Image-based Study

Xylella fastidiosa is a xylem-limited plant pathogen infecting many crops globally and is the causeof the recent olive disease epidemic in Italy. One strategy proposed to mitigate losses is to replantsusceptible crops with resistant varieties. Several genetic, biochemical and biophysical traitsare associated to X. fastidiosa disease resistance. However, mechanisms underpinning resistanceare poorly understood. We hypothesize olive cultivars' susceptibility to infection will correlate toxylem vessel diameters, with narrower vessels being resistant to air embolisms and having slowerflow rates limiting pathogen spread. To test this, we scanned stems from four olive cultivars of varying susceptibility to X. fastidiosa using X-ray Computed Tomography. Scans were processedvia a bespoke methodology that segmented vessels, facilitating diameter measurements. Thoughsignificant differences were not found comparing stem-average vessel section diameters amongcultivars, they were comparing diameter distributions. Moreover, the measurements indicatedthat though vessel diameter distributions may play a role regarding the resistance of Leccino, it isunlikely they do for FS17®. Considering Young-Laplace and Hagen-Poiseuille equations, weinferred differences in embolism susceptibility and hydraulic conductivity of the vasculature. Ourresults suggest susceptible cultivars, having a greater proportion of larger vessels, are morevulnerable to air embolisms. In addition, results suggest that under certain pressure conditions,functional vasculature in susceptible cultivars could be subject to greater stresses than in resistantcultivars. These results support investigation into xylem morphological screening to help informolive re-planting. Furthermore, our framework could test the relevance of xylem geometry todisease resistance in other crops.