## 4 Discussion  
Esca and other grapevine trunk diseases are one of the major limiting factors for grape production throughout the world [40]. In esca, which affects the trunk vascular tissues, information about the morphology of the xylem among grapevine cultivars will allow better understanding of the mechanisms of vascular pathogens. Previous studies have shown that the xylem vessel diameter differs among grapevine cultivars, and that these features might predict the degree of susceptibility of any given cultivar to vascular diseases [37]. Applying the methodologies used by [38], we investigated the main anatomical traits in xylem vessels in a wide range of grapevine genotypes, as 24 red-berried and 27 white-berried cultivars with Italian national and international importance in viticulture. Some  
Figure 9: Comparisons between the xylem characteristics of vessel diameter (columns), and vessel density (circle) (**A**) and disease incidence (**B**) of the red-berried and white-berried cultivars of the grapevines. Data are means \(\pm\) standard deviation. Different letters above column indicate significant differences between cultivars ([PERSON]’s tests; \(p\leq 0.05\)). of these are spread worldwide, as reported by the International Organisation of Vine and Wine (OIV) [41]. The information gained here provides novel knowledge about the anatomical characteristics of xylem tissue in grapevine cultivars. In particular, our data showed high variability in both vessel diameter and vessel density within and among red-berried and white-berried cultivars grown in the field, emphasizing that the vessel dimensions can be affected by cultivar characteristics. The differences in vessel sizes and frequencies observed among the grapevine cultivars were in agreement with data reported in previous studies, which involved small numbers of cultivars grown in greenhouses. In particular, we observed similar vessel densities for the same cultivars previously analysed, i.e., 'Merlot', 'Chardonnay' and 'Cabernet Sauvignon' [34; 37]. However, it should be emphasized that several studies carried out in a list of species have showed that the variation in xylem vessel size within genotypes changed across different environments [42; 43; 44; 45]. These investigations showed that smaller vessels are advantageous to avoid freezing-induced embolism and to minimise the impact of drought-induced cavitation. In peach rootstock, larger vessel sizes were found in the more vigorous rootstock than in the most dwarfing rootstock, and these characteristics were associated with the different hydraulic conductances estimated among these rootstocks [47]. Seasonal variations in vessel diameter are highly plastic and change with summer moisture availability, affecting plant growth [48; 49]. However, several comparative studies underline that conductive path length is by far the main driver of variation in mean vessel diameter, suggesting that basipetal vessel size provides a useful methodological means of standardizing samples [50; 51]. The present study analysed a large number of cultivars grown on the same experimental field, grafted onto the same rootstock, and trained with the same trellis system. For this reason, we can assume that external environmental factors, such as agricultural practices and soil and weather conditions, did not affect the results, which could instead be associated with grapevine genotypes. Interestingly, in this study, different distributions for the diameter classes were observed. In particular, there was a higher average vessel diameter in the white-berried cultivars compared to the average value detected in the red-berried cultivars. As might be predicted, a negative correlation was observed among the vessel diameters and vessel densities in both the white-berried and red-berried cultivars. Moreover, the lowest correlation coefficient was observed for the red-berried cultivars, which suggests less uniformity among this group regarding vessel size.