Is There Variability for Xylem Vulnerability to Cavitation in Walnut Tree Cultivars and Species (Juglans spp.)?

Drought-tolerance selection is a current challenge for breeding programs to ensure agrosystem resilience, particularly with intensity and frequency of drought increasing worldwide. In tree species, xylem vulnerability to cavitation is among the most important traits on which drought-induced mortality depends. It can be rapidly evaluated, enabling large-scale screening for drought resistance. Genetic variability and phenotypic plasticity for this trait have been studied in natural populations, but not yet for cultivated tree species. In this work, the genetic variability in xylem vulnerability to cavitation of six Persian walnuts (Juglans regia L.), six hybrid walnuts (J. regia × Juglans nigra), and seven walnut species was investigated. In the first step, the method for measuring xylem vulnerability to cavitation using the “Cavitron” centrifuge technique was improved on walnut samples to obtain more accurate results. The Cavitron technique was found well suited to assessing xylem vulnerability to cavitation on this species using a 0.38 m rotor, which is large enough to analyze samples with intact vessels. Despite differences in wood anatomical traits, xylem vulnerability to cavitation among the Persian walnuts studied was similar. Very narrow variations in xylem vulnerability to cavitation were also found among hybrid walnuts. Only slight differences were observed on comparing species among which some have shown differences in various traits. These results suggest uniform selection, leading to canalization in cavitation resistance for cultivated Juglans.