Annual patterns of xylem embolism in high-yield apple cultivars

Temperate angiosperm species show pronounced annual patterns in xylem embolism. In this study, we investigated whether high-yield cultivars of Malus domestica Borkh. growing under optimised soil water conditions follow similar patterns to wild-type plants, and evaluated crucial factors for the formation of winter embolism and the subsequent restoration of the hydraulic system in spring. In five different cultivars growing at three different sites, various hydraulic and microclimatic parameters were monitored over three successive years. In all cultivars on all sites and in all years, the percentage loss of hydraulic conductivity (PLC) increased in autumn with freeze-thaw events and accumulated over winter. Maximum values were reached in late winter and differed significantly among cultivars. In spring, the hydraulic system was restored and PLC remained negligible during summer. Embolism formation in autumn was significantly correlated with the occurrence of freeze-thaw events, whereas further conductivity losses over winter were related to winter desiccation and influenced by climatic and cultivar-specific parameters. Restoration of the hydraulic system in spring was strongly linked to a decrease in the starch content of wood and buds, and soil temperature. Despite high soil water availability, hydraulic recovery took several weeks and was not completed before bud break. Spring is thus a critical phase for temperate angiosperms, especially for high-yield cultivars with risky hydraulic strategies.