Advancements in the study of the grapevine hydraulic strategy under water deficit in relation to the formation of embolism

In recent years, the validity of embolism quantification methods has been questioned, especially for long-vesseled plants, as some studies have suggested that cutting xylem while under tension (even under water) might generate artificial cavitation. Accordingly, a rehydration procedure prior to hydraulic measurements has been recommended to avoid this artefact. On the other hand, concerns have been raised that xylem refilling might occur when samples are rehydrated. Recently we have proposed an improved method for reliable quantification of this phenomenon in grapevine petioles in which the inclusion of a short-time (20 min) xylem tension relaxation prior to the hydraulic measurement resulted in reliable quantification of native embolism. Much longer periods of hydration (overnight) were required before xylem refilling was observed to occur. This method yielded xylem vulnerability curves similar to the ones obtained using direct microCT visualization and the values were lower than the ones found in samples harvested under native tensions. The new protocol revealed cavitation in grapevine petioles occurs only after significant stomatal conductance reduction, contrary to what was previously hypothesized. In field-grown vines, over 85% of stomatal closure occurred at less negative xylem tension than that required to induce 12% loss of conductivity (PLC). These findings indicate that grapevines regulate their stomata to prevent xylem embolism and would even shed a large proportion of their leaves in order to avoid cavitation tensions. This pattern of stomatal behavior is more consistent with a strategy of cavitation resistance and avoidance, rather than a paradigm of tolerance featuring diurnal cycles of cavitation accumulation and repair.