No evidence of xylem embolism refilling during recovery from drought stress in intact sunflowers

Refilling of xylem embolism during recovery from drought has been studied for decades, although the legitimacy of supporting evidence has been debated in recent years due to potentially widespread methodological artifacts associated with destructive sampling. Evidence from multiple destructive/indirect measurements of embolism suggests sunflower can refill, but more recent evidence suggests this species is particularly susceptible to artifacts that cause the appearance of refilling. Here, we investigated drought-induced xylem embolism and its potential reversibility in intact sunflowers using low-radiation micro-computed tomography (µCT). While plants were drying, turgor loss and stomatal closure occurred at ca. -1.0 MPa, whereas comparable losses of photosystem II efficiency as well as leaf and stem xylem embolism occurred at ca. -1.5 MPa. After re-watering, xylem embolism that accumulated in stems and leaves during the drought event did not reverse despite significant recovery of transpiration and photosynthesis. We found no evidence of xylem embolism refilling in sunflowers, highlighting the potential widespread nature of methodological artifacts and the need to revisit conclusions of "refilling" in other species as well as physiological and developmental consequences of irreversible embolism.