Xylem embolism repair in the stem of intact Populus balsamifera saplings : single-scan and time-series microCT experiments

Increasing frequency and severity of drought events pose significant risks to native North American tree species such as balsam poplar (Populus balsamifera L.). Drought induces excessive xylem tension in the main stem, leading to air embolism that disrupts long-distance water transport. Although embolism repair following soil rehydration has been observed in some woody species, its duration and mechanism remain unclear in balsam poplar. This study investigated xylem refilling over time in intact P. balsamifera L. saplings using X-ray microtomography (microCT) to visualize and quantify the water- and air-filled status of xylem vessels and fibers. Two experiments were conducted: (1) a single-scan experiment comparing droughted saplings and those 7 days after rehydration, and (2) a time-series experiment in which individual saplings were repeatedly imaged over 50 hours. In the single-scan experiment, embolized vessel counts decreased by an average of 76%, accompanied by reductions in air-filled vessel and fiber areas (83% and 50%, respectively), and recovery of stem water potential (Ψstem) from approximately −15 to −5 bar. In contrast, the time-series experiment showed increases in embolized vessel counts (51%) and air-filled areas of vessels and fibers (39% and 16%, respectively) over 50 hours despite recovery in Ψstem. Water droplets and short water columns were initially observed within embolized vessels (~5 hours) but did not expand and instead disappeared over time. Anatomical analysis showed that approximately 40% of vessel wall area consisted of vessel-associated cell (VAC) pits across vessels of varying diameter and radial position, suggesting that initial water entry into embolized vessels may be linked to metabolic activity and solute export from adjacent living cells. However, the failure of embolism repair and formation of air pockets near the cambial region in time-series scans indicate that repeated X-ray exposure likely compromised cellular integrity and interfered with the repair process. Overall, these results provide evidence that embolized xylem vessels and fibers can refill following soil rehydration in intact P. balsamifera L. saplings, supporting recovery of water transport and storage over longer timescales. However, frequent microCT imaging under the conditions used here disrupts embolism repair and is not suitable for resolving its spatiotemporal dynamics.