Synchrotron X-ray microtomography of xylem embolism in Sequoia sempervirens saplings during cycles of drought and recovery.

The formation of emboli in xylem conduits can dramatically reduce hydraulic capacity and represents one of the principal mechanisms of drought-induced mortality in woody plants. However, our understanding of embolism formation and repair is constrained by a lack of tools to directly and nondestructively measure these processes at high spatial resolution. Using synchrotron-based microcomputed tomography (microCT), we examined embolism in the xylem of coast redwood (Sequoia sempervirens) saplings that were subjected to cycles of drought and rewatering. Embolism formation was observed occurring by three different mechanisms: as tracheids embolizing in wide tangential bands; as isolated tracheids in seemingly random events; and as functional groups connected to photosynthetic organs. Upon rewatering, stem water potential recovered to predrought stress levels within 24 h; however, no evidence of embolism repair was observed even after a further 2 wk under well-watered conditions. The results indicate that intertracheid air seeding is the primary mechanism by which embolism spreads in the xylem of S. sempervirens, but also show that a small number of tracheids initially become gas-filled via another mechanism. The inability of S. sempervirens saplings to reverse drought-induced embolism is likely to have important ecological impacts on this species.