Rachele Gamba

Background and Aims: Climate change is increasing the frequency and intensity of droughts, challenging the growth, hydraulic function and survival of woody plants. Poplar is a fast-growing species widely cultivated for biomass and cellulose production, but it is also highly sensitive to water deficit, with substantial variation among species and clones. Methods: This study compared the drought responses of two Populus × canadensis clones, ‘Tucano’ and ‘San Martino’, under gradually imposed, prolonged water stress. Physiological traits and intrinsic wood anatomical characteristics revealed different strategies between clones for coping with drought. Key Results: Compared with the ‘San Martino’ clone, the ‘Tucano’ clone exhibited faster stomatal closure under moderate stress and a greater resistance to embolism formation. The resistance was further supported by specific intrinsic wood anatomical traits, including higher vessel density, increased vessel clustering, greater theoretical hydraulic conductivity, thicker cell walls and higher wood density. Conversely, the delayed stomatal closure, the less negative P50 value and the distinct xylem anatomical structure observed in ‘San Martino’ suggest a significantly higher vulnerability to prolonged water deficit. Conclusions: ‘Tucano’ adopts an effective drought-avoidance strategy, integrating stomatal regulation, intrinsic wood anatomy and carbon allocation to balance hydraulic safety with growth and wood quality. The study highlights specific traits as key determinants of drought resistance and supports the selection of a specific clone for poplar cultivation in future climate scenarios characterized by water scarcity.

Summary: Short‐term recovery from drought‐induced vessel embolism is an energy‐dependent biological process that requires a water source and solutes, both likely supplied by parenchyma cells. Despite fibers primarily providing structural support, their functional role as a reservoir of unbound water during and after stress remains unclear. In this study, Populus nigra plants were exposed to two drying regimes (slow and fast developing stress). At the end of the drought treatments and after stress relief, nondestructive structural observations were performed in vivo using synchrotron X‐ray microCT. Different drought progression rates did not affect the final extent of vessel embolism, but poplars subjected to slower drought development exhibited higher levels of air‐filled fibers. Following stress relief, faster hydraulic recovery was observed in plants exposed to rapid drought, which displayed lower occurrences of water‐depleted fibers. We suggest a novel functional role for xylem fibers during drought and recovery. We hypothesize that parenchyma cells can access water stored in completely mature fibers via pits, enhancing their survival during drought. Upon xylem tension relief, this stored water may be mobilized by living cells from fibers to vessels, facilitating the recovery of their transport function.