Fibers beyond structure: do they contribute to embolism reversal after drought relief in poplar?

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.