Rethinking post-drought hydraulic recovery: from consensus to controversy and mechanistic insights into embolism refilling

Abstract

Restoring xylem water transport function after drought is central to plant drought adaptation. Embolism refilling, a key mechanism for recovering water transport capacity, remains debated regarding its generality and mechanistic basis. This review synthesizes four decades of research on xylem embolism refilling, tracing developments from early theoretical constraints and positive-pressure refilling models to phloem-mediated refilling under low tension. Classical hydraulic assays have suggested that embolism refilling is widespread among vascular plants, but high-resolution imaging has revealed that it is not a universal phenomenon. An analysis of 44 imaging-based studies shows that approximately 57% of species can refill embolized conduits to varying degrees, and that refilling depends on three concurrent conditions: sufficient relief of xylem tension to near 0 MPa, metabolically active parenchyma cells supplying energy, and adequate non-structural carbohydrate and water reserves in tissues surrounding conduits. Building on these conditions, we propose that ethylene may function as a key regulator, promoting embolism refilling through multiple coordinated pathways: enhancing root hydraulic conductance to increase root pressure, maintaining stomatal closure to alleviate xylem tension, and stimulating respiration to boost energy supply, thereby driving radial sugar transport and establishing osmotic gradients. This framework integrates aboveground and belowground functions and redefines embolism refilling as a coordinated whole-plant response. Future studies should further define the boundary conditions under which embolism refilling occurs and validate the potential regulatory role of ethylene.