Summary Global warming is expected to dramatically accelerate forest mortality as temperature and drought intensity increase. Predicting the magnitude of this impact urgently requires an understanding of the process connecting atmospheric drying to plant tissue damage. Recent episodes of forest mortality worldwide have been widely attributed to dry conditions causing acute damage to plant vascular systems. Under this scenario vascular embolisms produced by water stress are thought to cause plant death, yet this hypothetical trajectory has never been empirically demonstrated. Here we provide foundational evidence connecting failure in the vascular network of leaves with tissue damage caused during water stress. We observe a catastrophic sequence initiated by water column breakage under tension in leaf veins which severs local leaf tissue water supply, immediately causing acute cellular dehydration and irreversible damage. By highlighting the primacy of vascular network failure in the death of leaves exposed to drought or evaporative stress our results provide a strong mechanistic foundation upon which models of plant damage in response to dehydration can be confidently structured.

The interesting species-specific relationship between maximum daily shrinkage (MDS) and stem water potential ( stem ) was investigated in order to understand the mechanisms involved. Vulnerability curves of Vitis, Citrus, Olea and Prunus were analysed and the air entry  stem in the xylem (P e ) was calculated. The water relations parameters of the living tissue of the bark were also studied by building P-V curves. A strong linear correlation was obtained between the  stem at which MDS occurs ( MDS ) and P e in the studied species. Furthermore, P-V curves showed that, the species with the highest resistance to cavitation, i.e., Prunus, had the lowest osmotic pressure and the largest bulk elastic modulus. The conclusions of this study reinforce the idea of a tight correlation among plant capacitance, transpiration and maintenance of water status of the plant. These results illustrate the importance of studying the integration of long-distance transport systems in plants. INTRODUCTIONMonitoring of trunk diameter variations (TDV) on the bark of woody plants is becoming a useful tool for managing irrigation in commercial orchards. In addition to being a valuable feature for agronomists, these sensors supply important information about the physiological mechanisms that allow the plant to transpire under a wide range of environmental conditions. Currently, it is not possible to mechanistically explain the daily and seasonal dynamics of transpiration without considering the role played by water storage in trunks, branches and even leaves (Steppe et al., 2006). Trunk diameter fluctuations have been reported to reflect the addition of four components: irreversible radial growth, reversible living-cell dehydration/rehydration, thermal expansion and contraction, and expansion of dead conducting elements due to the increase and relaxation of internal tensions. Therefore, the interaction between both the living parenchyma in the cortex and the xylem is important to understand the dynamics of TDV and its consequences on the water relations observed in the plant. Maximum daily shrinkage (MDS) is one of the most used indicators of water stress in fruit trees (Fernandez and Cuevas, 2010). When plotted against stem water potential ( stem ) MDS

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Knowledge of temporal variations in radial profiles of sap flux density (J s ) and its relation to canopy water vapour exchange would help to improve our understanding of plant water relations. In this study we aimed to test if there is a consistent radial profile of J s in olive trees under optimal soil water availability and severe water stress conditions. Furthermore, if the radial profile of J s is not consistent through time we aimed to assess whether this radial variability can be explained through differential water exchange response to evaporative demand (D) of sun-exposed, new foliage and shade, old foliage. We measured sap flux density in the trunk of 6-year-old olive trees under two different irrigation treatments: a full irrigation treatment and a treatment replacing 30% of the of irrigation needs. We related the hourly radial profiles characterized with the Beta probability distribution function with independent water vapour exchange measurements at different exposures in the tree canopy (sun-exposed, young foliage and shade, old foliage). Results showed that under well irrigated conditions the shape of the radial profile of J s hardly varied, but under water deficit situations most of the sap appears to be conducted deeper into the sapwood and J s is also more variable throughout the sapwood. Accordingly, results demonstrated that in shade, old foliage water vapour exchange varied less than in sun-exposed new foliage. As a result, in days under severe water deficit, the contribution of shade old foliage to the whole tree transpiration was greater. Radial profile changes and canopy measurements were found to respond in a similar way to D. During conditions of low D and soil water deficit it was observed a greater contribution of the inner xylem to total stem flow and shade, old leaves to canopy gas exchange.