Plant hydraulics at the heart of plant, crops and ecosystem functions in the face of climate change.

Plant hydraulics is crucial for assessing the plants' capacity to extract and transport water from the soil up to their aerial organs. Along with their capacity to exchange water between plant compartments and regulate evaporation, hydraulic properties determine plant water relations, water status and susceptibility to pathogen attacks. Consequently, any variation in the hydraulic characteristics of plants is likely to significantly impact various mechanisms and processes related to plant growth, survival and production, as well as the risk of biotic attacks and forest fire behaviour. However, the integration of hydraulic traits into disciplines such as plant pathology, entomology, fire ecology or agriculture can be significantly improved. This review examines how plant hydraulics can provide new insights into our understanding of these processes, including modelling processes of vegetation dynamics, illuminating numerous perspectives for assessing the consequences of climate change on forest and agronomic systems, and addressing unanswered questions across multiple areas of knowledge.

On the path from xylem hydraulic failure to downstream cell death.

Xylem hydraulic failure (HF) has been identified as a ubiquitous factor in triggering drought-induced tree mortality through the damage induced by the progressive dehydration of plant living cells. However, fundamental evidence of the mechanistic link connecting xylem HF to cell death has not been identified yet. The main aim of this study was to evaluate, at the leaf level, the relationship between loss of hydraulic function due to cavitation and cell death under drought conditions and discern how this relationship varied across species with contrasting resistances to cavitation. Drought was induced by withholding water from potted seedlings, and their leaves were sampled to measure their relative water content (RWC) and cell mortality. Vulnerability curves to cavitation at the leaf level were constructed for each species. An increment in cavitation events occurrence precedes the onset of cell mortality. A variation in cells tolerance to dehydration was observed along with the resistance to cavitation. Overall, our results indicate that the onset of cellular mortality occurs at lower RWC than the one for cavitation indicating the role of cavitation in triggering cellular death. They also evidenced a critical RWC for cellular death varying across species with different cavitation resistance.

Hydraulic failure and tree mortality: from correlation to causation.

Xylem hydraulic failure has been recognized as a pervasive factor in the triggering of drought-induced tree mortality. However, foundational evidence of the mechanistic link connecting hydraulic failure with living cell damage and tree death has not been identified yet, compromising our ability to predict mortality events. Meristematic cells are involved in the recovery of trees from drought, and focusing on their vitality and functionality after a drought event could provide novel information on the mechanistic link between hydraulic failure and drought-induced tree mortality. In this Opinion, we focus on the cell's critical hydration status for tree recovery from drought and how it links with the membrane integrity of the meristems.

The interplay of hydraulic failure and cell vitality explains tree capacity to recover from drought.

Global climatic models predict an increment in the frequency and intensity of drought events, which have important consequences on forest dieback. However, the mechanisms leading to tree mortality under drought conditions and the physiological thresholds for recovery are not totally understood yet. This study aimed to identify what are the key physiological traits that determine the tree capacity to recover from drought. Individuals of a conifer (Pseudotsuga menziesii M.) and an angiosperm (Prunus lusitanica L.) species were exposed to drought and their ability to recover after rehydration monitored. Results showed that the actual thresholds used for recovery from drought based on percentage loss of conductance (PLC) (i.e., 50% for conifers, 88% for angiosperms) do not provide accurate insights about the tree capacity for surviving extreme drought events. On the contrary, differences in stem relative water content (RWCStem ) and the level of electrolytes leakage (EL) were directly related to the capacity of the trees to recover from drought. This was the case for the conifer species, P. menziesii, for which higher RWCStem and lower EL values were related to the recovery capacity. Even if results showed a similar trend for the angiosperm P. lusitanica as for the conifers, differences between the two traits were much more subtle and did not allow an accurate differentiation between trees able to recover and those that were not. RWCStem and EL could work as indicators of tree capacity to recover from drought for conifers but more studies are required to confirm this observation for angiosperms.