Which stem anatomical characters drive drought resistance? Hydraulic failure is one of the prime mechanisms underlying drought-induced mortality in plants, and corresponds to the disruption of water transport in embolized xylem conduits when plants face drought.
As the proportion of gas embolism in xylem conduits generally enhances with increasing drought stress, the hydraulic conductivity decreases until a critical threshold, potentially leading to plant death. Plant resistance to embolism is estimated using so-called vulnerability curves, from which P50, i.e. the xylem pressure inducing 50% loss of hydraulic conductivity, can be estimated. P50 measurements have been carried out for hundreds of woody species and show that the species from dry environments are generally more resistant to embolism (more negative P50) than species from wet climates. This enables us to use P50 as a reliable proxy for drought stress resistance, and search for wood anatomical features that explain the variation in P50 across species based on light microscopy, scanning electron microscopy and transmission electron microscopy. One of the wood features that has proven to be hydraulically relevant in woody plants is the ultrastructure of the interconduit pits (see picture), as it forms a direct link for the propagation of embolisms within the 3D conduit network via a process called air-seeding.