Why are leaves hydraulically vulnerable?

As plant tissues dehydrate, water transport efficiency declines, a process typically attributed to air obstruction (embolism) in the xylem. Trifiló et al. (pages 5029– 5039) dissect leaf hydraulic vulnerability and show that both xylem and living tissues may be important. If confirmed and clarified, an important role for outside-xylem hydraulic decline will change our understanding of how plants transport water and control biosphere carbon and water fluxes. The study of leaf hydraulics has taken off exponentially in the past decade, after many decades of very sparse attention, especially compared with stem hydraulics. The obstacle was that leaves seemed impossibly complex. Whereas in stems, water flows through the xylem, in leaves water flows not only through the xylem within the complex venation network but also across living tissues outside the xylem, and evaporates somewhere in the leaf, before diffusing through the stomata. No part of that system was well understood. The first step was to establish methods to reliably quantify the hydraulic conductance of the whole leaf (K leaf), defined as the flow rate divided by a given gradient in water potential (Ψ) from petiole to mesophyll (reviewed by Sack and Tyree, 2005). Subsequent experiments on leaves with severed veins allowed the dissection of leaf xylem hydraulic conduct-ance (K x) from outside-xylem conductance (K ox) and showed that the xylem and outside-xylem compartments contribute strongly and similarly to total hydraulic resistance on average across species, with the relative contributions highly variable among species (reviewed by Sack and Holbrook, 2006). Now each component of the leaf hydraulic system could be illuminated by creative and exciting research, with major breakthroughs on the role of the venation network (reviewed by Sack and Scoffoni, 2013), the properties of the living tissues , including aquaporins, which affect leaf water transport (Maurel et al., 2015), and the vapor phase transport pathways (Rockwell et al., 2014; Buckley, 2015). These studies further showed that K leaf is responsive to internal and external factors that affect both xylem and living tissues, such as temperature , light and water status. In this issue, Trifiló et al. (2016) extend the dissection approach to clarify the response of K leaf to dehydration. The decline of K leaf with dehydration is known as 'vulnerabil-ity', by analogy with concepts established for stem hydraulics. Because water flows through the xylem under tension, it is susceptible to interruption by air, which can fill the xylem conduit and …