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Modelling the outside-xylem hydraulic conductance: towards a new understanding of leaf water relations.

One of the most tantalizing unresolved questions in plant hydraulics is: where does water evaporate within the leaf? While several studies have explored experimental routes to answer this question (e.g. Sheriff & Meidner 1974; Farquhar & Raschke 1978), none have been able to provide a clear answer. In this issue of Plant, Cell & Environment, Tom Buckley has presented a new mathematical model incorporating anatomical and biophysical parameters to estimate hydraulic conductance in the apoplast, symplast and gas phase pathways outside the leaf xylem (Buckley 2014). Buckley reports that the apoplastic route was the most conductive flow path for water, but that gas phase pathways can represent a substantial portion of water movement, especially when high temperature gradients exist within the leaves (from high temperature at the leaf veins to lower temperature values at the epidermis), a result previously described in another modelling paper earlier this year (Rockwell et al. 2014). Buckley’s model provides, for the first time, equations to estimate outside-xylem hydraulic conductance based on real leaf anatomy and found that the proportion of total water movement in the different pathways was strongly dependent on mesophyll anatomy. This new mathematical model has the potential to finally answer the longstanding question of where water evaporates in leaves of a given species. Knowing the site of water evaporation within the leaf is crucial to our understanding of water movement outside the xylem. Leaf hydraulic conductance (Kleaf) is a measure of the capacity of water to move through the leaf and a major determinant of plant growth (Sack & Holbrook 2006). Unlike in stems, and just like in roots, water moves through both xylem and living cells outside the xylem. Understanding water flow through the outside-xylem pathways remains the largest challenge in the field of leaf hydraulics. Three main hypotheses of where most of the water evaporates from within the leaf have been proposed based on empirical data and models (Fig. 1a,b,c): (1) water mainly evaporates from the liquid phase once it reaches the substomatal cavities and diffuses through stomata (e.g. Sheriff & Meidner 1974; Tyree & Yianoulis 1980; Fig. 1a); (2) water flows through the bundle sheath cells and principally evaporates there (Boyer 1985; Fig. 1b); and (3) water evaporates from the mesophyll and epidermal cells, with an appreciable part evaporating near the stomata (Farquhar & Raschke 1978; Fig. 1c). Based on a novel analytical framework, Buckley (2014) offers a new view of where water evaporates, partially reconciling the previous hypotheses described above and in accordance with what Rockwell et al. (2014) proposed earlier this year (Fig. 1d). According to this new view, water will move in parallel through both liquid and gas phases until it reaches the stomata. In this model, the intercellular airspaces in the leaf are viewed as conduits for water transport, and thus, after moving through the bundle sheath cells, water can either continue flowing as liquid through the apoplastic and symplastic pathways, evaporating near stomata, or it can evaporate at the bundle sheath, and diffuse as gas until it reaches stomata (Fig. 1d). Because both liquid and gas phases share the same low water potential sink (airspaces near the stomata), they also share the same water potential gradient along the way, thus no evaporation would occur from mesophyll cells along the liquid path because the liquid water would be at the same water potential as the gas phase in the adjacent intercellular airspaces.The site of evaporation in the leaf would thus depend on where the bulk of the flow occurs, whether as vapour diffusion from near the bundle sheath, or liquid phase through the mesophyll and epidermis, evaporating at the most distal sites along the pathway. Buckley’s model allows us to determine where evaporation would occur, given the knowledge of species’ leaf anatomy. Because hydraulic conductance through both liquid and vapour phase pathways was highly dependent on leaf anatomy and temperature gradients within the leaf, different species could exhibit different evaporation sites. For example, species with very thick cell walls and low temperature gradients within the leaf could potentially have the bulk of its evaporation take place close to the stomata (such as shown in Fig. 1a). That would be expected because apoplastic conductance would be high and evaporation potential would be similar throughout the leaf. On the other hand, species with very thin cell walls and large temperature gradients could potentially have the bulk of its evaporation take place at the bundle sheath (such as shown in Fig. 1b). That would be expected because apoplastic conductance would be low and evaporation would be higher within the leaf, where the temperature is highest. Testing of this model against empirical data should show very promising results towards a better understanding of water movement outside the xylem. Buckley’s model can also give us great insight into the possible causes of leaf hydraulic decline with dehydration. Indeed, Kleaf has been shown to decline with dehydration, with drought-sensitive species usually declining rapidly at mild water stress (Sack & Holbrook 2006; Scoffoni et al. 2014). While the decline of hydraulic conductance inside the xylem (Kx) with dehydration is fairly well understood (cavitation – embolizing conduits – and/or xylem cell wall collapse), few studies have looked at how dehydration impacts outside-xylem hydraulic conductance (Kox) and how this would affect Kleaf. Knowing the impact of Kox on Kleaf decline Correspondence: C. Scoffoni. e-mail: cscoffoni@ucla.edu Plant, Cell and Environment (2014) doi: 10.1111/pce.12433 bs_bs_banner