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Over three decades ago, it was first observed that the hydraulic resistance of perfused stem segments of Acer twigs was significantly decreased when deionized water was accidentally replaced by tap water containing a low concentration of metal ions (Zimmermann 1978). Initially, the effect of ions on xylem hydraulic resistance was seen as a methodological problem of limited biological importance rather than an evolved xylem trait (Sperry et al. 1988). The recognition that this phenomenon might substantially contribute to the regulation of water flow through plants came much later (van Ieperen et al. 2000, Zwieniecki et al. 2001). These reports challenged the classical paradigm of vessels being purely passive in water transport (Zimmermann and Milburn 1982, Zimmermann 1983, Gartner 1995). Recently, a new view has emerged: that these tubes can control/regulate sap flow in response to changing plant physiological status (Zwieniecki et al. 2001, Nardini et al. 2011). These follow-up studies were aimed at understanding the biophysical basis for the ion-mediated variable resistance of xylem. Later studies demonstrated the potential for a variable conductance in regulating water distribution in large dendritic structures that can help to optimize plant resource utilization or mitigate the effects of environmental perturbations (see the opinion by van Ieperen 2007). The initial biophysical focus on variable xylem resistance had a positive effect on refining our understanding of the central role that bordered pits play in plant water transport. It was proposed that the porosity of the bordered pit membranes can be actuated by swelling/de-swelling activity of pectin hydrogels that cross-link the fibrous membrane structure (Zwieniecki et al. 2001). The role of hydrogel in the variable xylem resistance phenomen was indicated by the response of xylem resistance to ion concentration, pH and non-polar solutions (ethanol), where all treatments resulted in reduced resistance. However, the hypothesis that de-swelling opens micro channels in the bordered pit membrane (Zwieniecki et al. 2001, van Ieperen 2007) had to be modified to reflect the recent results from direct observations of conformational changes of the bordered pit structure using an atomic force microscope (AFM) (Lee et al. 2012). Lee and co-authors showed that while the general idea of hydrogels being responsible for the hydraulic properties of bordered pits did not change, a decrease in membrane thickness is behind the decrease of the hydraulic resistance rather than a change in membrane porosity. Atomic force microscope analysis delivered a clear image of the change from a featureless surface of the membrane in deionized water to a rigid, sharp and fibrous surface in 50 mmol KCl (Figure 1). No submicron pores were observed in the membrane, suggesting that water flow occurs via a hydrated hydrogel matrix rather than through the pores between cellulose fibers (Figure 1). We feel that these recent findings clarified the basic understanding of variable xylem resistance and allow for a re-focus from questions such as ‘how does it happen?’ to the analysis of the physiological role of the variable hydraulic resistance of xylem in plants (Holbrook et al. 2002, Zwieniecki et al. 2004, van Ieperen 2007, Trifilo et al. 2011). There is no doubt that the ever-growing dendritic structure of the tree requires an efficient water distribution system. It was shown that the xylem distribution system is often optimized following Murray’s law in which xylem resistance is minimized for a given investment in transport tissue (McCulloh et al. 2004, Sperry et al. 2008) or optimized with a fractal geometry that allows for vascular design where resistance is independent from path length (West et al. 1999). Once in Commentary

Getting variable xylem hydraulic resistance under control: interplay of structure and function