Optimized redistribution – autonomous control of fluxes in xylem

We have recently shown that increasing concentrations of ions flowing through the xylem produce rapid, substantial, and reversible decreases in hydraulic resistance. Changes in hydraulic resistance in response to solution ion concentration, pH, and non-polar solvents are consistent with this process being mediated by hydrogels. The effect is localized to intervessel bordered pits, suggesting that micro-channels in the pit membranes are altered by the swelling/de-swelling of pectins. This local activity depends entirely on properties of xylem sap that can be locally changed by living cells, thus leading to a functional interaction between the parallel transport systems of xylem and phloem. Although such a link was previously hypothesized, no mechanism was proposed to link the two pathways, nor was any physiological “meaning” ascribed to such interactions. Our goal is to understand how this variable resistance helps large trees to redistribute water in the way that it will be available to most productive branches in the tree crown. This research venue requires detailed analysis of xylem and phloem anatomy at large longitudinal scales (the intellectual descendents of the early anatomical movies made by Martin Zimmermann), detailed analysis of xylem and phloem sap chemistry, measurements of changes in xylem resistance, and whole tree level water redistribution.