eLife http://doi.org/bj2d (2016)

While the movement of water through xylem tissue is relatively well understood, the movement of solute-laden sap through phloem is not. One possibility is the ‘pressure flow hypothesis’ proposed by the German plant physiologist Ernst Münch in 1930. By making careful measurements of the physical properties of morning glory (Ipomoea nil), Michael Knoblauch at Washington State University, USA and colleagues have shown that Münch's hypothesis is physically plausible even for large plants measuring tens of metres or more.

Münch proposed that high concentrations of solutes, especially sugars, in the phloem of source tissues such as leaves would draw water into those vessels from surrounding cells by osmosis, producing large turgor pressures. Conversely, sinks, where sugars were in demand, would have lower osmosis-sustained turgors. Material would flow from source to sink by mass transport.

Knoblauch et al. used I. nil vines from which lateral shoots were removed daily, leaving a very long plant with only two major sinks at the roots and shoot tip. These could be rolled up and carried from lab to lab: a ‘portable tree’. They measured physical and anatomical properties such as turgor pressure, conductance and sap flow with a sophisticated toolkit including tiny pressure meters dubbed ‘pico-gauges’. Both changes in anatomy and flow parameters scaled with the length of the vines, as Münch's hypothesis predicts.