Direct measurements of sieve element hydrostatic pressure reveal strong regulation after pathway blockage

According to the Münch hypothesis, solution flow through the phloem is driven by a hydrostatic pressure gradient. At the source, a high hydrostatic pressure is generated in the collection phloem by active loading of solutes, which causes a concomitant passive flow of water, generating a high turgor pressure. At the sink, solute unloading from the phloem keeps the turgor pressure low, generating a source-to-sink hydrostatic pressure gradient. Localised changes in loading and unloading of solutes along the length of the transport phloem can compensate for small, short-term changes in phloem loading at the source, and thus, maintain phloem flow to the sink tissue. We tested directly the hydrostatic pressure regulation of the sieve tube by relating changes in sieve tube hydrostatic pressure to changes in solute flow through the sieve tube. A sudden phloem blockage was induced (by localised chilling of a 1-cm length of stem tissue) while sieve-tube-sap osmotic pressure, sucrose concentration, hydrostatic pressure and flow of recent photosynthate were observed in vivo both upstream and downstream of the block. The results are discussed in relation to the Münch hypothesis of solution flow, sieve tube hydrostatic pressure regulation and the mechanism behind the cold-block phenomenon.