Scaling phloem transport: information transmission

Sieve tubes are primarily responsible for the movement of solutes over long distances, but they also conduct informa- tion about the osmotic state of the system. Using a previ- ously developed dimensionless model of phloem transport, the mechanism behind the sieve tube’s capacity to rapidly transmit pressure/concentration waves in response to local changes in either membrane solute exchange or the magni- tude and axial gradient of apoplastic water potential is demonstrated. These wave fronts can move several orders of magnitude faster than the solution itself when the sieve tube’s axial pressure drop is relatively small. Unlike the axial concentration drop, the axial pressure drop at steady state is independent of the apoplastic water potential gra- dient. As such, the regulation of whole-sieve tube turgor could play a vital role in controlling membrane solute exchange throughout the translocation pathway, making turgor a reliable source of information for communicating change in system state.