Scaling phloem transport: Elasticity and pressure–concentration waves

Earlier theoretical analyses of the rate of propagation of pressure–concentration waves in the phloem were performed without adequate attention to the elastic expansion of sieve tube walls. Here, it is shown that the rate of propagation of pressure–concentration waves in phloem sieve tubes is not significantly impeded by wall elasticity, but rather, as previously implicated, by the ratio of sap osmotic pressure to the axial drop in sap hydrostatic pressure. It is also shown that pressure–concentration waves move equally well in both the upstream and downstream directions. These results permit future models to ignore elastic effects, and lend additional theoretical support to the “osmoregulatory flow” hypothesis, which argues that efficient molecular control of the phloem is permitted by maintaining sieve sap hydrostatic pressure at a value that is spatially nearly constant, which in turn permits changes in sieve tube state to be rapidly transmitted throughout the sieve tube via pressure–concentration waves.