Water Flow Across the Sieve-Tube Boundary: Estimating Turgor and Some Implications for Phloem Loading and Unloading. II. Phloem in the Stem

Confirming a previous analysis by Lang (1974), it is concluded that in tree trunks, phloem turgor and turgor gradients may be estimated from osmotic pressure and osmotic-pressure gradients, respectively. The present analysis is an improvement because it is based on observed osmotic-pressure gradients rather than supposed turgor gradients, and allowance is made for sucrose unloading and gradients of external water potential. It is concluded that the rate of sucrose unloading in tree trunks must be less than 50 nmol m−2 S m−1. In small plants, higher rates of unloading (100 nmol m m−2 S m−1) and steeper concentration gradients will lead to larger errors, but turgor pressures can still be estimated with acceptable accuracy. One should be more cautious when considering turgor gradients in small plants, although it seems likely that reasonable estimates will still be obtained. Assuming plasmodesmatal transport through an unconstricted cytoplasmic annulus, it is concluded that the sieve elements and their associated cells will sustain very similar turgor and osmotic pressures. Convection and diffusion can both contribute significantly to plasmodesmatal sucrose unloading. Similarly, the plasmodesmatal volume flux will reflect a combination of pressure flow and osmosis. Water fluxes across the sieve element plasmalemma and through the plasmodesmata can be in opposite directions. It may be possible to assess the extent of hydraulic coupling between the sieve elements and their associated cells from studies of phloem water relations