Post-phloem transport: principles and problems

The movement of assimilates from the sieve element/companion cell complex to sites of utilization has been examined in an extensive array of sinks possessing diverse anatomies. This work has been reviewed with respect to the pathways taken, the conductances and driving forces for movement along the pathways, and interaction between the apoplast and symplast. Most investigations to date have been concerned primarily with determining the pathway of assimilate movement. A symplastic pathway is followed in the great majority of cases studied. However, available methods are less suited for demonstrating apoplastic transport in those instances where it occurs. Far less information is available on quantitative aspects of post-phloem transport. Only a very limited number of observations are available on the diffusive or hydraulic conductances of the apoplast or symplast. In some cases, symplastic conductance appears to be enhanced by a larger-than-usual size exclusion limit for cell-to-cell transport. Measurements of the driving forces for post-phloem transport (i.e. gradients in concentration and/or pressure) are also very few in number nor, to date, are they always readily interpretable. Evaluation of solute movement is complicated by interactions between the apoplastic and symplastic pathways, including water relations effects and solute exchange. The presence of apoplastic domains or, simply, high resistance to movement in the apoplast, can lead to steep water relations gradients within sinks, with important implications for transport. To understand how import into sinks is controlled, many more quantitative measurements are needed. This will require considerable experimental ingenuity.