It has been shown that long-distance transport of solutes through the phloem within plants is driven by an osmotically generated pressure gradient, with associated radial exchange of water in the source and sink regions. However, there is also water and solute exchanges along the long-distance pathway, but their magnitudes are poorly known and their physiological role has rarely been investigated, especially in mathematical models of phloem transport. Therefore, this study investigated the magnitude of these fluxes in stems, and what can regulate them, by both theory and experiment. A steady state model of phloem transport developed using Navier-Stokes and convection-diffusion equations with allowance for water and solute exchange along the pathway showed that radial water exchange affects the pressure gradient. Solute exchange, dependent on the phloem cells permeability, also affects the pressure gradient by modifying water exchange. This result is significantly different from Hagen-Poiseuille flow which has been used so far in most mechanistic descriptions of phloem transport, not considering solute radial exchange. The experimental approach to investigate the importance of radial exchange of water and solutes on phloem transport made use of 11C to non-invasively trace sugars, and transfer-function analysis to calculate tracer transport and unloading in squash and wheat plants, in response to treatments of the stem apoplast by perfusion with test solutions. In squash, effects of treatments with sucrose or mannitol on tracer unloading were similar at concentrations up to 300 mM. At 500 mM mannitol caused a transient stoppage of phloem transport, unlike sucrose. Application of mannitol may have caused a more abrupt osmotic shock than sucrose because of a higher permeability into the tissue, corresponding to its lower molecular size. In squash, the loss of tracer increased in the presence of PCMBS, which inhibits membrane transport, suggesting that there was phloem reloading of sugar via membrane transport from the apoplast. The observed response to apoplastic treatments was interpreted with a simple compartmental model: changes in the apoplast water potential and solute exchange greatly affected phloem transport, in agreement with the experimental work. Both the theoretical and experimental approaches showed that radial solute and water exchange in pathway regions between sources and sinks have to be recognised for a better understanding of phloem transport to be possible.
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