FLUXES AND DEPOSITION RATES OF SOLUTES IN GROWING ROOTS OF ZEA-MAYS

The equations and experimental methods are reviewed for calculating so lute deposition rates and fluxes in growing parts of roots. Then the e xpression for the growth-sustaining sucrose flux in the meristem is pr esented and evaluated to show the sucrose flux required to produce the empirically observed biomass deposition rate in primary maize roots. Two independent methods (one based on dye transport measurements and t he second based on Stokes radii) are used to estimate the symplasmic c onductivity for diffusion. The concentration gradients required to dri ve a growth-sustaining symplasmic diffusion of sucrose are found to be physiologically unreasonable. Either our current understanding of pla smodesmatal ultrastructure is flawed, or alternative mechanisms must e xist for sucrose transport to the meristem.