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

The equations and experimental methods are reviewed for calculating solute deposition rates and fluxes in growing parts of roots. Then the expression for the growth-sustaining sucrose flux in the meristem is presented 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 the second based on Stokes radii) are used to estimate the symplasmic conductivity for diffusion. The concentration gradients required to drive a growth-sustaining symplasmic diffusion of sucrose are found to be physiologically unreasonable. Either our current understanding of plasmodesmatal ultrastructure is flawed, or alternative mechanisms must exist for sucrose transport to the meristem.