NONVASCULAR, SYMPLASMIC DIFFUSION OF SUCROSE CANNOT SATISFY THE CARBON DEMANDS OF GROWTH IN THE PRIMARY ROOT-TIP OF ZEA-MAYS L

Nonvascular, symplasmic transport of sucrose (Sue) was investigated th eoretically in the primary root tip of maize (Zea mays L. cv WF9 x Mo 17) seedlings. Symplasmic diffusion has been assumed to be the mechani sm of transport of Suc to cells in the root apical meristem (R.T. Giaq uinta, W. Lin, N.L. Sadler, V.R. Franceschi [1983] Plant Physiol 72: 3 62-367), which grow apical to the end of the phloem and must build all biomass with carbon supplied from the shoot or kernel. We derived an expression for the growth-sustaining Suc flux, which is the minimum lo ngitudinal flux that would be required to meet the carbon demands of g rowth in the root apical meristem. We calculated this flux from data o n root growth velocity, area, and biomass density, taking into account construction and maintenance respiration and the production of mucila ge by the root cap. We then calculated the conductivity of the symplas mic pathway for diffusion, from anatomical data on cellular dimensions and the frequency and dimensions of plasmodesmata, and from two estim ates of the diffusive conductance of a plasmodesma, derived from indep endent data. Then, the concentration gradients required to drive a gro wth-sustaining Suc flux by diffusion alone were calculated but were fo und not to be physiologically reasonable. We also calculated the hydra ulic conductivity of the plasmodesmatal pathway and found that mass fl ow of Suc solution through plasmodesmata would also be insufficient, b y itself, to satisfy the carbon demands of growth. However, much of th e demand for water to cause cell expansion could be met by the water u nloaded from the phloem while unloading Suc to satisfy the carbon dema nds of growth, and the hydraulic conductivity of plasmodesmata is high enough that much of that water could move symplasmically. Either our current understanding of plasmodesmatal ultrastructure and function is flawed, or alternative transport mechanisms must exist for Suc transp ort to the meristem.