Calcium transport between tissues and its distribution in the plant
Abstract. The low cytosol concentration of free Ca2+ makes the symplast of roots an ineffective pathway for the supply of the calcium needed for healthy growth in the aerial parts of plants. Ca2+ moves rapidly across the cortical apoplast by diffusion and mass flow but is probably diverted across the plasmamembranes of endodermal cells by Casparian bands. A proposal is made to account for the movement of calcium across the endodermis and it is estimated that Ca-fluxes are likely to be appreciably greater than in the regulation of cell Ca level by cortical cells. 
 
Ca transport in the xylem occurs by mass flow of free Ca2+, and some organically complexed Ca, and by chromatographic movement along Ca-exchange sites in the xylem walls. Delivery of Ca to transpiring leaves and to weakly transpiring meristematic zones is discussed in relation to the two modes of Ca movement in the xylem. Competition between sinks is intensified when [Ca2+] in xylem is low and transpiration is great. 
 
Tropic growth responses involve pumping of vacuolar calcium into the apoplast followed by its migration along gradients of electrical potential which develop in the apoplast after geo-stimulation. An attempt is made to estimate plasmalemma efflux during this process. 
 
Redistribution from mature tissues to meristems in the pholem is likely to be small, if it occurs at all, since sieve tubes cannot have more than micro-molar concentrations of free-Ca2+ in them.