Experimental evidence clearly indicates that K, Mg and Ca are required in
the water column for the 'normal' growth of at least a few submerged
aquatic plants. The reasons for this are less clear but can be discussed
on the basis of their chemical properties and physiological/biochemical
roles. As mentioned, potassium, while a mobile element, is required for
ionic balance and so must be present both internally and external to the
cell membrane ... remember that aquatic plants, unlike terrestrial
plants, have their apoplasm (that part of the plant external to the
membrane) in direct contact with a solution into which ions can diffuse.
Calcium is also necessary external to the cell membrane for normal cell
wall development ... internal Ca requirements are actually quite low and
in fact ionic Ca is extremely toxic to plant cell function. Both Ca and
Mg in land plants do not 'move' as readily in the xylem as other ions.
The hypothesis is that because the movement of water through aquatic
plants is less than in land plants, Mg requirements cant be satisfied by
upward movement. At no time have I said that Ca and Mg cant move within
the xylem of aquatic plants ... in fact experimental evidence clearly
shows that at least Ca can move up or down in aquatic plants tissues
(just not at a rapid enough rate to satisfy external requirements).