I think we can agree that roots allow for the absorption/uptake of water, right?
So then if we consider that water enters (most often for our purposes) through the root system
and exits through the stomata on the leaves via transpiration as a vapor...
then I'm at a loss (currently) to understand how the discussion can begin and end
with the physiology of the roots. Water (it seems) acts as a host vehicle for the mobility
of anything that is both soluble in water and capable of traveling through the plant at all.
Which nutrients obviously are (of any kind) or they would have no effect on the plant at
all. So if the nutrients are mobile enough to travel in, why aren't they then mobile enough
to travel out? Even if they are in some way broken down or absorbed by the plant in some
way that disallows their departure, that is certainly well down the line from the roots.