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RE: Nutrient Uptake: Roots v. Leaves
> On Wed, 13 Oct 1999, MBCREATIVE wrote:
> > > It is my understanding that aquatic plants DO NOT absorb nutrients
> through
> > > their roots, that the roots are only for anchoring and that the
> vast bulk of
> > > nutrient intake is in the plant above the roots. Anyone care to
> comment?
>
> > Wed, 13 Oct 1999 "Roger S. Miller" <rgrmill at rt66_com> replied:
> > That is wrong.
> >
> > Aquatic plants can take in nutrients at either roots or leave. I've
> read
> > that that aquatic plants switch from root feeding to foliar feeding
> when
> > dissolved nutrients are high enough to supply the plant.
> >
> > Taken literally this means that even if the substrate is very rich
> the
> > plant won't use nutrients in the substrate if there are sufficient
> > nutrients in the water column to supply the plants. I'm not sure
> that was
> > the author's intent in writing that passage, but that's how it was
> > written.
>
> Mon, 11 Oct 1999 Ed Street <br at ldl_net> also replied:
> > this is simply NOT true for all aquatic plants. While some fall
> into this
> > catagory like java moss which has modified leafs (if you can call it
> that)
> > to cling to surfaces you also have others like the sword family
> which DOES
> > have standard roots as we think of them.
>
We all know that our Anacharis canadensis can't
possibly support it's fifteen feet of growth from
those two or three wimpy roots that it put out.
However, it does have little roots all the way up
its stem in time to assist in uptake to support
the increasingly inefficient growth of aging tissue.
The primary issue in nutrient adsorption is the
gradient between the plant and the environment.
Of course, there is a lot the plant can do to more
efficiently capitalize on the gradient... but in general,
if it's in the water column, it will permeate the
foliage. If it's in the substrate, it will permeate the
roots (assuming the ion is small enough and the
plant doesn't select against it). If the nutrient
is in both, it can be taken up from both foliage
and root system. However, it may be more
"efficient" to uptake a lower concentration in the
water column directly through a more inefficient
mechanism in the foliage than to uptake a relatively
higher concentration in the substrate through a
relatively more efficient mechanism in the
roots, with the requirement that the nutrient is
mobilized systemically for eventual use at the
actual site of growth.
Thus, aquatic plants can and probably usually
do both. Depending on the relative concentrations
of nutrients in the water column and substrate and
target site of nutrient need, as well as the aquatic
plant's morphology (are they "true" roots or not),
the plant will reach some ratio of column/substrate
uptake. The total uptake rate between foliage
and root system will determine plant growth rate
(assuming no limiting nutrient ratios).
Remember that the environments are very different,
though. Terrestrial plants usually have a protective
epidermis that decreases evapotransipiration (water)
loss, provides resistence to abrasion, weathering, or
toxins, may make the foliage unpalatable to
critters, etc. This function is often decreased in
aquatic plants so aquatic plants are often more
successful at uptaking nutrients from the foliage.
Also, it is less typical for terrestrial plants to
have nutrient access at the foliage level other than
CO2, sulfur, and whatever else your acid rain brings
you, (and that's a very inefficient uptake as well,
unless you live next to a smoke stack) so the roots
play an increased role in terrestrial plants. (I have
read that nitrogen fixation in the atmosphere does
occur from lightening storms, and since that's often
the limiting terrestrial nutrient, that's why grass is
so green after the thunderstorm.)
Also remember that nutrient uptake in terrestrial
plants is usually driven by transpiration... water
taken in at the roots, and pulled up the plant
through the osmotic gradient (evaporation at the
leaves leave a lower gradient there, so the water
is pulled up). If you don't transpire your water,
it's pretty hard to get those nutrients up there.
That's the "real" reason plant growth stops on
hot days... the plant can't absorb water faster than
it loses it, the stomata close at the foliage (they
dry out and shut), and the nutrients aren't going
up to where they need to be. Because aquatic
plants typically don't have the osmotic gradient like
terrestrial plants (it's hard to have evaporation
when under water), they are less efficient uptaking
nutrients from the substrate, and need to rely more
heavily on foliage uptake.
Of course, this is a much greater problem
if you are 40-meter oak tree, with terminal
meristems and growth far above the ground.
You've got a long way to go to get those nutrients
to the mitotic (growth) site. Our aquatic stem
plants (Cabomba, Anacharis, Bacopa, etc.)
have terminal meristems like trees do, and grow
from the tip. However, there are far more
rosette type plants like our Echinodorus,
Anubias, and Crypts that have a basal (not terminal)
meristem, like grass: They grow from the
base. Thus, foliage uptake and root uptake
only need to get the nutrients to the "middle"
of the plant, at the base of ground level where
the growth actually occurs (most "growth" of
leaves on our leafy plants that we perceive outside
the plant's "base" is actually not "growth",
but cellular elongation... the cells are all there
when the leaf moves outside the "base" of the
plant [at the basal meristem], but they are very
small; the cells simply stretch and fill out
to make the leaf increase in size. That's why
the most nutricious part of the plant is at the
site of growth... dense DNA, more proteins.)
Sorry this is long... I just get excited about
some topics here.
Isn't this cool stuff?
--charley
charleyb at cytomation_com