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Re:uptake of nutrients
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>From: Stephen.Pushak at hcsd_hac.com
Stephen Pushak writes, Date: Tue, 29 Aug 95 2:20:37 PDT
>We know about three mechanisms that roots use to absorb nutrients:
>
>1> root interception: by growth, coming in direct contact with nutrients
>
>2> flow into the plants vascular system (by terrestrial plants which
>use evaporation at the leaf and capillary action; don't know if aquatic
>plants have a similar process)
>
Aquatic plants do have a tranpiration stream, even though they are entirely
submerged. Arber, Hutchinson, and others discuss observations that
indicate a considerable flow rate.
>3> diffusion via concentration gradients in the local regions around
>the rootlets.
>
>How big are those rootlets? How fast do they grow?
Rootlets are just small roots coming off of larger ones. Some plants like
Vallisneria, Elodea, Myriophyllum or Hydrilla have no rootlets.
Ceratophyllum even has no roots, but is usually well anchored by shoots
that grow into the mud. You can pull up your plants, wash away the soil or
gravel, and see the structure of the root system, including the rootlets.
What you can't see is the root hairs. These can get quite long in many
species of aquatic plants, up to 1 cm or possibly even longer. They
undoubtedly have an important role in absorbing nutrients.
>
>Can these rootlets still use capillary action to induce flow even
>under water? What makes capillary action work?
>
Capillary action pulls up water in very narrow spaces. The surfaces of the
speces have to attract water, and then surface tension pulls up the
meniscus, thereby filling up the space. The meniscus is the dividing line
between air and water. Capillary action wouldn't be a factor in the mud
under the water. The mechanism that causes the plant to have a
transpiration stream is the important factor, here. This mechanism
probably involves the production of higher concentrations of sugars or
other dissolved substances in the roots than in the leaves and stems. This
would cause water to enter the roots by the process of osmosis (Osmosis is
the diffusion of water from a higher concentration to a lower concentration
across a cell membrane. In the roots there would be a higher concentration
of dissolved substances and a lower concentration of water than outside the
roots. The water can diffuse through the cell membranes, and the dissolved
substances can not. Therefore, the water diffuses in, and the pressure
goes up in the cells until the pressure prevents more water from coming in.
Plants have tricks for moving water around inside themselves that involve
secretion of ions into the xylem until the water concentration in the xylem
solution is lower than that of surrounding cells. Then water diffuses from
these cells into the xylem. The ions in the xylem solution are taken back
higher up in the shoot.
>If the mechanism for transport into the rootlet is diffusion, wouldn't
>this diffusion take place at the same rate as diffusion elsewhere in
>the substrate? Does the rate of diffusion depend upon concentration
>of ions?
>
Water is transported by diffusion, but many of the needed mineral nutrients
are taken up by active transport mechanisms which can transport them
against the concentration gradient---that is, from lower concentration
outside the cell to a higher concentration inside the cell. The active
transport mechanisms for phosphorus, for example, are known the be very
efficient in aquatic plants and algae. Research has shown that some lakes
have phosphorus turnover times of around 1/2 hour. That means that all the
soluble phosphorus in the water is removed by the plants in half an hour
and is replaced by an equivalent amount released from decay processes. In
other words, a phosphate ion released by decay has at most a half an hour
to be in solution before it is snapped up by some plant.
>If the edge of an aluminum silicate crystal has a negative charge,
>could this act as a conduit for a stream of cations? Imagine a series
>of steel bearings attached to a magnetic rod. As you pull one off the
>end of the chain, the whole chain shuffles forward.
>
Clay particles or vermiculite have a lot of cation exchange sites. That
means that they tend to hold on to positively charged ions, such ans NH4+
or H+. They don't, however, hold on to them so tightly that there isn't an
equilibrium between the bound ions and ions in solution. If all the ions
in solution are taken away, then the bound cations, such as ammonium can
come off and be replaced by H+.
>How does oxygen get transported into the root system? What function
>does oxygen perform on behalf of plants in the substrate? Is this
>something all aquatic plants do? What about terrestrial plants?
Oxygen gets transported to the roots by way of air channels that are
continuous throughout the entire plant. Oxygen diffuses through air
100,000 times faster than it diffuses through water. The air channel
system serves as a rapid transport system for oxygen. Even at night when
the plant isn't making any oxygen, the O2 can diffuse from the water into
the air channels in the stems and leaves and then rapidly diffuse down to
the roots in the mud.
Paul Krombholz