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Nitrogen uptake in aquatic plants.
- To: Aquatic-Plants at actwin_com (Receipt Notification Requested)
- Subject: Nitrogen uptake in aquatic plants.
- From: Christopher.Holloway at hr-m_b-m.defence.gov.au
- Date: 05 Jan 98 13:48:37 +0000
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Hi Folks ...
Following a brief discussion with the local aquarium shop owner I found
myself confused as to aquatic plants' preferred sources of nitrogen.
On this subject, the bulk of the postings to APD over the last couple of
years suggest that the plants will take up NH4+ in preference to NH3,
NO2 and NO3. However the Fish Shop Guy swears that NO3 is the preferred
source of N for aquatic plants. His argument, which consisted in the
main of simple assertion, was less than compelling.
Unhappily my browsings through the APD back issues has not provided a
hard line of argument to take back to the FSG. I have however gleaned
1. NH3 and NH4+ exist in equilibrium, and this equilibrium is affected
by pH and temperature.
2. Colloidal clays offer binding sites for cations (i.e., NH4+). As
George Booth has it: "The negative sites [CEC sites offered by colloidal
clays] attract and hold the ammonium ions ... until a plant root hair
exchanges another [cation] for the ammonium (adsorption) and takes it in
to metabolise into amino acids and ultimately protein."
3. Another writer (apologies, should have kept the references ...) noted
that plants contained an enzyme which allows NH4+ ions to be attached to
C 'skeletons' to form amino acids. The writer also noted that plants
[all plants? some plants?] could reduce NO3- first to NO2- and then to
NH4+ by way of enzyme activity. However this nitrogen reduction process
is said to be energy expensive and is bypassed if NH4+ is immediately
available. A second caveat in my mind is that this writer sourced his /
her comments from a plant physiology text that dealt with terrestrial
plants. Do aquatic plants use a different set of reductions?
4. I am also aware that Diana Walstead wrote an article in a
peer-reviewed journal that (as I understand it) reported on experiments
involving Spirodela that revealed the plant's preference for N delivered
as NH4+. Does anyone have a source for this article?
I have a number of other questions.
Is there an authoritative text with detailed information on this topic
(N uptake in aquatic plants)?
Do aquatic plants take up NH4+ solely as a function of cation exchange?
Is this function limited to activity at the substrate level or do the
plants (or some plants) have an ability to take NH4+ (or nitrogen in
another form) straight from the water column? If so, what is the
machinery the plant uses to do this?
Do aquatic plants take up NH3 at all? NH3 doesn't seem to figure in the
account of the nitrogen reduction process mentioned above. Is the
beneficial effect of plants on N levels centred on moving N from an
occurrence as NH3 to an occurrence as NH4+ (given that the two species
exist in equilibrium)?
Thank-you for your patience, gentle reader.