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Re: Nitrogen uptake in aquatic plants.

Chris Holloway asks a lot of good questions about N uptake preferences
by aquatic plants. I'd like to read some responses on the theories of
uptake mechanics from our botanist and limnologist readers but I can
answer one question. Diana Walstad's article are in TAG 5:6 and 6:1. The
list of scientific references is extremely lengthy. The conclusion is
that the vast majority of plants prefer ammonium over nitrate/nitrite.

>      Do aquatic plants take up NH4+ solely as a function of cation exchange?

I believe not. Even rooted plants are also able to absorb ammonium
through their leaf tissues although I believe less efficiently than
through roots. I think this may be because of higher ammonium
concentrations in interstitial water than in free water. Jim Kelly
mentioned three nutrient induction methods: 1) diffusion as a result of
concentration gradients 2) mass flow as a result of the plant
transpiration system and 3) direct contact by the root hairs. Cation
exchange via secreted organic acids probably plays a role in the latter.
I believe that Dave Huebert mentioned the transpiration stream in
aquatic plants but I cannot locate the reference in my archives. The
hour grows late so I'll stop here and spare y'all. :-)

>      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)?

I'm curious about these questions and the physiological transport
mechanisms for N and P.