I was thumbing through a well known, but older Limnology text
(Hutchinson, 1975, Vol 3 Limnological Botany pg 351-357) when I saw a
nice graph showing growth rate vs NO3 concentration.

It seems Paul had found that 20-80 ppm and above was the ideal range
for submersed plant growth(Vallisneria americana) back in the
1960's(1966). I suppose I redisocovered this range(20-75ppm)
independently some 30 years later thropugh trial and errors.

What is interesting is that we both arrived at the same range. At
progressively high concentrations, this high rate of growth slowly
decline, but very slowly.....even at 100ppm etc.

This also mirrors my own observations when I did longer term NO3 at
75ppm for several weeks.

What is really interesting is how rapid the growth rate increases when
the level is maintained.

For example:
At 5 ppm the rate of growth is greatly reduce, about 2.2/0.7= 3.14
times less growth (dry weight mass).
At 10ppm, the growth was about 1/2, 2.2/1.1 = 2x less growth than at
20-80ppm.
After 20ppm, the plant's growth is no longer nitrogen limited.

Fast forward to the molecular age of plant biology.
Why might these plants show this pattern? How would they control it?
Given what is known about LAT and HAT transportors for NO3, it may now
be suggested that when plants have all their constitutive and
inducible transporters upregulated and maintained, they grow faster
and have non limited growth.

In order for the plants to do this, 20-30ppm of NO3 needs to be
present in the medium(the water column). Now we have a plant that is
healthy and can grow at a maximum rate. If the NO3 levels varies
between say 2-15ppm, then the various transporters will be degraded
and more efficient transports(the HATs) specific to low NO3 levels
will be put in their place. As a result, the plants growth rate will
be reduced.

It takes more energy to concentrate nutrients when there is less in
the external environment. So at higher levels, the plants use
different transportors that take full advantage of the higher NO3
levels and grow faster as result.

PO4 data was also discussed. But no such graph was provided, other
than tissue analysis for PO4.Still if one assumes a ratio for PO4,
then a 6:1 to 10:1 relationship would suggest about 2ppm or higher for
PO4(Conversion from N:P to NO3:PO4 is addressed FYI).

Which is about what we find to be optimal for growth in the water
column.
Seems the data was and has been there all along, just no one bothered
to listen to Paul, nor look stuff up.

He looked at many lakes and plants and did a lot of tissue analysis
beside this as well. Below is my personal favorite of Paul at the
plant fest, he should stick to plants:-)

Reference:

Gerloff, G.C., and Krombholz, P.H., 1966. Tissue analysis as measure
of nutrient availability for the growth of aquatic plants.
Limnological Oceanography, 11:529-537. (Hutchinson, 351-357)


Regards,
Tom Barr

www.BarrReport.com
www.sfbaaps.com
www.gregwatson.com