Nitrate uptake in plants

Robert Knox knox at
Fri Jan 3 17:58:55 EST 1992

I wrote: 
 >3) Many wild (non-agricultural) plants not only thrive in
 >environments with very low levels of detectable nitrate in 
 >the soil, but take up ammonium in preference to nitrate if
 >both are present.     .
 >Some useful references:
 >Nobel, P.S. 1991. Physiochemical and environmental plant 
 >physiology.  Academic Press, San Diego.
 >Fitter, A.H. and R.K.M. Hay. 1987. Environmental physiology
 >of plants, 2nd Ed.  Academic Press, London.
 >Charles-Edwards, D.A., D. Doley, G.M. Rimmington. 1986. 
 >Modeling plant growth and development. Academic Press,
 >Syndey. --this one is closest to having a suitable simple 
 >uptake model (pp. 144-146).
 >Haynes, R.J., K.C. Cameron, K.M. Goh, R.R. Sherlock. 1986.
 >Mineral nitrogen in the plant--soil system. Academic Press,
 >Orlando. --see chapter 6: Uptake and assimilation of mineral
 >nitrogen by plants, text pp. 303-358, references pp. 358-378.
Cameron Laird writes: 
|> You have me curious.  Which plants prefer
|> ammonium?  Is there a causal, or coinci-
|> dental, relation with domestication?  Does
|> one of these references particularly ad-
|> dress itself to agronomic concerns?
|> --

I won't attempt a review of these books for agronomic content.
(I have another book review overdue as it is.)  But I'll try to  
partially answer your query:

The relation with domestication probably has more to do with the 
requirements for domestication than the effects of domestication.
Plants suitable for intensive agriculture almost uniformly have
ancestors with high growth rates and native to highly productive
habitats (e.g. annual grasses from strongly seasonal climates, 
root and tree crops from tropical gaps or floodplains, etc.).  
In nitrogen rich habitats, bacteria oxidize a large fraction of 
the available ammonium to nitrate.  In most soils, nitrate also 
diffuses much more readily to roots than ammonium.  Ergo, agronomic 
plants tend to readily utilize nitrate.

Reducing nitrate to ammonium requires NADH and reduced ferredoxin,
as well as production of high levels of nitrate reductase.  This 
process requires energy, competing with growth and carbohydrate
storage.  If ammonium is the predominant available form, available
more less continuously from exchange sites, whereas nitrate is
is only available when pluses of N-mineralization are tracked by
high populations of nitrifying bacteria, there would only be modest 
selection for high levels of nitrate reductase in roots.  Ergo most
native plants from unproductive habitats (i.e. most habitats) 
"prefer" ammonium, in that they don't maintain high levels of the 
necessary enzymes.

Fitter & Hay (above) have a few pages on forms of nitrogen and the
differences in baseline and inducible levels of nitrate reductase in
plants from various sorts of habitats--emphasizing correlations with
soil pH.

|> Cameron Laird				+1 713-579-4613
|> cl at ( at	+1 713-996-8546 

- Robert G. Knox, Research Scientist
- Biospheric Sciences Branch, Mail Code 923.0
- NASA/Goddard Space Flight Center, Greenbelt, Maryland  20771 
- knox at         | BITNET: knox at ricevm1

DISCLAIMER:  NASA researchers sometimes claim to be able to estimate 
leaf nitrogen content from space, but measuring soil nitrogen 
remotely remains beyond the state-of-the-art.  For now, they don't
care what I say about it.

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