Changes in growth and nutrient uptake in Brassica oleracea exposed to atmospheric ammonia.
OBJECTIVE
Plant shoots form a sink for NH3, and are able to utilize it as a source of N. NH3 was used as a tool to investigate the interaction between foliar N uptake and root N uptake. To what extent NH3 can contribute to the N budget of the plant or can be regarded as a toxin, was investigated in relation to its concentration and the N supply in the root environment.


METHODS
Brassica oleracea was exposed to 0, 4 and 8 microL L(-1) NH3, with and without nitrate in the nutrient solution. Growth, N compounds, nitrate uptake rate, soluble sugars and cations were measured.


RESULTS
In nitrate-sufficient plants, biomass production was not affected at 4 microL L(-1) NH3, but was reduced at 8 microL L(-1) NH3. In nitrate-deprived plants, shoot biomass was increased at both concentrations, but root biomass decreased at 8 microL L(-1) NH3. The measured nitrate uptake rates agreed well with the plant's N requirement for growth. In nitrate-sufficient plants nitrate uptake at 4 and 8 microL L(-1) NH3 was reduced by 50 and 66 %, respectively.


CONCLUSIONS
The present data do not support the hypothesis that NH3 toxicity is caused by a shortage of sugars or a lack of capacity to detoxify NH3. It is unlikely that amino acids, translocated from the shoot to root, are the signal metabolites involved in the down-regulation of nitrate uptake, since no relationship was found between changes in nitrate uptake and root soluble N content of NH3-exposed plants.
Castro
Ana
A
Laboratory of Plant Physiology, University of Groningen, PO Box 14, 9750 AA Haren, The Netherlands.
Stulen
Ineke
I
Posthumus
Freek S
FS
De Kok
Luit J
LJ
D000596:Amino Acids / Q000032:analysis; D000641:Ammonia / Q000378:metabolism / Q000494:pharmacology* / Q000633:toxicity; D001272:Atmosphere; D001937:Brassica / Q000187:drug effects / Q000254:growth & development* / Q000378:metabolism; D002241:Carbohydrates / Q000032:analysis; D002412:Cations / Q000032:analysis; D050901:Nitrate Reductase / Q000378:metabolism; D009566:Nitrates / Q000032:analysis / Q000378:metabolism; D009584:Nitrogen / Q000378:metabolism*; D018515:Plant Leaves / Q000378:metabolism; D018517:Plant Roots / Q000378:metabolism; D013431:Sulfates / Q000378:metabolism; D013455:Sulfur / Q000032:analysis
D000596:Amino Acids; D002241:Carbohydrates; D002412:Cations; D009566:Nitrates; D013431:Sulfates; D013455:Sulfur; D000641:Ammonia; D050901:Nitrate Reductase; D009584:Nitrogen