Changes in Growth and Nutrient Uptake in Brassica oleracea Exposed to Atmospheric Ammonia

Background and Aims 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 (mu)L L–1 NH3, with and without nitrate in the nutrient solution. Growth, N compounds, nitrate uptake rate, soluble sugars and cations were measured. Key Results In nitrate-sufficient plants, biomass production was not affected at 4 (mu)L L–1 NH3, but was reduced at 8 (mu)L L–1 NH3. In nitrate-deprived plants, shoot biomass was increased at both concentrations, but root biomass decreased at 8 (mu)L L–1 NH3. The measured nitrate uptake rates agreed well with the plantʹs N requirement for growth. In nitrate-sufficient plants nitrate uptake at 4 and 8 (mu)L 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.