Changes in growth and nutrient uptake in Brassica oleracea exposed to atmospheric ammonia
2006
Castro, A. | Stulen, I. | Posthumus, F.S. | Kok, L.J de
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 [micro]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 [micro]L L⁻¹ NH₃, but was reduced at 8 [micro]L L⁻¹ NH₃. In nitrate-deprived plants, shoot biomass was increased at both concentrations, but root biomass decreased at 8 [micro]L 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 [micro]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.
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