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The role of ammonia in plant physiology Ineke Stulen, Ana Castro, and Luit J. De Kok Laboratory of Plant Physiology University of Groningen The Netherlands N is essential – amino acids and proteins N uptake and assimilation NH3 nitrate ammonium N assimilation - root { nitrate ammonium NO3- and NH4+ actively taken up by transporters N assimilation - root { { nitrate ammonium NO3- and NH4+ actively taken up by transporters NO3- reduced, and transported N assimilation - root { { { nitrate ammonium NO3- and NH4+ actively taken up by transporters NO3- reduced, and transported NH4+ attached to a C skeleton (GS) N assimilation - root { { { { { nitrate ammonium NO3- and NH4+ actively taken up by transporters NO3- reduced, and transported NH4+ attached to a C skeleton (GS) amino acids formed used in protein synthesis N assimilation - leaf { { NO3- reduced NH4+ attached to a C skeleton N assimilation - leaf { { { { NO3- reduced NH4+ attached to a C skeleton amino acids formed used in protein synthesis Atmospheric NH3 deposition NH3 taken up dissolved in apoplast NH4+ formed NH4+ assimilated by glutamine synthetase(s) Atmospheric NH3 deposition NH3 taken up dissolved in apoplast NH4+ formed NH4+ assimilated by glutamine synthetase(s) M. Pérez-Soba, 1995 Atmospheric NH3 as N source NH3 proteins amino acids substomatal cavity mesophyll GOGAT GS NH4+ GS - glutamine synthetase – high affinity for NH4+ Atmospheric NH3 as N source underlying physiological processes series of experiments with Brassica oleracea (curly kale) exposed to a range of NH3 concentrations (0 - 8 µl l-1) for one week controlled environmental conditions in nutrient solution, with and without nitrate Atmospheric NH3 as N source underlying physiological processes { { contribution to the N requirement for growth? effect on root NO3+ uptake ? A. Castro, 2006 Atmospheric NH3 as N source underlying physiological processes Total nitrogen content [N] μmolg FW Increase in total N and amino acids up to the highest [NH3] d 1250 c -1 1000 b 750 b a 500 b a 250 a a a Plants grown with nitrate in the nutrient solution 0 0.0 -1 4.0 6.0 “Metabolic detoxification” – capacity of the GS/GOGAT system to assimilate NH4+ - d 100 c 75 b 50 25 8.0 Free amino acid content 125 μmol g FW 2.0 a is high over the whole range a a a a a a 0 0.0 2.0 4.0 -1 [NH3] μl l 6.0 8.0 Atmospheric NH3 as N source underlying physiological processes Increase in fresh weight / 7 days Total nitrogen content b d 1250 [N] μmolg FW 7.5 c c b 750 gFW -1 1000 b a 500 a 250 a a a 5.0 2.5 b b b b a a b a a 0.0 0 0.0 2.0 4.0 6.0 8.0 0.0 2.0 4.0 6.0 8.0 -1 [NH3] μl l -1 μmol g FW d 100 c 75 b 50 25 growth (increase in fresh weight) is reduced at 6 and 8 µl l-1 NH3 Free amino acid content 125 a a a a a a a 0 0.0 2.0 4.0 -1 [NH3] μl l 6.0 8.0 although “detoxification” is still possible, shoot and root growth are reduced! Atmospheric NH3 as N source contribution to N nutrition Control 4 μl l-1 NH3 + NO3- - NO3- + NO3- - NO3- Biomass S 1.19c 0.29a 1.17bc 1.03c Biomass R 0.24b 0.35cd 0.10a 0.35abcd DW % S 8.9a 17d 12bc 12a Total N S 618c 414a 917e 803d AA S 14b 9a 73d 64d Total N R 317c 202a 348c 339c AA R 11a 13a 24c 25c Experiments with B. oleracea, grown in nutrient solution, + and – NO3-, fumigated with 4 µl l-1 NH3 (Castro, 2006) S shoot; R root; biomass production in g; N and amino acids (AA) in µmol g-1 FW Plants can grow well with NH4+ as sole N source Atmospheric NH3 as N source contribution to N requirement for growth N requirement for growth can be defined as Nrequirement = (RGR) x Ncontent in which RGR is the relative growth rate g g-1 day-1 4 µl l-1 NH3 as sole N source can contribute considerably (70-80%) to the N requirement for growth in agreement with theoretical calculations (Stulen et al, 1998) and Raven (1998) Atmospheric NH3 as N source effect on root uptake Does fumigation with 4 µl l-1 NH3 reduce root NO3- uptake? reduced N compound Root NO3- uptake Atmospheric NH3 as N source effect on root uptake Net nitrate uptake rate -1 -1 (μmol g FW day ) Does fumigation with 4 µl l-1 NH3 reduce root NO3- uptake? 150 125 (a) c 100 75 b 50 a 25 75 60 c 45 b 30 a 15 Net sulphate uptake rate -1 -1 (μmol g FW day ) 15 N Nitrate uptake rate -1 -1 (μmol g FW day ) 0 (b) 30 b b 24 18 a 12 6 0 0 0 4 8 0 4 8 -1 [NH3] μl l [NH3] μl l-1 signal – specific AA Atmospheric NH3 as N source underlying physiological processes { { { { contribution to the N requirement √ effect on root uptake √ nutrient vs toxin? both mechanisms of NH4+ toxicity? Atmospheric NH3 proposed mechanisms of toxicity – evidence? 1. Insufficient “detoxification” capacity – no evidence 2. Shortage of sugars? Atmospheric NH3 shortage of sugars? Shoot soluble sugar -1 (μmol g FW) 6 (a) 5 b 4 3 a 2 a a a a 1 0 (b) Root soluble sugar -1 (μmol g FW) 2.0 1.5 abc 1.0 abc c c 0.5 b a 0.0 0 4 The impact of NH3 on soluble sugar content in shoot (a) and roots (b). + NO3- and – NO3treatments are given in dark and light-grey bars, respectively. 8 -1 [NH3] μl l 2. “Shortage of sugars? – no evidence Atmospheric NH3 proposed mechanisms of toxicity 1. Insufficient “detoxification” capacity no evidence 2. Lack of sugars for the assimilation of NH4+ no evidence 3. Cation imbalances – impaired root uptake ? 4. Ethylene response experiments in nutrient solution [NH3] 0 – 8 µl l-1 yearly average concentration: 6.6 µg m3 – 9.3 nl l-1 5.1 µg m3 – 8.0 nl l-1 peak concentrations: > 2 µl l-1 soil/root compartment Atmospheric NH3 – nutrient or toxin? nutrient detoxification inhibition of growth toxin gradual - depending on plant species The role of NH3 in plant functioning With the current knowledge on the impact of atmospheric NH3 on plant functioning it is hard to establish cause-effects relations. The physiological basis the phytotoxicity of atmospheric NH3 is still largely unknown. There is no clear-cut transition in the level of metabolism of the absorbed atmospheric NH3 and its phytotoxicity. The effect of atmospheric NH3 on plants might be strongly dependent on the soil nutrient status. The role of NH3 in plant functioning With the current knowledge on the impact of atmospheric NH3 on plant functioning it is hard to establish cause-effects relations. The physiological basis of the phytotoxicity of atmospheric NH3 is still largely unknown. There is no clear-cut transition in the level of metabolism of the absorbed atmospheric NH3 and its phytotoxicity. The effect of atmospheric NH3 on plants might be strongly dependent on the soil nutrient status. The role of NH3 in plant functioning With the current knowledge on the impact of atmospheric NH3 on plant functioning it is hard to establish cause-effects relations. The physiological basis of the phytotoxicity of atmospheric NH3 is still largely unknown. There is no clear-cut transition in the level of metabolism of the absorbed atmospheric NH3 and its phytotoxicity. The effect of atmospheric NH3 on plants might be strongly dependent on the soil nutrient status. The role of NH3 in plant functioning With the current knowledge on the impact of atmospheric NH3 on plant functioning it is hard to establish cause-effects relations. The physiological basis of the phytotoxicity of atmospheric NH3 is still largely unknown. There is no clear-cut transition in the level of metabolism of the absorbed atmospheric NH3 and its phytotoxicity. The effect of atmospheric NH3 on plants might be strongly dependent on the soil nutrient status.