title: Transpiration, potassium uptake and flow in tobacco as affected by nitrogen forms and nutrient levels.
abstract: OBJECTIVE
  Ammonium can result in toxicity symptoms in many plants when it is supplied as the sole source of N. In this work, influences of different nitrogen forms at two levels (2 and 15 mm N) on growth, water relations and uptake and flow of potassium were studied in plants of Nicotiana tabacum 'K 326'.
  
  
  METHODS
  Xylem sap from different leaves was collected from 106-d-old tobacco plants cultured in quartz sand by application of pressure to the root system. Whole-shoot transpiration for each of the treatments was measured on a daily basis by weight determination.
  
  
  RESULTS
  Total replacement of NO(3)(-)N by NH(4)(+)-N caused a substantial decrease in dry weight gain, even when plants grew under nutrient deficiency. Increasing nutrient concentration resulted in a greater net dry weight gain when nitrogen was supplied as NO(3)(-) or NH(4)NO(3), but resulted in little change when nitrogen was supplied as NH(4)(+). NH(4)(+)-N as the sole N-source also caused reduction in transpiration rate, changes in plant WUE (which depended on the nutrient levels) and a decrease in potassium uptake. However, the amount of xylem-transported potassium in the plants fed with NH(4)(+) was not reduced: it was 457 % or 596 % of the potassium currently taken up at low or high nutrient level, respectively, indicating a massive export from leaves and cycling of potassium in the phloem.
  
  
  CONCLUSIONS
  Ammonium reduces leaf stomatal conductance of tobacco plants. The flow and partitioning of potassium in tobacco plants can be changed, depending on the nitrogen forms and nutrient levels.
authors.affiliation: The Key Laboratory of Plant Nutrition, Ministry of Agriculture, Key Laboratory of Plant-Soil Interactions, Ministry of Education, China Agricultural University, Beijing 100094, China.
authors.forename: Y X
authors.initials: YX
authors.lastname: Lu
authors.forename: C J
authors.initials: CJ
authors.lastname: Li
authors.forename: F S
authors.initials: FS
authors.lastname: Zhang
mesh_terms: D001692:Biological Transport; D009566:Nitrates / Q000378:metabolism; D009584:Nitrogen / Q000378:metabolism*; D018526:Plant Transpiration / Q000502:physiology*; D011188:Potassium / Q000378:metabolism*; D000644:Quaternary Ammonium Compounds / Q000378:metabolism; D012639:Seeds / Q000502:physiology; D014026:Nicotiana / Q000502:physiology*
chemical_list: D009566:Nitrates; D000644:Quaternary Ammonium Compounds; D009584:Nitrogen; D011188:Potassium