title: Diurnal variation in uptake and xylem contents of inorganic and assimilated N under continuous and interrupted N supply to Phleum pratense and Festuca pratensis.
abstract: Compensation by dark-period uptake of NH(4)(+) and NO(3)(-) in the grasses Phleum pratense L. and Festuca pratensis Huds. following N deprivation during the preceding light period was investigated in flowing solution culture under an artificial 10/14 h light/dark cycle. N was supplied as either NO(3)(-), NH(4)(+) or NH(4)NO(3) at 20+/-5 mmol m(-3), available continuously or only during the dark period, for 5-10 d. Intermittent N supply did not affect total daily N uptake, growth rate or net partitioning of dry matter. Net uptake and influx of NO(3)(-) varied similarly throughout the diurnal cycle when NO(3)(-) was supplied continuously, with a marginal contribution by NO(3)(-) efflux. Influx was significantly higher and efflux slightly higher following interruption of NO(3)(-) supply during the light period. Nitrate accounted for 80% of N in xylem exudate except between hours 6-9 of the light period when the amino acid concentration increased 3-fold, primarily as glutamine. Diurnal variation in relative NO(3)(-) uptake exhibited five phases of constant acceleration/deceleration, described reasonably well assuming NO(3)(-) influx was subject to metabolic co-regulation by NO(3)(-) and amino acid levels in the cytoplasmic compartment of the roots. Accordingly, influx is determined by variation in root NO(3)(-) levels throughout the dark period and the first half of the light period, but is down-regulated by increased amino acid levels during the second half of the light period. The sharp light/dark transitions affect transpiration rate and hence xylem N flux which, in turn, affect NO(3)(-) levels in the cytoplasmic compartment of the roots and the rate of NO(3)(-) assimilation in the shoot.
authors.affiliation: Institute of Grassland and Environmental Research, Aberystwyth Research Centre, Aberystwyth SY23 3EB, UK. james.macduff@bbsrc.ac.uk
authors.forename: J H
authors.initials: JH
authors.lastname: Macduff
authors.forename: A K
authors.initials: AK
authors.lastname: Bakken
mesh_terms: D002940:Circadian Rhythm; D031751:Festuca / Q000378:metabolism*; D008027:Light; D009566:Nitrates / Q000378:metabolism; D009584:Nitrogen / Q000378:metabolism*; D031784:Phleum / Q000378:metabolism*; D000644:Quaternary Ammonium Compounds / Q000378:metabolism
chemical_list: D009566:Nitrates; D000644:Quaternary Ammonium Compounds; D009584:Nitrogen