Journal of Experimental Botany, Vol. 54, No. 381, pp. 431-444,
January 2, 2003
© 2003 Oxford University Press
Diurnal variation in uptake and xylem contents of inorganic and assimilated N under continuous and interrupted N supply to Phleum pratense and Festuca pratensis
Received 8 May 2002; Accepted 19 September 2002
1 Institute of Grassland and Environmental Research, Aberystwyth Research Centre, Aberystwyth SY23 3EB, UK
2 Kvithamar Research Centre, N-7500 Stjørdal, Norway
3 To whom correspondence should be addressed. Fax: +44 (0)1970 828357. E-mail: james.macduff{at}bbsrc.ac.uk
Compensation by dark-period uptake of NH4+ and NO3– 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 NO3–, NH4+ or NH4NO3 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 NO3– varied similarly throughout the diurnal cycle when NO3– was supplied continuously, with a marginal contribution by NO3– efflux. Influx was significantly higher and efflux slightly higher following interruption of NO3– 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 NO3– uptake exhibited five phases of constant acceleration/deceleration, described reasonably well assuming NO3– influx was subject to metabolic co-regulation by NO3– and amino acid levels in the cytoplasmic compartment of the roots. Accordingly, influx is determined by variation in root NO3– 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 NO3– levels in the cytoplasmic compartment of the roots and the rate of NO3– assimilation in the shoot.
Key words: Amino acids, ammonium, diurnal variation, efflux, Gramineae, influx, ion uptake, nitrate, potassium, xylem composition.
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