Plant Physiol, January 2001, Vol. 125, pp. 476-487
Comparison of Gas Exchange and Bioassay Determinations of the
Ammonia Compensation Point in Luzula sylvatica (Huds.)
Gaud.1
Paul W.
Hill,2*
John A.
Raven,
Benjamin
Loubet,
David
Fowler, and
Mark A.
Sutton
Department of Biological Sciences, University of Dundee,
Dundee, Scotland DD1 4HN, United Kingdom (P.W.H., J.A.R.); Centre for
Ecology and Hydrology, Edinburgh Research Station, Bush Estate,
Penicuik, Midlothian, Scotland EH26 0QB, United Kingdom (P.W.H., D.F.,
M.A.S.); and Institut National de la Recherche Agronomique, 147, rue de
l'Université, 75 338 Paris, cedex 07, France (B.L.)
Determinations of the NH3 compensation point
for the understory plant of semi-natural woodlands Luzula
sylvatica (Huds.) Gaud. were carried out by measurements of gas
exchange and by calculation from the NH4+
concentration and pH of extracts of the foliar apoplast. Compensation points determined by gas exchange measurements were among the lowest
yet reported (0.51-1.10 µg NH3 m
3) and
those calculated from apoplast extracts were lower than any yet
reported (0.017-0.54 µg NH3 m3). Those
determined by gas exchange were consistently found to be between 2 and
30 times higher than those determined from apoplast extracts.
Consideration of possible causes of this discrepancy, which is not
confined to this investigation, showed that all likely errors would
result in an increase in the discrepancy, or were insufficient to
account for observed differences. It is suggested that spatial
variability of pH and NH4+ concentration within
the foliar apoplast represents the most promising line for further
investigation. It is also shown that the foliar apoplast of L.
sylvatica is sufficiently buffered to eliminate the need for
correction of H+ concentration for dilution during
extraction, but that it is necessary to correct the
NH4+ concentration of apoplast extracts for dilution.
1
This work was supported by a Cooperative Awards
in Sciences of the Environment Studentship (no. GT4/95/64/T) from the
Natural Environment Research Council and funds from the UK Department of Environment Transport and Regions (UMBRELLA project).
2
Present address: Institute of Environmental
Science, University of Wales, Bangon, Robinson Building, Deiniol Road,
Bangon, Gwynedd LL57 2UW, UK.
*
Corresponding author; e-mail pwhill{at}ceh.ac.uk; fax
44-01314453943.
© 2001 American Society of Plant Physiologists
