Agricultural and Forest Meteorology 105 2000 371–383
Stomatal compensation points for ammonia in oilseed rape plants under field conditions
Søren Husted
a,∗
, Jan K. Schjoerring
a
, Kent H. Nielsen
a
, Eiko Nemitz
b
, Mark A. Sutton
b
a
Plant Nutrition Laboratory, The Royal Veterinary and Agricultural University, Thorvaldsensvej 40, 1871 Frederiksberg C, Copenhagen, Denmark
b
Centre for Ecology and Hydrology, Edinburgh, Bush Estate, Penicuik, Midlothian EH26 OQB, UK Received 1 February 1999; received in revised form 19 May 2000; accepted 20 June 2000
Abstract
Compensation points for gaseous exchange of ammonia NH
3
between stomata and the atmosphere were determined in an oilseed rape Brassica napus canopy by analysing the concentrations of NH
4 +
and H
+
in leaf apoplastic solution. This bioassay approach was applied for the first time in the field, allowing the first intercomparison with compensation points
derived from micrometeorological measurements. Apoplastic NH
4 +
and H
+
concentrations differed between leaf heights but values were relatively stable over time, both diurnally and during a 2-week period. Stomatal NH
3
compensation points calculated on the basis of apoplastic NH
4 +
and H
+
concentrations and corrected for ambient leaf temperatures were found to correlate positively with the net NH
3
emission from the canopy estimated by micrometeorological measurements. As there was little diurnal variability in apoplastic concentrations,
this correlation was largely due to the effect of temperature on NH
3
solubility and NH
4 +
dissociation in the apoplast, together with similar effects of temperature on the net NH
3
flux. Very high NH
4 +
concentrations were also found in extracts of fallen litter and resulted in NH
3
partial pressures significantly exceeding NH
3
levels in the atmosphere close to the ground. By comparison of vertical atmospheric NH
3
concentration profiles in the plant canopy with the stomatal NH
3
compensation points determined here at three different plant heights, as well as NH
3
partial pressures in the litter, it is shown that plant residues on the soil surface would have been the primary NH
3
source while attached leaves acted as an NH
3
sink. Although it was not possible to measure apoplastic concentrations of siliques seed cases, bulk tissue NH
4 +
H
+
concentrations and vertical atmospheric NH
3
concentration profiles indicate that these may have acted as an NH
3
source. © 2000 Elsevier Science B.V. All rights reserved.
Keywords: Stomatal compensation points; Oilseed rape plants; Brassica napus; Apoplast; Ammonium
1. Introduction
Ammonia NH
3
is an important atmospheric pol- lutant with major impacts on atmospheric chemistry,
acidification of ecosystems and on the stability and
∗
Corresponding author. E-mail address: shukvl.dk S. Husted.
biodiversity of terrestrial oligotrophic biotopes Bob- bink et al., 1992. Quantification of the NH
3
flux be- tween vegetation and atmosphere is probably the most
uncertain factor in the compilation of a global NH
3
budget Dentener and Crutzen, 1994. Vegetation can act both as a source of or a sink for
atmospheric NH
3
. The direction of the NH
3
flux is controlled by the canopy compensation point for NH
3 0168-192300 – see front matter © 2000 Elsevier Science B.V. All rights reserved.
PII: S 0 1 6 8 - 1 9 2 3 0 0 0 0 2 0 4 - 5
372 S. Husted et al. Agricultural and Forest Meteorology 105 2000 371–383
Sutton et al., 1995, which is a composite parame- ter expressing the emission and deposition potential
of NH
3
fromto the canopy as influenced by the dif- ferent pathways, i.e. stomatal exchange, NH
3
sorption on leaf surfaces and liberation from degrading litter
material. Ammonia emission from plant leaves oc- curs if the mole fraction of NH
3
in the atmosphere is smaller than the mole fraction of gaseous NH
3
above the leaf apoplastic water phase, while in the oppo-
site case NH
3
absorption takes place Farquhar et al., 1980; Husted and Schjoerring, 1996. Plants grow-
ing in low N input ecosystems like moorlands and forests generally appear to have NH
3
compensation points close to zero and tend to absorb NH
3
from the atmosphere Sutton et al., 1992; Langford and Fehsen-
feld, 1992; Kesselmeier et al., 1993. However, agri- cultural crop species often have compensation points
from 1 to 6 nmol NH
3
mol
− 1
air 0.7–4.2 mg m
− 3
under field conditions Dabney and Bouldin, 1990; Schjoerring et al., 1993; Sutton et al., 1995; Yamulki
et al., 1996 and significant losses of NH
3
may occur. The NH
3
flux is linked to plant ontogeny and N supply Husted et al., 1996 and substantial NH
3
losses have been observed in several agricultural crops, especially
following flowering, where nitrogen compounds are remobilised from source organs leaves and stems to
seed capsules and seeds Schjoerring, 1997.
Controlled environment studies have clearly shown that NH
3
emission from plants are controlled by the stomatal NH
3
compensation point Husted and Schjoerring, 1996, but there is very little information
available on how stomatal NH
3
compensation points influence NH
3
fluxes from plants under field condi- tions. Thus, the aim of the present study was to apply a
recently developed vacuum infiltration technique that enables analysis of the apoplastic H
+
and NH
4 +
con- centrations with very little cytoplasmic contamination
2 Husted and Schjoerring, 1995. On the basis of apoplastic H
+
and NH
4 +
concentrations measured at different leaf heights above the ground, the stom-
atal NH
3
compensation points were calculated and compared with micrometeorological NH
3
flux mea- surements and within-canopy NH
3
concentrations. The field experiments were made in an oilseed
rape B. napus field in central Scotland, UK, dur- ing June 1995 as part of the EU EXAMINE project
North Berwick Experiment Sutton et al., 2000a. The measurements here represent the first occasion
that micrometeorological data on NH
3
exchange have been directly related to stomatal NH
3
compensation points calculated on the basis of apoplastic H
+
and NH
4 +
concentrations.
2. Materials and methods
