380 S. Husted et al. Agricultural and Forest Meteorology 105 2000 371–383
Fig. 5. Diurnal time course of the relationship between the net NH
3
emission from oilseed rape and the stomatal NH
3
compensation points estimated on the basis of apoplastic NH
4 +
and H
+
for the canopy during the two campaigns: A 9–10 June 1995 and B 14–15 June 1995.
s
NH
3
compensation points at leaf temperature;
d
net NH
3
emission from the canopy. All NH
3
compensation points were calculated using Eqs. 1–6 and corrected for temperature effects by Eq. 7. No in-canopy temperature profiles were available for the
period 14–15 June and consequently all compensation points were adjusted using the canopy temperature profiles measured during the period from 9 to 10 June.
The relationship between NH
3
fluxes and stomatal NH
3
compensation points showed a linear correlation r = 0.887; slope significantly different from zero,
P 0.05, with an intercept of 0.34 nmol NH
3
mol
− 1
air. This analysis was only done on data from the pe- riod 9–10 June 1995, because NH
3
flux measurements were lacking in some parts of the campaign from 14
to 15 June 1995 Fig. 5B.
4. Discussion
The ionic strength was estimated at 31.4 mM in the leaf apoplast, which is at the upper limit of values from
similar investigations on Pisum sativum and Spinacea oleracea, where estimates ranged from 18 to 32 mM
Speer and Kaiser, 1991. The ratio of analysed pos- itive to negative charges was in excess to 1.0 in the
leaf apoplast solution. Speer and Kaiser 1991 and Ruan et al. 1996 also observed a similar difference
and suggested that the discrepancy was due to an un- derestimation of bicarbonate and carboxylates. Only
trace amounts of these compounds were found in the present experiment, although it should be noted that
amino acids were not analysed and that they may in- fluence the ratio between anions and cations.
Comparing the periods 9–10 June and 14–15 June, there was no significant differences in apoplastic
NH
4 +
and H
+
concentrations P 0.05, which indicates stable physiological conditions in the plant
canopy during the field campaign. This was further confirmed by results obtained in the long-term exper-
iment from 8 to 15 June 1995, where standard errors were 10 Fig. 1.
The canopy profiles of both bulk tissue and apoplas- tic NH
4 +
concentrations were similar with decreas- ing concentrations from bottom to top leaves. Mean
apoplastic NH
4 +
concentrations of 0.28 ± 0.04 mM
S. Husted et al. Agricultural and Forest Meteorology 105 2000 371–383 381
and mean apoplastic pH values of 6.04 ± 0.13 in the attached leaves of the canopy were of the same magni-
tude as found in a previous study by Husted and Schjo- erring 1996, where values of 0.3–0.6 mM NH
4 +
and pH values of 5.3–5.9 were observed in oilseed rape.
The ratio between bulk tissue and apoplastic NH
4 +
concentrations was on average 8.0±0.7, which is com- parable with ratios between 3 and 16 observed around
anthesis in the previous study by Husted and Schjo- erring 1996. For plants growing under controlled
environmental conditions in growth chambers, the ratio between bulk tissue and apoplastic NH
4 +
de- clined 3–8-fold with increasing plant nitrogen status
and external N availability Mattsson et al., 1998; Husted et al., 2000. Further investigations are re-
quired to evaluate the functional relationship between apoplastic NH
4 +
concentrations and leaf tissue NH
4 +
concentrations. The stomatal ammonia compensation point de-
creased sharply from the bottom to the top of the canopy Fig. 3, see also Nemitz et al., 2000, which
Nemitz et al. 2000 showed to be due to NH
3
lib- eration in decomposing fallen plant residues on the
soil surface. Denmead et al. 1976 and Schjoerring et al. 1993 likewise found a sharp decrease in NH
3
concentration with height above the soil surface in a mixed ryegrassclover canopy and barley canopies,
respectively. In contrast, Sutton et al. 1993 observed smaller NH
3
concentrations near the ground within the canopy 0.8 m of an intensive short rotation
grassland. What is of interest for the oilseed rape here is that the near surface concentrations were so large,
often exceeding 10 nmol NH
3
mol
− 1
air. Comparison of calculated stomatal NH
3
compen- sation points based on apoplastic NH
4 +
and H
+
concentrations with vertical profiles of atmospheric NH
3
mole fractions in the canopy showed that fo- liar stomatal compensation points were smaller than
within-canopy air concentrations. In contrast, air concentrations in equilibrium with the decomposing
plant residues were an order of magnitude larger. The larger NH
3
air concentrations measured imme- diately above the litter reflected this, indicating that
the plant residues were the main source of NH
3
within the canopy. In contrast, attached leaves acted as sinks for atmospheric NH
3
. However, it should be noted that these data are mean diurnal values and
it is possible that temporary fluctuations may occur in which the sourcesink pattern may change due to
short-term temperature fluctuations. Using the inverse Lagrangian technique, Nemitz et al. 2000 confirmed
that plant residues on the soil surface were the major source of NH
3
in the canopy air. However, it was further suggested that NH
3
originating from the soil surface was recaptured within the lowest 0.7 m and
that the observed net NH
3
emission from the canopy was due to a source at the canopy top. This points to
the siliques as being a possible source. The apoplas- tic extraction technique has not yet been applied to
B. napus siliques and the importance of this fraction cannot be determined at present. However, bulk tissue
NH
4 +
in this fraction may be 3–5 times higher than in attached leaf fractions around anthesis Husted,
1997. Thus, it is not unlikely that siliques can act as an important source of atmospheric NH
3
. No diurnal trend in stomatal NH
3
compensation points calculated at 25
◦
C on the basis of the ratio of apoplastic NH
4 +
to H
+
concentrations, was ob- served for neither intact leaves nor plant residues on
the soil surface. This indicates that the physiological conditions were similar during the two diurnal pe-
riods 9–10 and 14–15 June 1995 and that no sig- nificant accumulationdepletion of NH
3
took place in the apoplast when the NH
3
flux changed. Recent in- vestigations using the stable isotope
15
N have shown that the apoplastic solution in B. napus constitutes
a highly dynamic NH
4 +
pool Nielsen and Schjoer- ring, 1998. Ammonium is constantly added to this
pool via NH
3
NH
4 +
efflux from the mesophyll cells, and is constantly retrieved by a system involving a
transporter with channel-like properties which is able to respond very rapidly 1
1 2
min to perturbations in apoplastic NH
4 +
concentration, thereby maintain- ing apoplastic NH
4 +
homeostasis. Investigations into
15
NH
4 +
uptake into B. napus leaf protoplasts J.N. Pearson, J. Finnemann and J.K. Schjoerring, unpub-
lished results have revealed the presence of an addi- tional high-affinity NH
4 +
uptake system similar in ki- netics to the AMT1 transporter observed in Arabidop-
sis, tomato and rice Ninnemann et al., 1994. When stomatal NH
3
compensation points were adjusted to the air temperature in each of the four
sampling heights and weighted stomatal NH
3
com- pensation points for the whole canopy were calculated
by correcting for the relative leaf areas in each height, a clear diurnal variation in compensation points was
382 S. Husted et al. Agricultural and Forest Meteorology 105 2000 371–383
observed, which agreed with the diurnal trend for the NH
3
flux Fig. 5A and B. Regression analysis re- vealed that a close linear relationship existed between
NH
3
flux and stomatal NH
3
compensation points adjusted for temperature effects R = 0.887. This
demonstrates the importance of canopy temperature as a major determinant of the compensation point
and net exchange fluxes of NH
3
. Since there was no diurnal variation in the ratio of [NH
4 +
] to [H
+
], it is clear that the temperature effect dominates the diurnal
variation in the stomatal compensation point. This is an important finding for developing models of NH
3
exchange, since the temperature response Eq. 7 is well known and easy to implement.
The correlation in Fig. 5 might be considered for- tuitous, since it has been noted above that the rape
leaves were generally a sink for rather than a source of ammonia. However, while emissions from the decom-
posing rape leaves might be expected to have a com- plex interaction with temperature and wetness gov-
erning mineralization and evaporation as Nemitz et al. 2000 have shown, most of these emissions would be
recaptured by the live oilseed rape leaves. In contrast, where siliques provide the main source of net emission
above the canopy, these would be subject to the same temperature dependence of solubility and dissociation
as the leaves. The close relationship between the net flux and the stomatal compensation point would there-
fore suggest that diurnal variability in [NH
4 +
][H
+
] is also unimportant for siliques.
5. Concluding remarks
