S. Husted et al. Agricultural and Forest Meteorology 105 2000 371–383 375
Henry’s law constant K
H
of 10
− 1.76
is not affected by the ionic strength. All equilibrium constants listed
above are given at 25
◦
C 298.13 K and at a pressure of 1 atm and valid only at I = 0 Wagman, 1968.
The temperature dependence of the stomatal NH
3
compensation point can be described by the Clausius–Clapeyron equation Husted and Schjoer-
ring, 1996:
ln χ
s2
χ
s1
= 1H
dis
+ 1H
vap
R 1
T
1
− 1
T
2
7 where χ
s1
is the actual stomatal NH
3
compensation point at the temperature T
1
and χ
s2
is the requested NH
3
compensation point at a new temperature T
2
. R denotes the gas constant 8.31 J K
− 1
mol
− 1
and the enthalpies of dissociation 1H
dis
and vaporisation 1H
vap
are 52.21 and 34.18 kJ mol
− 1
, respectively. The stomatal NH
3
compensation point for the whole plant canopy was calculated by weighting data from
the leaf fractions in 0.50, 0.75 and 1.25 m height ac- cording to the leaf area for each level. Moreover,
stomatal NH
3
compensation points were corrected to the mean diurnal temperature Eq. 7 for each plant
height and compared with the net NH
3
flux obtained by micrometeorological techniques see below.
2.6. Determination of NH
3
flux and NH
3
concentration profiles in canopy air The NH
3
concentrations at different heights above the canopy were continuously measured with two dif-
ferent techniques viz. replicated continuous wet de- nuders of the type described by Wyers et al. 1993
and filter packs see Sutton et al., 2000b in order to obtain the net NH
3
flux. However, during the last di- urnal campaign 14–15 June 1995 denuders and fil-
ter packs were placed at three different heights above the soil surface in the canopy h = 1.38 m, whereas
two additional denuders were used to measure the net NH
3
flux above the canopy. 2.7. Statistical analysis
The ratio between apoplastic concentrations of NH
4 +
and H
+
was used to calculate the stomatal compensation point for NH
3
see Eq. 3. The com- bined standard error for this ratio see Figs. 1 and 2
was derived from the following equation:
S [NH
4 +
] H
+
= s
1 H
+ 2
S
2 [NH
4 +
]
+ −[NH
4 +
] H
+ 2
S
2 [H
+
]
8
3. Results
3.1. Ion composition in the apoplast An ionic strength of 31.4 mM was obtained for the
apoplast solution in leaves at 0.75 m height above soil surface Table 2. This value changes pK
d
for the dis- sociation of NH
4 +
from 9.25 to 9.32. The estimated amount of cation equivalents exceeded anion equiva-
lents by 31. 3.2. Apoplastic NH
4 +
and H
+
concentrations throughout the campaign
Values for both apoplastic NH
4 +
and H
+
concen- trations were relatively constant in the long-term mea-
surements running from 8 to 15 June 1995 Fig. 1. The apoplastic NH
4 +
concentration in leaves at 0.50, 0.75 and 1.25 m above soil surface was 0.41 ± 0.03,
Table 2 Ion concentrations in the leaf apoplast of oilseed rape. Concen-
trations are given in mM and the total ion concentration in meq l
− 1
. Values are means of quadruplicate measurements ± S.E. Cations
Ca
2+
7.29 ± 1.53 Mg
2+
3.01 ± 0.85 K
+
0.76 ± 0.02 Na
+
0.78 ± 0.02 NH
4 +
0.25 ± 0.02 Amines
Trace Total equivalents
22.4 meq l
− 1
Anions NO
2 −
nd
a
NO
3 −
nd H
2
PO
4 −
1.34 ± 0.32 SO
4 2−
1.44 ± 0.06 Cl
−
12.70 ± 0.15 Malate and HCO
3 −
Trace Total equivalents
17.1 meq l
− 1
a
Not detected.
376 S. Husted et al. Agricultural and Forest Meteorology 105 2000 371–383
Fig. 1. A Daily means of apoplastic NH
4 +
concentrations, B apoplastic pH values and C ratio of NH
4 +
H
+
measured at three heights above the ground
d
0.50 m,
j
0.75 m,
m
1.25 m in the period 8–15 June 1995. Error bars are standard errors of 16 values four replicates at four locations.
0.26 ± 0.03 and 0.38 ± 0.06 mM, respectively, and apoplastic pH values at the corresponding heights were
5.66 ± 0.05, 6.15 ± 0.04 and 6.05 ± 0.04 means ± S.E. The lowest apoplastic pH was always measured
at 0.50 m, while the other leaf heights were very sim- ilar Fig. 1. The trends for apoplastic NH
4 +
were less clear, but concentrations in leaves at 0.75 m were
significantly smaller than those at the other two leaf heights P 0.05. There was a large variability in
the ratio of [NH
4 +
][H
+
] throughout the experiment, particularly for the 0.75 and 1.25 m sampling heights.
The relative standard errors RSE of the ratio be- tween NH
4 +
and H
+
concentrations were calculated on the basis of Eq. 8 and ranged from 7 to 86.
The errors associated with the f
dil
, used to correct the apoplastic NH
4 +
and H
+
concentrations, only con-
S. Husted et al. Agricultural and Forest Meteorology 105 2000 371–383 377
Fig. 2. Canopy profiles of A bulk tissue NH
4 +
concentrations, B apoplastic NH
4 +
concentrations, C apoplastic H
+
concentrations and D [NH
4 +
][H
+
] ratios in oilseed rape apoplastic solution during the two diurnal campaign periods 9–10 and 14–15 June 1995. Plant material was sampled from 3 to 4 different plant heights every 2–4 h n = 4.
378 S. Husted et al. Agricultural and Forest Meteorology 105 2000 371–383
tributed marginally 10 RSE to the overall error calculated by Eq. 8. Despite the considerable error
associated with the estimates of the NH
4 +
H
+
ratio, there was some indication of a small increase 30
with time in the mean NH
4 +
H
+
ratio. 3.3. Canopy profiles of NH
4 +
and H
+
concentrations Bulk tissue NH
4 +
concentrations were measured at four different leaf positions and apoplastic NH
4 +
and H
+
concentrations were measured at three different leaf positions every 2–4 h in two diurnal campaign pe-
riods 9–10 and 14–15 June 1995 Fig. 2A–C. For both periods the largest bulk tissue NH
4 +
concentra- tions were found in the dropped leaves on the soil
surface 17.2 ± 8.3 and 23.0 ± 2.3 mM, S.E.. Bulk tissue NH
4 +
concentrations in the same campaign pe- riods did not deviate significantly among the attached
leaves ranging from 2.3 ± 0.5 to 3.3 ± 0.7 mM. In both diurnal campaign periods, a general de-
crease in apoplastic NH
4 +
concentrations was ob- served from the bottom to the top of the canopy
Fig. 2B. Because no uncontaminated apoplastic fluid could be extracted from senescent leaves on
the soil surface, as revealed by high MDH activities data not shown, pH in a 1:1 suspension of tissue
Fig. 3. Comparison of stomatal NH
3
compensation points determined on the basis of apoplastic NH
4 +
and H
+
concentrations horizontal bars and measured atmospheric NH
3
concentrations
d
at different heights above the ground within the oilseed rape canopy. Similarly, the mole fractions of NH
3
in senescent leaves on the soil surface was calculated on the basis of pH and NH
4 +
in leaf extracts. The apoplast and bulk tissue NH
4 +
concentrations were analysed every 2–4 h during two diurnal periods 9–10 and 14–15 June 1995. Atmospheric NH
3
concentrations in five different heights were determined every 10 min on 14–15 June 1995. All NH
3
compensation points were adjusted to the actual air temperature at the various heights in the canopy by Eq. 7.
and sorbitol suspension was measured. The pH in leaf extract suspensions varied from 5.1 to 5.4 data not
shown, whereas apoplastic pH values increased from 5.4 in the lowest leaf position to 6.3 in top leaves
Fig. 2C.
The ratio between NH
4 +
and H
+
Fig. 2D, which is linearly related to the gaseous NH
3
mole fraction above the apoplast the stomatal NH
3
compensation point, Eq. 3, followed the same pattern as observed
for apoplastic pH, i.e. the highest values occurred in the mid-leaf fraction Fig. 2C. In lower attached
leaves, a ratio of 136–190 was found, increasing to 400–503 in the mid-position leaves.
3.4. Ammonia compensation points and canopy profiles of atmospheric NH
3
Stomatal compensation points for NH
3
at 25
◦
C were calculated for all leaf positions Eq. 6 and
corrected for temperature effects Eq. 7 using the diurnal mean leaf temperatures of 13.9, 14.4, 14.8
and 14.9
◦
C at 0.04, 0.50, 0.75 and 1.25 m plant height Fig. 3. The calculated mole fraction of gaseous
NH
3
in dropped leaves was in general much higher than the stomatal compensation points for NH
3
in attached leaves. Among the latter, leaves at 0.5 m
S. Husted et al. Agricultural and Forest Meteorology 105 2000 371–383 379
Fig. 4. Ratios of NH
4 +
and H
+
concentrations at three different heights above the ground
d
0.50 m;
j
0.75 m;
m
1.25 m for diurnal campaigns on A 9–10 June 1995 and B 14–15 June 1995. The relative standard errors of the ratios were on average 31 for both periods.
height had the significantly lowest P 0.05 NH
3
compensation point. The stomatal NH
3
compensation points were com- pared with canopy profiles of atmospheric NH
3
mea- sured at similar plant heights. The stomatal NH
3
com- pensation points only exceeded atmospheric NH
3
con- centrations in the dropped leaf fraction on the soil
surface, while NH
3
compensation points were signifi- cantly smaller in all the attached leaf positions Fig. 3.
3.5. Diurnal variations in NH
3
compensation points and NH
3
flux At all three leaf positions there was no distinct di-
urnal trend in the ratio between NH
4 +
and H
+
during both diurnal campaigns Fig. 4. For the second diur-
nal campaign, the ratio was larger on 14 June than on 15 June. However, values were largest in the afternoon
and evening, rather than showing variations between day and night.
The ratio between NH
4 +
and H
+
in the leaves at 0.50 m height was in both diurnal campaign periods
generally lower than that in the leaf fractions at 0.75 and 1.25 m height, respectively P 0.05. No signif-
icant difference was observed between leaves at 0.75 and 1.25 m Fig. 4.
The net NH
3
emission and stomatal NH
3
compen- sation points showed a clear diurnal trend in both di-
urnal campaign periods Fig. 5. Stomatal NH
3
com- pensation points peaked around mid-day 0.98 and
1.35 nmol NH
3
mol
− 1
air in the periods 9–10 June and 14–15 June 1995, respectively and gradually de-
creased during the afternoon. Nocturnal values sta- bilized below 0.5 nmol NH
3
mol
− 1
air 9–10 June 1995 or below 0.25 nmol NH
3
mol
− 1
air 14–15 June 1995.
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
