Tree Physiology 21, 931–940
© 2001 Heron Publishing—Victoria, Canada
Atmospheric carbon dioxide concentration, nitrogen availability,
temperature and the photosynthetic capacity of current-year Norway
spruce shoots
PETER ROBERNTZ
Department for Production Ecology, Swedish University of Agricultural Sciences, P.O. Box 7042, SE-750 07 Uppsala, Sweden
Received August 18, 2000
Summary Branches of field-grown Norway spruce (Picea
abies (L.) Karst.) trees were exposed to either long-term ambient or to elevated CO2 concentrations ([CO2]) using the branch
bag technique. The light-saturated photosynthetic rates (Amax)
of current-year shoots differing in nitrogen (N) status were
measured at various temperatures and at either ambient
(360 µmol mol –1, AMB) or elevated (ambient + 350 µmol
mol –1, EL) [CO2]. The value of Amax was determined at various
intercellular [CO2]s (A/Ci curves) and used to normalize photosynthetic rates to the mean treatment Ci values, which were
200 µmol mol –1 (AMB) and 450 µmol mol –1 (EL), respectively. Needle N status and temperature strongly affected Amax.
The response to N increased with temperature, and the photosynthetic temperature optimum increased with N status. This
was assumed to be a result of reduced mesophyll CO2 conductance. The relative increase of Amax in the EL treatment compared to the AMB treatment varied from 15 to 90%, and increased with temperature, but decreased with N status.
Nevertheless, the absolute photosynthetic response to EL increased with shoot N status. The relative increase in the instantaneous response of Amax to elevated [CO2] was about 20%
higher than the long-term response, i.e., there was downward
acclimation in Amax in response to elevated [CO2]. The photosynthetic temperature optimum increased 4 °C with either a
short- or a long-term increase in [CO2]. The bag treatment itself
increased Amax by approximately 16% and the temperature optimum of Amax by approximately 3 °C.
Keywords: Amax, climate change, Picea abies, temperature optimum.
Introduction
To estimate future effects of elevated CO2 concentration
([CO2]) on trees, results from laboratory experiments with
seedlings must be verified by long-term field trials (Eamus and
Jarvis 1989, Mousseau and Saugier 1992, Körner 1995, Saxe
et al. 1998). In the field, physiological processes such as photosynthesis show great variability, primarily because of variability in climatic conditions, but also because of variation in
soil water and nutrient availability.
Temperature affects respiration rate, electron transport and
carboxylation/oxygenation by ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco), which in turn determine net
photosynthetic rate (Farquhar et al. 1980, Farquhar and von
Caemmerer 1982). When the internal [CO2] is low and irradiance is saturating, photosynthesis is primarily limited by
Rubisco activity (Farquhar et al. 1980). As temperature rises,
the solubility of CO2 relative to O2 declines, as does the specificity of Rubisco (Brooks and Farquhar 1985, Long 1991).
Subsequently, O2 competition for the primary Calvin cycle acceptor, ribulose-1,5-bisphosphate (RuBP), increases and lowers photosynthetic light-use efficiency. Simultaneously, daytime respiration has a large negative effect on net carbon
accumulation (A). In contrast, a rise in [CO2] promotes carboxylation, which not only causes the relative enhancement of
photosynthesis at elevated [CO2] to increase with temperature,
but may also increase both the photosynthetic temperature optimum (Farquhar et al. 1980, Idso and Idso 1994, Eamus et al.
1995) and the maximum temperature for net photosynthesis
(Idso et al. 1995). Modeling exercises (Long 1991, McMurtrie
and Wang 1993) have shown that when atmospheric [CO2] is
increased by 300–350 µmol mol –1, canopy photosynthesis increases by 10–110%, depending on temperature. The models
also showed an increase of 5–10 °C in photosynthetic temperature optimum with increasing [CO2]. Consistent with this
finding, plants in a warm climate are reported to be more responsive to elevated [CO2] than plants in a cold climate (Idso
and Idso 1994, Drake et al. 1997).
Photosynthesis is generally reported to be less responsive to
elevated [CO2] under nutrient-limited conditions (Cure and
Acock 1986, Pettersson and McDonald 1994, Sage 1994,
Curtis 1996, Drake et al. 1997). In contrast, Idso and Idso
(1994) argued that the relative enhancement of photosynthesis
and growth at elevated [CO2] may be greatest when there is a
shortage of resources such as mineral nutrients, light or water.
Lloyd and Farquhar (1996) found “little theoretical justification or experimental evidence” that photosynthesis in nutrientlimited plants responds less to elevated [CO2] than plants
grown at high nutrient availability. This difference in opinion
is surprising, considering the number of published studies on
932
ROBERNTZ
plant growth responses to [CO2]. However, the uncertainty
may indicate variation in response among species.
The nutrient most frequently found to correlate with photosynthetic capacity is nitrogen (N), reflecting the large proportion of leaf N incorporated in thylakoid proteins and Calvin
cycle enzymes (Evans 1989). For instance, 10–30% of leaf N
is bound in Rubisco (Evans 1989). Nitrogen may affect leaf
structure and internal CO2 diffusivity (von Caemmerer and
Evans 1991, Pääkkönen and Holopainen 1995). If N increases
mesophyll conductance, it may lower O2 sensitivity (von
Caemmerer and Evans 1991), and alter the photosynthetic response to variation in temperature and [CO2].
The goal of this study was to evaluate the effect of temperature and leaf N status on photosynthetic responses to increases
in instantaneous and long-term [CO2] of Norway spruce
(Picea abies (L.) Karst.).
Materials and methods
Experimental site
The study was performed during a long-term, nutrient optimization experiment at Flakaliden (64°07′ N, 19°27′ E, 310 m
a.s.l.) in northern Sweden (Linder and Flower-Ellis 1992). The
experiment was established in 1986 in a young Norway spruce
(Picea abies) stand, planted in 1963 after clearcutting, followed by prescribed burning and scarification. The treatments,
which began in 1987, included untreated control plots, irrigated plots, and two nutrient optimization treatments. In the
present study, only control (C) and irrigated-fertilized (IL)
plots were included. For further details about these treatments,
see Linder (1995).
The climate at the site is characterized by long cold winters
and cool summers. Monthly mean air temperatures vary from
–9 °C in February to 14 °C in July. Snow cover is often established in October and usually remains until mid-May. Mean
annual precipitation is 600 mm, and soil water does not normally limit tree growth (cf. Bergh et al. 1999). The weather
station located at the site recorded hourly maximum, minimum and mean temperatures.
Branch bags and CO2 control system
Long-term exposure of branches to elevated [CO2] was
achieved by means of branch bags (BB) and a control system
for raising atmospheric [CO2] similar to that described by
Barton et al. (1993). In July 1992, six BBs were installed on a
control plot (C) and six on a plot with optimized water and nutrient supply (IL). One year later (1993), the number of BBs
was increased to 12 per treatment. The BBs set up in 1993
were run in a short-term trial, then moved up one branch whorl
in April 1994 until the termination of the experiment in October 1995. Pairs of trees were randomly selected among
codominants of similar appearance, and BBs were installed on
the fourth branch whorl from the top. Because of the small size
of the trees, only one BB was mounted per tree. A non-bagged
control branch was selected for each BB branch in the same
whorl and tree. For each pair of trees, one BB was flushed with
ambient air (seasonal mean 360 µmol CO2 mol –1, AMB) and
one with ambient air plus 337 ± 30 µmol CO2 mol –1 for 93% of
the duration of the experiment (EL) (see Roberntz 1998). The
BBs were removed from mid-November until mid-April each
year.
In 1992, the stands had mean stem diameters (at breast
height) of 5.3 (C) and 7.2 cm (IL), mean height was 3.9 (C)
and 5.0 m (IL), and leaf area index (LAI, projected) was 1.6
(C) and 2.6 (IL), respectively. When the branch bag experiment was discontinued in October 1995, the stem diameters
were 6.6 (C) and 10.5 cm (IL), heights were 4.7 (C) and 6.7 m
(IL), and LAI values were 2.4 (C) and 5.5 (IL), respectively.
The BBs consisted of a wire frame covered with PVC plastic film (cf. Barton et al. 1993). The chambers initially had volumes of 85 (C) and 360 dm 3 (IL) but were increased as the
branches increased in length. The air flow through the chambers was 7–9 air changes per minute, resulting in a wind speed
of 0.2–0.3 m s –1.
Air temperature in the bags was measured with copper-constantan thermocouples placed at the open end of the BBs.
Thermocouples were protected by ventilated radiation shields
and connected to a data logger (Campbell CR10, Campbell
Scientific Inc., Logan, UT), and hourly means were stored.
The seasonal 24-h mean temperature in the BBs was 1.3 (C)
and 1.1 °C (IL) and the daytime mean (0600–1800 h) was
1.9 (C) and 1.6 °C (IL) higher than ambient air temperature.
Occasionally the temperature in the BBs exceeded ambient
temperatures by as much as 6 °C. The variation in temperature
between ambient and elevated BBs was within 0.2 °C. A more
complete description of the details and performance of the
branch bag system may be found in Roberntz (1998).
Gas exchange measurements
One current-year shoot was selected on the BB and control
branches of all studied trees. To seal the gas exchange cuvette
around the shoots, a 3–5 mm band of needles was removed
14 days before measurements started. Measurements were
made with a Li-Cor 6200 portable infrared gas analyzer
(Li-Cor, Inc., Lincoln, NE) with illumination provided by a
100 W artificial light source (> 1000 µmol m –2 s –1) (Figure 1).
Relative humidity was maintained close to 50% for all measurements. The leaf-to-air vapor pressure difference in the
cuvette was usually less than 1 kPa. Needle temperature, measured beneath the shoot, averaged about 0.5 °C above cuvette
air temperature. Terostat plastic putty (Teroson GmbH, Heidelberg, Germany) sealed the cuvette around the shoot axis.
The photosynthetic response to the calculated intercellular
[CO2] was measured at the beginning of September 1993,
1994 and 1995. By the end of August, current-year shoots
were similar in photosynthetic capacity to shoots of the previous year’s growth (Roberntz 1999). Each A/Ci curve measurement took 40–60 minutes to complete; the assimilation rate
was measured at 7–8 [CO2] values according to a procedure
described by McDermitt et al. (1989). No artificial cooling
was used, but the cuvette temperature during A/Ci curve measurements varied by only ± 0.6 °C. Tests were performed in
TREE PHYSIOLOGY VOLUME 21, 2001
CLIMATIC EFFECTS ON PHOTOSYNTHESIS IN NORWAY SPRUCE
Figure 1. The relative increase of photosynthesis to PPFD at a CO2
concentration of 350 µmol CO2 mol –1. One hundred percent light saturation of photosynthesis was assumed to occur at 2000 µmol photons
m –2 s –1. Circles indicate measurements with standard error of the
mean (n = 8). Measurements in this study were performed to the right
of the arrow.
1995 to estimate the error that resulted when the A/Ci measurements were not corrected for leaks. The error consisted of
overestimates of about 10% when the initial slope of the A/Ci
curve was 0.05 mol m –2 s –1, and about 0.5% when the initial
slope was 0.12 mol m –2 s –1. Measurements in 1993 and 1994
could not be corrected, as no leak tests were made. Only uncorrected data were used in this analysis. However, the [CO2]
gradient was low for the other photosynthetic variables presented in this paper, and the resultant overestimation of initial
slope is less than 2%.
After gas exchange measurements, the projected leaf area
was measured with a Li-Cor leaf-area meter (Model 3100), after which the needles were frozen in liquid nitrogen and stored
at –20 °C for later analysis.
933
where Ci is intercellular [CO2], A is rate of net photosynthesis
at a given Ci, θ is convexity, gm is initial slope or carboxylation
efficiency, Asat is CO2 saturated rate of net photosynthesis, and
R is the model y-intercept. The net photosynthetic rate at Ci
concentrations of 200 and 450 µmol mol –1 (A200i and A450i) was
calculated from the fitted convexity equation. These concentrations were the mean Ci values for the [CO2] treatments.
Photosynthetic capacity at the Ca concentrations of 350 and
700 µmol mol –1 was also measured (A350 and A700). To improve
the precision of the estimate of gm, measured points below a Ci
of 200 µmol mol –1 were analyzed by linear regression. This is
an estimate of Rubisco activity (von Caemmerer and Farquhar
1981, Evans 1989, Sage 1994).
Effects of the quantitative variables, N concentration and
needle temperature, and the qualitative variables, [CO2] (EL
and AMB) and bag (bagged and non-bagged) treatments were
evaluated by analysis of variance with the SAS statistical software package (SAS Institute Inc., Cary, NC). Type III error
was used, and variables were retained in the model when the
level of significance was P < 0.10. Because repeated measurements were made on the same branches in 1993, 1994 and
1995, we tested whether a branch effect existed or whether the
shoots could be treated as individual observations. We also
tested for an effect of the duration of exposure to the elevated
[CO2] or bag treatments. The number of shoots measured each
year, the duration of exposure to CO2, the duration of enclosure in bags and the range in needle temperature and N concentration are given in Table 1. Needle temperature (T) and N
content (N) were also tested for nonlinearity by introducing
second order functions, i.e., T 2 and N 2.
The analysis may be described in four steps:
Step 1: The quantitative variables needle temperature (T)
and N concentration (N) were tested. Variables in the model
were deleted step by step.
µ = α+ β1(N) + β2(T)+ β3(N 2)+ β4(T 2)+ β5(N × T) + ε.
Step 2: The qualitative variables [CO2] (CO2), bag (Bag),
branch (Branch) and exposure time (Time) were introduced.
As in Step 1, nonsignificant variables or interactions were removed.
µ = result (Step 1) + (Bag) + (CO2) + (Branch) +
Nitrogen analysis
Samples were dried at 85 °C for 48 h and ground. Nitrogen
analyses were carried out with an ANA 1500 automatic analyzer (Carlo Erba Strumentatzione, Milan, Italy). Nitrogen
concentration was expressed on the basis of projected needle
area.
Data analyses
The A/Ci curves were fitted to a non-rectangular hyperbola
(convexity equation, Leverenz 1987):
g C + Asat − (( gmCi + Asat ) 2 − 4 gmCi Asatθ)
A= m i
− R,
2θ
(Time) + (Bag × Time) + (CO2 × Time) + ε.
Step 3: The interaction between the quantitative and qualitative variables was introduced into the result from Step 2. As in
Steps 1 and 2, nonsignificant variables or interactions were removed stepwise.
µ = result (Step 2) + interaction between quantitative
and qualitative variables + ε.
Step 4: The quantitative variables deleted in Step 1 were
tested again for significance when reintroduced into the result
from Step 3.
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ROBERNTZ
Table 1. The number of shoots measured in each treatment and the duration of Bag and CO2 treatments. Also presented is the range of needle temperatures and shoot nitrogen concentrations. Abbreviations: EL refers to elevated [CO2]-treated shoots, AMB refers to bagged shoots ventilated
with ambient air and Control refers to non-bagged shoots.
Treated since
July 24, 1992
July 24, 1992
July 24, 1992
August 12, 1993
April 28, 1994
April 28, 1994
Measured since
September 1993
September 1994
September 1995
September 1993
September 1994
September 1995
Treatment length (days)
255
457
665
25
131
339
Results
Branch and exposure dependency
Neither the branch term nor the exposure term significantly interacted with CO2 or Bag treatments in the models. Hence, individual shoots were considered as independent measurements and shoots on branches exposed to the Bag and CO2
treatments for 25 or 665 days were considered to be equally affected by the treatments (see Table 1).
Nitrogen and needle temperature
The variation in A350, A700, A200i and A450i was best described by
a model including a linear relationship with N, a second-order
relationship with T and a strong interaction effect with N and T
(Table 2). There was no substantial difference in the results of
the model whether photosynthetic capacity was measured at a
common Ca (A350, A700) or a common Ci (A200i, A450i). Mean
stomatal conductance was 0.12 mol m –2 s –1, with no significant differences between Bag or CO2 treatments. Figure 2
shows the data and response surfaces for A200i and A450i. There
was little effect of N status on photosynthetic capacity at low
temperatures, whereas N had an increasing effect at higher
temperatures. An increase in temperature affected the slope of
A450i versus N more than it did the slope of A200i versus N.
Carboxylation efficiency (gm) was similarly affected by temperature and N (Table 2).
The statistical model derived showed an increase in temperature optimum for net photosynthesis at higher N availability.
From an N concentration of 0.2 to 0.45 mg cm –2, the temperature optimum increased linearly from 11.3 to 24.5 °C.
The bag effect
There was a significant Bag interaction with needle temperature (Table 2). Photosynthetic capacity was up to 16% higher
in shoots grown in bags (A200i, Table 3). The temperature optimum for bagged foliage was 3.9 °C higher on average for A200i,
and 2.7 °C higher for A350, compared with non-bagged foliage.
Short- and long-term [CO2] effects
Comparison of measurements of A700 with A350 (A700 /A350) or
A450i with A200i (A450i /A200i ) on the same shoot permitted esti-
Shoots in treatments
Range in needle
Bagged
Control
Temperature (°C)
N (mg cm –2)
12
12
12
12
8
11
9.9–15.5
14.4–22.2
16.4–23.6
8.3–17.4
16.4–18.5
11.7–24.8
0.27–0.49
0.20–0.36
0.27–0.46
0.26–0.51
0.25–0.43
0.32–0.49
EL
AMB
6
6
6
6
4
6
6
6
6
6
4
6
mation of the relative enhancement of net photosynthesis resulting from instantaneous exposure to elevated [CO2]. Both
quotients were strongly dependent on temperature, and on N
(Table 2, Figure 3a–b). An increase in T increased the relative
enhancement of net photosynthesis, whereas higher N availability reduced the relative enhancement of net photosynthesis. The ratio A700 /A350 ranged from 40 to 90%, whereas
A450i /A200i ranged from 35 to 110% (Figure 3b). The ratio
A450i /A200i in bagged shoots in the EL treatment was significantly higher (9%) than that in bagged shoots in the AMB
treatment (Tables 2 and 3).
The relative long-term enhancement of net photosynthesis
by elevated [CO2] (i.e., comparison of the model result of A700
or A450i for the EL shoots with the model result of A350 or A200i
for the AMB shoots) yielded a response surface similar to that
for the relative instantaneous response surface (Figure 3c), although the magnitude of the response was smaller. The EL
shoots had 20–75% higher photosynthetic rates for A700 compared with A350 for the AMB shoots, and 15–90% higher for
A450i compared with A200i in the AMB shoots. Hence, the
long-term [CO2] response was about 20% lower than the instantaneous [CO2] response (compare Figure 3c with 3b). The
negative effect of N on relative enhancement of photosynthesis was also apparent in the long-term [CO2] enhancement.
However, the absolute increase in net photosynthesis resulting
from elevated [CO2] was generally larger when N status was
higher (Figure 3d). The estimated absolute increase in net photosynthesis in EL relative to AMB ranged from 1.4 to
10.9 µmol m –2 s –1.
The net photosynthetic temperature optimum increased in
elevated [CO2]. The mean shift in photosynthetic temperature
optimum for A700 of EL shoots was 3.5 °C higher than that for
A350 in AMB shoots; whereas for A450i, the optimum was about
4.1 °C higher than that for A200i. The short-term response to elevated [CO2] showed a similar shift in temperature optimum.
When measured at the same Ca or Ci, the EL shoots had significantly lower photosynthetic capacity than AMB shoots
(Tables 2 and 3). The [CO2]-induced reduction was equal
across all shoot N concentrations and needle temperatures;
thus the intercept was significant and there was no interaction
TREE PHYSIOLOGY VOLUME 21, 2001
CLIMATIC EFFECTS ON PHOTOSYNTHESIS IN NORWAY SPRUCE
935
Table 2. The final model derived in the analyses of the different tested variables. Presented are the P values of the individual variables and the
model R 2 and number of samples. If the P value was > 0.1, a variable was considered nonsignificant (n.s.) and excluded from the model. Abbreviations: T denotes needle temperature and N denotes nitrogen concentration (mg cm –2). Note: the ratios A700 /A350 and A450i /A200i are the instantaneous response results.
Model input variables
N
T
N×T
T2
CO2
T 2 × Bag
T 2 × CO2
Model R 2
n
Variables tested
A350
A700
A700 /A350
A200i
A450i
A450i /A200i
gm
0.032
n.s.
< 0.001
< 0.001
0.002
0.001
n.s.
0.57
135
0.001
n.s.
< 0.001
< 0.001
0.004
< 0.001
n.s.
0.68
135
n.s.
< 0.001
< 0.001
0.070
n.s.
n.s.
n.s.
0.47
135
0.043
n.s.
< 0.001
< 0.001
< 0.001
< 0.001
n.s.
0.57
131
0.001
n.s.
< 0.001
< 0.001
0.007
< 0.001
n.s.
0.70
131
< 0.001
n.s.
n.s.
< 0.001
n.s.
n.s.
0.024
0.47
131
n.s.
0.080
< 0.001
0.019
0.002
0.001
n.s.
0.40
131
with T or N. The estimated reductions were 1.1 for A350, 1.5 for
A700 and 1.4 for both A200i and A450i (µmol m –2 s –1). The
carboxylation efficiency (gm) was reduced by about 0.011 mol
m –2 s –1. Subsequently, the relative reduction of all tested variables by EL was higher at low rates and lower at high rates.
The estimated relative reduction of A200i varied from 8–20%.
The significantly higher A450i /A200i (9%) of EL shoots compared to AMB shoots (Table 2 and 3) was thus a function of a
relatively larger reduction at A200i compared to A450i.
Discussion
The nitrogen and needle temperature effects
The model results showed a linear relationship between photosynthetic capacity and nitrogen status (N), a second-order relationship to needle temperature (T 2 ) and an interaction between N and T (N × T). This interaction may be seen as the
most controversial component of the models, because it
caused an increase in temperature optimum with increase in N
status. However, N × T was a fairly robust component in the
Figure 2. (a) The photosynthetic capacity at a Ci of 200 µmol m –2 s –1 and (b)
at a Ci of 450 µmol m –2 s –1. (c and d)
Response surface diagrams derived
from the statistical model results (Table 2). In these diagrams, the bag and
[CO2] effect have not been included
and thus remain a source of variation.
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936
ROBERNTZ
Table 3. Least square means of the different variables from the derived
models.
Variable
A350
A700
A200i
A450i
A700 /A350
A450i /A200i
gm
Units
µmol m –2 s –1
µmol m –2 s –1
µmol m –2 s –1
µmol m –2 s –1
%
%
mol m –2 s –1
Treatment
EL
AMB
Bag
Control
9.2
15.5
8.7
15.6
71
80
0.066
10.2
17.1
10.1
17.1
67
71
0.077
10.1
17.2
10.1
17.4
70
74
0.076
9.3
15.5
8.7
15.4
67
78
0.067
models (Table 2). For example, the omission of N × T, and the
introduction of N 2 and T, decreased R 2 for A350 from 0.57 to
0.52. Sheriff and Mattay (1995) constructed similar response
surfaces, based on over 500 measured shoots of Pinus radiata
D. Don seedlings. Their measurements were performed under
more controlled conditions than in our field study. In their
analyses, the temperature optimum was extremely broad, from
about 14 to 38 °C, and showed no shift with N status. Nevertheless, they found the slope of net photosynthesis versus N to
be temperature-dependent. Similarly, Kubiske et al. (1997) reported that the relationship between net photosynthesis and N
in Populus tremuloides Michx. became stronger as [CO2] was
increased. Recently, comparisons made across species give
supporting evidence for an increase in the slope of net photosynthesis versus N at elevated [CO2] (Peterson et al. 1999).
These observations agree with our results.
The results we present were obtained by combining measurements conducted in the same month in each of three different years. Thus, the foliage may have acclimated to differences
in climate in the previous year (Table 4). The temperature optimum for net photosynthesis of plants may differ if they are
grown under warmer or colder conditions (Seeman et al. 1984,
Veres and Williams 1984, Battaglia et al. 1996). This could
have been a source of variation that was not accounted for in
the models presented, and it may have caused artificial relationships such as N × T. There are, however, plausible explanations for a true N-induced shift in the temperature optimum of
net photosynthesis. Increased N availability not only correlates with the investment of proteins in the Calvin cycle and
thylakoids (Evans 1989), but also alters leaf anatomy: e.g., less
xeromorphic Pinus sylvestris L. needles (Jokela et al. 1995)
and larger mesophyll cells with longer chloroplasts (Palomäki
and Holopainen 1995), increases in the intercellular space of
Betula pendula Roth leaves (Pääkkönen and Holopainen
1995), and greater internal surface area of chloroplasts exposed to intercellular airspaces and increases in the internal
transfer conductance of CO2 from the substomatal cavity to
the carboxylation site (gi) in Triticum aestivum L. leaves (von
Caemmerer and Evans 1991). Photosynthetic capacity is correlated with gi (von Caemmerer and Evans 1991, Evans et al.
1994, Makino et al. 1994, Syvertsen et al. 1995). From this, it
Figure 3. (a) A three-dimensional plot
of the relative instantaneous [CO2] enhancement when comparing measurements of A450i and A200i on the same
shoot. (b) The surface diagram derived
from the statistical model results of
(a). (c) The relative long-term [CO2]
enhancement derived from the comparison of the model result of A450i of
the elevated [CO2]-treated shoots with
the result of A200i of the ambient
[CO2]-treated shoots. (d) The absolute
long-term CO2 enhancement derived
by subtracting the model result of
A200i of the ambient [CO2]-treated
shoots from the model result of A450i
of the elevated [CO2]-treated shoots.
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Table 4. Mean climatic conditions from 0600 to 1800 h, 2 weeks before measurements. Relative humidity (RH) and photon flux density
(PPFD) were measured at a climate station, i.e., not within the bags.
Year
Control temp.
(°C)
Bag temp.
(°C)
RH
(%)
PPFD
(µmol m –2 s –1)
1993
1994
1995
8.7
12.1
11.2
10.1
13.8
13.0
80
71
69
389
445
631
follows that leaves with low N status and low gi may also have
greater sensitivity to O2 (von Caemmerer and Evans 1991),
which would result in a lower temperature optimum. A biochemical factor that may also influence gi is the presence of
carbonic anhydrase, which increases the diffusion rate of carbon in the chloroplasts and perhaps in the plasmalemma
(Makino et al. 1992, Evans et al. 1994). Carbonic anhydrase
has been found to increase with N status in some C3 plants
(Makino et al. 1992). The existence of an effect of N on gi is an
intriguing hypothesis, which is not contradicted by the observations in this study.
The instantaneous and long-term [CO2] effect
The relative effect of elevated [CO2] on photosynthetic capacity (Figure 3) was greatest at low N status and high needle temperatures. Photorespiratory losses are lower at elevated [CO2],
and this effect is amplified at higher temperatures (Farquhar et
al. 1980). The linear relationship between temperature and the
relative enhancement of long-term elevated [CO2] has been reported in Chenopodium album L. (Sage et al. 1995), Pinus
taeda L. (Lewis et al. 1996, Myers et al. 1999) and Glycine
max (L.) Merrill (Vu et al. 1997). Although the relative enhancement for these species was similar to that reported here
(10–50, –2–110 and 32–95%, respectively, compared with
20–75% in this study) the temperature ranges for the enhancement in these species were much higher (16–38, 16–35 and
28–40 °C, respectively, compared to 8–25 °C in this study). In
contrast, Teskey (1997) found a constant relative photosynthetic response to elevated [CO2] in Pinus taeda needles over a
temperature range between 12.6 to 28.2 °C. This indicates climatic adaptation, species differences in temperature dependency of net photosynthesis to elevated [CO2], or both. The shift
in temperature optimum as [CO2] doubled was estimated to be
approximately 4 °C , which is 1 °C lower than that predicted
by Long (1991) and that measured on Eucalyptus tetrodonta
F.J. Muell. (Eamus et al. 1995).
The finding of a stronger relative CO2 enhancement at low
N status is in accordance with Idso and Idso (1994). If foliage
with low N status has lower gi, then the relative effect as [CO2]
increases should be larger than that for high-N foliage. However, the absolute increase in net photosynthesis will be much
greater when N status in the foliage is high (Figure 3d). Therefore, it is important to distinguish between the relative and absolute increase at elevated [CO2].
The relative effect of long-term elevated [CO2] (EL) was
937
about 20% lower than the relative effect of instantaneous
[CO2] enrichment (Figure 3a). This was caused by a significant downward acclimation of A450i of about 1.4 µmol m –2 s –1.
In addition, A200i and gm were significantly lower in EL compared with AMB (Table 2). The relatively greater reduction in
photosynthetic capacity at lower Ci and a significantly lower
gm in the elevated [CO2] treatment indicate a decrease in
Rubisco activity. A reduction in Rubisco activity and content
is probably one of the most frequently observed biochemical
changes associated with elevated [CO2]. For example, van
Oosten et al. (1992) found the Rubisco activity in Norway
spruce foliage to decrease by 27–40% in elevated [CO2].
There was no difference in acclimation between current
shoots on branches treated for 25 days, and current shoots on
branches treated for 665 days. It is relevant that current foliage
by definition cannot grow in EL for more than one season, although the needle primordia are formed in the previous year.
However, acclimation of net photosynthesis within 1 month of
CO2 treatment has been reported for Norway spruce seedlings
(Roberntz 1998) and other species (Koike et al. 1996, Sims et
al. 1998). The finding of a constant absolute value of CO2 acclimation without an interaction with needle temperature or N
was surprising. This, however, results in a greater relative
downward acclimation at low temperatures and N availabilities (Sage 1994, Curtis 1996, Drake et al. 1997). Roberntz and
Stockfors (1998) estimated a downward acclimation of A200i of
8–32%, depending on N status. In the present analysis, when
considering the temperature effect, it was estimated to be between 8 and 20%. The carboxylation efficiency was reduced
by about 14%. These results are in agreement with a recent
meta-analysis by Medlyn et al. (1999). In contrast, other CO2
experiments with Norway spruce show a stronger acclimation
response, in the range of 23–37% (Laitat and Boussard 1995,
Marek et al. 1995, Lippert et al. 1996, 1997), but these studies
may include a reduction of needle N caused by elevated [CO2]
(e.g., Marek et al. 1995). However, recent findings on Norway
spruce trees indicate that data from branch-bag trials, i.e., a
low source to sink ratio, may underestimate the long-term acclimation process of net photosynthesis at elevated [CO2] as
opposed to data from experiments using whole-tree treatments
(P. Roberntz and G. Wallin, Botanical Institute, University of
Gothenburg, Sweden, unpublished data). In addition, the relatively larger response to elevated [CO2] at a low N concentration found in this study (Figure 3c), may not apply in wholetree CO2 treatments, where confounding effects between N
and [CO2] on the source–sink relationship may be more important.
The bag effect
The increase in temperature optimum in the bag treatment was
probably a result of acclimation to the elevated bag temperature, which was on average 1.5 °C above ambient, but which
occasionally reached 6 °C. The 16% increase in photosynthetic capacity in the AMB shoots compared with control
shoots may also have been a result of improved internal CO2
conductance (gi) caused by the altered growth conditions in
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938
ROBERNTZ
the bags. Elevated growth temperature has been shown to increase photosynthetic capacity in rice by improving gi in the
leaves (Makino et al. 1994). The bag treatment caused increases in needle length and shoot structure (Roberntz 1999).
These external morphological changes may mimic internal
morphological adjustments, which in turn affect gi and consequently photosynthetic capacity. However, other studies on
conifers have shown no effect or a decrease in net photosynthesis at elevated temperatures (Wang et al. 1995, Teskey
1997). Thus, although elevated temperature seems to be the
most likely explanation for the bag-induced increase in net
photosynthesis, it may also be the combined effect of several
other altered environmental factors, i.e., wind speed, light and
vapor pressure deficit.
Conclusion
The strong temperature dependency of the enhancement of
photosynthetic capacity at elevated [CO2] illustrates the importance of making measurements over an appropriate range
of temperatures. For instance, the average daytime temperature at the site during the growing season was about 12 °C,
which would stimulate light-saturated net photosynthesis by
40% at twice the ambient [CO2], which is representative of the
lower range of our results. Carbon dioxide experiments using
branch-bags may alter photosynthetic capacity in elevated
[CO2] to an extent that is similar to the mean stimulation and
acclimation responses found in many reviews and metaanalyses (e.g., Saxe et al. 1998, Norby et al. 1999, Medlyn et
al. 1999). However, extrapolating results from branch-bag experiments to whole tree or canopy CO2 responses should be
avoided until prediction methods have been validated.
Acknowledgments
This study was made possible by financial support from the Swedish
Council of Forestry and Agricultural Research and by the European
Community through the Environment R&D Programme (ECOCRAFT, Contract No. EV57-CT92-0127). I am grateful to J. Leverenz, S. Linder and B.D. Sigurðsson for valuable discussions and
constructive criticism during the preparation of the manuscript. Many
thanks to J. Parsby and M. Lindberg for their technical assistance in
the field, Gunnar Ekbohm for statistical advice and J. Flower-Ellis for
language editing. This work contributes to the Global Change and
Terrestrial Ecosystems (GCTE) Core Project of the International
Geosphere–Biosphere Program (IGBP).
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