Directory UMM :Data Elmu:jurnal:T:Tree Physiology:vol17.1997:
Tree Physiology 17, 655--661
© 1997 Heron Publishing----Victoria, Canada
Effect of elevated carbon dioxide concentration and root restriction on
net photosynthesis, water relations and foliar carbohydrate status of
loblolly pine seedlings
R. E. WILL and R. O. TESKEY
School of Forest Resources, University of Georgia, Athens, GA 30602, USA
Received July 24, 1996
Summary To determine the effects of CO2-enriched air and
root restriction on photosynthetic capacity, we measured net
photosynthetic rates of 1-year-old loblolly pine seedlings
grown in 0.6-, 3.8- or 18.9-liter pots in ambient (360 µmol
mol −1) or 2× ambient CO2 (720 µmol mol −1) concentration for
23 weeks. We also measured needle carbohydrate concentration and water relations to determine whether feedback inhibition or water stress was responsible for any decreases in net
photosynthesis. Across all treatments, carbon dioxide enrichment increased net photosynthesis by approximately 60 to
70%. Net photosynthetic rates of seedlings in the smallest pots
decreased over time with the reduction occurring first in the
ambient CO2 treatment and then in the 2× ambient CO2 treatment. Needle starch concentrations of seedlings grown in the
smallest pots were two to three times greater in the 2× ambient
CO2 treatment than in the ambient CO2 treatment, but decreased net photosynthesis was not associated with increased
starch or sugar concentrations. The reduction in net photosynthesis of seedlings in small pots was correlated with decreased
needle water potentials, indicating that seedlings in the small
pots had restricted root systems and were unable to supply
sufficient water to the shoots. We conclude that the decrease in
net photosynthesis of seedlings in small pots was not the result
of CO2 enrichment or an accumulation of carbohydrates causing feedback inhibition, but was caused by water stress.
Keywords: feedback inhibition, needle starch, needle sugar,
Pinaceae, Pinus taeda, water stress.
Introduction
Photosynthetic down-regulation in response to elevated CO2
concentration is a common finding in CO2 enrichment studies.
Gunderson and Wullschleger (1994) noted in their review that
in experiments with 20 tree species grown in twice-ambient
CO2 concentration, 12 species exhibited at least a 10% decrease in net photosynthetic rate compared to plants grown in
ambient CO2 concentration when measured at a common CO2
concentration. In general, trees growing in small pots (i.e.,
< 0.5 l) are more likely to exhibit photosynthetic down-regulation than trees growing in large pots (Arp 1991, Gunderson and
Wullschleger 1994, Sage 1994). The reason for this is unclear
but possible explanations include decreased nutrient or water
availability (Coleman et al. 1993), physical restrictions of
growth leading to a hormonal response (Sage 1994), or a build
up of carbohydrates in foliage as a result of decreased belowground sink strength causing feedback inhibition (Arp 1991).
The causes of decreased net photosynthesis in plants grown
in small pots have important ramifications for predicting the
effect of increasing atmospheric CO2 concentration on fieldgrown plants. For instance, if the decrease in net photosynthesis is caused by a physical restriction to growth, then it is
mainly an experimental artifact and will not occur in the field
except perhaps in cases of extreme soil compaction. However,
if the decrease in net photosynthesis occurs as a result of
nutrient or water limitations, then plants may not benefit from
elevated atmospheric CO2 concentrations when growing in
environments with low resource availability. Finally, if the
decrease in net photosynthesis of plants grown in small pots is
the result of accumulation of carbohydrates in leaves causing
feedback inhibition, then carbon acquisition is in part controlled by carbon use (sink strength) and the effect of elevated
CO2 concentration on photosynthesis will depend on a plant’s
ability to consume photosynthate.
Loblolly pine (Pinus taeda L.) is the most important commercial tree species, and an important component of many
natural stands, in the southeastern United States. Although net
photosynthesis of loblolly pine did not decrease in response to
elevated CO2 concentration in field grown trees (Ellsworth
et al. 1995, Liu and Teskey 1995, Teskey 1995, Tissue et al.
1996) or potted seedlings supplied with adequate water and
nutrients (Fetcher et al. 1988), the photosynthetic capacity of
loblolly pine seedlings grown in elevated CO2 declined when
nutrient availability was severely limited (Tissue et al. 1993,
Thomas et al. 1994). Whether feedback inhibition or water
relations also affect photosynthetic rates in this species when
grown in elevated CO2 concentrations has not been determined.
The goal of this study was to determine if net photosynthetic
rates decrease when loblolly pine seedlings are grown in elevated CO2 concentrations and whether this decrease only occurs when seedlings are grown in small pots. We also tested
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WILL AND TESKEY
three hypotheses: (1) decreased net photosynthesis resulting
from either the CO2 enrichment or the small pot volume treatment is related to needle carbohydrate status or water relations;
(2) an accumulation of starch or sugars in needles causes net
photosynthesis to decrease as a result of feedback inhibition;
and (3) water stress resulting from either root restriction or
CO2 enrichment decreases net photosynthesis.
Materials and methods
Plant material and treatments
On March 22, 1994, 1-year-old bare root loblolly pine seedlings (Weyerhaeuser family NCC 08-0074) were planted in
3.8-liter pots filled with Fafard 3B potting mix (Conrad Fafard
Inc., Agawam, MA). Seedlings were grown for 11 weeks in a
greenhouse where they were watered and fertilized as needed.
On June 27--28, all seedlings were removed from their pots and
their root systems trimmed to fit inside a 0.6-liter pot. After
pruning, the seedlings were randomly assigned to one of three
pot volumes: small (0.6 l), medium (3.8 l) or large (18.9 l), and
placed under mist to avoid water stress while the roots became
established.
After 10 days (July 8), half the seedlings in each pot treatment were randomly assigned to either the 2× ambient
(720 µmol mol −1) or ambient (360 µmol mol −1) CO2 treatment
in two growth chambers (Environmental Growth Chambers,
Chagrin Falls, OH) for 23 weeks. Except for CO2 concentration, chambers were maintained under similar conditions
(25/20 °C day/night temperature, 70% relative humidity and a
10-h photoperiod). After 3 weeks, the photoperiod was adjusted to 12 h. Light was supplied by incandescent and highoutput fluorescent bulbs so that photosynthetic photon flux
density (PPFD) was between 650 and 750 µmol m −2 s −1 near
the tops of the seedlings. Seedlings were rotated within chambers twice per week and between chambers once per week to
minimize potential confounding effects caused by within or
between chamber differences.
To maintain similar total nutrient availabilities among the
pot treatments and avoid confounding nutrient availability
with rooting volume, all seedlings were fertilized with an
excess of nutrients. The small pots were fertilized daily, the
medium pots twice per week, and the large pots once per week
with a solution containing 0.30 g l −1 N, 0.26 g l −1 P, 0.49 g l −1
K, 0.0006 g l −1 B, 0.0015 g l −1 Cu, 0.003 g l −1 Fe, 0.0015 g l −1
Mn, 0.0015 g l −1 Zn and 0.000015 g l −1 Mo supplied as Miller
Nutrileaf (Miller Chemical, Hanover, PA) as well as an additional 0.02 g l −1 chelated Fe (Miller Iron Chelate DP, Miller
Chemical). Between fertilizations, seedlings were watered beyond saturation to prevent salt accumulation. During the second half of the experiment, the plants in the small pots had
severely compacted root systems that decreased their ability to
absorb water and decreased the ability of water to percolate
through the potting medium. Therefore, these pots were placed
in 125-ml containers of water to provide a constant water
supply to the lower portion of the root system.
Biomass determinations
Seedling root, stem and needle biomass were destructively
measured before imposing the CO2 treatments (n = 3) and after
23 weeks (n = 5).
Gas exchange and water potential measurements
Leaf gas exchange was periodically measured (Weeks 1, 7, 13,
19, and 22) in the chambers at the growth CO2 concentrations
with an LI-6200 portable photosynthesis system equipped
with a 0.25-l cuvette (Li-Cor, Inc., Lincoln, NE). Fascicles
from the first needle flush of 1994 were measured throughout
the experiment. Gas exchange measurements were made at a
PPFD of approximately 485 µmol m −2 s −1 with the mean
PPFD differing by an average of 13 µmol m −2 s −1 between
chambers. Needle water potentials were measured with a pressure chamber (PMS, Corvallis, OR) at Week 22. Water potentials were measured immediately after gas exchange and
reflected the water status of the plants under the growth conditions.
At Weeks 14, 18 and 22, gas exchange of all seedlings was
measured at both 720 and 360 µmol mol −1 CO2 concentration
(reciprocal measurements) to determine whether seedlings
from the different treatment groups differed in their relative
photosynthetic response to CO2 concentration. The seedlings
from both CO2 treatments were randomized between the two
chambers to minimize any chamber effect. Gas exchange was
measured at 720 µmol mol −1 CO2 in one chamber and 360
µmol mol −1 CO2 in the other after which the CO2 concentration
was adjusted and seedlings were measured at the reciprocal
concentration. At least 90 min was allowed for seedlings to
adjust to new CO2 concentrations before measuring gas exchange.
Nutrient analysis
Needles were collected for nutrient analysis at the end of the
experiment (Week 23, December 19). Nitrogen concentration
was determined with a Technicon AutoAnalyzer II (Bran and
Luebbe, Inc., Buffalo Grove, IL) following the methods of
Issac and Johnson (1976). Phosphorus was measured by
atomic absorption spectrophotometry (Model 560, Perkin-Elmer, Inc., Norwalk, CT) as described by Chapman and Pratt
(1961).
Carbohydrate analysis
Nonstructural carbohydrate concentrations were measured at
Weeks 7 and 22. Five fascicles per seedling were collected
within 30 min of measuring gas exchange. Needles were immediately placed on dry ice and later that day stored in a
freezer at −50 °C until they were freeze dried. Samples were
analyzed by gas chromatography as described by Rieger and
Marra (1994) except that the sugars were oximated for better
separation of fructose from organic acids (Chapman and Horvat 1989).
Statistical analysis
There were six treatments consisting of a factorial cross between CO2 concentration (ambient = 360 µmol mol −1 and 2×
TREE PHYSIOLOGY VOLUME 17, 1997
ELEVATED CO2, ROOT RESTRICTION AND PHOTOSYNTHESIS
ambient = 720 µmol mol −1) and pot volume (small = 0.6 l,
medium = 3.8 l and large = 18.9 l). Every treatment group
consisted of six seedlings (n = 6). Data were log transformed
when necessary. Data were analyzed by two-way ANOVA with
pot volume and growth CO2 concentration as the main effects.
For the reciprocal photosynthesis measurements, measurement CO2 concentration was added to the model resulting in a
three-way ANOVA.
Results
Initial needle, stem and root mass were 18.0, 10.7 and 3.8 g,
respectively. By the final harvest, a significant pot volume
effect (P < 0.0001) and CO2 effect (P < 0.0001) had developed
for root mass (Figure 1). Seedlings grown in 2× ambient CO2
had larger root systems (90.8 versus 65.4 g) than seedlings
grown in ambient CO2. Among the pot treatments, seedlings
grown in large pots had the largest root systems (108.4 g),
whereas seedlings grown in small pots had the smallest root
systems (53.4 g). There were no statistically significant treatment effects on needle or stem mass (Figure 1); however, pot
volume (P < 0.02) and CO2 concentration (P < 0.007) were
significant for total mass. All seedlings had high nutrient status
Figure 1. Effects of pot volume (S = 0.6 l, M = 3.8 l and L = 18.9 l) on
mean final needle, stem and root mass of loblolly pine seedlings grown
in ambient (360 µmol mol −1) or 2× ambient (720 µmol mol −1) CO2
concentration. Error bars depict the standard error. The effect of CO 2
concentration and pot volume were significant (P < 0.05) for root
mass. Differences in stem and needle mass were not significant. Initial
needle, stem and root mass were 18.0, 10.7 and 3.8 g, respectively.
657
with foliar concentrations of at least 2.0% nitrogen and 0.1%
phosphorus.
Carbon dioxide enrichment increased net photosynthetic
rates by about 60% (P < 0.0001) (Figure 2). From Week 13
onward, there was also an effect of pot volume on net photosynthetic rate (P < 0.01) that was first observed as a reduction
in net photosynthetic rates of seedlings grown in small pots in
ambient CO2 concentration. Overall, net photosynthetic rates
of seedlings grown in the small pots were reduced 12, 28 and
39% at Weeks 13, 19 and 22, respectively. At Week 13, there
was an interaction (P < 0.04) between pot volume and CO2
treatment because the small pot treatment decreased net photosynthetic rates of seedlings grown in ambient CO2 concentration by 25% compared with the mean of seedlings grown in
medium and large pots in ambient CO2, whereas net photosynthetic rates of seedlings grown in 2× ambient CO2 was not
decreased by the small pot treatment (Figure 2). This interaction did not occur at Week 19 or 22 because net photosynthetic
rate of seedlings grown in 2× ambient CO2 was also decreased
by the small pot treatment. Compared to the mean net photosynthetic rate of seedlings grown in medium and large pots at
Figure 2. Effects of pot volume (Small = 0.6 l, Medium = 3.8 l and
Large = 18.9 l) on mean photosynthetic rate and needle conductance
of loblolly pine seedlings grown in ambient (360 µmol mol −1) or 2×
ambient (720 µmol mol −1) CO2 concentration. All plants were measured at the growth CO2 concentration. Error bars depict the standard
error; C indicates a significant effect (P < 0.05) of CO2 concentration,
P indicates a significant pot volume effect and an asterisk indicates a
significant pot volume × CO2 interaction.
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WILL AND TESKEY
Week 19, net photosynthetic rate of seedings grown in small
pots was decreased 24 and 35% in the ambient and 2× ambient
CO2 treatments, respectively (Figure 2). At Week 22, the corresponding decreases were 34 and 42% (Figure 2).
Needle conductance of all seedlings was low one week after
transplanting but increased dramatically between Weeks 1
and 7. At Weeks 13, 19 and 22, needle conductance was significantly lower for seedlings grown in 2× ambient CO2 than
for seedlings grown in ambient CO2 (P < 0.0001) (Figure 2).
From Week 13 onward, the small pot treatment caused significant decreases in needle conductance (P < 0.001) (Figure 2).
There was an interaction between pot size and CO2 treatment
at Week 13 because needle conductance of seedlings grown in
small pots in ambient CO2 was reduced, whereas needle conductance of seedlings grown in small pots in 2× ambient CO2
was not (P < 0.02) (Figure 2). This interaction did not occur at
Weeks 19 or 22 because needle conductances of the seedlings
grown in small pots were reduced in both CO2 treatments.
Gas exchange results from the three sets of reciprocal CO2
measurements were similar and so only the last set (Week 22)
is presented (Figure 3). Carbon dioxide enrichment during
measurement increased net photosynthetic rate by approxi-
Figure 3. Effects of pot volume (Small = 0.6 l, Medium = 3.8 l and
Large = 18.9 l) on mean photosynthetic rate of loblolly pine seedlings
grown in ambient (360 µmol mol −1) or 2× ambient (720 µmol mol −1)
CO2 concentration and measured at both ambient (360 µmol mol −1)
and 2× ambient (720 µmol mol −1) CO2 concentration after approximately 5 months of treatment exposure. Error bars depict the standard
error. The effects of measurement CO2 concentration and pot volume
were significant (P < 0.05), but growth CO2 concentration and all
interactions were not significant.
mately 70% for all seedlings regardless of growth CO2 concentration (Figure 3). Seedlings grown in small pots had significantly lower net photosynthetic rates than seedlings grown in
medium or large pots, but the effect of pot volume on net
photosynthetic rate was independent of growth and measurement CO2 concentrations (i.e., there were no pot volume × CO2
interactions). Thus, although seedlings grown in small pots in
2× ambient CO2 had the lowest absolute photosynthetic rates
when measured at a CO2 concentration of 360 or 720 µmol
mol −1, the relative photosynthetic stimulation due to CO2 enrichment was similar to that for seedlings in all of the other
treatments (Figure 3).
At Week 22, seedlings grown in small pots had more negative water potentials and lower rates of net photosynthesis than
seedlings grown in medium or large pots. When data for
seedlings in all of the pot treatments were plotted, the shape of
the water potential--net photosynthesis relationship was curvilinear and similar for seedlings in both CO2 treatments (Figure 4), indicating that low water potentials were closely
correlated with the low gas exchange rates in seedlings grown
in small pots at the end of the experiment.
Needle starch concentrations were significantly higher in
seedlings grown in 2× ambient CO2 concentration than in
seedlings grown in ambient CO2 concentration. However, at
both Weeks 7 and 22, there was a significant interaction between pot volume and CO2 concentration (P < 0.05) for starch
because seedlings grown in small pots in 2× ambient CO2
concentration had greater starch concentrations than seedlings
in other treatments (Figure 5). Needle starch concentrations
were lower at Week 22 than at Week 7 for seedlings in all
Figure 4. Effects of pot volume (Small = 0.6 l, Medium = 3.8 l and
Large = 18.9 l) on the relationship between net photosynthesis and
needle water potential for loblolly pine seedlings grown in ambient
(360 µmol mol −1) or 2× ambient (720 µmol mol −1) CO2 concentration
after approximately 5 months of exposure.
TREE PHYSIOLOGY VOLUME 17, 1997
ELEVATED CO2, ROOT RESTRICTION AND PHOTOSYNTHESIS
659
Figure 5. Effects of pot volume (S = 0.6 l, M = 3.8 l and L = 18.9 l) on
foliar total sugar and starch concentrations of loblolly pine seedlings
grown in ambient (360 µmol mol −1) or 2× ambient (720 µmol mol −1)
CO2 concentration. Error bars depict the standard error.
treatments except seedlings grown in medium and small pots
in ambient CO2 concentration (Figure 5). Although starch
concentrations increased in seedlings grown in small pots in
ambient CO2, at Week 22 these seedlings had starch concentrations that were lower than or similar to those of seedlings in
other treatments that did not exhibit a decrease in photosynthetic rate (e.g., the plants grown in medium and large pots in
the 2× ambient CO2 treatment).
Plants grown in 2× ambient CO2 concentration had significantly greater (P < 0.0001) needle sucrose concentration (Figure 6) and therefore greater total sugar concentrations (glucose
+ fructose + sucrose) (Figure 5) than plants grown in ambient
CO2 concentration. Although there was a significant interaction between pot volume and CO2 concentration for sucrose
and total sugar at Week 7, the effect of CO2 concentration was
unambiguous because the lowest mean sucrose and total sugar
concentrations of seedlings in any 2× ambient CO2 treatment
group was greater than the highest mean concentrations of
seedlings in any ambient CO2 treatment group. Sucrose concentrations of seedlings grown in small pots increased slightly
from Week 7 to Week 22, but they were always lower than
those of seedlings grown in medium and large pots.
Although glucose concentration was higher in seedlings
grown in small pots than in seedlings grown in medium or
large pots at Week 7 (P < 0.03) and fructose concentration was
higher in seedlings grown in ambient CO2 concentration than
in seedlings grown in 2× ambient CO2 concentration, no consistent trends between pot volume or CO2 concentration were
evident (Figure 6). Both fructose and glucose concentrations
were lower at Week 22 than at Week 7 (Figure 6).
Figure 6. Effects of pot volume (S = 0.6 l, M = 3.8 l and L = 18.9 l) on
mean foliar fructose, glucose and sucrose concentrations of loblolly
pine seedlings grown in ambient (360 µmol mol −1) or 2× ambient (720
µmol mol −1) CO2 concentration. Error bars depict the standard error.
Discussion
Net photosynthetic rates were higher for seedlings measured
in elevated CO2 than for seedlings measured in ambient CO2
regardless of growth CO2 concentration or pot volume. Seedlings in all treatments exhibited the same relative response to
CO2 concentration during the reciprocal measurements indicating that the effect of CO2 enrichment was similar for seedlings in all treatments. Photosynthetic rates of seedlings grown
in small pots in the 2× ambient CO2 treatment decreased during
the second half of the experiment; however, photosynthetic
rates of seedlings grown in small pots in ambient CO2 decreased earlier, indicating that the decreases in photosynthetic
capacity were caused by root confinement rather than CO2
enrichment.
Root restriction did not change the relative response of
photosynthesis to CO2 enrichment indicating that CO2 enrichment had a stimulatory effect regardless of pot size. Results
from studies examining the interaction between CO2 concentration and pot volume in other species are variable. For example, Thomas and Strain (1991) found that cotton seedlings
(Gossypium hirsutum L.) planted in small pots and exposed to
CO2 enrichment exhibited a decrease in photosynthetic capacity and an accumulation of starch and concluded that decreased
sink strength imposed by root restriction reduced the photosynthetic response to CO2. However, more recent studies on
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WILL AND TESKEY
cotton show that photosynthetic rate decreased in response to
CO2 enrichment regardless of whether plants were grown in
large or small pots (Barrett and Gifford 1995a, 1995b). Samuelson and Seiler (1992) observed a decrease in net photosynthesis of Fraser fir (Abies fraseri (Pursh.) Poir.) plants
grown in small pots in CO2 enrichment when measured at
346 µmol mol −1 or 796 µmol mol −1 CO2 concentration and a
decrease in net photosynthesis of plants grown in large pots in
CO2 enrichment when measured at 796 µmol mol −1 CO2 but
not 346 µmol mol −1. Thus, the effects of CO2 enrichment and
rhizosphere restriction on photosynthesis appear to be strongly
species specific.
Carbohydrate concentrations either decreased during the
experiment or were lower in plants grown in small pots than in
plants grown in medium or large pots. Thus, our carbohydrate
data provided no indication that carbohydrate accumulation in
foliage of seedlings grown in small pots caused the observed
reduction in net photosynthesis. Therefore, we conclude that
carbohydrate-induced feedback inhibition did not cause the
decrease in net photosynthesis in plants grown in small pots. It
is possible that feedback inhibition did not occur in these
loblolly pine seedlings as a result of the recurrent flushing
pattern of the species. New shoot growth and continued stem
diameter growth may have provided adequate photosynthate
sinks so that needle carbohydrate concentrations never reached
inhibitory levels. An alternative explanation is that photosynthesis of loblolly pine seedlings is insensitive to high carbohydrate concentration regardless of the concentration of CO2
exposure.
Although our loblolly pine plants grown in small pots exhibited a decrease in photosynthetic capacity without a corresponding increase in carbohydrate concentration, other studies
have shown that decreased net photosynthesis is associated
with carbohydrate accumulation in plants grown in small pots.
For example, cucumber plants (Cucumis sativus L. cv Calypso) grown in small pots exhibited decreased photosynthetic
rates and increased starch concentrations when starch was
measured over a 24-h period (Robbins and Pharr 1988). Similarly, peach trees (Prunus persica L.) grown in small pots
showed a decrease in photosynthetic rate along with an accumulation of starch and sorbitol (Rieger and Marra 1994). In a
study of the effects of root restriction in fruiting and nonfruiting peach trees, Mandre et al. (1995) found a negative relationship between net photosynthesis and leaf carbohydrate
concentration; however, this relationship was inconsistent
among treatment groups and led the authors to conclude that
something other than feedback inhibition was contributing to
the decrease in net photosynthesis in the nonfruiting trees.
We observed a strong negative correlation between water
potential and net photosynthesis as well as a decrease in needle
conductance in seedlings grown in small pots. These findings,
coupled with the observation that water potentials of seedlings
grown in small pots were lower than those of seedlings grown
in the medium and large pots and low enough to decrease net
photosynthesis (Seiler and Johnson 1985, Teskey et al. 1986,
Seiler and Johnson 1988), led us to conclude that the decrease
in net photosynthetic rates of plants grown in small pots was
probably caused by water stress because the small and restricted root systems were unable to absorb enough water even
though a continuous supply was available. Similarly,
Tschaplinski and Blake (1985) found that root restriction
caused a decrease in net photosynthesis, an increase in stomatal resistance and a decrease in leaf water potential in Alnus
glutinosa (Gaertn.) and concluded that water stress was the
mechanism that caused the decrease in net photosynthesis.
Because the Alnus glutinosa plants were grown in solution
culture, water stress was not caused by lack of available water,
but resulted from an inability of the restricted roots to take up
an adequate amount of water.
Seedling water status may also explain why net photosynthesis of seedlings grown in small pots decreased earlier in the
ambient CO2 treatment than in the 2× ambient CO2 treatment.
At Week 13, when net photosynthetic rates of seedlings grown
in small pots in ambient CO2 were lower than those of seedlings grown in small pots in 2× ambient CO2, needle conductance was significantly lower in seedlings in the 2× ambient
CO2 treatment. Thus, seedlings grown in small pots in 2×
ambient CO2 were losing less water on a per needle area basis
than seedlings grown in small pots in ambient CO2. Because
needle areas of seedlings grown in ambient or 2× ambient CO2
were similar, this implies that seedlings grown in small pots in
2× ambient CO2 used less water and would therefore experience water stress later in the experiment than seedlings grown
in small pots in ambient CO2.
Our results support the conclusion that loblolly pine plants
will benefit from a high CO2 environment because they do not
decrease their photosynthetic capacity when exposed to longterm CO2 enrichment (Ellsworth et al. 1995, Liu and Teskey
1995, Teskey 1995, Tissue et al. 1996). An exception may be
plants growing under conditions of nutrient stress, because
Tissue et al. (1993) and Thomas et al. (1994) were able to
induce down-regulation in loblolly pine by supplying low
nutrient concentrations to potted seedlings. The plants that
exhibited decreased net photosynthetic rates were deficient in
foliar nitrogen with concentrations of approximately 0.5 to
0.7%, which is well below the typical foliar nitrogen concentrations reported on nutrient-poor sites (1.07%, Van Lear et al.
1984; and 0.90%, Valentine and Allen 1990). The inability to
force feedback inhibition by severe root restriction indicates
that loblolly pine is unlikely to show a decrease in photosynthetic capacity in response to carbohydrate accumulation induced by elevated CO2 environments.
Acknowledgments
We thank Dr. Mark Rieger from the Department of Horticulture at
University of Georgia for the use of his laboratory for the carbohydrate
analysis, Paula Litvin for assistance during the carbohydrate analysis,
Patsy Mason and Susan Wilson of the USDA Phytochemical research
unit in Athens, GA for analyzing the carbohydrate samples and William Jennings (Jay) Brown for his aid and expertise in growing
loblolly pine.
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Physiol. 14:947--960.
Thomas, R.B. and B.R. Strain. 1991. Root restriction as a factor in
photosynthetic acclimation of cotton seedlings grown in elevated
carbon dioxide. Plant Physiol. 96:627--634.
Tissue, D.T., R.B. Thomas and B.R. Strain. 1993. Long-term effects
of elevated CO2 and nutrients on photosynthesis and Rubisco in
loblolly pine seedlings. Plant Cell Environ. 16:859--865.
Tissue, D.T., R.B. Thomas and B.R. Strain. 1996. Growth and photosynthesis of loblolly pine (Pinus taeda) after exposure to elevated
CO2 for 19 months in the field. Tree Physiol. 16:49--59.
Tolley, L.C. and B.R. Strain. 1985. Effects of CO 2 enrichment and
water stress on gas exchange of Liquidambar stryraciflua and Pinus
taeda seedlings grown under different irradiance levels. Oecologia
65:166--172.
Tschaplinski, T.J. and T.J. Blake. 1985. Effects of root restriction on
growth correlations, water relations and senescence of alder seedlings. Physiol. Plant. 64:167--176.
Valentine, D.W. and H.L. Allen. 1990. Foliar responses to fertilization
identify nutrient limitation in loblolly pine. Can. J. For. Res.
20:144--151.
Van Lear, D.H., J.B. Waide and M.J. Teuke. 1984. Biomass and
nutrient content of a 41-year-old loblolly pine (Pinus taeda L.)
plantation on a poor site in South Carolina. For. Sci. 30:395--404.
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© 1997 Heron Publishing----Victoria, Canada
Effect of elevated carbon dioxide concentration and root restriction on
net photosynthesis, water relations and foliar carbohydrate status of
loblolly pine seedlings
R. E. WILL and R. O. TESKEY
School of Forest Resources, University of Georgia, Athens, GA 30602, USA
Received July 24, 1996
Summary To determine the effects of CO2-enriched air and
root restriction on photosynthetic capacity, we measured net
photosynthetic rates of 1-year-old loblolly pine seedlings
grown in 0.6-, 3.8- or 18.9-liter pots in ambient (360 µmol
mol −1) or 2× ambient CO2 (720 µmol mol −1) concentration for
23 weeks. We also measured needle carbohydrate concentration and water relations to determine whether feedback inhibition or water stress was responsible for any decreases in net
photosynthesis. Across all treatments, carbon dioxide enrichment increased net photosynthesis by approximately 60 to
70%. Net photosynthetic rates of seedlings in the smallest pots
decreased over time with the reduction occurring first in the
ambient CO2 treatment and then in the 2× ambient CO2 treatment. Needle starch concentrations of seedlings grown in the
smallest pots were two to three times greater in the 2× ambient
CO2 treatment than in the ambient CO2 treatment, but decreased net photosynthesis was not associated with increased
starch or sugar concentrations. The reduction in net photosynthesis of seedlings in small pots was correlated with decreased
needle water potentials, indicating that seedlings in the small
pots had restricted root systems and were unable to supply
sufficient water to the shoots. We conclude that the decrease in
net photosynthesis of seedlings in small pots was not the result
of CO2 enrichment or an accumulation of carbohydrates causing feedback inhibition, but was caused by water stress.
Keywords: feedback inhibition, needle starch, needle sugar,
Pinaceae, Pinus taeda, water stress.
Introduction
Photosynthetic down-regulation in response to elevated CO2
concentration is a common finding in CO2 enrichment studies.
Gunderson and Wullschleger (1994) noted in their review that
in experiments with 20 tree species grown in twice-ambient
CO2 concentration, 12 species exhibited at least a 10% decrease in net photosynthetic rate compared to plants grown in
ambient CO2 concentration when measured at a common CO2
concentration. In general, trees growing in small pots (i.e.,
< 0.5 l) are more likely to exhibit photosynthetic down-regulation than trees growing in large pots (Arp 1991, Gunderson and
Wullschleger 1994, Sage 1994). The reason for this is unclear
but possible explanations include decreased nutrient or water
availability (Coleman et al. 1993), physical restrictions of
growth leading to a hormonal response (Sage 1994), or a build
up of carbohydrates in foliage as a result of decreased belowground sink strength causing feedback inhibition (Arp 1991).
The causes of decreased net photosynthesis in plants grown
in small pots have important ramifications for predicting the
effect of increasing atmospheric CO2 concentration on fieldgrown plants. For instance, if the decrease in net photosynthesis is caused by a physical restriction to growth, then it is
mainly an experimental artifact and will not occur in the field
except perhaps in cases of extreme soil compaction. However,
if the decrease in net photosynthesis occurs as a result of
nutrient or water limitations, then plants may not benefit from
elevated atmospheric CO2 concentrations when growing in
environments with low resource availability. Finally, if the
decrease in net photosynthesis of plants grown in small pots is
the result of accumulation of carbohydrates in leaves causing
feedback inhibition, then carbon acquisition is in part controlled by carbon use (sink strength) and the effect of elevated
CO2 concentration on photosynthesis will depend on a plant’s
ability to consume photosynthate.
Loblolly pine (Pinus taeda L.) is the most important commercial tree species, and an important component of many
natural stands, in the southeastern United States. Although net
photosynthesis of loblolly pine did not decrease in response to
elevated CO2 concentration in field grown trees (Ellsworth
et al. 1995, Liu and Teskey 1995, Teskey 1995, Tissue et al.
1996) or potted seedlings supplied with adequate water and
nutrients (Fetcher et al. 1988), the photosynthetic capacity of
loblolly pine seedlings grown in elevated CO2 declined when
nutrient availability was severely limited (Tissue et al. 1993,
Thomas et al. 1994). Whether feedback inhibition or water
relations also affect photosynthetic rates in this species when
grown in elevated CO2 concentrations has not been determined.
The goal of this study was to determine if net photosynthetic
rates decrease when loblolly pine seedlings are grown in elevated CO2 concentrations and whether this decrease only occurs when seedlings are grown in small pots. We also tested
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WILL AND TESKEY
three hypotheses: (1) decreased net photosynthesis resulting
from either the CO2 enrichment or the small pot volume treatment is related to needle carbohydrate status or water relations;
(2) an accumulation of starch or sugars in needles causes net
photosynthesis to decrease as a result of feedback inhibition;
and (3) water stress resulting from either root restriction or
CO2 enrichment decreases net photosynthesis.
Materials and methods
Plant material and treatments
On March 22, 1994, 1-year-old bare root loblolly pine seedlings (Weyerhaeuser family NCC 08-0074) were planted in
3.8-liter pots filled with Fafard 3B potting mix (Conrad Fafard
Inc., Agawam, MA). Seedlings were grown for 11 weeks in a
greenhouse where they were watered and fertilized as needed.
On June 27--28, all seedlings were removed from their pots and
their root systems trimmed to fit inside a 0.6-liter pot. After
pruning, the seedlings were randomly assigned to one of three
pot volumes: small (0.6 l), medium (3.8 l) or large (18.9 l), and
placed under mist to avoid water stress while the roots became
established.
After 10 days (July 8), half the seedlings in each pot treatment were randomly assigned to either the 2× ambient
(720 µmol mol −1) or ambient (360 µmol mol −1) CO2 treatment
in two growth chambers (Environmental Growth Chambers,
Chagrin Falls, OH) for 23 weeks. Except for CO2 concentration, chambers were maintained under similar conditions
(25/20 °C day/night temperature, 70% relative humidity and a
10-h photoperiod). After 3 weeks, the photoperiod was adjusted to 12 h. Light was supplied by incandescent and highoutput fluorescent bulbs so that photosynthetic photon flux
density (PPFD) was between 650 and 750 µmol m −2 s −1 near
the tops of the seedlings. Seedlings were rotated within chambers twice per week and between chambers once per week to
minimize potential confounding effects caused by within or
between chamber differences.
To maintain similar total nutrient availabilities among the
pot treatments and avoid confounding nutrient availability
with rooting volume, all seedlings were fertilized with an
excess of nutrients. The small pots were fertilized daily, the
medium pots twice per week, and the large pots once per week
with a solution containing 0.30 g l −1 N, 0.26 g l −1 P, 0.49 g l −1
K, 0.0006 g l −1 B, 0.0015 g l −1 Cu, 0.003 g l −1 Fe, 0.0015 g l −1
Mn, 0.0015 g l −1 Zn and 0.000015 g l −1 Mo supplied as Miller
Nutrileaf (Miller Chemical, Hanover, PA) as well as an additional 0.02 g l −1 chelated Fe (Miller Iron Chelate DP, Miller
Chemical). Between fertilizations, seedlings were watered beyond saturation to prevent salt accumulation. During the second half of the experiment, the plants in the small pots had
severely compacted root systems that decreased their ability to
absorb water and decreased the ability of water to percolate
through the potting medium. Therefore, these pots were placed
in 125-ml containers of water to provide a constant water
supply to the lower portion of the root system.
Biomass determinations
Seedling root, stem and needle biomass were destructively
measured before imposing the CO2 treatments (n = 3) and after
23 weeks (n = 5).
Gas exchange and water potential measurements
Leaf gas exchange was periodically measured (Weeks 1, 7, 13,
19, and 22) in the chambers at the growth CO2 concentrations
with an LI-6200 portable photosynthesis system equipped
with a 0.25-l cuvette (Li-Cor, Inc., Lincoln, NE). Fascicles
from the first needle flush of 1994 were measured throughout
the experiment. Gas exchange measurements were made at a
PPFD of approximately 485 µmol m −2 s −1 with the mean
PPFD differing by an average of 13 µmol m −2 s −1 between
chambers. Needle water potentials were measured with a pressure chamber (PMS, Corvallis, OR) at Week 22. Water potentials were measured immediately after gas exchange and
reflected the water status of the plants under the growth conditions.
At Weeks 14, 18 and 22, gas exchange of all seedlings was
measured at both 720 and 360 µmol mol −1 CO2 concentration
(reciprocal measurements) to determine whether seedlings
from the different treatment groups differed in their relative
photosynthetic response to CO2 concentration. The seedlings
from both CO2 treatments were randomized between the two
chambers to minimize any chamber effect. Gas exchange was
measured at 720 µmol mol −1 CO2 in one chamber and 360
µmol mol −1 CO2 in the other after which the CO2 concentration
was adjusted and seedlings were measured at the reciprocal
concentration. At least 90 min was allowed for seedlings to
adjust to new CO2 concentrations before measuring gas exchange.
Nutrient analysis
Needles were collected for nutrient analysis at the end of the
experiment (Week 23, December 19). Nitrogen concentration
was determined with a Technicon AutoAnalyzer II (Bran and
Luebbe, Inc., Buffalo Grove, IL) following the methods of
Issac and Johnson (1976). Phosphorus was measured by
atomic absorption spectrophotometry (Model 560, Perkin-Elmer, Inc., Norwalk, CT) as described by Chapman and Pratt
(1961).
Carbohydrate analysis
Nonstructural carbohydrate concentrations were measured at
Weeks 7 and 22. Five fascicles per seedling were collected
within 30 min of measuring gas exchange. Needles were immediately placed on dry ice and later that day stored in a
freezer at −50 °C until they were freeze dried. Samples were
analyzed by gas chromatography as described by Rieger and
Marra (1994) except that the sugars were oximated for better
separation of fructose from organic acids (Chapman and Horvat 1989).
Statistical analysis
There were six treatments consisting of a factorial cross between CO2 concentration (ambient = 360 µmol mol −1 and 2×
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ELEVATED CO2, ROOT RESTRICTION AND PHOTOSYNTHESIS
ambient = 720 µmol mol −1) and pot volume (small = 0.6 l,
medium = 3.8 l and large = 18.9 l). Every treatment group
consisted of six seedlings (n = 6). Data were log transformed
when necessary. Data were analyzed by two-way ANOVA with
pot volume and growth CO2 concentration as the main effects.
For the reciprocal photosynthesis measurements, measurement CO2 concentration was added to the model resulting in a
three-way ANOVA.
Results
Initial needle, stem and root mass were 18.0, 10.7 and 3.8 g,
respectively. By the final harvest, a significant pot volume
effect (P < 0.0001) and CO2 effect (P < 0.0001) had developed
for root mass (Figure 1). Seedlings grown in 2× ambient CO2
had larger root systems (90.8 versus 65.4 g) than seedlings
grown in ambient CO2. Among the pot treatments, seedlings
grown in large pots had the largest root systems (108.4 g),
whereas seedlings grown in small pots had the smallest root
systems (53.4 g). There were no statistically significant treatment effects on needle or stem mass (Figure 1); however, pot
volume (P < 0.02) and CO2 concentration (P < 0.007) were
significant for total mass. All seedlings had high nutrient status
Figure 1. Effects of pot volume (S = 0.6 l, M = 3.8 l and L = 18.9 l) on
mean final needle, stem and root mass of loblolly pine seedlings grown
in ambient (360 µmol mol −1) or 2× ambient (720 µmol mol −1) CO2
concentration. Error bars depict the standard error. The effect of CO 2
concentration and pot volume were significant (P < 0.05) for root
mass. Differences in stem and needle mass were not significant. Initial
needle, stem and root mass were 18.0, 10.7 and 3.8 g, respectively.
657
with foliar concentrations of at least 2.0% nitrogen and 0.1%
phosphorus.
Carbon dioxide enrichment increased net photosynthetic
rates by about 60% (P < 0.0001) (Figure 2). From Week 13
onward, there was also an effect of pot volume on net photosynthetic rate (P < 0.01) that was first observed as a reduction
in net photosynthetic rates of seedlings grown in small pots in
ambient CO2 concentration. Overall, net photosynthetic rates
of seedlings grown in the small pots were reduced 12, 28 and
39% at Weeks 13, 19 and 22, respectively. At Week 13, there
was an interaction (P < 0.04) between pot volume and CO2
treatment because the small pot treatment decreased net photosynthetic rates of seedlings grown in ambient CO2 concentration by 25% compared with the mean of seedlings grown in
medium and large pots in ambient CO2, whereas net photosynthetic rates of seedlings grown in 2× ambient CO2 was not
decreased by the small pot treatment (Figure 2). This interaction did not occur at Week 19 or 22 because net photosynthetic
rate of seedlings grown in 2× ambient CO2 was also decreased
by the small pot treatment. Compared to the mean net photosynthetic rate of seedlings grown in medium and large pots at
Figure 2. Effects of pot volume (Small = 0.6 l, Medium = 3.8 l and
Large = 18.9 l) on mean photosynthetic rate and needle conductance
of loblolly pine seedlings grown in ambient (360 µmol mol −1) or 2×
ambient (720 µmol mol −1) CO2 concentration. All plants were measured at the growth CO2 concentration. Error bars depict the standard
error; C indicates a significant effect (P < 0.05) of CO2 concentration,
P indicates a significant pot volume effect and an asterisk indicates a
significant pot volume × CO2 interaction.
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WILL AND TESKEY
Week 19, net photosynthetic rate of seedings grown in small
pots was decreased 24 and 35% in the ambient and 2× ambient
CO2 treatments, respectively (Figure 2). At Week 22, the corresponding decreases were 34 and 42% (Figure 2).
Needle conductance of all seedlings was low one week after
transplanting but increased dramatically between Weeks 1
and 7. At Weeks 13, 19 and 22, needle conductance was significantly lower for seedlings grown in 2× ambient CO2 than
for seedlings grown in ambient CO2 (P < 0.0001) (Figure 2).
From Week 13 onward, the small pot treatment caused significant decreases in needle conductance (P < 0.001) (Figure 2).
There was an interaction between pot size and CO2 treatment
at Week 13 because needle conductance of seedlings grown in
small pots in ambient CO2 was reduced, whereas needle conductance of seedlings grown in small pots in 2× ambient CO2
was not (P < 0.02) (Figure 2). This interaction did not occur at
Weeks 19 or 22 because needle conductances of the seedlings
grown in small pots were reduced in both CO2 treatments.
Gas exchange results from the three sets of reciprocal CO2
measurements were similar and so only the last set (Week 22)
is presented (Figure 3). Carbon dioxide enrichment during
measurement increased net photosynthetic rate by approxi-
Figure 3. Effects of pot volume (Small = 0.6 l, Medium = 3.8 l and
Large = 18.9 l) on mean photosynthetic rate of loblolly pine seedlings
grown in ambient (360 µmol mol −1) or 2× ambient (720 µmol mol −1)
CO2 concentration and measured at both ambient (360 µmol mol −1)
and 2× ambient (720 µmol mol −1) CO2 concentration after approximately 5 months of treatment exposure. Error bars depict the standard
error. The effects of measurement CO2 concentration and pot volume
were significant (P < 0.05), but growth CO2 concentration and all
interactions were not significant.
mately 70% for all seedlings regardless of growth CO2 concentration (Figure 3). Seedlings grown in small pots had significantly lower net photosynthetic rates than seedlings grown in
medium or large pots, but the effect of pot volume on net
photosynthetic rate was independent of growth and measurement CO2 concentrations (i.e., there were no pot volume × CO2
interactions). Thus, although seedlings grown in small pots in
2× ambient CO2 had the lowest absolute photosynthetic rates
when measured at a CO2 concentration of 360 or 720 µmol
mol −1, the relative photosynthetic stimulation due to CO2 enrichment was similar to that for seedlings in all of the other
treatments (Figure 3).
At Week 22, seedlings grown in small pots had more negative water potentials and lower rates of net photosynthesis than
seedlings grown in medium or large pots. When data for
seedlings in all of the pot treatments were plotted, the shape of
the water potential--net photosynthesis relationship was curvilinear and similar for seedlings in both CO2 treatments (Figure 4), indicating that low water potentials were closely
correlated with the low gas exchange rates in seedlings grown
in small pots at the end of the experiment.
Needle starch concentrations were significantly higher in
seedlings grown in 2× ambient CO2 concentration than in
seedlings grown in ambient CO2 concentration. However, at
both Weeks 7 and 22, there was a significant interaction between pot volume and CO2 concentration (P < 0.05) for starch
because seedlings grown in small pots in 2× ambient CO2
concentration had greater starch concentrations than seedlings
in other treatments (Figure 5). Needle starch concentrations
were lower at Week 22 than at Week 7 for seedlings in all
Figure 4. Effects of pot volume (Small = 0.6 l, Medium = 3.8 l and
Large = 18.9 l) on the relationship between net photosynthesis and
needle water potential for loblolly pine seedlings grown in ambient
(360 µmol mol −1) or 2× ambient (720 µmol mol −1) CO2 concentration
after approximately 5 months of exposure.
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ELEVATED CO2, ROOT RESTRICTION AND PHOTOSYNTHESIS
659
Figure 5. Effects of pot volume (S = 0.6 l, M = 3.8 l and L = 18.9 l) on
foliar total sugar and starch concentrations of loblolly pine seedlings
grown in ambient (360 µmol mol −1) or 2× ambient (720 µmol mol −1)
CO2 concentration. Error bars depict the standard error.
treatments except seedlings grown in medium and small pots
in ambient CO2 concentration (Figure 5). Although starch
concentrations increased in seedlings grown in small pots in
ambient CO2, at Week 22 these seedlings had starch concentrations that were lower than or similar to those of seedlings in
other treatments that did not exhibit a decrease in photosynthetic rate (e.g., the plants grown in medium and large pots in
the 2× ambient CO2 treatment).
Plants grown in 2× ambient CO2 concentration had significantly greater (P < 0.0001) needle sucrose concentration (Figure 6) and therefore greater total sugar concentrations (glucose
+ fructose + sucrose) (Figure 5) than plants grown in ambient
CO2 concentration. Although there was a significant interaction between pot volume and CO2 concentration for sucrose
and total sugar at Week 7, the effect of CO2 concentration was
unambiguous because the lowest mean sucrose and total sugar
concentrations of seedlings in any 2× ambient CO2 treatment
group was greater than the highest mean concentrations of
seedlings in any ambient CO2 treatment group. Sucrose concentrations of seedlings grown in small pots increased slightly
from Week 7 to Week 22, but they were always lower than
those of seedlings grown in medium and large pots.
Although glucose concentration was higher in seedlings
grown in small pots than in seedlings grown in medium or
large pots at Week 7 (P < 0.03) and fructose concentration was
higher in seedlings grown in ambient CO2 concentration than
in seedlings grown in 2× ambient CO2 concentration, no consistent trends between pot volume or CO2 concentration were
evident (Figure 6). Both fructose and glucose concentrations
were lower at Week 22 than at Week 7 (Figure 6).
Figure 6. Effects of pot volume (S = 0.6 l, M = 3.8 l and L = 18.9 l) on
mean foliar fructose, glucose and sucrose concentrations of loblolly
pine seedlings grown in ambient (360 µmol mol −1) or 2× ambient (720
µmol mol −1) CO2 concentration. Error bars depict the standard error.
Discussion
Net photosynthetic rates were higher for seedlings measured
in elevated CO2 than for seedlings measured in ambient CO2
regardless of growth CO2 concentration or pot volume. Seedlings in all treatments exhibited the same relative response to
CO2 concentration during the reciprocal measurements indicating that the effect of CO2 enrichment was similar for seedlings in all treatments. Photosynthetic rates of seedlings grown
in small pots in the 2× ambient CO2 treatment decreased during
the second half of the experiment; however, photosynthetic
rates of seedlings grown in small pots in ambient CO2 decreased earlier, indicating that the decreases in photosynthetic
capacity were caused by root confinement rather than CO2
enrichment.
Root restriction did not change the relative response of
photosynthesis to CO2 enrichment indicating that CO2 enrichment had a stimulatory effect regardless of pot size. Results
from studies examining the interaction between CO2 concentration and pot volume in other species are variable. For example, Thomas and Strain (1991) found that cotton seedlings
(Gossypium hirsutum L.) planted in small pots and exposed to
CO2 enrichment exhibited a decrease in photosynthetic capacity and an accumulation of starch and concluded that decreased
sink strength imposed by root restriction reduced the photosynthetic response to CO2. However, more recent studies on
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WILL AND TESKEY
cotton show that photosynthetic rate decreased in response to
CO2 enrichment regardless of whether plants were grown in
large or small pots (Barrett and Gifford 1995a, 1995b). Samuelson and Seiler (1992) observed a decrease in net photosynthesis of Fraser fir (Abies fraseri (Pursh.) Poir.) plants
grown in small pots in CO2 enrichment when measured at
346 µmol mol −1 or 796 µmol mol −1 CO2 concentration and a
decrease in net photosynthesis of plants grown in large pots in
CO2 enrichment when measured at 796 µmol mol −1 CO2 but
not 346 µmol mol −1. Thus, the effects of CO2 enrichment and
rhizosphere restriction on photosynthesis appear to be strongly
species specific.
Carbohydrate concentrations either decreased during the
experiment or were lower in plants grown in small pots than in
plants grown in medium or large pots. Thus, our carbohydrate
data provided no indication that carbohydrate accumulation in
foliage of seedlings grown in small pots caused the observed
reduction in net photosynthesis. Therefore, we conclude that
carbohydrate-induced feedback inhibition did not cause the
decrease in net photosynthesis in plants grown in small pots. It
is possible that feedback inhibition did not occur in these
loblolly pine seedlings as a result of the recurrent flushing
pattern of the species. New shoot growth and continued stem
diameter growth may have provided adequate photosynthate
sinks so that needle carbohydrate concentrations never reached
inhibitory levels. An alternative explanation is that photosynthesis of loblolly pine seedlings is insensitive to high carbohydrate concentration regardless of the concentration of CO2
exposure.
Although our loblolly pine plants grown in small pots exhibited a decrease in photosynthetic capacity without a corresponding increase in carbohydrate concentration, other studies
have shown that decreased net photosynthesis is associated
with carbohydrate accumulation in plants grown in small pots.
For example, cucumber plants (Cucumis sativus L. cv Calypso) grown in small pots exhibited decreased photosynthetic
rates and increased starch concentrations when starch was
measured over a 24-h period (Robbins and Pharr 1988). Similarly, peach trees (Prunus persica L.) grown in small pots
showed a decrease in photosynthetic rate along with an accumulation of starch and sorbitol (Rieger and Marra 1994). In a
study of the effects of root restriction in fruiting and nonfruiting peach trees, Mandre et al. (1995) found a negative relationship between net photosynthesis and leaf carbohydrate
concentration; however, this relationship was inconsistent
among treatment groups and led the authors to conclude that
something other than feedback inhibition was contributing to
the decrease in net photosynthesis in the nonfruiting trees.
We observed a strong negative correlation between water
potential and net photosynthesis as well as a decrease in needle
conductance in seedlings grown in small pots. These findings,
coupled with the observation that water potentials of seedlings
grown in small pots were lower than those of seedlings grown
in the medium and large pots and low enough to decrease net
photosynthesis (Seiler and Johnson 1985, Teskey et al. 1986,
Seiler and Johnson 1988), led us to conclude that the decrease
in net photosynthetic rates of plants grown in small pots was
probably caused by water stress because the small and restricted root systems were unable to absorb enough water even
though a continuous supply was available. Similarly,
Tschaplinski and Blake (1985) found that root restriction
caused a decrease in net photosynthesis, an increase in stomatal resistance and a decrease in leaf water potential in Alnus
glutinosa (Gaertn.) and concluded that water stress was the
mechanism that caused the decrease in net photosynthesis.
Because the Alnus glutinosa plants were grown in solution
culture, water stress was not caused by lack of available water,
but resulted from an inability of the restricted roots to take up
an adequate amount of water.
Seedling water status may also explain why net photosynthesis of seedlings grown in small pots decreased earlier in the
ambient CO2 treatment than in the 2× ambient CO2 treatment.
At Week 13, when net photosynthetic rates of seedlings grown
in small pots in ambient CO2 were lower than those of seedlings grown in small pots in 2× ambient CO2, needle conductance was significantly lower in seedlings in the 2× ambient
CO2 treatment. Thus, seedlings grown in small pots in 2×
ambient CO2 were losing less water on a per needle area basis
than seedlings grown in small pots in ambient CO2. Because
needle areas of seedlings grown in ambient or 2× ambient CO2
were similar, this implies that seedlings grown in small pots in
2× ambient CO2 used less water and would therefore experience water stress later in the experiment than seedlings grown
in small pots in ambient CO2.
Our results support the conclusion that loblolly pine plants
will benefit from a high CO2 environment because they do not
decrease their photosynthetic capacity when exposed to longterm CO2 enrichment (Ellsworth et al. 1995, Liu and Teskey
1995, Teskey 1995, Tissue et al. 1996). An exception may be
plants growing under conditions of nutrient stress, because
Tissue et al. (1993) and Thomas et al. (1994) were able to
induce down-regulation in loblolly pine by supplying low
nutrient concentrations to potted seedlings. The plants that
exhibited decreased net photosynthetic rates were deficient in
foliar nitrogen with concentrations of approximately 0.5 to
0.7%, which is well below the typical foliar nitrogen concentrations reported on nutrient-poor sites (1.07%, Van Lear et al.
1984; and 0.90%, Valentine and Allen 1990). The inability to
force feedback inhibition by severe root restriction indicates
that loblolly pine is unlikely to show a decrease in photosynthetic capacity in response to carbohydrate accumulation induced by elevated CO2 environments.
Acknowledgments
We thank Dr. Mark Rieger from the Department of Horticulture at
University of Georgia for the use of his laboratory for the carbohydrate
analysis, Paula Litvin for assistance during the carbohydrate analysis,
Patsy Mason and Susan Wilson of the USDA Phytochemical research
unit in Athens, GA for analyzing the carbohydrate samples and William Jennings (Jay) Brown for his aid and expertise in growing
loblolly pine.
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