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2 FiguresDirect inhibition of maintenance respiration in western hemlock roots exposed to ambient soil carbon dioxide concentrations
Abstract
Root respiration often exhibits a direct and immediate decline with increasing concentrations of ambient soil carbon dioxide
concentration ([CO2]), and recent evidence suggests this decline may be attributable to a decline in maintenance respiration within the root.
If true, this concept could provide a clue to the biochemical process underlying respiratory inhibition as well as improve
our knowledge of the timing and degree to which this inhibition occurs in nature. To test the hypothesis that maintenance
respiration exhibits a direct, negative response to increasing [CO2], we measured total respiration in intact root systems of western hemlock (Tsuga heterophylla (Raf.) Sarg.) seedlings grown at different relative growth rates and exposed to soil [CO2]s ranging from 91 to 7008 μmol mol−1. Analysis of covariance was used to separate maintenance from total respiration. Total respiration declined exponentially
with increasing [CO2]. Maintenance respiration, which comprised 85% of total respiration over all treatments, also declined exponentially with
increasing [CO2]. Growth respiration was not inhibited at any [CO2]. These findings may explain why roots of some fast-growing species do not show [CO2] inhibition.
Figures
Summary Root respiration often exhibits a direct and imme-
diate decline with increasing concentrations of ambient soil
carbon dioxide concentration ([CO
2
]), and recent evidence
suggests this decline may be attributable to a decline in main-
tenance respiration within the root. If true, this concept could
provide a clue to the biochemical process underlying respira-
tory inhibition as well as improve our knowledge of the timing
and degree to which this inhibition occurs in nature. To test the
hypothesis that maintenance respiration exhibits a direct, nega-
tive response to increasing [CO
2
], we measured total respira-
tion in intact root systems of western hemlock (Tsuga
heterophylla (Raf.) Sarg.) seedlings grown at different relative
growth rates and exposed to soil [CO
2
]s ranging from 91 to
7008 µmol mol
−1
. Analysis of covariance was used to separate
maintenance from total respiration. Total respiration declined
exponentially with increasing [CO
2
]. Maintenance respiration,
which comprised 85% of total respiration over all treatments,
also declined exponentially with increasing [CO
2
]. Growth
respiration was not inhibited at any [CO
2
]. These findings may
explain why roots of some fast-growing species do not show
[CO
2
] inhibition.
Keywords: maintenance respiration, Pinaceae, root respira-
tion, Tsuga heterophylla.
Introduction
Net carbon gain in plants is the balance between photosyn-
thetic gains and respiratory use of carbon; hence both can
significantly affect plant net primary productivity. Root respi-
ration is a variable and often high carbon cost, consuming
between 2 and 52% of gross photosynthesis annually (Linder
and Lohammar 1981, Ewel et al. 1987, Behera et al. 1990,
Ryan et al. 1994, Lambers et al. 1996). Because of variability
and potential size, root respiration is often included in models
of ecosystem productivity (Ryan et al. 1996a, Eissenstat and
Yanai 1997). The root respiration values used for model para-
meterization are usually obtained from studies of the func-
tional controls over in situ root respiration rates (e.g., Van der
Werf et al. 1989, Cropper and Gholz 1991, Zogg et al. 1996).
To measure root respiration, such studies must either overcome
methodological difficulties or assume disturbance of the soil to
access roots has no effect on respiration rates (Vogt et al. 1989).
Respiration is often inhibited by high carbon dioxide con-
centrations ([CO
2
]) (see reviews by Reuveni et al. 1993,
Amthor 1994a). Two effects of elevated [CO
2
] on respiration
have been identified: (a) a direct immediate effect where the
observed rate reversibly declines with increasing [CO
2
], and
(b) an indirect acclimation effect where plant tissues grown in
elevated [CO
2
] (> 365 µmol mol
−1
) usually respire less than
plant tissues grown in lower [CO
2
], when measured at similar
[CO
2
] (Amthor 1991, 1994a). The direct effect of elevated
[CO
2
] on root respiration has been documented in cacti (Palta
and Nobel 1989), conifer seedlings (Qi et al. 1994), mature
deciduous trees (Burton et al. 1997) and in isolated mitochon-
dria from roots of crop plants (Reuveni et al. 1995, Gonzàlez-
Meler et al. 1996).
Carbon dioxide concentrations observed in forest soils are
commonly an order of magnitude greater than aboveground
[CO
2
], (Kiefer and Amey 1992) and exhibit great seasonal and
spatial variability (Mattson 1995, N. McDowell unpublished
observations). Current knowledge of respiratory inhibition by
CO
2
and the nearly ubiquitous existence of elevated and vari-
able soil [CO
2
] provide compelling evidence for the impor-
tance of accounting for the respiratory response to soil [CO
2
]
when estimating in situ root respiration. Most field studies of
root respiration have been conducted on roots that were re-
moved from the soil and measured at aboveground ambient or
unknown [CO
2
] (but see Ryan et al. 1996b, Zogg et al. 1996).
Plant respiration is commonly divided into two functional
components, maintenance (R
m
) and growth (R
g
) respiration
(Amthor 1989). Maintenance respiration results from the gen-
eration of energy used to maintain living biomass, and varies
primarily with temperature and protein content of the tissue,
whereas growth respiration is used for the construction of new
biomass and varies primarily with growth rate and chemical
composition of the tissue (Penning de Vries et al. 1974,
Amthor 1989). The annual fraction of forest stand respiration
associated with R
m
ranges from 30 to 80% (Ryan et al. 1994,
Edwards and Hanson 1996, Ryan et al. 1996b). Root R
m
is
directly inhibited by elevated [CO
2
] in Douglas-fir seedlings
(Pseudotsuga menziesii var. glauca [Beissn.] Franco) grown at
the light compensation point (Qi et al. 1994). In contrast, total
Direct inhibition of maintenance respiration in western hemlock roots
exposed to ambient soil carbon dioxide concentrations
NATE G. MCDOWELL,
1,2
JOHN D. MARSHALL,
1
JINGEN QI
1
and KIM MATTSON
1
1
Department of Forest Resources, University of Idaho, Moscow, ID 83844-1133, USA
2
Present address: Department of Forest Science, Oregon State University, Corvallis, OR 97331, USA
Received June 5, 1998
Tree Physiology 19, 599--605
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root respiration (R
t
) of some species shows little response to
[CO
2
] over the range tested by Qi et al. (< 300--2000 µmol
mol
−1
; Nobel and Palta 1989, Palta and Nobel 1989, Bouma et
al. 1997a, 1997b). Roots from those studies were presumably
growing faster than the roots of shaded Douglas-fir seedlings
used by Qi et al. (Bouma et al. 1997a). Because faster-growing
tissues should have proportionally more growth respiration, it
has been suggested that R
m
may be more sensitive to [CO
2
]
than R
g
(Qi et al. 1994, Bouma et al. 1997a). This hypothesis
was recently tested by Bouma et al. (1997a) in bean roots
(Phaseolus vulgaris L.), in which elevated [CO
2
] caused no
significant inhibition of either R
m
or R
g
; they suggest that the
respiratory response of roots may be species specific. The
effects of [CO
2
] on R
m
, R
g
and R
t
need clarification before we
can understand and estimate root respiration in situ. We con-
ducted an experiment to examine the direct or short-term
effects of [CO
2
] on root maintenance, growth and total respi-
ration in western hemlock seedlings (Tsuga heterophylla
[Raf.] Sarg.).
Materials and methods
Growth conditions
Seedlings of western hemlock were grown from seed collected
from a 40-year-old stand in the western Cascade Range near
Scio, Oregon. Seedlings were grown in 66-ml tube containers
at the University of Idaho Research Nursery glasshouse from
April through August 1993 under standard irrigation and fer-
tilization regimes in non-sterilized commercial potting soil
(Wenny and Dumroese 1992). After five months, seedling
roots were carefully washed with distilled water and the plants
transferred to individual root boxes. Mycorrhizae were not
apparent at the time of transfer nor at any time during the
experiment. Plants were supplied with nutrient solution every
other day (Lu 1994).
The root boxes used in this experiment were a modification
of an original design by Neufeld et al. (1989). Detailed descrip-
tions of the structure and function of the root boxes are given
in Qi (1994) and Qi et al. (1994). The boxes were constructed
by milling a piece of plastic to include a central cavity and an
outer groove for an O-ring. The cavity was covered by a sheet
of nylon fabric and the root system was inserted between the
fabric and a glass plate. The plate was tightened against the
O-ring forming a seal. Root boxes (152 × 25.4 × 508 mm) were
filled with silica sand to minimize heterotrophic activity. The
roots were undisturbed during the study. The barbed hose
fittings that served as outlets were left open except when boxes
were measured. Root box [CO
2
] was measured by inserting a
syringe through the outlet to extract a gas sample. The sample
was extracted from the middle of the box near the middle of
the eight-week growth period and analyzed by gas chromatog-
raphy (Model 3700, Varian Instruments, Sugar Land, TX). The
gas chromatograph was equipped with a flame ionization de-
tector, and used a 180-cm long, 7-mm diameter stainless steel
column packed with Porapak QS. Nitrogen was used as the
carrier gas. Carbon dioxide was reduced to CH
4
by a ruthe-
nium catalytic converter.
Partitioning maintenance and growth respiration
Because R
m
and R
g
are biochemically indistinct, we induced
different relative growth rates and then used regression analy-
sis to estimate R
m
and R
g
. Different relative growth rates (RGR)
were induced with layers of shade cloth. The seedlings were
randomly assigned to different light treatments (approximately
53, 20, 11, 5 and 3% of full sun). The RGR (g g
−1
day
−1
) for
seedling roots was determined by tracing the new area of root
growth along the Plexiglas wall of the root box onto acetate
sheets. We measured RGR at 0, 2, 4 and 6 weeks after trans-
planting to the root boxes, and immediately before harvest
after 8 weeks. Root respiration was measured midway between
the third and fourth root area measurements on most seedlings.
All RGR values presented were calculated from the root area
measurements before and after the respiration measurements.
New root biomass (dry weight) was measured on five ran-
domly harvested seedlings after six weeks and on all seedlings
at harvest. Change in root area was measured by digitizing the
new root growth from the acetate sheets with a digital image
analysis system (JAVA, Jandel Scientific/SPSS Inc., Chicago,
IL). Regression analysis was used to predict new root biomass
from the root area and biomass sampling.
Respiration measurements
Root respiration was measured five times on a single day for
each seedling. On the day of measurement, we applied five
treatment [CO
2
]s: 0, 78, 350, 1565 and 7000 µmol mol
−1
.
Treatments were applied randomly, though there was no effect
of progressively increasing versus progressively decreasing
[CO
2
] on root respiration rate at a given concentration (Qi et
al. 1994). Although Qi et al. (1994) found no hysteresis in
response, we allowed two to three hours of exposure to a given
[CO
2
] before measurement to avoid variation caused by time-
dependent respiration responses (Gonzàlez-Meler et al. 1996)
and to allow soil [CO
2
] pools to equilibrate. One seedling (all
CO
2
concentrations) was measured per day. Respiration meas-
urements were made in the laboratory at 23.0 ± 1.0 °C. Relative
humidity inside the box was maintained at 100% by passing
the inlet air through a humidifier before it entered the root
boxes. All seedlings were maintained under similar conditions
during the measurement period.
The CO
2
-exchange measuring system, detailed in Qi et al.
(1994), is designed to measure CO
2
production from the root
box by precisely balancing it against a known CO
2
addition.
Carbon dioxide
was added to the reference cell with a cali-
brated mass-flow meter at a rate that caused the infrared gas
analyzer to read zero. Under these conditions, the root box
respiration rate and the reference cell CO
2
addition rate were
equal, and the respiration rate could be inferred from the
mass-flow meter reading. The advantage of this system is that
it maintains accurate calibration throughout the treatment CO
2
concentration range. Root box air leakage, measured as the
proportion of flow lost between the inlet and outlet, averaged
15% across all of the root boxes. A test of the effect of root box
leakage on observed respiration rates was conducted through
linear regression of the ratio of respiration at 7000 µmol mol
−1
and at 350 µmol mol
−1
against root box specific leak rates (data
600 MCDOWELL, MARSHALL, QI AND MATTSON
TREE PHYSIOLOGY VOLUME 19, 1999
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not shown). Both x- and y-axis values were logarithmically
transformed before regression. No relationship was found be-
tween the respiration ratio and leakage (n = 25, R
2
< 0.01, P >
0.85). An independent test was conducted by applying each
[CO
2
] treatment to an empty root box and measuring apparent
respiration rates. Variation in the ratio of root box respiration
to mean root respiration (for a given [CO
2
], ln-transformed)
showed no correlation with ln-transformed [CO
2
] (n = 7, R
2
=
0.16, P = 0.2). Therefore, root box leakage of CO
2
was not
correlated with observed variation in respiration among [CO
2
]
treatments, and thus was not responsible for apparent respira-
tory responses.
Respiratory flux within each root box increased the [CO
2
]
within the root boxes slightly above the applied inlet [CO
2
].
Mean outlet [CO
2
] was 96, 151, 395, 1585 and 7009 µmol
mol
−1
at inlet treatment concentrations of 0, 78, 350, 1565 and
7000 µmol mol
−1
. We assumed that the soil air was well mixed
and therefore the outlet values are the true concentrations
surrounding the roots during measurement of respiration rates.
These values were used in the statistical analysis, and will be
presented henceforth. After the experiment, the seedlings were
harvested and dried for 48 h at 70 °C. Because root box
background respiration was previously found to be less than
15% of total CO
2
flux, we assumed that the observed respira-
tion was entirely caused by roots (Qi et al. 1994). This method
assumes that the short-term [CO
2
] treatments have no instan-
taneous effect on RGR. This assumption has rarely been tested;
however, Bouma et al. (1997a) found no effect of [CO
2
] on
root growth. Based on data collected from 10 published stud-
ies, Poorter and Villar (1997) found a strong correlation be-
tween R
g
estimated by the regression approach and calculated
from construction costs based on the amount of glucose re-
quired to construct a gram of tissue. Therefore, the regression
approach appears valid.
We were concerned that roots may differentially acclimate
to the root box [CO
2
] experienced during growth (termed
‘‘indirect effects,’’ see Introduction). Relative growth rate was
correlated with growth period [CO
2
] (n = 25, r = 0.58, P <
0.01), a consequence of higher respiration rates associated
with rapidly growing roots. Regression of total respiration at
1585 ppm (the treatment [CO
2
] closest to the observed grow-
ing period [CO
2
]) versus growing period [CO
2
] exhibited a
positive correlation (n = 25, r = 0.52, P < 0.02). However,
according to the definition of indirect effects (Amthor 1994a),
respiration should decline, not increase, with increases in
growing period [CO
2
]. We conclude that any indirect effects of
growing period [CO
2
] were overwhelmed by the higher rates
of respiration associated with increasing RGR.
Statistical analyses
Analysis of covariance (ANCOVA) was used to test for effects
of [CO
2
] on R
m
and R
g
(Thornley 1970, Szaniawski and
Kielkiewicz 1982, Amthor 1989). In this method the slope of
each individual regression line equals respiration attributable
to R
g
, and the intercepts equal R
m
(see Figure 1). The test for
treatment effects on R
g
was conducted by analyses of the linear
regression slopes of each treatment; if the slopes are signifi-
cantly different from parallel then treatment [CO
2
] had signifi-
cant effects on R
g
. The test for treatment effects on R
m
compares the y-intercepts of the regression line for each CO
2
treatment. All assumptions of ANCOVA were tested as re-
quired, including normality, homogeneity, autocorrelation and
parallel slopes (Ott 1988, Neter et al. 1990). Analyses were
done with the SYSTAT 5.03 (SPSS Inc.) statistical package
with a level of significance (α) of 0.05.
Results
Root system relative growth rate and soil CO
2
concentrations
At the end of the experiment, mean total weight of the western
hemlock seedlings was 1.67 g (SE = 0.18 g) and mean root
weight was 0.76 g (SE = 0.09 g). Projected surface area of new
roots was strongly correlated with dry mass of new roots (R
2
=
0.98); mean ratio of new root area to new root weight (specific
root area) was 1.2 mm
2
g
−1
. Estimates of relative growth rate
(RGR) for the roots ranged from 0.0004 to 0.0154 g g
−1
day
−1
(n = 25, mean = 0.003, SD = 0.004 g g
−1
day
−1
). Root box
[CO
2
] ranged from 974 to 4542 µmol mol
−1
(n = 25, mean =
2196, SD = 974 µmol mol
−1
); these values are typical of
[CO
2
]s found in representative soils in the Pacific Northwest
(Mattson 1995, N. McDowell, unpublished observations).
Total, maintenance and growth respiration
Mean root respiration rates at 23 °C and [CO
2
]s of 96, 151,
395, 1585 and 7008 µmol mol
−1
were 0.055, 0.041, 0.025,
0.011, and 0.005 g C g
−1
day
−1
respectively. Total root respi-
ration rates exhibited a significant relationship to RGR (P =
0.02, Table 1; Figure 1) and were strongly influenced by [CO
2
]
treatment (P < 0.01, Table 1; Figure 1). There was no variation
in the slope of the relationship between R
g
and RGR (P = 0.92);
the influence of [CO
2
] was exclusively on R
m
. The power to
detect a change in slopes with a resulting change in model R
2
Table 1. Analysis of covariance for total respiration rate compared to relative growth rate at varyious [CO
2
]s. Maintenance respiration is estimated
as the intercept of each line relating respiration to RGR at each [CO
2
]. Growth respiration is estimated as the mean slope of all lines. The interaction,
which tests for differences in slope among [CO
2
] treatments with respect to RGR, was not significant (P = 0.92).
Source Sum-of-squares df Mean-square F-ratio P
RGR (R
g
) 0.001 1 0.001 5.645 0.02
[CO
2
] (R
m
) 0.037 4 0.009 71.271 0.00
Error 0.013 99 0.000 -- --
CARBON DIOXIDE INHIBITION OF ROOT MAINTENANCE RESPIRATION 601
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of 0.05 (at α = 0.05) was 0.98 (NCSS-PASS v.1, 1991).
Therefore, it is unlikely that a significant effect of [CO
2
]
treatment on slopes would have passed undetected by our analy-
sis. We therefore collapsed the slopes of the respiration versus
RGR curves and estimated growth respiration as 0.68 g C (g dry
weight)
−1
, which converts to 1.36 g C g C
−1
if one assumes a
root carbon concentration of 0.5 g C (g dry weight)
−1
.
Mean maintenance respiration rates at 23°C and [CO
2
]s of
96, 151, 395, 1585 and 7008 µmol mol
−1
were 0.054, 0.039,
0.024, 0.009, and 0.003 g C g
−1
day
−1
, respectively (Figure 2).
Maintenance respiration comprised 85% (SE = 7%) of total
respiration across [CO
2
] treatments. Maintenance respiration
rates exhibited an exponential decline of approximately 37%
for every doubling of CO
2
concentration (Figure 2). The re-
gression equation describing this relationship explained more
that 99% of the variation in the means.
A general model of root respiration can be generated from
the regression equation predicting maintenance respiration
rates from [CO
2
]:
R
m
= exp(0.18157 − 0.6712 ln([CO
2
])), (1)
where R
m
is the maintenance respiration rate in g C g
−1
day
−1
,
and [CO
2
] is the soil carbon dioxide concentration in µmol
mol
−1
. Equation 1 can be combined with the equation predict-
ing total respiration (g C g
−1
):
R
t
= 0.68RGR + R
m
, (2)
where R
t
is the total respiration rate in g C g
−1
day
−1
and RGR
is the relative growth rate (g g
−1
day
−1
).
Discussion
The temperature response (Q
10
) of conifer roots is often 2.0 or
slightly lower (Ryan et al. 1994), so we have used the Q
10
value
of Cropper and Gholz (1991) of 1.94 to adjust our observed
root respiration rates to a temperature of 15 °C for comparison
with published estimates. The mean root respiration rate of
0.015 g C g
−1
day
−1
at 15 °C and 395 µmol mol
−1
for western
hemlock seedlings is similar to other published estimates for
conifer roots of similar age at a similar temperature and [CO
2
].
Root respiration rates from species of Pinus, Tsuga, Pseudot-
suga, and Picea ranged from 0.002 to 0.024 g C g
−1
day
−1
, with
seedlings consistently having higher rates than mature trees
(Krueger and Ruth 1969, Qi et al. 1994, Ryan et al. 1994).
Total root respiration rates at 1585 and 7009 µmol mol
−1
equaled 45 and 19% of that at 395 µmol mol
−1
, respectively.
This reduction in total respiration within hours of exposure to
elevated [CO
2
] constitutes direct inhibition of root respiration
(Amthor 1994a). It appears that the direct response occurs in
both seedlings and mature trees, and in inert sand and field
soils. A reduction in fine-root respiration at 1500 µmol mol
−1
to 23% of the rate at 400 µmol mol
−1
was found in mature,
field-grown Pinus radiata roots using a [CO
2
] correction equa-
tion similar to Equation 4 (see later discussion) for mature
conifer roots (Ryan et al. 1996b). Similar responses have been
reported for deciduous trees (Acer saccharum Marsh, Burton
et al. 1997), and at higher [CO
2
]s in cacti (Palta and Nobel
1989), but not in citrus (Citrus volkameriana Tan. & Pasq.) and
bean (Bouma et al. 1997a, 1997b).
Maintenance respiration rates at 1585 and 7009 µmol mol
−1
were approximately 40 and 12% of R
m
at 395 µmol mol
−1
.
Maintenance respiration in Douglas-fir seedlings at 7015 µmol
mol
−1
was reduced to 41% of the rate at 350 µmol mol
−1
(Qi
et al. 1994). The lower rates and greater inhibition of mainte-
nance respiration for western hemlock may be associated with
the low metabolic activity (Amthor 1994b) of this late-succes-
Figure 2. Maintenance respiration rates relative to ambient soil
[CO
2
]s. The mean of the standard errors was 0.0026 (n = 25).
Figure 1. Respiration rates measured at various [CO
2
]s, as a function
of relative growth rate. Each line is associated with a common [CO
2
]
treatment, where the y-intercept estimates maintenance respiration and
the slope estimates the change in growth respiration with respect to
RGR. Each line was generated from the analysis of covariance. Values
of R
m
are: 0.054, 0.039, 0.024, 0.009 and 0.003 g C g
−1
day
−1
at 96
(s), 151 (j), 395 (n), 1585 (d) and 7009 (u) µmol mol
−1
, respec-
tively.
602 MCDOWELL, MARSHALL, QI AND MATTSON
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sional, shade-tolerant species that typically grows more slowly
than Douglas-fir.
Mean growth respiration rates were 0.68 g C g
−1
, which is
higher than estimates from studies of the cost of synthesis
(Penning de Vries et al. 1974) and those that separate ion
uptake from growth-associated respiration (for example,
Bouma et al. 1996, Mata et al. 1996). Respiration associated
with ion uptake is sometimes considered separately from
maintenance and growth; however, in this study we used a
regression approach that includes ion uptake respiration in the
growth respiration estimate, and assumes that ion uptake res-
piration varies linearly with R
g
. This may explain why our R
g
estimate is high compared with values obtained in studies that
separate ion-associated flux. Ion uptake respiration can consti-
tute over 40% of total root respiration in some herbaceous
species; however, herbs generally exhibit RGRs tenfold higher
than those of conifers (Lambers et al. 1996). Because ion
uptake is correlated with growth, ion uptake respiration will
constitute a much smaller proportion of R
t
in conifers than in
herbaceous species. This is particularly true in late-succes-
sional conifers, such as western hemlock, that assimilate al-
most no nitrate and hence have low nitrate reduction costs
(e.g., Kronzucker et al. 1997), and assimilate relatively low
amounts of nitrogen (in any form) because of relatively slow
growth rates. Although we suspect that very little ion uptake
respiration occurred, that which did occur may have increased
our growth respiration coefficient (T. Bouma, Netherlands
Institute for Ecology, Maarsen, the Netherlands, personal com-
munication).
Dark CO
2
fixation in roots may play a role in respiratory
decline with increasing [CO
2
], particularly in roots with high
RGR. Amthor (1997) suggests that increased fixation by phos-
phoenolpyruvate carboxylase (PEPC) following short-term in-
creases in CO
2
in the dark may contribute to the observed
respiratory inhibition. However, Burton et al. (1997) con-
cluded that there was no refixation in mature sugar maple roots
because the ratio of CO
2
efflux to O
2
consumption did not vary
among short-term [CO
2
] treatments ([CO
2
]s ranging from 350
to 20,000 µmol mol
−1
). If re-fixation had caused a reduction in
CO
2
efflux, then the rate of CO
2
production would have de-
clined relative to O
2
consumption at the higher [CO
2
]s. Based
on this finding in roots, and the lack of consensus in the
literature on the role of PEPC (Amthor 1997), we do not
believe that refixation explains the observed respiratory de-
cline with elevated CO
2
.
It was previously speculated that increasing [CO
2
] reduced
activity of the alternative non-phosphorylating (cyanide resis-
tant) pathway relative to the cytochrome pathway of respira-
tion (Amthor 1991, Qi et al. 1994); however, the evidence for
this concept is mixed. The alternative pathway consistently
showed no effect of elevated [CO
2
], whereas the cytochrome
pathway was strongly inhibited in studies with isolated mito-
chondria from Glycine max (L.) Merrill roots (Gonzàlez-Meler
et al. 1996), and in callus cells of carnation (Palet et al. 1991).
However, the cytochrome pathway was unaffected in isolated
mitochondria from Medicago sativum L. roots (Reuveni et al.
1995). In most studies, the reduction in respiration appears to
be related to inhibition of the cytochrome pathway rather than
alternative pathway activity. We note that most previous stud-
ies of the alternative path have involved the use of chemical
inhibitors; a technique that has recently been questioned (Mil-
lar et al. 1995).
Could the alternative pathway be more active as RGR in-
creases, thus decreasing respiratory inhibition? It has been
suggested that greater demand for carbon skeletons in fast-
growing tissues may be responsible for increased activity of
the alternative pathway during periods of growth (Amthor
1994a). The alternative pathway has been shown to operate at
maximal rates in young, actively growing roots of carrot (Dau-
cus carota L.) and white spruce (Picea glauca [Moench] Voss),
whereas the cytochrome pathway was dominant when growth
ceased (Steingrover 1981, Johnson-Flanagan et al. 1986).
Likewise, Collier and Cummins (1989) found that leaves from
fast-growing weeds exhibited greater activity and capacity for
alternative pathway respiration compared to slower-growing
understory plants. These results lead to the hypothesis that
declining R
m
/R
t
causes decreasing respiratory [CO
2
] inhibition
because it increases the alternative pathway:cytochrome path-
way activity ratio.
The differential responses of R
m
and R
g
to short-term expo-
sure to elevated [CO
2
] observed in this study lend support to
the original hypothesis that maintenance and growth respira-
tion are differentially controlled by [CO
2
] (Qi et al. 1994). The
lack of similar results in citrus and bean support the hypothesis
that respiratory inhibition may be taxon-specific (Bouma et al.
1997a, 1997b). This hypothesis will be further supported if
variation in the activity and capacity of the two respiratory
pathways is found to be coordinated with both respiratory
inhibition and R
m
/R
t
among species. However, the appropriate
technique for estimation of CO
2
production by the alternative
and cytochrome pathway must first be established. Oxygen
isotopes offer a promising option for estimating the contribu-
tion of the alternative path to root respiration (Guy et al. 1989),
and may be used in place of, or in conjunction with, inhibitors
to determine the relationship of the two pathways with R
m
/R
t
.
In conifers, we suspect that as the ratio of R
m
/R
t
increases the
error in measurement of in situ root respiration at low atmos-
pheric [CO
2
] increases. Past studies that have measured respi-
ration without correcting for soil [CO
2
] during periods of rapid
root growth should have less error than studies of roots that
were not growing.
A final equation can be generated that provides estimates of
root respiration during periods of diverse RGR, temperature
and soil [CO
2
] conditions. Maintenance respiration can be
determined with a Q
10
measured on site or from the literature:
R
m
∗
= R
m
Q
10
((T−t)/10)
, (3)
where R*
m
is the maintenance respiration rate at any tempera-
ture T, R
m
is the respiration rate at measurement temperature t
(23 °C in this study) from Equation 1, Q
10
is the change in
respiration with a 10 °C change in temperature, and T is the
temperature in °C. Combining Equations 1, 2, and 3 yields an
equation that will predict total respiration rate for a hemlock
root under any given [CO
2
], soil temperature (T), and RGR:
CARBON DIOXIDE INHIBITION OF ROOT MAINTENANCE RESPIRATION 603
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R
t
∗
= (0.68RGR) +
[exp(0.182 − 0.672 ln[CO
2
])Q
10
((T−t)/10)
], (4)
where R*
t
is the total respiration rate in g C g
−1
day
−1
.
It is plausible that CO
2
inhibition is only indirectly related
to the ratio of R
m
/R
t
, as R
m
and R
g
may be more tightly coupled
than previously believed (Amthor 1989, Lavigne and Ryan
1997). The long-term acclimation response of root tissues
deserves more attention because of the variation in [CO
2
] of
the soil in which roots grow. Although the short-term response
has a significant effect on measurement accuracy and provides
information on the mechanism of respiratory inhibition, it is
the long-term response of R
m
, R
g
and R
t
to [CO
2
] that will
ultimately control root carbon budgets.
Conclusions
Maintenance respiration in hemlock roots is strongly control-
led by the ambient [CO
2
]. Growth respiration, in contrast,
exhibited no response to changing [CO
2
]. We propose that, as
the ratio of R
m
/R
t
decreases, the relative inhibition of root
respiration decreases. This inhibition should change with sea-
sonal changes in root growth and soil [CO
2
]. Results from the
literature raise the possibility that the biochemical control of
the observed inhibition lies within the partitioning of respira-
tion between the cytochrome and alternative pathways. Future
research on root respiration should account for the [CO
2
] at
which roots are measured, and autotrophic carbon budgets will
require estimates of soil [CO
2
].
Acknowledgments
This research was supported by USDA competitive grant No. 91-
37101-6856. We thank the University of Idaho Forest Research Nurs-
ery for supplying tree seedlings and greenhouse space; Dr. Gerald E.
Edwards (Washington State University) for advice on construction of
the respiration measurement system; Geneva E. Pym and Dr. Jianwei
Zhang (University of Idaho) for laboratory help; and Dr. Shengjun Lu
(Oregon State University) for building the root boxes. Drs. Mike Ryan
(USDA Forest Service) and Tjeerd Bouma (Netherlands Institute for
Ecology) provided helpful comments on an earlier version of this
manuscript.
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- ... The effect of changes in ambient environment on root respiration rate and δ 13 CO 2 is inherently difficult to study; the necessity to separate roots and soil in order to measure their unique respired δ 13 CO 2 values represents the major conundrum for efflux partitioning studies, which must accept the compromise between maintaining the natural, undisturbed root-soil system and achieving accurate measurements of root and SOM δ 13 CO 2 end-members. High chamber [CO 2 ] suppresses the CO 2 efflux from attached roots of cultivated Douglas fir and western hemlock seedlings (Qi et al. 1994; McDowell et al. 1999) but has no effect on attached roots of citrus seedlings or on excised fine roots of nine adult North American tree species (Buoma et al. 1997; Burton and Pregitzer 2002). Our method of sampling root CO 2 for isotope analysis requires that the chamber is flushed to remove CO 2 and this could potentially induce an artificially high rate of CO 2 diffusion (respiration) out of the roots. ...
- ... Until now, little information has been available on how the different types of sealant affect soil gas diffusivity or how urban trees react to soil aeration deficiencies. Against the background of existing investigations, soil aeration efficiencies hamper root respiration (Qi et al. 1994; McDowell et al. 1999; Gaertig et al. 2002), leading to the functional loss of fine roots (Rickman et al. 1965). Recent research in Kassel, Germany, showed that the lowest gas diffusivities and respiration rates were found at sealed sites, and the highest values were measured at vegetated sites such as lawn or flower beds (Weltecke and Gaertig 2012). ...
- ... If soil pore volume and soil pore continuity are reduced by compaction or sealing, roots cannot be supplied with oxygen and, conversely, CO 2 emissions from the soil are inhibited (Hildebrand, 1987; Herbauts et al., 1996; Horn et al., 2007). Against the background of existing investigations, soil aeration deficiencies hamper root respiration (Qi et al., 1994; McDowell et al., 1999; Gaertig et al., 2002), leading to the functional loss of fine roots (Rickman et al., 1965; Gaertig et al., 1999). As a consequence of a reduced root system, the tree may fail to absorb sufficient water and nutrients, resulting in the deterioration of crown structure and declining growth (Aslanboga, 1976; Hetsch et al., 1990; Gaertig, 2001; Uhl, 2008; Gaertig et al., 2010 ). ...
- ... cytochrome oxidase and succinate dehydrogenase) (Gonzàlez-Meler et al., 1997) and reduced respiratory cost for tissue synthesis under elevated C a (Wullschleger et al., 1994; Amthor, 1997). Reductions in R d in different forest tree species have been observed in several studies (Stewart & Hoddinott, 1993; Karnosky et al., 1999; McDowell et al., 1999). However, CO 2 -induced reduction of R d is not as conclusive as the decrease of R p because there is also evidence against a significant reduction of R d (Saxe et al., 1998; Ceulemans et al., 1999; Amthor, 2000). ...
- ... Respiration rates are often linearly related to relative growth rates in roots of many plants ( Lambers 1979). In some studies, high CO 2 around roots has been shown to inhibit or alter the total root respiration rate ( Burton et al. 1997) and maintenance respiration rate ( Qi et al. 1994;McDowell et al. 1999). The influence of CO 2 enrichment on root respiration has yielded variable results ( Davey et al. 2004). ...
- ... These decreases occur in many different kinds of tissues including roots, leaves, stems, and soil bacteria and are probably caused by inhibition of enzymes of the mitochondrial electron transport system by CO 2 (Drake et al. 1997; Lambers et al. 1998 ). For tree roots, declines in respiration with increasing CO 2 have been reported recently in Pseudotsuga (Qi et al. 1994), Tsuga (McDowell et al. 1999), Pinus (Crookshanks et al. 1998; Clinton and Bose 1999), Acer (Burton et al. 1997), Fraxinus, and Quercus (Crookshanks et al. 1998 ). However, not all tree roots behave in this way, because in Citrus volkameriana Tan. ...
- ... CO 2 concentrations above 1% in top soils indicate insufficient aeration (GAERTIG et al., 2002; SCHACK-KIRCHNER and HILDEBRAND, 1998). Higher CO 2 concentrations could lead to reduced respiration and functional loss in the rooting space (BURTON et al., 1997; GAERTIG et al., 2002; MCDOWELL et al., 1999; QI et al., 1994). ...... E-mail: gaertig@hawk-hhg.de increase in CO 2 concentrations in the rooting space (GAERTIG et al., 2002; MCDOWELL et al., 1999; QI et al., 1994). GAERTIG et al. (2002) have presumed that elevated CO 2 concentrations directly delay root metabolism. ...... Beside the increasing mechanical impedance with increasing soil compaction, rooting might decline with increasing deformation level because high soil CO 2 concentrations, which are highly correlated with the deformation level, may affect root metabolism (GAERTIG et al., 2002). Although the detrimental effects of high carbon dioxide in soil air on roots are debated in the literature (BOUMA et al., 1997; BURTON and PREGITZER, 2002), there are several indicators that root growth is hampered by increasing soil CO 2 concentration (GAERTIG et al., 2002; MCDOWELL et al., 1999; QI et al., 1994; SMIT and STACHOWIAK, 1988). The use of soil CO 2 concentration as an indicator for aeration deficiencies is challenging because of the difficulty of standardisation of values. ...
- ... There are three potential reasons for the slightly nonlinear rate of increase in CO 2 concentration in the chamber: (1) leaks, for which we corrected our data; (2) the effect of chamber CO 2 concentration on diffusion through the root tissues; and (3) a direct inhibition of cellular respiration at high ambient CO 2 concentration, which has been reported by several authors in the 1990s (Ryan et al. 1996, Burton et al. 1997, Clinton and Vose 1999, McDowell et al. 1999 ). No recent work has confirmed these results, however, and it has since been shown that leaks in the measurement systems could be responsible for this apparent inhibitory effect of high CO 2 concentration (Amthor et al. 2001, Burton and). ...
- ... Daily averages ranged from 1.72 to 0.51 mol CO 2 m 2 s 1 , which is similar to respiration rates observed by Seiler (2004a). As Rakonczay et al. (1997) hypothesized, these rates may reflect the upper limits due to roots being removed from a much higher ambient soil CO 2 concentration and measured at a lower ambient atmospheric CO 2 concentration (Qi et al. 1994, Clinton and Vose 1999, McDowell et al. 1999). Other studies have found no change or a decrease in respiration rates when roots are removed from the soil (Bouma et al. 1997, Burton and Pregitzer 2002, Lipp and Andersen 2003). ...... Daily averages ranged from 1.72 to 0.51 mol CO 2 m 2 s 1 , which is similar to respiration rates observed by Gough and Seiler (2004a). As Rakonczay et al. (1997) hypothesized, these rates may reflect the upper limits due to roots being removed from a much higher ambient soil CO 2 concentration and measured at a lower ambient atmospheric CO 2 concentration ( Qi et al. 1994, Clinton and Vose 1999, McDowell et al. 1999). Other studies have found no change or a decrease in respiration rates when roots are removed from the soil ( Bouma et al. 1997, Burton and Pregitzer 2002, Lipp and Andersen 2003. ...
- ... Studies on the effects of elevated atmospheric CO 2 (e.g. up to 2000 ppm, or 0.2% v/v) on plant respiration suggest little to no direct inhibitory effect at the tissue level (Gonzalez-Meler and Taneva, 2005), noting that early reports of respiratory inhibition in leaves and whole shoots included serious measurement artefacts (Jahnke and Krewitt, 2002). Reports of reduced root respiration under high levels of CO 2 within the soil (0.1–1.0% v/v; McDowell et al., 1999) have been similarly questioned (Burton and Pregitzer, 2002). Oxygen levels within woody stems are at a minimum during the growing season (2–8% v/v) and closer to atmospheric levels (15–20%) throughout dormancy (Eklund, 1990Eklund, , 1993 Pruyn et al., 2002b). ...... Studies on the effects of elevated atmospheric CO 2 (e.g. up to 2000 ppm, or 0.2% v/v) on plant respiration suggest little to no direct inhibitory effect at the tissue level (Meler and Taneva, 2005), noting that early reports of respiratory inhibition in leaves and whole shoots included serious measurement artefacts (Jahnke and Krewitt, 2002). Reports of reduced root respiration under high levels of CO 2 within the soil (0.1–1.0% v/v; McDowell et al., 1999) have been similarly questioned (Burton and Pregitzer, 2002). Oxygen levels within woody stems are at a minimum during the growing season (2–8% v/v) and closer to atmospheric levels (15–20%) throughout dormancy (Eklund, 1990Eklund, , 1993 Pruyn et al., 2002b). ...
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