Annals of Botany 77 : 487–496, 1996
Growth and Partitioning in Pascopyrum smithii (C3) and Bouteloua gracilis (C4) as
Influenced by Carbon Dioxide and Temperature
J. J. R E A D and J. A. M O R G A N *
USDA-ARS, Rangeland Resources Research Unit, Crops Research Laboratory, 1701 Center AŠe., Fort Collins,
CO 80526-2083, USA
Received : 2 August 1995
Accepted : 24 November 1995
This study investigated how CO and temperature affect dry weight (d.wt) accumulation, total nonstructural
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carbohydrate (TNC) concentration, and partitioning of C and N among organs of two important grasses of the
shortgrass steppe, Pascopyrum smithii Rydb. (C ) and Bouteloua gracilis (H.B.K.) Lag. ex Steud. (C ). Treatment
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combinations comprised two temperatures (20 and 35 °C) at two concentrations of CO (380 and 750 µmol mol−"),
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−
and two additional temperatures of 25 and 30 °C at 750 µmol mol " CO . Plants were maintained under favourable
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nutrient and soil moisture and harvested following 21, 35, and 49 d of treatment. CO -induced growth enhancements
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were greatest at temperatures considered favourable for growth of these grasses. Compared to growth at 380 µmol
mol−" CO , final d.wt of CO -enriched P. smithii increased 84 % at 20 °C, but only 4 % at 35 °C. Final d.wt of B.
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gracilis was unaffected by CO at 20 °C, but was enhanced by 28 % at 35 °C. Root : shoot ratios remained relatively
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constant across CO levels, but increased in P. smithii with reduction in temperature. These partitioning results were
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adequately explained by the theory of balanced root and shoot activity. Favourable growth temperatures led to CO #
induced accumulations of TNC in leaves of both species, and in stems of P. smithii, which generally reflected responses
of above-ground d.wt partitioning to CO . However, CO -induced decreases in plant tissue N concentrations were
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more evident for P. smithii. Roots of CO -enriched P. smithii had greater total N content at 20 °C, an allocation of
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N below-ground that may be an especially important adaptation for C plants. Tissue N contents of B. gracilis were
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unaffected by CO . Results suggest CO enrichment may lead to reduced N requirements for growth in C plants and
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lower shoot N concentration, especially at favourable growth temperatures. # 1996 Annals of Botany Company
Key words : Acclimation to CO , blue grama, Bouteloua gracilis, carbohydrate, climate change, global change, grass,
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growth, growth temperature optima, nitrogen, N uptake, Pascopyrum smithii, western wheatgrass.
INTRODUCTION
Because CO limits photosynthesis more in C than C
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plants, rising atmospheric CO is predicted to affect growth
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more in the former, and may already have been involved in
the encroachment of C shrubs into C grasslands (Polley,
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Johnson and Mayeux, 1994 ; however, see Archer, Schimel
and Holland, 1995). However, C plants may be better
%
adapted to future CO -enriched environments if significant
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warming ensues (Seastedt et al., 1994), or if water limitations
are important (Knapp, Hamerlynck and Owensby, 1993 ;
Owensby et al., 1993 a).
The short-grass steppe of eastern Colorado is a mixedgrass community, containing both C and C species. It is
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dominated by the C grass, Bouteloua gracilis, but contains
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many important C grasses as well, including Pascopyrum
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smithii (Monson, Littlejohn and Williams, 1983). Photosynthesis (Morgan et al., 1994 a) and growth (Reichers and
Strain, 1988 ; Morgan et al., 1994 b ; Hunt et al., 1996) of
these species are enhanced at CO concentrations above
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current ambient levels. However, their photosynthetic
* For correspondence.
Mention of a trademark, proprietary product, or vendor does not
constitute a guarantee or warrantee of the product by the U.S. Department of Agriculture and does not imply its approval to the exclusion of other products or vendors that also may be suitable.
0305-7364}96}050487­10 $18.00}0
capacities are reported to decline when grown under longterm CO enrichment at temperatures 4 °C above current
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field-average temperatures (Morgan et al., 1994 a). Acclimation of photosynthesis to CO enrichment has com#
monly been reported in C species (Allen, 1990 ; Woodrow,
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1994), and has been attributed to : (a) photosynthetic
feedback inhibition due to carbohydrate accumulation (Stitt,
1991) ; (b) a reallocation of N away from carboxylating
enzymes to support sink and nutrient acquisition activities
and}or light reactions (Stulen and den Hertog, 1993 ;
Woodrow, 1994) ; and (c) an overall decline in plant tissue
[N] (Conroy, 1992). All of these reflect alterations in plant
metabolism which serve to balance source–sink activities of
the plant and stabilize the relative concentrations of plant
resources needed for growth (Farrar and Williams, 1991 ;
Stitt, 1991). Although these adjustments to growth at
elevated CO can reduce photosynthetic and growth
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responses in the short term, they are adaptive in that they
permit an optimal plant response in the long-term
(Woodrow, 1994). For instance, the commonly-reported
increased root : shoot ratio of CO -enriched plants enhances
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their ability to obtain soil nutrients and water (Billes,
Rouhier and Bottner, 1993 ; Stulen and den Hertog, 1993),
and allows a more protracted photosynthetic and growth
response to CO in systems where nutrients and soil water
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are limiting.
# 1996 Annals of Botany Company
488
Read and Morgan—Growth and Partitioning in Range Grasses
Plant metabolic adjustments to long-term CO enrichment
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should be greater in C plants since their carboxylation
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system is less CO -saturated (Allen, 1990 ; Morgan et al.,
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1994 a ; Polley et al., 1994) and they invest considerably
more N in carboxylating enzymes than C plants (Conroy,
%
1992). Nevertheless, some studies have shown a reduction in
tissue N concentration of C grasses when CO enrichment
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led to significant growth responses (Owensby, Coyne and
Allen, 1993 b ; Morgan et al., 1994 b). Such reductions in C
%
species could be due to a similar feedback of C metabolism
on photosynthetic enzymes, but more likely involve an
inability of roots to assimilate soil N at rates sufficient to
meet the increased demand of CO -enriched plants for N.
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This soils-based response to CO would apply to all
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photosynthetic types, and would most likely occur under
conditions of low soil N.
Little is known of how CO and temperature interact to
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affect growth of P. smithii and B. gracilis. Long (1991)
suggested CO enrichment should stimulate growth and
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photosynthesis in C plants more as temperatures rise
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because photorespiration becomes more dominant at higher
temperature. We found that this response may be off-set by
a reduction in photosynthetic capacity in P. smithii plants
grown long-term at elevated temperatures (Morgan et al.,
1994 a). In a growth chamber study comparing responses of
unfertilized, deficit-watered native sods of P. smithii and B.
gracilis obtained from the shortgrass steppe of eastern
Colorado, Hunt et al. (1996) observed 17 and 20 % increases
in growth of P. smithii and B. gracilis plants, respectively,
grown for 2 years at 700 µmol mol−" CO compared to
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plants grown at 350 µmol mol−", with no interaction of CO
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and temperature on growth of either species. However,
CO -induced growth stimulation of shoot tissues was greater
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at current ambient temperatures in B. gracilis, but greater in
P. smithii at temperatures 4 °C warmer than current ambient
conditions.
This present study evaluates how growth of these two
grass species at different CO and temperature regimes
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influences their d.wt accumulation and concentrations of
total nonstructural carbohydrates (TNC) and N in leaves,
stems and roots. In contrast to previous work with these
species (Morgan et al., 1994 a, b), plants were grown under
favourable nutrient and soil moisture conditions so that we
could examine the direct effects of CO on plant growth and
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resource partitioning, without confounding the results with
indirect effects of CO on plant water relations and nutrient
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availability (Stulen and den Hertog, 1993). We predicted
concentrations of TNC would be increased and N concentrations reduced in CO -enriched P. smithii plants, but these
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traits would be less affected by CO regime in B. gracilis due
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to its predicted low photosynthetic response to CO (above
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current ambient levels), and its relatively low N requirement
for carboxylation compared to C plants. Since all plants
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were grown with readily available N and water, we
hypothesized no effect of CO on plant partitioning of dry
#
matter or N.
MATERIALS AND METHODS
Plant materials, growth conditions and statistical design
Ten seedlings of ‘ Arriba ’ western wheatgrass [P. smithii
(Rybd.) Love] or ‘ Lovington ’ blue grama [B. gracilis
(H.B.K.) Lag ex Steud.] were established in a greenhouse in
6 l plastic pots (20 cm diameter¬20 cm deep) containing a
1 : 1 mixture (v}v) of sand and Ascalon fine sandy loam.
Sixteen days after planting, the pots were transferred to
high-light-intensity growth chambers (EGC, Chagrin Falls,
Ohio, USA) which supplied a photosynthetic photon flux of
1000 µmol m−# s−" at plant height. The concentration of CO
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was controlled day and night at either 380 or 750 µmol mol−".
Air temperature was controlled at either 20, 25, 30, or
35 °C by day, and 15 °C by night for all treatments. The
photoperiod was 12 h. Relative humidity was maintained
near 60 % during the day and close to saturation at night.
Pots were watered to saturation every other day and
fertilized once a week with 400 ml of modified half-strength
Hoagland’s nutrient solution containing 400 µl l−" N. The
ten plants in each pot were harvested during a 2-h period
corresponding to 1200 h following 21, 35 and 49 d of
treatment.
Time and growth chamber space constraints did not
permit a complete factorial combination of the two CO
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concentrations and four temperature regimes. Therefore, we
designed the experiment to allow a complete factorial
comparison of plants grown at a cool (20 °C) and warm
(35 °C) daytime temperature and two CO concentrations,
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380 and 700 µmol mol−". Additional daytime growth
temperatures of 25 and 30 °C were imposed only on plants
grown at elevated CO , bringing to six the total number of
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treatment combinations of CO and temperature. These six
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treatment combinations were randomly assigned to three
growth chambers (each representing a single CO and
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temperature combination), and were repeated once (three
chambers per run) in order to consider chamber effects.
Within each chamber, ten pots (pseudoreplicates) of each
species were arranged randomly. Three replicate pots were
harvested following 21 and 35 d of treatment, and four
replicate pots were harvested following 49 d of treatment.
Separate analyses of variance and linear regressions were
conducted for each grass using GLM procedures in SAS
(SAS Institute, 1985). The experiment was analysed as a
2¬2 block design for CO and temperature, and also as a
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linear arrangement of treatments for CO -enriched plants
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grown at 20, 25, 30 and 35 °C. Repeated chamber variation
(true replication) was tested against ‘ pseudoreplicates ’ for
pooling purposes. Consequently, statistical tests are conservative for most traits, as appropriate F tests were
typically performed at the level of true error rather than at
the level of ‘ pseudoreplicates ’.
Leaf area and dry weight accumulation
Shoots of four of the ten plants were cut at the soil
surface. Leaf blades were excised from the stems, and leaf
area (LA) was determined using a LI-3000 area meter (LICOR, Inc., Lincoln, NE, USA). Stem (including leaf sheaths)
and leaf tissues were dried separately at 60 °C to a constant
489
Read and Morgan—Growth and Partitioning in Range Grasses
A
D
4
4
3
3
2
2
1
1
0
0
Plant dry weight (g per pot)
*
*
*
B
E
20
20
15
15
10
**
*
10
**
5
5
*
0
0
C
50
40
F
50
**
40
**
30
20
10
0
30
t
**
20
10
**
Am El
20
El
25
*
Am El
El Am El
30
35
20
CO2 /temperature regime (°C)
0
t
El
25
El Am El
30
35
F. 1. Dry weight accumulation in shoots and roots of Pascopyrum smithii (C ) (A,B,C) and Bouteloua gracilis (C ), (D,E,F) following 21
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(A,D), 35 (B,E), and 49 (C,F) d of growth at 20 and 35 °C and either near current ambient (Am : 380 µmol mol−") or elevated (El : 750 µmol mol−")
concentrations of CO , and also at two additional temperatures of 25 and 30 °C at elevated CO . Values for the 20 and 35 °C regimes on day
#
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21 represent means of three observations, otherwise values represent means of six observations on days 21 and 35, and means of eight observations
on day 49. t, *, ** adjacent to a plant component or above the column for total dry weight comparison indicate significant difference between
CO treatments at the 10, 5, and 1 % levels of probability, respectively, according to an F test.*, Shoot d.wt ; +, root d.wt.
#
weight, and d.wt was expressed on a pot basis. Shoot d.wt
was expressed on a pot basis by summing values for leaves
and stems of four plants (above), leaf and stem tissues
analysed for nonstructural carbohydrate and N (below),
and remaining shoot tissue from the other six plants.
Immediately following above-ground harvests, roots (including crown tissues) of all plants were carefully removed
from the soil using a low-pressure washing. Root tissue was
rapidly dehydrated in an oven at 90 °C for 1 h to minimize
respiratory loss or conversion of carbohydrates, then dried
to a constant weight at 60 °C for 48 h and weighed.
Nonstructural carbohydrates and nitrogen analyses
Total nonstructural carbohydrates (TNC) were determined in leaf and stem tissue of two plants sampled for leaf
gas exchange (Read et al., unpubl. res.). Two uppermost,
fully expanded laminae were excised from several culms,
and then the culms were cut at a 5 cm height. Stem
tissue was obtained by excising culms from subtending
root tissue at the crown-root interface, and removing any
lamina tissue. Leaf and stem tissues were separated, cut into
pieces (1 cm length), transferred to small paper bags, quickfrozen in liquid N , and stored at ®20 °C. Samples were
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subsequently lyophilized, weighed, ground to pass a 0±5 mm
screen in a Tecator-Cyclotec tissue grinder (40 mesh), and
stored at ®20 °C. Leaf and stem TNC was determined
according to methods of Chatterton et al. (1987) using a
Technicon Autoanalyzer II. Samples (50 mg) were boiled in
a small volume of water to stop any endogenous enzyme
activity, and then digested for 24 h at 38 °C using a
commercial amylase preparation (0±1 % Clarase 40 000).
The Clarase extract was analysed for reducing sugar content
following hydrolysis with 0±6  HCl using potassium
ferricyanide as a colour reagent. Oven-dried roots were
ground to pass a 0±5 mm screen and analysed for TNC
concentration according to methods of Smith, Paulsen and
Raguse (1964). Samples (500 mg) were extracted with 0±2 
sulphuric acid and TNC of the extract determined on a
glucose equivalent basis. Carbohydrate concentrations are
expressed on a structural dry weight (mg g−" SDW) basis
(dry weight minus TNC) to avoid errors associated with
490
Read and Morgan—Growth and Partitioning in Range Grasses
1.0
1.0
Root : shoot dry weight ratio
A
D
0.8
0.8
0.6
0.6
0.4
0.4
0.2
0.2
*
0.0
0.0
B
0.6
E
0.8
0.8
a
0.6
b
0.4
b
0.4
c
0.2
0.2
0.0
0.0
C
F
0.8
0.8
a
0.6
0.6
b
c
0.4
0.4
*
d
0.2
0.2
0.0
El
25
Am El
20
0.0
Am El
El Am El
30
35
20
CO2 /temperature regime (°C)
El
25
El Am El
30
35
F. 2. Dry weight root : shoot ratios of Pascopyrum smithii (C ), (A,B,C) and Bouteloua gracilis (C ), (D,E,F) following 21 (A,D), 35 (B,E), and
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49 (C,F) d of growth at 20 and 35 °C and either near current ambient (Am : 380 µmol mol−") or elevated (El : 750 µmol mol−") concentrations of
CO , and also at two additional temperatures of 25 and 30 °C at elevated CO . Letters above columns of El treatment indicate significant (P !
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#
0±05 for day 35, P ! 0±06 for day 49) temperature effects in plants grown under elevated CO , with no differences in mean comparisons of the
#
same letter (as determined by l.s.d. at the 0±05 level of significance). * above the column indicates significant difference for total dry weight between
CO treatments at the 5 % level of probability according to an F test.
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simultaneous changes in carbohydrate content and dry
weight (Chatterton et al., 1987).
Oven-dried samples of leaf, stem and root tissues were
analysed for total Kjeldahl N according to methods of
Schuman, Stanley and Knudsen (1973). Samples (0±25 g)
were digested in sulphuric acid and a copper sulphate
catalyst at 360 °C for 3 h, and subsequently analysed
colorimetrically for ammonia using a Technicon TRAACS
800 system. Ammonium sulphate was used as a standard.
Root N concentration was determined following 35 and
49 d of treatment ; analysis for root N concentration was
not possible on day 21 due to small sample size. Values for
TNC were subtracted from tissue d.wt to express tissue N
on a SDW basis so that treatment changes in N concentration were not confounded by treatment effects on
TNC.
RESULTS
Dry weight accumulation and partitioning
Significant CO -induced enhancements of plant or organ
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d.wt were evident only at temperatures considered favourable for growth (Fig. 1), that is, 20 °C in the cool-season
grass, P. smithii, and 35 °C in the warm-season grass, B.
gracilis (Kemp and Williams, 1980). Total d.wt of P. smithii
grown at 20 °C was unresponsive to CO on day 21, but in#
creased 54 % on day 35, and 84 % on day 49 due to CO
#
enrichment (Fig. 1 A–C). Root : shoot ratio of P. smithii did
not differ between CO treatments. However, linear re#
gression analysis performed across the four temperatures
among plants grown under elevated CO indicated root :
#
shoot ratio increased as temperature decreased from 35 to
20 °C on days 35 and 49 (Fig. 2 B, C). A factorial analysis
of variance involving the two temperature extremes and two
CO concentrations also revealed greater root : shoot ratios
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491
Read and Morgan—Growth and Partitioning in Range Grasses
A
D
3
3
2
2
t
1
Shoot dry weight (g per pot)
1
*
0
0
B
E
12
12
9
9
*
6
6
3
0
3
t
**
*
0
C
F
30
30
*
**
20
20
**
10
0
10
**
**
Am El
20
El
25
0
Am El
El Am El
30
35
20
CO2 /temperature regime (°C)
El
25
El Am El
30
35
F. 3. Dry weight accumulation in leaves and stems. *, leaf d.wt ; +, stem d.wt.
in plants of P. smithii grown at 20 °C compared to 35 °C on
days 35 (P ! 0±01) and 49 (P ! 0±06). This response to
temperature occurred because root d.wt increased as
temperature decreased from 35 to 20 °C (P ! 0±01), while
shoot d.wt was similar among plants grown at 20, 25 and
30 °C (Fig. 1 B, C).
Significant CO -induced increases in total d.wt of B.
#
gracilis were detected only on day 49 in plants grown at
35 °C, when CO -enriched plants weighed 28 % more than
#
ambient-grown plants (Fig. 1 F). However, trends were
evident on earlier dates (Fig. 1 D, E) suggesting CO
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enhanced growth in B. gracilis plants grown at 35 °C by
48 % at 21 d and 49 % at 35 d. Further, significant growth
enhancements from CO were detected in shoot tissues on
#
days 35 and 49. CO enrichment resulted in a 33 % reduction
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in the root : shoot ratio in plants grown at 20 °C at 21 d, and
a 13 % reduction at 35 °C at 49 d (Fig. 2 D, F). Among CO #
enriched plants, both shoot and root d.wt increased
significantly as temperature increased on days 35 (P ! 0±10)
and 49 (P ! 0±01) (Fig. 1 E, F). Unlike P. smithii, root : shoot
ratio of B. gracilis was not affected by growth temperature.
CO -induced growth enhancements were realized in both
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stem (days 35 and 49) and leaf (day 49) tissues in P. smithii
(Fig. 3 B, C). In contrast, only leaf tissues were significantly
enhanced by growth at elevated CO in B. gracilis, although
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T     1. Effect of CO concentration on leaf area (cm# per
#
pot) of Pascopyrum smithii and Bouteloua gracilis following
21, 35, and 49 d of growth at two CO concentrations and two
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daytime temperature regimes
Growth parameter
Species
P. smithii
Temperature
(°C)
20
35
B. gracilis
20
35
CO
#
(µmol
−
mol ")
Leaf area
21 d
35 d
49 d
380
750
380
750
380
750
380
750
198
210
65
68
112
65
155b
210a
642
678
158
190
275
400
378b
680a
1015b*
1505a
490b
612a
722
690
1000b
1355a
* Means within a column and growth temperature followed by a
different superscript letter are significantly different for CO treatment
#
effects by F test at the 5 % level of probability, otherwise, no significant
difference. See text for discussion of temperature effects.
trends suggested higher stem weights in CO -enriched plants
#
of this species on all sampling days (Fig. 3 D–F). CO
#
enrichment at 35 °C led to increased leaf area of the C grass
%
492
Read and Morgan—Growth and Partitioning in Range Grasses
Leaves
Stems
A
180
150
day 21
G
*
[TNC] (mg g–1 SDW)
**
**
*
*
60
B
180
E
**
H
150
120
*
90
60
360
300
240
180
120
60
day 49
C
I
F
**
**
**
20
*
25
30
20
35
**
40
25
30
35
20
25
30
[N] (mg g–1 SDW)
35
t
*
30
day 35
day 49
D
120
90
day 35
Roots
20
10
J
L
N
0
40
30
**
**
**
20
10
0
K
20
25
30
35
M
20
25
30
35
Growth temperature regime (°C)
O
20
25
30
35
F. 4. Concentrations of total nonstructural carbohydrate (TNC), (A–I) and [N], (J–O) in leaves (A–C,J,K), stems (D–F,L,M) and roots
(G–I,N,O) of Pascopyrum smithii (C ) following 21 (TNC only) (A,D,G), 35 (B,E,H,J,L,N) and 49 (C,F,I,K,M,O) d of growth at 20 and 35 °C
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and either 380 (D) or 750 (E) µmol mol−" CO , and also at two additional temperatures of 25 and 30 °C at elevated CO (E). Values for the
#
#
20 and 35 °C regimes on day 21 represent means of three observations, otherwise values represent means of six observations on days 21 and 35,
and means of eight observations on day 49. t, *, ** indicate significant difference between CO treatments at the 10 %, 5 %, and 1 % levels of
#
probability, respectively, according to an F test. Bar intervals indicate³maximal SEM for plants grown at 750 µmol mol−" CO .
#
on each sampling date (Table 1), and increased leaf dry
weight on day 49 (Fig. 3 F).
Tissue carbohydrate concentrations
CO enrichment often resulted in higher midday TNC in
#
P. smithii (Fig. 4) and B. gracilis (Fig. 5), although these
higher levels were generally more pronounced and were
observed more frequently in P. smithii. The influence of CO
#
on TNC was sometimes conditioned by growth temperature
and plant age. The most consistent pattern was observed in
P. smithii at 20 °C, as these plants exhibited large CO #
induced increases in leaf and stem TNC concentrations on
day 21, and in all three tissues on day 49 (Fig. 4 A–I). With
one exception (day 49 at 20 °C), P. smithii stems contained
from 30 to 60 mg g−" greater TNC concentration than corresponding leaves in all CO and temperature treatments.
#
Root tissue also contained relatively high TNC concentration, and exhibited large CO -induced accumulations of
#
TNC on each sampling date at 35 °C. Among CO -enriched
#
P. smithii plants, TNC concentration decreased as temperature increased in roots on day 49, and in leaves and
stems on each sampling date (P ! 0±05).
Growth of B.gracilis at 35 °C led to significant CO #
induced increases in TNC of leaves on days 21 and 35, and
increased TNC of roots on day 35 (Fig. 5 A–I). Further,
B. gracilis leaves contained from 15 to 80 mg g−" more TNC
than corresponding stems, and from 2 to 40 mg g−" more
493
Read and Morgan—Growth and Partitioning in Range Grasses
Leaves
Stems
Roots
A
180
D
G
E
H
150
day 21
120
day 35
[TNC] (mg g–1 SDW)
90
**
t
60
B
180
150
120
90
t
*
60
C
180
F
I
150
day 49
120
90
60
20
25
30
35
20
25
30
35
20
25
30
35
40
day 49
[N] (mg g–1 SDW)
30
day 35
20
10
J
L
N
M
O
0
40
30
*
20
t
10
0
K
20
25
30
35
20
25
30
35
Growth temperature regime (°C)
20
25
30
35
F. 5. Concentrations of total nonstructural carbohydrate (TNC) and N in leaves, stems and roots of Bouteloua gracilis (C ). Legend as in Fig. 4.
%
TNC than corresponding roots. However, like the C grass,
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TNC concentration in leaves and stems of CO -enriched
#
plants decreased as temperature increased on day 49 (P !
0±05).
Nitrogen concentration and partitioning
Significant CO -induced decreases in N concentration
#
(expressed on a structural dry weight basis) were evident in
leaves, stems and roots of the C grass, P. smithii, grown at
$
20 °C on the final sampling date (Fig. 4 K, M, O), the same
treatment that led to large increases in d.wt (Fig. 1 B, C) and
TNC (Fig. 4 C). Decreased N concentration at elevated CO
#
was also evident in leaves and stems on day 35 at 35 °C,
whereas CO -enriched roots exhibited increased N concen#
tration (Fig. 4 J, L, N). Tissue N concentration of CO #
enriched P. smithii was generally greatest at 35 °C, and
decreased with reduction in growth temperature regime in
leaves, stems and roots on day 49 (P ! 0±01), and in roots
on day 35 (P ! 0±05).
In contrast to the C grass, tissue N concentration of B.
$
gracilis was generally unaffected by CO (Fig. 5). However,
#
on the final sampling date, N concentration in CO -enriched
#
stems decreased 20 % at 20 °C and 13 % at 35 °C as
compared to controls grown at 380 µmol mol−" CO
#
(Fig. 5 M). Among CO -enriched B. gracilis plants on the
#
final sampling date, N concentration in stems and roots was
greatest at 20 °C and decreased as temperature increased
(P ! 0±05).
In large part, changes in tissue N contents reflected plant
growth responses to the environment, with greater N
generally occurring when growth was greatest (Fig. 6).
Compared to controls, growth of P. smithii at 20 °C led to
significant CO -induced increases in N content of roots
#
(45 %) and stems (70 %) on day 35 (Fig. 6 A), and roots
(37 %) on day 49 (Fig. 6 B). Total plant N content also
494
Read and Morgan—Growth and Partitioning in Range Grasses
C
N content (mg per pot)
A
600
600
400
400
200
0
200
**
*
t
B
0
600
600
400
400
200
0
D
200
**
t
Am El
20
El
25
0
El Am El
Am El
30
35
20
CO2/temperature regime (°C)
El
25
El Am El
30
35
F. 6. Nitrogen content in leaf (9), stem (*) and root (+) tissues of Pascopyrum smithii (C ), (A,B) and Bouteloua gracilis (C ), (C,D) following
$
%
35 (A,C) and 49 (B,D) d of growth at 20 and 35 °C and either near current ambient (Am : 380 µmol mol−") or elevated (El : 750 µmol mol−")
concentrations of CO , and also at two additional temperatures of 25 and 30 °C at elevated CO . Values represent means of six observations on
#
#
day 35, and means of eight observations on day 49. t, *, ** adjacent to a plant component indicate significant difference between CO treatments
#
at the 10, 5 and 1 % levels of probability, respectively, according to an F test.
T     2. Root : shoot N content per ratio for Pascopyrum
smithii and Bouteloua gracilis following 35 and 49 d of
growth at either near current ambient (Am : 380 µmol mol−")
or eleŠated (El : 750 µmol mol−") concentrations of CO . CO #
#
enriched plants were grown at four daytime temperatures,
whereas, current ambient plants were grown at 20 and 35 °C
Root : shoot N content ratio
Temperature
regime
(°C)
Pascopyrum smithii
20
25
30
35
Bouteloua gracilis
20
25
30
35
35 d
Am
0±39
nd
nd
0±23
0±54
nd
nd
0±23
49 d
El
0±50a†
0±35b
0±41b
0±25c
0±33
0±27
0±29
0±25
Am
El
0±60*
nd
nd
0±34
0±76a†
0±50b
0±44bc
0±27c
0±31
nd
nd
0±30
0±30a†
0±25b
0±27ab
0±24b
* Significantly different from paired observation immediately to the
right (i.e. Šs. elevated CO ) by F test at the 5 % level of probability.
#
† For CO -enriched plants, means followed by the same letter are
#
not significantly different by l.s.d. test at the 5 % level of probability.
increased at elevated CO on day 49 at 20 °C (P ! 0±10).
#
Trends (P ! 0±10) indicating smaller CO -induced increases
#
in N content were also evident for roots (42 % on day 35)
and stems (51 % on day 49) when the C grass was grown at
$
35 °C. Tissue N content of CO -enriched P. smithii plants
#
decreased as temperature increased on both sampling dates
(P ! 0±05).
Unlike those of the C grass, B. gracilis tissues did no
$
differ in N content between CO levels (Fig. 6 C, D). Nitro#
gen content in CO -enriched stems and roots of B. gracilis
#
was greatest at either 30 or 35 °C, and increased as
temperature increased on both sampling dates (P ! 0±05).
Partitioning of N between roots and shoots was influenced
by CO only for P. smithii grown at 20 °C on day 49, when
#
proportionately more N was partitioned to roots in plants
grown at 700 µmol mol−" CO (Table 2). Root N partitioning
#
was enhanced in both species as growth temperatures
declined, although the response was greater and more
evident in P. smithii than B. gracilis.
DISCUSSION
CO enrichment enhanced dry weight accumulation in both
#
species (C and C , Fig. 1). Contrary to reports of greater
$
%
potential benefit for CO responses at warmer temperatures
#
(Allen, 1990 ; Imai and Okamoto-Sato, 1991 ; Long, 1991),
growth enhancements were greatest at temperatures considered optimal for these respective species (Monson et al.,
1983). Stimulation of dry weight in B. gracilis under wellwatered conditions supports evidence that CO -induced
#
growth enhancements may occur from direct stimulations
of photosynthesis, as well as from indirect effects resulting
from stomatal closure and improved water relations (Sionit
and Patterson, 1984 ; Reichers and Strain, 1988 ; Morgan
et al., 1994 a).
Root : shoot ratios were generally little influenced by CO ,
#
Read and Morgan—Growth and Partitioning in Range Grasses
but increased in CO -enriched P. smithii with reductions in
#
growth temperature ; no such temperature response was
observed in B. gracilis. These results, involving variable
species, temperature, and CO are adequately explained by
#
the theory of balanced shoot and root activity (Davidson,
1969 ; Hilbert and Reynolds, 1991), which proposes that
shoot and root activities maintain a homeostatic equilibrium
to balance the internal resource demands of the plant ; a
stimulation of shoot CO uptake necessarily involves
#
increased N uptake by roots. In assessing this lack of effect
of CO on root : shoot ratio, it is important to consider that
#
this study was conducted under favourable nutrients and soil
moisture. Although increased partitioning of fixed C to
roots is often reported in CO enrichment studies (Rogers,
#
Runion and Krupa, 1994), an increased demand for
nutrients due to enhanced shoot growth may be the principal
underlying mechanism, rather than a direct response to CO
#
alone (Larigauderie, Hilbert and Oeschel, 1988 ; Farrar and
Williams, 1991 ; Billes et al., 1993 ; Cipollini, Drake and
Whigham, 1993 ; Stulen and den Hertog, 1993). We
previously found CO enrichment can induce a substantial
#
increase in the root : shoot ratio of B. gracilis in unfertilized,
deficit-irrigated soil (Morgan et al., 1994 b). Adequate root
nutrient and water supply in the present study, insured by
frequent fertilizations and watering, apparently precluded
substantial below-ground partitioning responses to CO
#
fertilization.
Differences in the presence (P. smithii) or absence (B.
gracilis) of a temperature-sensitive partitioning response
may reflect different strategies in balancing above- and
below-ground growth between a cool- and warm-season
grass. Increases in root : shoot ratios with declining temperatures are often reported (Farrar and Williams, 1991), and
appear to be an adaptation to reduced root function at low
soil temperatures (Davidson, 1969), when relatively more
root tissue is required to maintain soil nutrient uptake. This
was apparently true for P. smithii in the present study as the
greatest growth rates were realized at the low temperature
regimes. In contrast, growth of the warm season B. gracilis
was greatest at the warmest temperatures, 30 and 35 °C.
Lack of a shift in root : shoot ratio of B. gracilis across the
four growth temperatures suggests that increased root
activity, induced by warmer temperatures, was sufficient to
meet the requirement for enhanced nutrient uptake to
support greater plant growth.
Increases in the concentration of soluble carbohydrate in
tissue are a common feature of CO enrichment studies, and
#
reflect an inability of sink activities to keep pace with
enhanced photosynthetic rates (Stitt, 1991 ; Thomas and
Strain, 1991). In the present study, CO -induced increases in
#
soluble carbohydrate were most evident at the growth
temperatures which resulted in the greater CO responses of
#
both species (P. smithii, 20 °C ; B. gracilis, 35 °C). In the case
of B. gracilis, these soluble carbohydrate accumulations
were greatest in leaf tissues where growth responses to CO
#
were most evident in this species. While they were not as
great or distributed among as many tissues as in the C
$
species, they provide additional evidence of the direct
photosynthetic response of B. gracilis to CO enrichment
#
above current ambient levels.
495
In P. smithii, reductions in N concentration were observed
in all tissues where CO -induced increases in soluble
#
carbohydrate were greatest, and as they were expressed on
a structural dry weight basis, could not be attributed to a
dilution effect caused by high carbohydrate levels. Because
of frequent fertilizations and consequent high soil N
availability, reductions in leaf N in CO -enriched P. smithii
#
plants were likely a plant-regulated phenomenon
(Woodrow, 1994). Recent evidence suggests synthesis of
mRNA transcripts for RuBP carboxylase}oxygenase
(Rubisco) in C species may be regulated by soluble sugars
$
accumulating in CO -enriched leaves. This might account
#
for reductions of N in tissues exposed to elevated CO for
#
prolonged periods (Van Oosten and Besford, 1994). This
regulation is expected to be more important in C species as
$
they invest substantially more N in carboxylating protein
than C species (Conroy, 1992). Our results support that
%
hypothesis, as leaf N concentration of B. gracilis was
unresponsive to growth CO concentration. Although there
#
are reports of reduced N concentration in C plants grown
%
in enriched CO atmospheres, those plants were either
#
grown without fertilizer and only limited water additions
(Owensby et al., 1993 b ; Morgan et al., 1994 b), or with only
periodic fertilizer additions (Owensby, Allen and Coyne,
1994). Under these kinds of nutrient- or water-limiting
conditions, reduced tissue nutrient concentrations may
result from a dilution effect due to CO -induced growth
#
enhancement. If CO enrichment also leads to increased C
#
translocation below-ground, then immobilization of soil N
may contribute towards reduced tissue N concentration
under CO enrichment (Morgan et al., 1994 b).
#
The partitioning of N above- and below-ground was
similar to biomass partitioning in that cooler temperatures
resulted in greater root : shoot N ratios. This further supports
our earlier contention that resources (biomass and N) were
preferentially partitioned below-ground when temperatures
were least favourable for nutrient uptake in order to achieve
balanced activity (Davidson, 1969).
Consistent with this evidence, but in contrast to biomass
partitioning which was not responsive to CO in P. smithii,
#
a trend developed at 35 d (and reached significance at
49 d) indicating greater N partitioning below-ground in
CO -enriched P. smithii grown at 20 °C. A shift in N
#
resources away from C fixation towards the acquisition of
other required plant resources may occur when CO #
enrichment leads to a substantial growth response
(Woodrow, 1994). This adjustment is not necessarily limited
to C species, as Morgan et al. (1994 b) reported a similar
$
shift of N resources below-ground in CO -enriched B.
#
gracilis plants. This kind of shift was not observed in the
present study, apparently because growth responses of B.
gracilis to CO under the well-irrigated and nutrient-supplied
#
conditions were relatively less than when plants were deficitirrigated and nutrient-limited in a previous study (Morgan
et al., 1994 b). However, a change in root : shoot N
partitioning may be more likely in C species since they
$
invest considerably more shoot N in the construction of
photosynthetic machinery than C species. This potentially
%
large investment of protein N in carboxylating enzymes of
C plants may explain why CO -induced increases in
$
#
496
Read and Morgan—Growth and Partitioning in Range Grasses
root : shoot partitioning were observed for N, but not for
biomass.
The results suggest that variation in photosynthetic C
metabolism and growth-temperature optima exhibited by
these cool- and warm-season species will be heavily featured
in their adaptation to future climate change scenarios.
While both species demonstrate an ability to respond
directly to CO enrichment, our results, and others from the
#
literature, indicate the final outcome will be conditioned by
species, temperature, and the availability of nutrients and
water.
A C K N O W L E D G E M E N TS
We thank N. J. Chatterton and P. A. Harrison for their
assistance with TNC analyses, and Dan LeCain for technical
assistance and manuscript preparation. We also thank J. P.
Conroy and R. H. Skinner for helpful comments on the
manuscript. This research was supported in part by award
number 92-37100-7670 of the National Research Initiative
Competitive Grants Program of the USDA.
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