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Is Partitioning of Dry Weight and Leaf Area Within Dactylis glomerata Affected by N and CO2Enrichment?
Annals of Botany 86: 833±839, 2000 doi:10.1006/anbo.2000.1243, available online at http://www.idealibrary.com on
Is Partitioning of Dry Weight and Leaf Area Within Dactylis glomerata Aected by N and CO2 Enrichment? H . H A R ME N S *{} , C . M . ST I R L IN G{ {, C . M A R S H A L L } and J . F. FA R R A R } {Centre for Ecology and Hydrology Bangor, University of Wales, Bangor, Gwynedd LL57 2UP, UK and }School of Biological Sciences, University of Wales, Bangor, Gwynedd LL57 2UW, UK Received: 5 April 2000 Returned for revision: 15 May 2000 Accepted: 22 June 2000 Published electronically: 14 August 2000 We examined changes in dry weight and leaf area within Dactylis glomerata L. plants using allometric analysis to determine whether observed patterns were truly aected by [CO2] and N supply or merely re¯ect ontogenetic drift. Plants were grown hydroponically at four concentrations of NO3ÿ in controlled environment cabinets at ambient (360 ml l ÿ1) or elevated (680 ml l ÿ1) atmospheric [CO2]. Both CO2 and N enrichment stimulated net dry matter production. Allometric analyses revealed that [CO2] did not aect partitioning of dry matter between shoot and root at high N supply. However, at low N supply there was a transient increase in dry matter partitioning into the shoot at elevated compared to ambient [CO2] during early stages of growth, which is inconsistent with predictions based on optimal partitioning theory. In contrast, dry matter partitioning was aected by N supply throughout ontogeny, such that at low N supply dry matter was preferentially allocated to roots, which is in agreement with optimal partitioning theory. Independent of N supply, atmospheric CO2 enrichment resulted in a reduction in leaf area ratio (LAR), solely due to a decrease in speci®c leaf area (SLA), when plants of the same age were compared. However, [CO2] did not aect allometric coecients relating dry weight and leaf area, and eects of elevated [CO2] on LAR and SLA were the result of an early, transient stimulation of whole plant and leaf dry weight, compared to leaf area production. We conclude that elevated [CO2], in contrast to N supply, changes allocation patterns only transiently during early stages # 2000 Annals of Botany Company of growth, if at all. Key words: Allometric growth, carbon dioxide enrichment, Cocksfoot, Dactylis glomerata L., dry weight partitioning, leaf area ratio, nitrogen supply, shoot : root ratio, speci®c leaf area.
I N T RO D U C T I O N Doubling atmospheric [CO2] stimulates the growth of C3 species by an average of 41 % (Poorter, 1993). The magnitude of this increase varies with the individual species (Hunt et al., 1991; Poorter, 1993), experimental duration and the availability of other resources such as N (Bazzaz, 1990). When N limits growth, root dry weight increases relatively more than shoot dry weight, perhaps maintaining a functional equilibrium that results in the balanced acquisition of carbon and nitrogen (Davidson, 1969; Reynolds and Thornley, 1982; Brouwer, 1983; Wilson, 1988). Atmospheric [CO2] might also aect the allocation of dry matter between shoots and roots, although contrasting results have been reported for the shoot to root ratio (S : R ratio, de®ned as the dry weight of shoot divided by the dry weight of root): CO2 enrichment may either increase, decrease or not aect S : R ratio (Hunt et al., 1991; Stulen and Den Hertog, 1993; Rogers et al., 1996). Although [CO2] and N supply would be expected to interact regarding dry matter production and partitioning (i.e. the distribution of dry weight within the plant), the outcome cannot be predicted (Lloyd and Farquhar, 1996). uk
* For correspondence. Fax 44 (0)1248 355365, e-mail [email protected].
{ Present address: School of Agricultural and Forest Sciences, University of Wales, Bangor, Gwynedd LL57 2UW, UK.
0305-7364/00/100833+07 $35.00/00
Comparisons of the eects of treatments on plant growth are often based on ratios such as S : R with plants of the same age. However, patterns of allocation between plant parts change during growth and development independent of resource availability (Pearsall, 1927; Bowler and Press, 1993; Farrar and Gunn, 1996). Treatments may aect growth and development and alter S : R ratio and morphological characteristics compared to the control either because of true treatment eects or ontogenetic drift, i.e. phenotypic traits of plants change over the course of plant growth and development (Evans, 1972; Coleman et al., 1994; Farrar and Gunn, 1998; McConnaughay and Coleman, 1999). Dierentiation between true eects of treatment and of ontogenetic drift is possible using allometry, i.e. the study of the growth and development of one part of the plant in relation to another (Pearsall, 1927; Troughton, 1955). A few studies have made allometric comparisons between CO2 treatments, usually restricted to the allometric coecient relating the net dry matter production of shoot and root (Bowler and Press, 1993; Baxter et al., 1994; Farrar and Gunn, 1996; Hibberd et al., 1996). More recently, some studies have included the allometric relationship of dry matter and leaf area (Farnsworth et al., 1996; Gebauer et al., 1996; Stirling et al., 1998; Gunn et al., 1999). In general, allometric coecients were not aected by [CO2]. In contrast, allocation of dry matter to roots decreased with increasing N supply (Gebauer et al., 1996; McConnaughay and Coleman, 1999). # 2000 Annals of Botany Company
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We suspect that many reports claiming that [CO2] alters net dry matter partitioning might be confounding eects of [CO2] with those of uncontrolled variables, especially when plants are grown in a solid substrate. For example, limited access to nutrients due to either growth limiting concentrations or limited ¯ux from a solid substrate to the roots, can mean that plants grown at elevated [CO2] become more nutrient de®cient than controls due to increased dry weight (Stitt and Krapp, 1999), resulting in a correspondingly lower S : R ratio. Recently it has become clear that soil water content can be another confounding variable (Samarakoon and Giord, 1995; Knapp et al., 1996), and one that is known to aect dry matter partitioning. Plants rooted in a solid substrate with a ®nite supply of water may have more favourable water status at elevated [CO2] due to a lower stomatal conductance for water vapour at elevated than ambient [CO2] (Drake et al., 1997). Accordingly, good control of nutrient and water supply is needed to distinguish between direct eects of [CO2] and indirect eects through nutrient and water status of soils. Here we examine the eects of elevated [CO2] and the interaction with N availability on growth, partitioning of dry weight, and dry weight±leaf area relationships in Dactylis glomerata L. (Cocksfoot). Plants were grown in controlled environments at either ambient (360 ml l ÿ1) or elevated (680 ml l ÿ1) [CO2] and four N concentrations (0.15, 0.6, 1.5 and 6.0 mM NO3ÿ ), ranging from growth limiting to optimal. Confounding eects of soil water status were minimized by growing plants in hydroponics. Comparisons between treatments were made both as a function of plant age and by applying allometry. It was hypothesized that elevated [CO2] aects net allocation of dry matter and distribution of dry matter and leaf area only through accelerated growth, but that N supply has a direct eect independent of changes in ontogeny.
et al., 2000). In order to reduce cabinet eects and eects of environmental heterogeneity within the cabinets, CO2 treatments were swapped between the two cabinets twice a week and troughs were randomized within each cabinet. Growth analysis Seven plants per treatment (three±four plants per trough) were harvested at 23, 28, 34 and 38 d after sowing. Plants were separated into leaf blades, leaf sheath stem, and roots. The area of leaf blades was determined using a digital leaf area meter (Delta T Ltd, Cambridge, UK) and plant parts were dried for at least 48 h at 65 8C and weighed. The following parameters were calculated: shoot : root ratio (dry weight), leaf area ratio (LAR; leaf area per plant dry weight), speci®c leaf area (SLA; leaf area per leaf dry weight) and leaf weight ratio (LWR; leaf dry weight per plant dry weight). Allometric relationships were determined using means per trough for each harvest and an ordinary linear regression equation (Pearsall, 1927; Troughton, 1955): ln y ln a k ln x where ln a is the y-intercept, k is the slope, and y and x are respectively: shoot and root dry weight; leaf dry weight and total plant dry weight; leaf area and total plant dry weight; and leaf area and leaf dry weight. In the majority of cases, the regression was linear as determined by linear and sequential polynomial regression. Subsequently, the slope (v) of the geometric mean regression was determined for all relationships (Ricker, 1984; Farrar and Gunn, 1996), as both y and x are dependent variables: v k=r
M AT E R I A L S A N D M E T H O D S Plant growth Seeds of Dactylis glomerata L., `Sylvan', were sown in propagators on moistened ®lter paper in controlled environment cabinets (Sanyo Gallenkamp, model SGC660/C/HQI, Loughborough, UK) at either ambient (360 ml l ÿ1) or elevated (680 ml l ÿ1) CO2 concentrations. Nine days after sowing, uniform-sized seedlings with one leaf were transferred to 3.5 l troughs, containing half-strength Long Ashton solution (Hewitt, 1966) with 10 mg l ÿ1 of sodium metasilicate. Supply of NO3ÿ was modi®ed to give four concentrations: 0.15, 0.6, 1.5 and 6.0 mM. At the smaller N supplies, the potassium and calcium concentrations were made up by adding K2SO4 and CaCl2 . The pH of the aerated solutions was controlled between 5.6±6.4 using 2 mM MES and KOH. Twenty seedlings were planted in each trough (two troughs per treatment) and after 1 week the nutrient solutions were replaced and the number of plants per trough was reduced to 15. Thereafter, nutrient solutions were replaced twice weekly and the number of plants per trough was reduced at every harvest. The conditions of growth were as described previously (Harmens
where r is the correlation coecient of the ordinary linear regression and v is the allometric coecient calculated by geometric mean regression. Where there was no signi®cant dierence between the slopes due to [CO2] a comparison of the elevations (as opposed to the y-intercepts) of the regressions (i.e. comparison of the vertical position of the lines) was carried out (Zar, 1996). Regression lines with the same slope and elevation coincide, whereas regression lines with the same slope but dierent elevation are parallel. Statistical analysis Data were analysed by analysis of variance (ANOVA) of the mean values of each trough (n 2 per treatment per harvest) using the Genstat statistical package (Lawes Agricultural Trust, Rothamsted, UK). Where indicated, data were ln-transformed prior to analysis to obtain homogeneity of variances. Eects of CO2 and N supply on the allometric coecient (v) and elevations of the regression were analysed by pairwise comparison using Student's t-test with 12 degrees of freedom. Unless indicated otherwise, signi®cant treatment eects refer to P 4 0.05.
Shoot:root ratio
Total dry wt (g)
Harmens et al.ÐPartitioning at Elevated CO2 and N 3
0.15 mM NO3−
0.6 mM NO3−
835
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6.0 mM NO3−
A
B
C
D
E
F
G
H
2 1 0 4 3 2 1 0
20
30
40
20
30
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20
30
40
20
30
40
Time (d) F I G . 1. Total dry weight (A±D) and shoot : root ratio (E±H) of D. glomerata grown at 360 (ÐdÐ) or 680 (- - -s - - -) ml l ÿ1 CO2 and varying [NO3ÿ ]. Each data point represents the mean of two troughs, three±four plants per trough (+s.e.).
R E S U LT S Growth and ratios at the same plant age Total plant dry weight was greater at elevated than ambient [CO2] (Fig. 1A±D; Table 1) and although the magnitude of the increase varied, no signi®cant interactions between [CO2] and time, or [CO2] and N supply were found. From 28 d onwards, total plant dry weight increased with increasing [NO3ÿ ]. Responses of shoot and root dry weight to elevated [CO2] and [NO3ÿ ] were similar to that of total plant dry weight (data not shown), except that N supply did not aect root dry weight until 34 d. The S : R ratio was generally not aected by [CO2] and increased with increasing N supply (Fig. 1E±H; Table 1). However, at T A B L E 1. Summary of statistical signi®cance of eects of date of harvest, [CO2] and N supply on growth and development parameters of D. glomerata Source of variation Parameter
Time
N
CO2
Time N
Wtotal Wshoot Wroot Wshoot : Wroot Leaf area Leaf area ratio Speci®c leaf area Leaf weight ratio
*** *** *** *** *** *** *** ***
*** *** ** *** *** ** *** ***
*** *** *** n.s. ** *** *** n.s.
*** *** *** *** *** *** *** ***
Number of f.e.l. main stem Total number of leaves Total number of tillers
*** *** ***
*** *** ***
n.s. n.s. n.s.
* *** ***
Data for dry weight (W) and leaf area were ln-transformed to obtain homogeneity of variance. A general linear ANOVA model was ®tted to the mean value of each trough, so n 2 per treatment at each harvest. n.s., Not signi®cant, *P 4 0.05, **P 4 0.01, ***P 4 0.001. `Time CO2', `N CO2' and `Time N CO2' interactions were not signi®cant; f.e.l., fully expanded leaves.
[NO3ÿ ] above 0.15 mM there was a tendency towards an increase in S : R ratio due to CO2 enrichment. Developmental parameters such as number of tillers, leaves and fully expanded leaves on the main stem were not aected by [CO2], but increased with increasing N availability (Table 1). LAR was signi®cantly higher at ambient than elevated [CO2] at all [NO3ÿ ] and it decreased with time (Fig. 2A±D; Table 1). SLA was lower in plants exposed to elevated than ambient [CO2], independent of [NO3ÿ ] (Fig. 2E±H; Table 1). At 0.15 mM NO3ÿ the SLA increased with time, but it decreased with time at 1.5 and 6.0 mM NO3ÿ , resulting in a higher SLA at the ®nal harvest at 0.15 mM NO3ÿ compared to 1.5 and 6.0 mM NO3ÿ . LWR was not aected by CO2 (Fig. 2I±L; Table 1), thus the decrease in LAR at elevated [CO2] was due solely to a decrease in SLA. At low N supply LWR decreased with time, whilst at high N supply it was nearly constant, resulting in a higher LWR at the ®nal harvest at high compared to low N availability. Clearly, most ratios changed with time and therefore with increasing plant dry weight. Allometric relationships Elevated [CO2] had no signi®cant eect on any of the allometric coecients (v) and therefore did not aect net dry matter partitioning and dry weight±leaf area relationships within the plants between the ®rst and ®nal harvest (Figs 3, 4; Table 2). However, CO2 enrichment had a signi®cant eect on some of the elevations of regression lines, indicating a transient change in the partitioning of dry matter and leaf area during the initial stages of growth (before the ®rst harvest). At low N supply the allocation of dry matter to the shoot or leaf blades was increased early on at elevated compared to ambient [CO2]. At high N supply CO2 enrichment decreased the production of leaf area relative to whole plant dry weight early in growth. Independent of N supply, CO2 enrichment reduced the
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Harmens et al.ÐPartitioning at Elevated CO2 and N
LAR (cm2 g−1)
0.6 mM NO3−
0.15 mM NO3−
200
1.5 mM NO3−
6.0 mM NO3−
A
B
C
D
E
F
G
H
I
J
K
L
100
0
SLA (cm2 g−1)
500
250
0
LWR
0.8 0.4
0.0
20
30
40
20
30
40
20
30
40
20
30
40
Time (d) F I G . 2. LAR (A±D), SLA (E±H) and LWR (I±L) of D. glomerata grown at 360 (ÐdÐ) or 680 (- - -s - - -) ml l ÿ1 CO2 and varying [NO3ÿ ]. Each data point represents the mean of two troughs, three±four plants per trough (+s.e.).
ln DW shoot (mg)
0.6 mM NO3−
0.15 mM NO3−
8
A
1.5 mM NO3−
B
6.0 mM NO3−
C
D
6 4 2 2
ln DW leaf (mg)
8
4
6
8
2
E
4
6
8 2 ln DW root (mg)
F
4
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2
G
4
6
8
H
6 4 2 2
4
6
8
10
2
4
6
8
10
2
4
6
8
10
2
4
6
8
10
ln DW total (mg) F I G . 3. The allometric relationship between shoot and root dry weight (A±D) and leaf and plant dry weight (E±H) of D. glomerata grown at 360 (ÐdÐ) or 680 (- - -s- - -) ml l ÿ1 CO2 and varying [NO3ÿ ]. Each data point represents the mean three±four plants per trough.
production of leaf area relative to leaf dry weight early in ontogeny. In contrast with [CO2], nitrogen supply signi®cantly aected most allometric coecients (Figs 3, 4; Table 2). At 0.15 and 0.6 mM NO3ÿ , dry matter was preferentially
partitioned to the root (v 5 1), whereas at 1.5 and 6.0 mM NO3ÿ it was allocated equally to shoot and root (v was not signi®cantly dierent from 1). Consequently, the increase in leaf weight per unit of plant dry weight was signi®cantly lower at low compared to high N supply. At
Harmens et al.ÐPartitioning at Elevated CO2 and N 0.6 mM NO3−
0.15 mM NO3−
6
A
837
1.5 mM NO3−
B
6.0 mM NO3−
C
D
4
ln leaf area (cm2)
2 0 0
2
4
6
8
10
0
2
4
6
8
10
0
2
4
6
8
10
0
2
4
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10
ln DW total (mg) 6
E
F
G
H
4 2 0 0
2
4
6
8
0
2
4
6
8
0
2
4
6
8
0
2
4
6
8
ln DW leaf (mg) F I G . 4. The allometric relationship between leaf area and plant dry weight (A±D) and leaf area and leaf dry weight (E±H) of D. glomerata grown at 360 (ÐdÐ) or 680 (- - -s- - -) ml l ÿ1 CO2 and varying [NO3ÿ ]. Each data point represents the mean three±four plants per trough.
T A B L E 2. Eects of [CO2] and N supply on the allometric coecient (v) relating dry weight (W) and leaf area in D. glomerata CO2 concentration (ml l ÿ1)
v [NO3ÿ concentration (mM)] 0.15
0.6
1.5
6.0
0.79b (q***) 0.78b
0.99c 0.99c
1.07c 1.05c
0.65a (q**) 0.59a
0.84b (q**) 0.83b
0.97c 0.95c
0.99c 0.98c
360 680
0.74a 0.77a
0.77a 0.80a
0.85b (Q***) 0.82a
0.90c (Q***) 0.88b
360 680
1.13a (Q**) 1.32a
0.91b (Q***) 0.96b
0.88b (Q***) 0.86c
0.91b (Q***) 0.91bc
x
y
Wroot
Wshoot
360 680
0.56a (q**) 0.52a
Wtotal
Wleaf blade
360 680
Wtotal
Leaf area
Wleaf
Leaf area
The natural logarithm of y was plotted against the natural logarithm of x for all harvests and an ordinary linear regression was ®tted. The allometric coecient v was then calculated from the slope of the regression (k) and the correlation coecient (r): v k=r (r 0.98±1.00). Eects of [CO2] and N supply on v were tested by pairwise comparison using Student's t-test (degrees of freedom 12). Dierent superscripts within rows indicate signi®cant dierences (P 4 0.05) between v at dierent N treatments. Although there were no signi®cant eects of CO2 on v, signi®cant eects of CO2 on the elevation of regressions were found as indicated within parentheses: Q or q indicate a signi®cant higher or lower elevation at 360 compared to 680 ml l ÿ1 CO2 , respectively; **P 4 0.01, ***P 4 0.001.
ambient [CO2] the increase in leaf area per unit of plant dry weight was signi®cantly less at 0.15 and 0.6 mM NO3ÿ than at higher [NO3ÿ ]. At 0.15 mM NO3ÿ , leaf area increased relatively more than leaf dry weight compared to higher [NO3]. DISCUSSION When growth was analysed allometrically, it was evident that elevated [CO2] in comparison with N supply had minimal eects on the partitioning of dry matter within D. glomerata. An increase in N availability aected partitioning throughout ontogeny, such that enhanced N supply
reduced allocation of dry matter to roots. Dependent on N availability, elevated [CO2] changed the partitioning of dry weight and dry weight±leaf area relationships within D. glomerata only during the initial stages of growth. Hence, eects of CO2 enrichment on S : R ratio, LAR and SLA in plants of the same age were sometimes still present when ontogenetic drift was taken into account. Ratios indicate the state of the plant at an instant in time, are the product of the plant's history and will be an insensitive measure of changes in partitioning throughout ontogeny (Farrar and Gunn, 1998). We applied allometry to identify eects of treatment independent of ontogeny over a period of growth; allometry has a clear biological
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Harmens et al.ÐPartitioning at Elevated CO2 and N
meaning and proved to be statistically more robust than comparing plants on the basis of similar total dry weights (Gunn et al., 1999). We found that dry weight and leaf area to dry weight ratios were subject to ontogenetic drift. Had we not accounted for this, we would have misjudged, in some treatments, the adjustments in allocation patterns. For example, the decrease in LAR due to CO2 enrichment at low N supply when plants of the same age were compared was simply the result of accelerated growth. On the other hand, a transient increase in allocation of dry matter to the shoot at elevated [CO2] and very low N supply during early stages of growth would not have been detected when only S : R ratios of plants of the same age were compared. Although other studies have shown that elevated [CO2] has little eect on allometric coecients (Bowler and Press, 1993; Baxter et al., 1994; Farnsworth et al., 1996; Gebauer et al., 1996; Stirling et al., 1998; Gunn et al., 1999), little attention has been paid to changes in the elevation of regressions (Gunn et al., 1999; Marriott, 1999). Whilst previously values for ln a have been used to quantify treatment eects on allometry early in growth, extreme care is required since the results of analyses will be highly dependent on the units used, so ln a has little biological signi®cance (Baxter et al., 1994; Stirling et al., 1998). This study clearly shows that both allometric coecients and elevations of regressions should be analysed to determine whether changes in partitioning occur throughout ontogeny or are restricted to a de®ned period in the plant's development. Whereas CO2 enrichment did not aect any of the allometric coecients (v), it did aect elevations of the regressions in most cases, indicating a change in partitioning during the initial stages of growth of D. glomerata. These early changes in partitioning are hard to detect, mainly due to the diculties associated with measuring very small plants and the large number of replicates that would be required to pick up small changes in v, which appear to occur in a short period of time. Optimal partitioning models and theory suggest that plants respond to variation in the environment by partitioning dry weight among plant organs to optimize the capture of resources in a manner that maximizes plant growth rate (Reynolds and Thornley, 1982; Brouwer, 1983; Wilson, 1988; McConnaughay and Coleman, 1999). In D. glomerata, optimal partitioning did apply to the range of N availability such that at limited N supply dry matter was preferentially allocated to roots. Whereas optimal partitioning models predict increased allocation of dry matter to roots at elevated CO2 , the partitioning of dry matter between shoots and roots in D. glomerata was little aected by CO2 enrichment. In contrast, a transient increase in the partitioning of dry matter to the shoot (and leaves) was observed at elevated [CO2] at low N supply at an early stage of plant development. Clearly, if the basic assumptions of optimal partitioning (i.e. that plants do alter dry weight distribution in response to changes in availability of resources to maximize growth rate) are incorrect, models attempting to predict the ecological outcome of environmental changes based on the optimal partitioning theory should be re-evaluated (McConnaughay and Coleman, 1999).
Reductions in SLA, and consequently in LAR, have been reported at elevated CO2 (Bazzaz, 1990; Den Hertog et al., 1993; Poorter, 1993; Baxter et al., 1994), and are often correlated with an increase in non-structural carbohydrates. Although the non-structural carbohydrate content was increased in the youngest fully expanded leaf of D. glomerata (Harmens et al., 2000), reduction in its SLA at elevated compared to ambient [CO2] was still evident when expressed per unit structural dry weight (Harmens, unpubl. res.). The current study indicates that decreases in SLA cannot simply be explained by accelerated growth at elevated [CO2], but are due to changes in the distribution of leaf dry weight and area during early stages of growth. In conclusion this work has shown the importance of assessing the eects of treatment on dry weight distribution and leaf area development using the dynamic allometric approach rather than by using ratios of plant components which change as a consequence of ontogeny. There is increasing evidence that CO2 enrichment does not aect allometric coecients during ontogeny, in contrast to resources such as N and light (McConnaughay and Coleman, 1999). Therefore, sequential harvesting in combination with allometry is a useful tool to distinguish between eects primarily due to enhanced [CO2] and confounding eects such as water and nutrient status of soils. AC K N OW L E D G E M E N T S This work was completed whilst H. H. was in receipt of a NERC Post-doctoral Fellowship. We thank Tim Sparks (ITE) for his help on statistical analyses and Ray Rafarel (ITE) for technical support. L I T E R AT U R E C I T E D Baxter R, Ashenden TW, Sparks TH, Farrar JF. 1994. Eects of elevated carbon dioxide on three montane grass species. I. Growth and dry matter partitioning. Journal of Experimental Botany 45: 305±315. Bazzaz FA. 1990. The response of natural ecosystems to the rising global CO2 levels. Annual Review of Ecology and Systematics 21: 167±196. Bowler JM, Press MC. 1993. Growth responses of two contrasting upland grass species to elevated CO2 and nitrogen concentration. New Phytologist 124: 515±522. Brouwer R. 1983. Functional equilibrium: sense or nonsense?. Netherlands Journal of Agricultural Science 31: 335±348. Coleman JS, McConnaughay KDM, Ackerly DD. 1994. Interpreting phenotypic variation in plants. Trends in Ecology and Evolution 9: 187±191. Davidson RL. 1969. Eect of root/leaf temperature dierentials on root : shoot ratios in some pasture grasses and clover. Annals of Botany 33: 561±569. Den Hertog J, Stulen I, Lambers H. 1993. Assimilation, respiration and allocation of carbon in Plantago major as aected by atmospheric CO2 levels. Vegetatio 104/105: 369±378. Drake BG, GonzaÁlez-Meler MA, Long SP. 1997. More ecient plants: a consequence of rising atmospheric CO2?. Annual Review of Plant Physiology and Plant Molecular Biology 48: 609±639. Evans GC. 1972. The quantitative analysis of plant growth. Oxford: Blackwell Scienti®c Publications. Farnsworth EJ, Ellison AM, Gong WK. 1996. Elevated CO2 alters anatomy, physiology, growth, and reproduction of red mangrove (Rhizophora mangle L.). Oecologia 108: 599±609.
Harmens et al.ÐPartitioning at Elevated CO2 and N Farrar JF, Gunn S. 1996. Eects of temperature and atmospheric carbon dioxide on source-sink relations in the context of climate change. In: Zamski E, Schaer AA, eds. Photoassimilate distribution in plants and crops. Source-sink relationships. New York: Marcel Dekker Inc., 389±406. Farrar JF, Gunn S. 1998. Allocation: allometry, acclimationÐand alchemy?. In: Lambers H, Poorter H, Van Vuuren MMI, eds. Inherent variation in plant growth. Physiological mechanisms and ecological consequences. Leiden: Backhuys Publishers, 183±198. Gebauer RLE, Reynolds JF, Strain BR. 1996. Allometric relations and growth in Pinus taeda the eect of elevated CO2 and changing N availability. New Phytologist 134: 85±93. Gunn S, Bailey SJ, Farrar JF. 1999. Partitioning of dry mass and leaf area within plants of three species grown at elevated CO2. Functional Ecology 13(Suppl. 1): 3±11. Harmens H, Stirling CM, Marshall C, Farrar JF. 2000. Does downregulation of photosynthetic capacity by elevated CO2 depend on N supply in Dactylis glomerata?. Physiologia Plantarum 108: 43±50. Hewitt EJ. 1966. Sand and water culture methods used in the study of plant nutrition. 2nd edn. Commonwealth Agricultural Bureaux of Technical Communications 22. Hibberd JM, Whitbread R, Farrar JF. 1996. Eect of 700 mmol mol ÿ1 CO2 and infection with powdery mildew on the growth and carbon partitioning of barley. New Phytologist 134: 309±315. Hunt R, Hand DW, Hannah MA, Neal AM. 1991. Responses to CO2 enrichment in 27 herbaceous species. Functional Ecology 5: 410±421. Knapp AK, Hamerlynck EP, Ham JM, Owensby CE. 1996. Responses in stomatal conductance to elevated CO2 in 12 grassland species that dier in growth form. Vegetatio 125: 31±41. Lloyd J, Farquhar GD. 1996. The CO2 dependence of photosynthesis, plant growth responses to elevated atmospheric CO2 concentrations and their interaction with soil nutrient status. I. General principles and forest ecosystems. Functional Ecology 10: 4±32.
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McConnaughay KDM, Coleman JS. 1999. Biomass allocation in plants: ontogeny or optimality? A test along three resource gradients. Ecology 80: 2581±2593. Marriott D. 1999. Growth of Urtica urens in elevated CO2. PhD Thesis, University of Wales, Bangor, Wales. Pearsall WH. 1927. Growth studies. VI. On the relative sizes of growing plant organs. Annals of Botany 151: 549±556. Poorter H. 1993. Interspeci®c variation in the growth response of plants to an elevated ambient CO2 concentration. Vegetatio 104/105: 77±97. Reynolds JF, Thornley JHM. 1982. A shoot : root partitioning model. Annals of Botany 49: 585±597. Ricker WE. 1984. Computation and uses of central trend lines. Canadian Journal of Zoology 62: 1897±1905. Rogers HH, Prior SA, Runion GB, Mitchell RJ. 1996. Root to shoot ratio of crops as in¯uenced by CO2. Plant and Soil 187: 229±248. Samarakoon AB, Giord RM. 1995. Soil water content under plants at high CO2 concentration and interactions with the direct CO2 eects: a species comparison. Journal of Biogeography 22: 193±202. Stirling CM, Heddell-Cowie M, Jones ML, Ashenden TW, Sparks TH. 1998. Eects of elevated CO2 and temperature on growth and allometry of ®ve native fast-growing annual species. New Phytologist 140: 343±354. Stitt M, Krapp A. 1999. The interaction between elevated carbon dioxide and nitrogen nutrition: the physiological and molecular background. Plant, Cell and Environment 22: 583±621. Stulen I, Den Hertog J. 1993. Root growth and functioning under atmospheric CO2 enrichment. Vegetatio 104/105: 99±115. Troughton A. 1955. The application of the allometric formula to the study of the relationship between the roots and shoots of young grass plants. Agricultural Progress 30: 59±65. Wilson JB. 1988. A review of evidence on the control of shoot : root ratio, in relation to models. Annals of Botany 61: 433±449. Zar JH. 1996. Biostatistical analysis. New Jersey: Prentice Hall Inc.
Is Partitioning of Dry Weight and Leaf Area Within Dactylis glomerata Aected by N and CO2 Enrichment? H . H A R ME N S *{} , C . M . ST I R L IN G{ {, C . M A R S H A L L } and J . F. FA R R A R } {Centre for Ecology and Hydrology Bangor, University of Wales, Bangor, Gwynedd LL57 2UP, UK and }School of Biological Sciences, University of Wales, Bangor, Gwynedd LL57 2UW, UK Received: 5 April 2000 Returned for revision: 15 May 2000 Accepted: 22 June 2000 Published electronically: 14 August 2000 We examined changes in dry weight and leaf area within Dactylis glomerata L. plants using allometric analysis to determine whether observed patterns were truly aected by [CO2] and N supply or merely re¯ect ontogenetic drift. Plants were grown hydroponically at four concentrations of NO3ÿ in controlled environment cabinets at ambient (360 ml l ÿ1) or elevated (680 ml l ÿ1) atmospheric [CO2]. Both CO2 and N enrichment stimulated net dry matter production. Allometric analyses revealed that [CO2] did not aect partitioning of dry matter between shoot and root at high N supply. However, at low N supply there was a transient increase in dry matter partitioning into the shoot at elevated compared to ambient [CO2] during early stages of growth, which is inconsistent with predictions based on optimal partitioning theory. In contrast, dry matter partitioning was aected by N supply throughout ontogeny, such that at low N supply dry matter was preferentially allocated to roots, which is in agreement with optimal partitioning theory. Independent of N supply, atmospheric CO2 enrichment resulted in a reduction in leaf area ratio (LAR), solely due to a decrease in speci®c leaf area (SLA), when plants of the same age were compared. However, [CO2] did not aect allometric coecients relating dry weight and leaf area, and eects of elevated [CO2] on LAR and SLA were the result of an early, transient stimulation of whole plant and leaf dry weight, compared to leaf area production. We conclude that elevated [CO2], in contrast to N supply, changes allocation patterns only transiently during early stages # 2000 Annals of Botany Company of growth, if at all. Key words: Allometric growth, carbon dioxide enrichment, Cocksfoot, Dactylis glomerata L., dry weight partitioning, leaf area ratio, nitrogen supply, shoot : root ratio, speci®c leaf area.
I N T RO D U C T I O N Doubling atmospheric [CO2] stimulates the growth of C3 species by an average of 41 % (Poorter, 1993). The magnitude of this increase varies with the individual species (Hunt et al., 1991; Poorter, 1993), experimental duration and the availability of other resources such as N (Bazzaz, 1990). When N limits growth, root dry weight increases relatively more than shoot dry weight, perhaps maintaining a functional equilibrium that results in the balanced acquisition of carbon and nitrogen (Davidson, 1969; Reynolds and Thornley, 1982; Brouwer, 1983; Wilson, 1988). Atmospheric [CO2] might also aect the allocation of dry matter between shoots and roots, although contrasting results have been reported for the shoot to root ratio (S : R ratio, de®ned as the dry weight of shoot divided by the dry weight of root): CO2 enrichment may either increase, decrease or not aect S : R ratio (Hunt et al., 1991; Stulen and Den Hertog, 1993; Rogers et al., 1996). Although [CO2] and N supply would be expected to interact regarding dry matter production and partitioning (i.e. the distribution of dry weight within the plant), the outcome cannot be predicted (Lloyd and Farquhar, 1996). uk
* For correspondence. Fax 44 (0)1248 355365, e-mail [email protected].
{ Present address: School of Agricultural and Forest Sciences, University of Wales, Bangor, Gwynedd LL57 2UW, UK.
0305-7364/00/100833+07 $35.00/00
Comparisons of the eects of treatments on plant growth are often based on ratios such as S : R with plants of the same age. However, patterns of allocation between plant parts change during growth and development independent of resource availability (Pearsall, 1927; Bowler and Press, 1993; Farrar and Gunn, 1996). Treatments may aect growth and development and alter S : R ratio and morphological characteristics compared to the control either because of true treatment eects or ontogenetic drift, i.e. phenotypic traits of plants change over the course of plant growth and development (Evans, 1972; Coleman et al., 1994; Farrar and Gunn, 1998; McConnaughay and Coleman, 1999). Dierentiation between true eects of treatment and of ontogenetic drift is possible using allometry, i.e. the study of the growth and development of one part of the plant in relation to another (Pearsall, 1927; Troughton, 1955). A few studies have made allometric comparisons between CO2 treatments, usually restricted to the allometric coecient relating the net dry matter production of shoot and root (Bowler and Press, 1993; Baxter et al., 1994; Farrar and Gunn, 1996; Hibberd et al., 1996). More recently, some studies have included the allometric relationship of dry matter and leaf area (Farnsworth et al., 1996; Gebauer et al., 1996; Stirling et al., 1998; Gunn et al., 1999). In general, allometric coecients were not aected by [CO2]. In contrast, allocation of dry matter to roots decreased with increasing N supply (Gebauer et al., 1996; McConnaughay and Coleman, 1999). # 2000 Annals of Botany Company
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Harmens et al.ÐPartitioning at Elevated CO2 and N
We suspect that many reports claiming that [CO2] alters net dry matter partitioning might be confounding eects of [CO2] with those of uncontrolled variables, especially when plants are grown in a solid substrate. For example, limited access to nutrients due to either growth limiting concentrations or limited ¯ux from a solid substrate to the roots, can mean that plants grown at elevated [CO2] become more nutrient de®cient than controls due to increased dry weight (Stitt and Krapp, 1999), resulting in a correspondingly lower S : R ratio. Recently it has become clear that soil water content can be another confounding variable (Samarakoon and Giord, 1995; Knapp et al., 1996), and one that is known to aect dry matter partitioning. Plants rooted in a solid substrate with a ®nite supply of water may have more favourable water status at elevated [CO2] due to a lower stomatal conductance for water vapour at elevated than ambient [CO2] (Drake et al., 1997). Accordingly, good control of nutrient and water supply is needed to distinguish between direct eects of [CO2] and indirect eects through nutrient and water status of soils. Here we examine the eects of elevated [CO2] and the interaction with N availability on growth, partitioning of dry weight, and dry weight±leaf area relationships in Dactylis glomerata L. (Cocksfoot). Plants were grown in controlled environments at either ambient (360 ml l ÿ1) or elevated (680 ml l ÿ1) [CO2] and four N concentrations (0.15, 0.6, 1.5 and 6.0 mM NO3ÿ ), ranging from growth limiting to optimal. Confounding eects of soil water status were minimized by growing plants in hydroponics. Comparisons between treatments were made both as a function of plant age and by applying allometry. It was hypothesized that elevated [CO2] aects net allocation of dry matter and distribution of dry matter and leaf area only through accelerated growth, but that N supply has a direct eect independent of changes in ontogeny.
et al., 2000). In order to reduce cabinet eects and eects of environmental heterogeneity within the cabinets, CO2 treatments were swapped between the two cabinets twice a week and troughs were randomized within each cabinet. Growth analysis Seven plants per treatment (three±four plants per trough) were harvested at 23, 28, 34 and 38 d after sowing. Plants were separated into leaf blades, leaf sheath stem, and roots. The area of leaf blades was determined using a digital leaf area meter (Delta T Ltd, Cambridge, UK) and plant parts were dried for at least 48 h at 65 8C and weighed. The following parameters were calculated: shoot : root ratio (dry weight), leaf area ratio (LAR; leaf area per plant dry weight), speci®c leaf area (SLA; leaf area per leaf dry weight) and leaf weight ratio (LWR; leaf dry weight per plant dry weight). Allometric relationships were determined using means per trough for each harvest and an ordinary linear regression equation (Pearsall, 1927; Troughton, 1955): ln y ln a k ln x where ln a is the y-intercept, k is the slope, and y and x are respectively: shoot and root dry weight; leaf dry weight and total plant dry weight; leaf area and total plant dry weight; and leaf area and leaf dry weight. In the majority of cases, the regression was linear as determined by linear and sequential polynomial regression. Subsequently, the slope (v) of the geometric mean regression was determined for all relationships (Ricker, 1984; Farrar and Gunn, 1996), as both y and x are dependent variables: v k=r
M AT E R I A L S A N D M E T H O D S Plant growth Seeds of Dactylis glomerata L., `Sylvan', were sown in propagators on moistened ®lter paper in controlled environment cabinets (Sanyo Gallenkamp, model SGC660/C/HQI, Loughborough, UK) at either ambient (360 ml l ÿ1) or elevated (680 ml l ÿ1) CO2 concentrations. Nine days after sowing, uniform-sized seedlings with one leaf were transferred to 3.5 l troughs, containing half-strength Long Ashton solution (Hewitt, 1966) with 10 mg l ÿ1 of sodium metasilicate. Supply of NO3ÿ was modi®ed to give four concentrations: 0.15, 0.6, 1.5 and 6.0 mM. At the smaller N supplies, the potassium and calcium concentrations were made up by adding K2SO4 and CaCl2 . The pH of the aerated solutions was controlled between 5.6±6.4 using 2 mM MES and KOH. Twenty seedlings were planted in each trough (two troughs per treatment) and after 1 week the nutrient solutions were replaced and the number of plants per trough was reduced to 15. Thereafter, nutrient solutions were replaced twice weekly and the number of plants per trough was reduced at every harvest. The conditions of growth were as described previously (Harmens
where r is the correlation coecient of the ordinary linear regression and v is the allometric coecient calculated by geometric mean regression. Where there was no signi®cant dierence between the slopes due to [CO2] a comparison of the elevations (as opposed to the y-intercepts) of the regressions (i.e. comparison of the vertical position of the lines) was carried out (Zar, 1996). Regression lines with the same slope and elevation coincide, whereas regression lines with the same slope but dierent elevation are parallel. Statistical analysis Data were analysed by analysis of variance (ANOVA) of the mean values of each trough (n 2 per treatment per harvest) using the Genstat statistical package (Lawes Agricultural Trust, Rothamsted, UK). Where indicated, data were ln-transformed prior to analysis to obtain homogeneity of variances. Eects of CO2 and N supply on the allometric coecient (v) and elevations of the regression were analysed by pairwise comparison using Student's t-test with 12 degrees of freedom. Unless indicated otherwise, signi®cant treatment eects refer to P 4 0.05.
Shoot:root ratio
Total dry wt (g)
Harmens et al.ÐPartitioning at Elevated CO2 and N 3
0.15 mM NO3−
0.6 mM NO3−
835
1.5 mM NO3−
6.0 mM NO3−
A
B
C
D
E
F
G
H
2 1 0 4 3 2 1 0
20
30
40
20
30
40
20
30
40
20
30
40
Time (d) F I G . 1. Total dry weight (A±D) and shoot : root ratio (E±H) of D. glomerata grown at 360 (ÐdÐ) or 680 (- - -s - - -) ml l ÿ1 CO2 and varying [NO3ÿ ]. Each data point represents the mean of two troughs, three±four plants per trough (+s.e.).
R E S U LT S Growth and ratios at the same plant age Total plant dry weight was greater at elevated than ambient [CO2] (Fig. 1A±D; Table 1) and although the magnitude of the increase varied, no signi®cant interactions between [CO2] and time, or [CO2] and N supply were found. From 28 d onwards, total plant dry weight increased with increasing [NO3ÿ ]. Responses of shoot and root dry weight to elevated [CO2] and [NO3ÿ ] were similar to that of total plant dry weight (data not shown), except that N supply did not aect root dry weight until 34 d. The S : R ratio was generally not aected by [CO2] and increased with increasing N supply (Fig. 1E±H; Table 1). However, at T A B L E 1. Summary of statistical signi®cance of eects of date of harvest, [CO2] and N supply on growth and development parameters of D. glomerata Source of variation Parameter
Time
N
CO2
Time N
Wtotal Wshoot Wroot Wshoot : Wroot Leaf area Leaf area ratio Speci®c leaf area Leaf weight ratio
*** *** *** *** *** *** *** ***
*** *** ** *** *** ** *** ***
*** *** *** n.s. ** *** *** n.s.
*** *** *** *** *** *** *** ***
Number of f.e.l. main stem Total number of leaves Total number of tillers
*** *** ***
*** *** ***
n.s. n.s. n.s.
* *** ***
Data for dry weight (W) and leaf area were ln-transformed to obtain homogeneity of variance. A general linear ANOVA model was ®tted to the mean value of each trough, so n 2 per treatment at each harvest. n.s., Not signi®cant, *P 4 0.05, **P 4 0.01, ***P 4 0.001. `Time CO2', `N CO2' and `Time N CO2' interactions were not signi®cant; f.e.l., fully expanded leaves.
[NO3ÿ ] above 0.15 mM there was a tendency towards an increase in S : R ratio due to CO2 enrichment. Developmental parameters such as number of tillers, leaves and fully expanded leaves on the main stem were not aected by [CO2], but increased with increasing N availability (Table 1). LAR was signi®cantly higher at ambient than elevated [CO2] at all [NO3ÿ ] and it decreased with time (Fig. 2A±D; Table 1). SLA was lower in plants exposed to elevated than ambient [CO2], independent of [NO3ÿ ] (Fig. 2E±H; Table 1). At 0.15 mM NO3ÿ the SLA increased with time, but it decreased with time at 1.5 and 6.0 mM NO3ÿ , resulting in a higher SLA at the ®nal harvest at 0.15 mM NO3ÿ compared to 1.5 and 6.0 mM NO3ÿ . LWR was not aected by CO2 (Fig. 2I±L; Table 1), thus the decrease in LAR at elevated [CO2] was due solely to a decrease in SLA. At low N supply LWR decreased with time, whilst at high N supply it was nearly constant, resulting in a higher LWR at the ®nal harvest at high compared to low N availability. Clearly, most ratios changed with time and therefore with increasing plant dry weight. Allometric relationships Elevated [CO2] had no signi®cant eect on any of the allometric coecients (v) and therefore did not aect net dry matter partitioning and dry weight±leaf area relationships within the plants between the ®rst and ®nal harvest (Figs 3, 4; Table 2). However, CO2 enrichment had a signi®cant eect on some of the elevations of regression lines, indicating a transient change in the partitioning of dry matter and leaf area during the initial stages of growth (before the ®rst harvest). At low N supply the allocation of dry matter to the shoot or leaf blades was increased early on at elevated compared to ambient [CO2]. At high N supply CO2 enrichment decreased the production of leaf area relative to whole plant dry weight early in growth. Independent of N supply, CO2 enrichment reduced the
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Harmens et al.ÐPartitioning at Elevated CO2 and N
LAR (cm2 g−1)
0.6 mM NO3−
0.15 mM NO3−
200
1.5 mM NO3−
6.0 mM NO3−
A
B
C
D
E
F
G
H
I
J
K
L
100
0
SLA (cm2 g−1)
500
250
0
LWR
0.8 0.4
0.0
20
30
40
20
30
40
20
30
40
20
30
40
Time (d) F I G . 2. LAR (A±D), SLA (E±H) and LWR (I±L) of D. glomerata grown at 360 (ÐdÐ) or 680 (- - -s - - -) ml l ÿ1 CO2 and varying [NO3ÿ ]. Each data point represents the mean of two troughs, three±four plants per trough (+s.e.).
ln DW shoot (mg)
0.6 mM NO3−
0.15 mM NO3−
8
A
1.5 mM NO3−
B
6.0 mM NO3−
C
D
6 4 2 2
ln DW leaf (mg)
8
4
6
8
2
E
4
6
8 2 ln DW root (mg)
F
4
6
8
2
G
4
6
8
H
6 4 2 2
4
6
8
10
2
4
6
8
10
2
4
6
8
10
2
4
6
8
10
ln DW total (mg) F I G . 3. The allometric relationship between shoot and root dry weight (A±D) and leaf and plant dry weight (E±H) of D. glomerata grown at 360 (ÐdÐ) or 680 (- - -s- - -) ml l ÿ1 CO2 and varying [NO3ÿ ]. Each data point represents the mean three±four plants per trough.
production of leaf area relative to leaf dry weight early in ontogeny. In contrast with [CO2], nitrogen supply signi®cantly aected most allometric coecients (Figs 3, 4; Table 2). At 0.15 and 0.6 mM NO3ÿ , dry matter was preferentially
partitioned to the root (v 5 1), whereas at 1.5 and 6.0 mM NO3ÿ it was allocated equally to shoot and root (v was not signi®cantly dierent from 1). Consequently, the increase in leaf weight per unit of plant dry weight was signi®cantly lower at low compared to high N supply. At
Harmens et al.ÐPartitioning at Elevated CO2 and N 0.6 mM NO3−
0.15 mM NO3−
6
A
837
1.5 mM NO3−
B
6.0 mM NO3−
C
D
4
ln leaf area (cm2)
2 0 0
2
4
6
8
10
0
2
4
6
8
10
0
2
4
6
8
10
0
2
4
6
8
10
ln DW total (mg) 6
E
F
G
H
4 2 0 0
2
4
6
8
0
2
4
6
8
0
2
4
6
8
0
2
4
6
8
ln DW leaf (mg) F I G . 4. The allometric relationship between leaf area and plant dry weight (A±D) and leaf area and leaf dry weight (E±H) of D. glomerata grown at 360 (ÐdÐ) or 680 (- - -s- - -) ml l ÿ1 CO2 and varying [NO3ÿ ]. Each data point represents the mean three±four plants per trough.
T A B L E 2. Eects of [CO2] and N supply on the allometric coecient (v) relating dry weight (W) and leaf area in D. glomerata CO2 concentration (ml l ÿ1)
v [NO3ÿ concentration (mM)] 0.15
0.6
1.5
6.0
0.79b (q***) 0.78b
0.99c 0.99c
1.07c 1.05c
0.65a (q**) 0.59a
0.84b (q**) 0.83b
0.97c 0.95c
0.99c 0.98c
360 680
0.74a 0.77a
0.77a 0.80a
0.85b (Q***) 0.82a
0.90c (Q***) 0.88b
360 680
1.13a (Q**) 1.32a
0.91b (Q***) 0.96b
0.88b (Q***) 0.86c
0.91b (Q***) 0.91bc
x
y
Wroot
Wshoot
360 680
0.56a (q**) 0.52a
Wtotal
Wleaf blade
360 680
Wtotal
Leaf area
Wleaf
Leaf area
The natural logarithm of y was plotted against the natural logarithm of x for all harvests and an ordinary linear regression was ®tted. The allometric coecient v was then calculated from the slope of the regression (k) and the correlation coecient (r): v k=r (r 0.98±1.00). Eects of [CO2] and N supply on v were tested by pairwise comparison using Student's t-test (degrees of freedom 12). Dierent superscripts within rows indicate signi®cant dierences (P 4 0.05) between v at dierent N treatments. Although there were no signi®cant eects of CO2 on v, signi®cant eects of CO2 on the elevation of regressions were found as indicated within parentheses: Q or q indicate a signi®cant higher or lower elevation at 360 compared to 680 ml l ÿ1 CO2 , respectively; **P 4 0.01, ***P 4 0.001.
ambient [CO2] the increase in leaf area per unit of plant dry weight was signi®cantly less at 0.15 and 0.6 mM NO3ÿ than at higher [NO3ÿ ]. At 0.15 mM NO3ÿ , leaf area increased relatively more than leaf dry weight compared to higher [NO3]. DISCUSSION When growth was analysed allometrically, it was evident that elevated [CO2] in comparison with N supply had minimal eects on the partitioning of dry matter within D. glomerata. An increase in N availability aected partitioning throughout ontogeny, such that enhanced N supply
reduced allocation of dry matter to roots. Dependent on N availability, elevated [CO2] changed the partitioning of dry weight and dry weight±leaf area relationships within D. glomerata only during the initial stages of growth. Hence, eects of CO2 enrichment on S : R ratio, LAR and SLA in plants of the same age were sometimes still present when ontogenetic drift was taken into account. Ratios indicate the state of the plant at an instant in time, are the product of the plant's history and will be an insensitive measure of changes in partitioning throughout ontogeny (Farrar and Gunn, 1998). We applied allometry to identify eects of treatment independent of ontogeny over a period of growth; allometry has a clear biological
838
Harmens et al.ÐPartitioning at Elevated CO2 and N
meaning and proved to be statistically more robust than comparing plants on the basis of similar total dry weights (Gunn et al., 1999). We found that dry weight and leaf area to dry weight ratios were subject to ontogenetic drift. Had we not accounted for this, we would have misjudged, in some treatments, the adjustments in allocation patterns. For example, the decrease in LAR due to CO2 enrichment at low N supply when plants of the same age were compared was simply the result of accelerated growth. On the other hand, a transient increase in allocation of dry matter to the shoot at elevated [CO2] and very low N supply during early stages of growth would not have been detected when only S : R ratios of plants of the same age were compared. Although other studies have shown that elevated [CO2] has little eect on allometric coecients (Bowler and Press, 1993; Baxter et al., 1994; Farnsworth et al., 1996; Gebauer et al., 1996; Stirling et al., 1998; Gunn et al., 1999), little attention has been paid to changes in the elevation of regressions (Gunn et al., 1999; Marriott, 1999). Whilst previously values for ln a have been used to quantify treatment eects on allometry early in growth, extreme care is required since the results of analyses will be highly dependent on the units used, so ln a has little biological signi®cance (Baxter et al., 1994; Stirling et al., 1998). This study clearly shows that both allometric coecients and elevations of regressions should be analysed to determine whether changes in partitioning occur throughout ontogeny or are restricted to a de®ned period in the plant's development. Whereas CO2 enrichment did not aect any of the allometric coecients (v), it did aect elevations of the regressions in most cases, indicating a change in partitioning during the initial stages of growth of D. glomerata. These early changes in partitioning are hard to detect, mainly due to the diculties associated with measuring very small plants and the large number of replicates that would be required to pick up small changes in v, which appear to occur in a short period of time. Optimal partitioning models and theory suggest that plants respond to variation in the environment by partitioning dry weight among plant organs to optimize the capture of resources in a manner that maximizes plant growth rate (Reynolds and Thornley, 1982; Brouwer, 1983; Wilson, 1988; McConnaughay and Coleman, 1999). In D. glomerata, optimal partitioning did apply to the range of N availability such that at limited N supply dry matter was preferentially allocated to roots. Whereas optimal partitioning models predict increased allocation of dry matter to roots at elevated CO2 , the partitioning of dry matter between shoots and roots in D. glomerata was little aected by CO2 enrichment. In contrast, a transient increase in the partitioning of dry matter to the shoot (and leaves) was observed at elevated [CO2] at low N supply at an early stage of plant development. Clearly, if the basic assumptions of optimal partitioning (i.e. that plants do alter dry weight distribution in response to changes in availability of resources to maximize growth rate) are incorrect, models attempting to predict the ecological outcome of environmental changes based on the optimal partitioning theory should be re-evaluated (McConnaughay and Coleman, 1999).
Reductions in SLA, and consequently in LAR, have been reported at elevated CO2 (Bazzaz, 1990; Den Hertog et al., 1993; Poorter, 1993; Baxter et al., 1994), and are often correlated with an increase in non-structural carbohydrates. Although the non-structural carbohydrate content was increased in the youngest fully expanded leaf of D. glomerata (Harmens et al., 2000), reduction in its SLA at elevated compared to ambient [CO2] was still evident when expressed per unit structural dry weight (Harmens, unpubl. res.). The current study indicates that decreases in SLA cannot simply be explained by accelerated growth at elevated [CO2], but are due to changes in the distribution of leaf dry weight and area during early stages of growth. In conclusion this work has shown the importance of assessing the eects of treatment on dry weight distribution and leaf area development using the dynamic allometric approach rather than by using ratios of plant components which change as a consequence of ontogeny. There is increasing evidence that CO2 enrichment does not aect allometric coecients during ontogeny, in contrast to resources such as N and light (McConnaughay and Coleman, 1999). Therefore, sequential harvesting in combination with allometry is a useful tool to distinguish between eects primarily due to enhanced [CO2] and confounding eects such as water and nutrient status of soils. AC K N OW L E D G E M E N T S This work was completed whilst H. H. was in receipt of a NERC Post-doctoral Fellowship. We thank Tim Sparks (ITE) for his help on statistical analyses and Ray Rafarel (ITE) for technical support. L I T E R AT U R E C I T E D Baxter R, Ashenden TW, Sparks TH, Farrar JF. 1994. Eects of elevated carbon dioxide on three montane grass species. I. Growth and dry matter partitioning. Journal of Experimental Botany 45: 305±315. Bazzaz FA. 1990. The response of natural ecosystems to the rising global CO2 levels. Annual Review of Ecology and Systematics 21: 167±196. Bowler JM, Press MC. 1993. Growth responses of two contrasting upland grass species to elevated CO2 and nitrogen concentration. New Phytologist 124: 515±522. Brouwer R. 1983. Functional equilibrium: sense or nonsense?. Netherlands Journal of Agricultural Science 31: 335±348. Coleman JS, McConnaughay KDM, Ackerly DD. 1994. Interpreting phenotypic variation in plants. Trends in Ecology and Evolution 9: 187±191. Davidson RL. 1969. Eect of root/leaf temperature dierentials on root : shoot ratios in some pasture grasses and clover. Annals of Botany 33: 561±569. Den Hertog J, Stulen I, Lambers H. 1993. Assimilation, respiration and allocation of carbon in Plantago major as aected by atmospheric CO2 levels. Vegetatio 104/105: 369±378. Drake BG, GonzaÁlez-Meler MA, Long SP. 1997. More ecient plants: a consequence of rising atmospheric CO2?. Annual Review of Plant Physiology and Plant Molecular Biology 48: 609±639. Evans GC. 1972. The quantitative analysis of plant growth. Oxford: Blackwell Scienti®c Publications. Farnsworth EJ, Ellison AM, Gong WK. 1996. Elevated CO2 alters anatomy, physiology, growth, and reproduction of red mangrove (Rhizophora mangle L.). Oecologia 108: 599±609.
Harmens et al.ÐPartitioning at Elevated CO2 and N Farrar JF, Gunn S. 1996. Eects of temperature and atmospheric carbon dioxide on source-sink relations in the context of climate change. In: Zamski E, Schaer AA, eds. Photoassimilate distribution in plants and crops. Source-sink relationships. New York: Marcel Dekker Inc., 389±406. Farrar JF, Gunn S. 1998. Allocation: allometry, acclimationÐand alchemy?. In: Lambers H, Poorter H, Van Vuuren MMI, eds. Inherent variation in plant growth. Physiological mechanisms and ecological consequences. Leiden: Backhuys Publishers, 183±198. Gebauer RLE, Reynolds JF, Strain BR. 1996. Allometric relations and growth in Pinus taeda the eect of elevated CO2 and changing N availability. New Phytologist 134: 85±93. Gunn S, Bailey SJ, Farrar JF. 1999. Partitioning of dry mass and leaf area within plants of three species grown at elevated CO2. Functional Ecology 13(Suppl. 1): 3±11. Harmens H, Stirling CM, Marshall C, Farrar JF. 2000. Does downregulation of photosynthetic capacity by elevated CO2 depend on N supply in Dactylis glomerata?. Physiologia Plantarum 108: 43±50. Hewitt EJ. 1966. Sand and water culture methods used in the study of plant nutrition. 2nd edn. Commonwealth Agricultural Bureaux of Technical Communications 22. Hibberd JM, Whitbread R, Farrar JF. 1996. Eect of 700 mmol mol ÿ1 CO2 and infection with powdery mildew on the growth and carbon partitioning of barley. New Phytologist 134: 309±315. Hunt R, Hand DW, Hannah MA, Neal AM. 1991. Responses to CO2 enrichment in 27 herbaceous species. Functional Ecology 5: 410±421. Knapp AK, Hamerlynck EP, Ham JM, Owensby CE. 1996. Responses in stomatal conductance to elevated CO2 in 12 grassland species that dier in growth form. Vegetatio 125: 31±41. Lloyd J, Farquhar GD. 1996. The CO2 dependence of photosynthesis, plant growth responses to elevated atmospheric CO2 concentrations and their interaction with soil nutrient status. I. General principles and forest ecosystems. Functional Ecology 10: 4±32.
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McConnaughay KDM, Coleman JS. 1999. Biomass allocation in plants: ontogeny or optimality? A test along three resource gradients. Ecology 80: 2581±2593. Marriott D. 1999. Growth of Urtica urens in elevated CO2. PhD Thesis, University of Wales, Bangor, Wales. Pearsall WH. 1927. Growth studies. VI. On the relative sizes of growing plant organs. Annals of Botany 151: 549±556. Poorter H. 1993. Interspeci®c variation in the growth response of plants to an elevated ambient CO2 concentration. Vegetatio 104/105: 77±97. Reynolds JF, Thornley JHM. 1982. A shoot : root partitioning model. Annals of Botany 49: 585±597. Ricker WE. 1984. Computation and uses of central trend lines. Canadian Journal of Zoology 62: 1897±1905. Rogers HH, Prior SA, Runion GB, Mitchell RJ. 1996. Root to shoot ratio of crops as in¯uenced by CO2. Plant and Soil 187: 229±248. Samarakoon AB, Giord RM. 1995. Soil water content under plants at high CO2 concentration and interactions with the direct CO2 eects: a species comparison. Journal of Biogeography 22: 193±202. Stirling CM, Heddell-Cowie M, Jones ML, Ashenden TW, Sparks TH. 1998. Eects of elevated CO2 and temperature on growth and allometry of ®ve native fast-growing annual species. New Phytologist 140: 343±354. Stitt M, Krapp A. 1999. The interaction between elevated carbon dioxide and nitrogen nutrition: the physiological and molecular background. Plant, Cell and Environment 22: 583±621. Stulen I, Den Hertog J. 1993. Root growth and functioning under atmospheric CO2 enrichment. Vegetatio 104/105: 99±115. Troughton A. 1955. The application of the allometric formula to the study of the relationship between the roots and shoots of young grass plants. Agricultural Progress 30: 59±65. Wilson JB. 1988. A review of evidence on the control of shoot : root ratio, in relation to models. Annals of Botany 61: 433±449. Zar JH. 1996. Biostatistical analysis. New Jersey: Prentice Hall Inc.