Does soil nitrogen influence growth,water transport and survival of snow gum (Eucalyptus pauciflora Sieber ex Sprengel.) under CO2 enrichment?

Eucalyptus pauciflora Sieber ex Sprengel. (snow gum) was grown under ambient (370  µ L L⁻¹) and elevated (700  µ L L⁻¹) atmospheric [CO₂] in open-top chambers (OTCs) in the field and temperature-controlled glasshouses. Nitrogen applications to the soil ranged from 0.1 to 2.75 g N per plant. Trees in the field at high N levels grew rapidly during summer, particularly in CO₂-enriched atmosphere, but suffered high mortality during summer heatwaves. Generally, wider and more numerous secondary xylem vessels at the root–shoot junction in CO₂-enriched trees conferred fourfold higher below-ground hydraulic conductance. Enhanced hydraulic capacity was typical of plants at elevated [CO₂] (in which root and shoot growth was accelerated), but did not result from high N supply. However, because high rates of N application consistently made trees prone to dehydration during heatwaves, glasshouse studies were required to identify the effect of N nutrition on root development and hydraulics. While the effects of elevated [CO₂] were again predominantly on hydraulic conductivity, N nutrition acted specifically by constraining deep root penetration into soil. Specifically, 15–40% shallower root systems supported marginally larger shoot canopies. Independent changes to hydraulics and root penetration have implications for survival of fertilized trees under elevated atmospheric [CO₂], particularly during water stress.     

Effect of enhanced atmospheric CO2 on mycorrhizal colonization and phosphorus inflow in 10 herbaceous species of contrasting growth strategies

1. Ten herbaceous species were grown over a 4-month period under ambient (360 μmol mol^–1) and elevated (610 μmol mol^–1) atmospheric CO₂ conditions. Plants were inoculated with the arbuscular mycorrhizal (AM) fungus Glomus mosseae and given a phosphorus (P) supply which was not immediately available to the plants.
2. Multiple harvests were taken in order to determine whether the effect of elevated CO₂ on mycorrhizal colonization and phosphorus inflow was independent of its effect on plant growth.
3. All species grew faster under elevated CO₂ and carbon partitioning was altered, generally in favour of the shoots. All species responded similarly to elevated CO₂.
4. Elevated CO₂ did not affect the percentage of root length colonized by AM fungi, but the total amount of colonized root length was increased, because the plants were bigger.
5. Elevated CO₂ increased total P content, but had little or no effect on P concentration. At a given age, P inflow was stimulated by elevated CO₂, but when root length was taken into account the CO₂ effect disappeared.
6. In these host species there is no evidence for a direct effect of elevated CO₂ on mycorrhizal functioning, because both internal mycorrhizal colonization and P inflow are unaffected.
7. Future research should concentrate on the potential for carbon flow to the soil via the external mycelial network.     

In situ responses to elevated CO2 in tropical forest understorey plants

1. Plants growing in deep shade and high temperature, such as in the understorey of humid tropical forests, have been predicted to be particularly sensitive to rising atmospheric CO₂. We tested this hypothesis in five species whose microhabitat quantum flux density (QFD) was documented as a covariable. After 7 (tree seedlings of Tachigalia versicolor and Beilschmiedia pendula ) and 18 months (shrubs Piper cordulatum and Psychotria limonensis, and grass Pharus latifolius ) of elevated CO₂ treatment ( c. 700 μl litre^–1) under mean QFD of less than 11 μmol m^–2 s^–1, all species produced more biomass (25–76%) under elevated CO₂.
2. Total plant biomass tended to increase with microhabitat QFD (daytime means varying from 5 to 11μmol m^–2 s^–1) but the relative stimulation by elevated CO₂ was higher at low QFD except in Pharus .
3. Non-structural carbohydrate concentrations in leaves increased significantly in Pharus (+ 27%) and Tachigalia (+ 40%).
4. The data support the hypothesis that tropical plants growing near the photosynthetic light compensation point are responsive to elevated CO₂. An improved plant carbon balance in deep shade is likely to influence understorey plant recruitment and competition as atmospheric CO₂ continues to rise.     

Effect of enhanced atmospheric CO2 on mycorrhizal colonization by Glomus mosseae in Plantago lanceolata and Trifolium repens

Plantago lanceolata L. and Trifolium repens L. were grown for 16 wk in ambient (360 μmol mol⁻¹) and elevated (610 μmol mol⁻¹) atmospheric CO₂. Plants were inoculated with the arbuscular mycorrhizal (AM) fungus Glomus mosseae (Nicol. & Gerd.) Gerdemann & Trappe and given a phosphorus supply in the form of bonemeal, which would not be immediately available to the plants. Seven sequential harvests were taken to determine whether the effect of elevated CO₂ on mycorrhizal colonization was independent of the effect of CO₂ on plant growth. Plant growth analysis showed that both species grew faster in elevated CO₂ and that P. lanceolata had increased carbon allocation towards the roots. Elevated CO₂ did not affect the percentage of root length colonized (RLC); although total colonized root length was greater, when plant size was taken into account this effect disappeared. This finding was also true for root length colonized by arbuscules. No CO₂ effect was found on hyphal density (colonization intensity) in roots. The P content of plants was increased at elevated CO₂, although both shoot and root tissue P concentration were unchanged. This was again as a result of bigger plants at elevated CO₂. Phosphorus inflow was unaffected by CO₂ concentrations. It is concluded that there is no direct permanent effect of elevated CO₂ on mycorrhizal functioning, as internal mycorrhizal development and the mycorrhizal P uptake mechanism are unaffected. The importance of sequential harvests in experiments is discussed. The direction for future research is highlighted, especially in relation to C storage in the soil.     

Acquisition and allocation of carbon and nitrogen by Danthonia richardsonii in response to restricted nitrogen supply and CO2 enrichment

Dry weight (DW) and nitrogen (N) accumulation and allocation were measured in isolated plants of Danthonia richardsonii (Wallaby Grass) for 37 d following seed imbibition. Plants were grown at ≈ 365 or 735 μ L L^–1 CO₂ with N supply of 0·05, 0·2 or 0·5 mg N plant^–1 d^–1. Elevated CO₂ increased DW accumulation by 28% (low-N) to 103% (high-N), following an initial stimulation of relative growth rate. Net assimilation rate and leaf nitrogen productivity were increased by elevated CO₂, while N concentration was reduced. N uptake per unit root surface area was unaffected by CO₂ enrichment. The ratio of leaf area to root surface area was decreased by CO₂ enrichment. Allometric analysis revealed a decrease in the shoot-N to root-N ratio at elevated CO₂, while the shoot-DW to root-DW ratio was unchanged. Allometric analysis showed leaf area was reduced, while root surface area was unchanged by elevated CO₂, indicating a down-regulation of total plant capacity for carbon gain rather than a stimulation of mineral nutrient acquisition capacity. Overall, growth in elevated CO₂ resulted in changes in plant morphology and nitrogen use, other than those associated simply with changing plant size and non-structural carbohydrate content.     

Elevated CO2 concentration, nitrogen use, and seed production in annual plants

Elevated atmospheric CO₂ concentration ([CO₂]) stimulates seed mass production in many species, but the extent of stimulation shows large variation among species. We examined (1) whether seed production is enhanced more in species with lower seed nitrogen concentrations, and (2) whether seed production is enhanced by elevated [CO₂] when the plant uses more N for seed production. We grew 11 annuals in open top chambers that have different [CO₂] conditions (ambient: 370 μmol mol⁻¹, elevated: 700 μmol mol⁻¹). Elevated [CO₂] significantly increased seed production in six out of 11 species with a large interspecific variation (0.84–2.12, elevated/ambient [CO₂]). Seed nitrogen concentration was not correlated with the enhancement of seed production by elevated [CO₂]. The enhancement of seed production was strongly correlated with the enhancement of seed nitrogen per plant caused by increased N acquisition during the reproductive period. In particular, legume species tended to acquire more N and produced more seeds at elevated [CO₂] than non-nitrogen fixing species. Elevated [CO₂] little affected seed [N] in all species. We conclude that seed production is limited primarily by nitrogen availability and will be enhanced by elevated [CO₂] only when the plant is able to increase nitrogen acquisition.     

Elevated CO2 and conifer roots: effects on growth, life span and turnover

Elevated CO₂ increases root growth and fine (diam. 2 mm) root growth across a range of species and experimental conditions. However, there is no clear evidence that elevated CO₂ changes the proportion of C allocated to root biomass, measured as either the root:shoot ratio or the fine root:needle ratio. Elevated CO₂ tends to increase mycorrhizal infection, colonization and the amount of extramatrical hyphae, supporting their key role in aiding the plant to more intensively exploit soil resources, providing a route for increased C sequestration. Only two studies have determined the effects of elevated CO₂ on conifer fine-root life span, and there is no clear trend. Elevated CO₂ increases the absolute fine-root turnover rates; however, the standing crop root biomass is also greater, and the effect of elevated CO₂ on relative turnover rates (turnover:biomass) ranges from an increase to a decrease. At the ecosystem level these changes could lead to increased C storage in roots. Increased fine-root production coupled with increased absolute turnover rates could also lead to increases in soil organic C as greater amounts of fine roots die and decompose. Although CO₂ can stimulate fine-root growth, it is not known if this stimulation persists over time. Modeling studies suggest that a doubling of the atmospheric CO₂ concentration initially increases biomass, but this stimulation declines with the response to elevated CO₂ because increases in assimilation are not matched by increases in nutrient supply.     

Responses to elevated temperature and CO2 in the perennial grass Agrostis curtisii in relation to population origin

1. Evolutionary responses to climate change will depend on the presence of heritable variation within species populations for traits that increase fitness under the changing conditions. Patterns of ecotypic differentiation in relation to latitude in some species suggest that such variation exists in relation to temperature responses. Response to elevated CO₂, whether heritable or not, is not expected to be related to latitudinal or climatic differences within temperate regions.
2. To test these ideas, seeds were collected from 10 populations of the outbreeding perennial grass Agrostis curtisii across its range in Europe from south Wales to Portugal. Plants were grown under ambient and elevated temperature and CO₂ conditions, in a factorial design, in solardomes; two half sibs from each population were planted in separate pots in each of the two replicate domes with each combination of treatments. One half sib was harvested at the end of the first summer, the second at the end of the second summer.
3. Survival was uniformly high and flowering uniformly low across treatments and populations.
4. Responses to temperature and CO₂ treatments varied over time for almost all populations. Treatment effects were not significant on plants harvested in year 1, although there was a trend towards higher shoot biomass under the elevated temperature and CO₂ treatment. In year 2 shoot biomass was significantly higher under the elevated temperature treatment across all populations and there was a strong trend towards decreased biomass under elevated CO₂.
5. There were no significant correlations of plant response to either CO₂ or temperature with climate at origin.
6. These results warn of the dangers of extrapolating evolutionary plant responses to CO₂ from short-term experiments.     

The influence of elevated CO2 on species phenology, growth and reproduction in a Mediterranean old-field community

We studied the effects on the phenology, growth and reproduction of 19 Mediterranean species, of elevating the atmospheric CO₂ concentration ([CO₂]) to twice-ambient. Intact monoliths were taken from an old-field and put, during a six month growing season, into growth chambers in which external climatic conditions were mimicked and [CO₂] was regulated. Fruit set time was significantly changed in six species under elevated [CO₂] and leaf and branch senescence accelerated in most species. Grasses had fewer leaves and legumes were more branched at peak production under elevated [CO₂] than under ambient. Plant seed number was not significantly changed under elevated [CO₂], whereas the reproductive effort of grasses was significantly depressed. Reproductive and vegetative characteristics showed related responses to [CO₂], as species with enhanced biomass had a hastened fruit set time, a higher number of fruits per plant and a higher reproductive biomass under elevated [CO₂] than under ambient conditions, while species with depressed biomass had a delayed fruit set time, a lower number of fruits per plant and a lower reproductive biomass. Our results also show a high interspecific variability in [CO₂] response, but some trends emerged at the family level: the production of vegetative and reproductive modules were depressed in grasses and slightly stimulated in legumes.     

Partitioning of dry mass and leaf area within plants of three species grown at elevated CO2

1. We tested the hypothesis that the net partitioning of dry mass and dry mass:area relationships is unaltered when plants are grown at elevated atmospheric CO₂ concentrations.
2. The total dry mass of Dactylis glomerata, Bellis perennis and Trifolium repens was higher for plants in 700 compared to 350 μmol CO₂ mol^–1 when grown hydroponically in controlled-environment cabinets.
3. Shoot:root ratios were higher and leaf area ratios and specific leaf areas lower in all species grown at elevated CO₂. Leaf mass ratio was higher in plants of B. perennis and D. glomerata grown at elevated CO₂.
4. Whilst these data suggest that CO₂ alters the net partitioning of dry mass and dry mass:leaf area relationships, allometric comparisons of the components of dry mass and leaf area suggest at most a small effect of CO₂. CO₂ changed only two of a total of 12 allometric coefficients we calculated for the three species: ν relating shoot to root dry mass was higher in D. glomerata , whilst ν relating leaf area to total dry mass was lower in T. repens .
5. CO₂ alone has very little effect on partitioning when the size of the plant is taken into account.     

Interspecific differences in the response of arbuscular mycorrhizal fungi to Artemisia tridentata grown under elevated atmospheric CO2

Arbuscular mycorrhizal (AM) fungi form mutualistic symbioses with the root systems of most plant species. These mutualisms regulate nutrient exchange in the plant–soil interface and might influence the way in which plants respond to increasing atmospheric CO₂. In other experiments, mycorrhizal responses to elevated CO₂ have been variable, so in this study we test the hypothesis that different genera of AM fungi differ in their response, and in turn alter the plant's response, to elevated CO₂. Four species from three genera of AM fungi were tested. Artemisia tridentata Nutt. seedlings were inoculated with either Glomus intraradices Schenck & Smith, Glomus etunicatum Becker & Gerdemann, Acaulospora sp. or Scutellospora calospora (Nicol. & Gerd.) Walker & Sanders and grown at either ambient CO₂ (350 ppm) or elevated CO₂ (700 ppm). Several significant inter-specific responses were detected. Elevated CO₂ caused percent arbuscular and hyphal colonization to increase for the two Glomus species, but not for Acaulospora sp. or S. calospora . Vesicular colonization was not affected by elevated CO₂ for any fungal species. In the extra-radical phase, the two Glomus species produced a significantly higher number of spores in response to elevated CO₂, whereas Acaulospora sp. and S. calospora developed significantly higher hyphal lengths. These data show that AM fungal taxa differ in their growth allocation strategies and in their responses to elevated CO₂, and that mycorrhizal diversity should not be overlooked in global change research.     

Effects of mycorrhizal colonization on biomass production and nitrogen fixation of black locust (Robinia pseudoacacia) seedlings grown under elevated atmospheric carbon dioxide

Interactive effects of elevated atmospheric CO₂ and arbuscular mycorrhizal (AM) fungi on biomass production and N₂ fixation were investigated using black locust ( Robinia pseudoacacia ). Seedlings were grown in growth chambers maintained at either 350 μmol mol⁻¹ or 710 μmol mol⁻¹ CO₂. Seedlings were inoculated with Rhizobium spp. and were grown with or without AM fungi. The ¹⁵N isotope dilution method was used to determine N source partitioning between N₂ fixation and inorganic fertilizer uptake. Elevated atmospheric CO₂ significantly increased the percentage of fine roots that were colonized by AM fungi. Mycorrhizal seedlings grown under elevated CO₂ had the greatest overall plant biomass production, nodulation, N and P content, and root N absorption. Additionally, elevated CO₂ levels enhanced nodule and root mass production, as well as N₂ fixation rates, of non- mycorrhizal seedlings. However, the relative response of biomass production to CO₂ enrichment was greater in non-mycorrhizal seedlings than in mycorrhizal seedlings. This study provides strong evidence that arbuscular mycorrhizal fungi play an important role in the extent to which plant nutrition of symbiotic N₂-fixing tree species is affected by enriched atmospheric CO₂.     

Soil microbial responses to increased concentrations of atmospheric CO2

Terrestrial ecosystems respond to an increased concentration of atmospheric CO₂. While elevated atmospheric CO₂ has been shown to alter plant growth and productivity, it also affects ecosystem structure and function by changing below-ground processes. Knowledge of how soil microbiota respond to elevated atmospheric CO₂ is of paramount importance for understanding global carbon and nutrient cycling and for predicting changes at the ecosystem-level. An increase in the atmospheric CO₂ concentration not only alters the weight, length, and architecture of plant roots, but also affects the biotic and abiotic environment of the root system. Since the concentration of CO₂ in soil is already 10–50 times higher than that in the atmosphere, it is unlikely that increasing atmospheric CO₂ will directly influence the rhizosphere. Rather, it is more likely that elevated atmospheric CO₂ will affect the microbe–soil–plant root system indirectly by increasing root growth and rhizodeposition rates, and decreasing soil water deficit. Consequently, the increased amounts and altered composition of rhizosphere-released materials will have the potential to alter both population and community structure, and activity of soil- and rhizosphere-associated microorganisms. This occurrence could in turn affect plant health and productivity and plant community structure. This review covers current knowledge about the response of soil microbes to elevated concentrations of atmospheric CO₂.     

Effect of elevated CO2 on carbon partitioning and exudate release from Plantago lanceolata seedlings

Plantago lanceolata L. seedlings were grown in sand microcosm units over a 43‐day experimental period under two CO₂ regimes (800 or 400 µmol mol⁻¹) to investigate the effect of elevated atmospheric CO₂ concentration on carbon partitioning and exudate release. Total organic carbon (TOC) content of the collected exudate material was measured throughout the experimental period. After 42 days growth the seedlings were labelled with [¹⁴C]‐CO₂ and the fate of the label within the plant and its release by the roots monitored. Elevated CO₂ significantly (P ≤ 0.001) enhanced shoot, root and total dry matter production although the R:S ratio was unaltered, suggesting no alteration in gross carbon partitioning. The cumulative release of TOC (in mg C) over 0‐42 days was unaltered by CO₂ treatment however, when expressed as a percentage of net assimilated C, ambient‐grown plants released a significantly (P≤ 0.001) higher percentage from their roots compared to elevated CO₂‐grown plants (i.e. 8 vs 3%). The distribution of ¹⁴C‐label was markedly altered by CO₂ treatment with significantly (P≤ 0.001) greater per cent label partitioned to the roots under elevated CO₂. This indicates increased partitioning of recent assimilate below‐ground under elevated CO₂ treatment although there was no significant difference in the percentage of ¹⁴C‐label released by the roots. Comparison of plant C budgets based on ¹⁴C‐pulse‐chase methodology and TOC measurements is discussed.     

Low vapour pressure deficit reduces the beneficial effect of elevated CO2 on growth of N2-fixing alfalfa plants

Plant responses to elevated CO₂ can be modified by many environmental factors, but very little attention has been paid to the interaction between CO₂ and changes in vapour pressure deficit (VPD). Thirty-day-old alfalfa plants ( Medicago sativa L. cv. Aragón), which were inoculated with Sinorhizobium meliloti 102F78 strain, were grown for 1 month in controlled environment chambers at 25/15°C, 14 h photoperiod, and 600 µmol m⁻² s⁻¹ photosynthetic photon flux (PPF), using a factorial combination of CO₂ concentration (400 µmol mol⁻¹ or 700 µmol mol⁻¹) and vapour pressure deficit (0.48 kPa or 1.74 kPa, which corresponded to relative humidities of 85% and 45% at 25°C, respectively). Elevated CO₂ strongly stimulated plant growth under high VPD conditions, but this beneficial effect was not observed under low VPD. Under low VPD, elevated CO₂ also did not enhance plant photosynthesis, and plant water stress was greatest for plants grown at elevated CO₂ and low VPD. Moreover, plants grown under elevated CO₂ and low VPD had a lower leaf soluble protein and photosynthetic activity (photosynthetic rate and carboxylation efficiency) than plants grown under elevated CO₂ and high VPD. Elevated CO₂ significantly increased leaf adaxial and abaxial temperatures. Because the effects of elevated CO₂ were dependent on vapour pressure deficit, VPD needs to be controlled in experiments studying the effect of elevated CO₂ as well as considered in the extrapolations of results to a warmer, high-CO₂ world.     

Nodulation and nitrogenase activity in nitrogen-fixing woody plants stimulated by CO2 enrichment of the atmosphere

The responses of three species of nitrogen-fixing trees to CO₂ enrichment of the atmosphere were investigated under nutrient-poor conditions. Seedlings of the legume, Robinia pseudoacacia L. and the actinorhizal species, Alnus glutinosa (L.) Gaertn. and Elaeagnus angustifolia L. were grown in an infertile forest soil in controlled-environment chambers with atmospheric CO₂ concentrations of 350 μl ⁻¹ (ambient) or 700 μl ⁻¹. In R. pseudoacacia and A. glutinosa , total nitrogenase (N₂ reduction) activity per plant, assayed by the acetylene reduction method, was significantly higher in elevated CO₂, because the plants were larger and had more nodule mass than did plants in ambient CO₂. The specific nitrogenase activity of the nodules, however, was not consistently or significantly affected by CO₂ enrichment. Substantial increases in plant growth occurred with CO₂ enrichment despite probable nitrogen and phosphorus deficiencies. These results support the premises that nutrient limitations will not preclude growth responses of woody plants to elevated CO₂ and that stimulation of symbiotic activity by CO₂ enrichment of the atmosphere could increase nutrient availability in infertile habitats.     

Shade‐tolerance as a predictor of responses to elevated CO2 in trees

Evidence from 10 studies comparing angiosperm trees and 5 studies comparing conifers of differing shade‐tolerance was analysed. The number of intraphyletic comparisons in which the more shade‐tolerant species showed the greater relative increase of biomass in elevated CO₂ was significantly higher than would be expected by chance alone. It is suggested that more shade‐tolerant species are inherently better disposed, in terms of plant architecture and partitioning of biomass and nitrogen, to utilise resources (light, water, nutrients) that are potentially limiting in elevated CO₂ and that these traits are responsible for the interaction between shade‐tolerance and CO₂ concentration. Compared with less shade‐tolerant angiosperm trees, more shade‐tolerant angiosperm species generally have a lower leaf area ratio in ambient CO₂ and show a smaller relative reduction in elevated CO₂. Furthermore, leaf nitrogen content is usually lower in more shade‐tolerant angiosperm species and tends to be more strongly reduced by elevated CO₂ in those species. Within angiosperm trees, more shade‐tolerant species showed a stronger stimulation of net leaf photosynthetic rate in most experiments, but this trend was not significant.     

Architectural and physiological mechanisms of reduced size inequality in CO2-enriched stands of common ragweed (Ambrosia artemisiifolia)

Testing whether and how subordinate individuals differ from dominants in the utilization of enriched CO₂ atmospheres is important for understanding future stand and community structure. We hypothesized that subordinate and dominant Ambrosia artemisiifolia L. (Asteraceae) (common ragweed) plants growing in dense stands would not equally acquire or utilize carbon gains from CO₂-enrichment, and that the resulting disproportionate growth gains to subordinates would reduce size inequalities in competing stands. We grew experimental stands of A. artemisiifolia in either ambient (360 μL L⁻¹) or twice ambient (720 μL L⁻¹) levels of atmospheric CO₂. We compared the relative growth, photosynthetic capacity, and architecture of subordinate and dominant plants in each treatment, and assessed size inequalities using the stand-level coefficient of variation (CV). In elevated CO₂, plants grew larger, but subordinate plants shifted more mass to upper stem allocation than dominants. Dominant plants demonstrated reduced leaf-level photosynthetic gains in elevated CO₂ compared with subordinate plants. Reduced CVs in plant size reflected smaller proportional growth gains by dominants over subordinates in elevated vs. ambient stands. We conclude that differences in the architectural and physiological responses of subordinate and dominant ragweed plants reduce competition and allow subordinate plants to catch up to dominants in elevated CO₂ conditions.     

The Effect of Elevated [CO2] on Uptake and Allocation of 13C and 15N in Beech (Fagus sylvatica L.) during Leafing

Abstract: A continuous dual ¹³CO₂ and ¹⁵NH₄¹⁵NO₃ labelling experiment was undertaken to determine the effects of ambient (350μmol mol^-1) or elevated (700μmol mol^-1) atmospheric CO₂ concentrations on C and N uptake and allocation within 3-year-old beech ( Fagus sylvatica L.) during leafing. After six weeks of growth, total carbon uptake was increased by 63 % (calculated on total C content) under elevated CO₂ but the carbon partitioning was not altered. 56 % of the new carbon was found in the leaves. On a dry weight basis was the content of structural biomass in leaves 10 % lower and the lignin content remained unaffected under elevated as compared to ambient [CO₂]. Under ambient [CO₂] 37 %, and under elevated [CO₂] 51 %, of the lignin C of the leaves derived from new assimilates. For both treatments, internal N pools provided more than 90 % of the nitrogen used for leaf-growth and the partitioning of nitrogen was not altered under elevated [CO₂]. The C/N ratio was unaffected by elevated [CO₂] at the whole plant level, but the C/N ratio of the new C and N uptake was increased by 32 % under elevated [CO₂].     

The effects of increased atmospheric carbon dioxide and temperature on carbon partitioning, source-sink relations and respiration

Abstract. Herbaceous C₃ plants grown in elevated CO₂ show increases in carbon assimilation and carbohydrate accumulation (particularly starch) within source leaves. Although changes in the partitioning of biomass between root and shoot occur, the proportion of this extra assimilate made available for sink growth is not known. Root:shoot ratios tend to increase for CO₂-enriched herbaceous plants and decrease for CO₂-enriched trees. Root:shoot ratios for cereals tend to remain constant. In contrast, elevated temperatures decrease carbohydrate accumulation within source and sink regions of a plant and decrease root:shoot ratios. Allometric analysis of at least two species showing changes in root: shoot ratios due to elevated CO₂ show no alteration in the whole-plant partitioning of biomass. Little information is available for interactions between temperature and CO₂. Cold-adapted plants show little response to elevated levels of CO₂, with some species showing a decline in biomass accumulation. In general though, increasing temperature will increase sucrose synthesis, transport and utilization for CO₂-enriched plants and decrease carbohydrate accumulation within the leaf. Literature reports are discussed in relation to the hypothesis that sucrose is a major factor in the control of plant carbon partitioning. A model is presented in support.