Modulation of carbon and nitrogen allocation in Urtica dioica and Plantago major by elevated CO2: Impact of accumulation of nonstructural carbohydrates and ontogenetic drift

Doubling the atmospheric CO² concentration from 350 to 700 μ1 1^?1 increased the relative growth rate (RGR) of hydroponically grown Urtica dioica L. and Plantago major ssp. pleiosperma Pilger only for the first 10–14 days. Previous experiments with P. major led to the conclusion that RGR did not respond in proportion to the rate of photosynthesis. The present paper is focussed on the analysis of the impact of changes in leaf morphology, dry matter partitioning, dry matter chemical composition and ontogenetic drift on this discrepancy. Soon after the start of the treatment, carbohydrate concentrations were higher at elevated CO²: a reaction that was largely due to starch accumulation. An increase in the percentage of leaf dry matter and decreases in the specific leaf area (SLA) and the shoot nitrogen concentration were correlated with an increase in the total nonstructural carbohydrate concentration (TNC). A combination of accumulation of soluble sugars and starch and ontogenetic drift explains the decrease in SLA at the elevated CO² level. A similar ontogenetic effect of elevated CO² was observed on the specific root length (SRL). Other variables such as shoot nitrogen concentration and percentage leaf dry matter were not affected by correction of data for TNC levels. The net diurnal fluctuation of the carbohydrate pool in P. major was equal for both CO² concentrations, indicating that the growth response to elevated CO² may be ruled by variables other than photosynthesis, as for instance sink strength. Elevated CO² did not greatly influence the partitioning of nitrogen between soluble and insoluble, reduced N and nitrate, nor the allocation of dry matter between leaf. stem and root. The finding that the root to shoot ratio (R/S) was not affected by elevated CO² implies that, in order to maintain a balanced activity between roots and shoot, no shift in partitioning of dry matter upon doubling of the atmospheric CO² concentration is required. Our data on R/S are in good agreement with the response of R/S to high CO² predicted by models based on such a theorem.     

Assimilation,respiration and allocation of carbon inPlantago major as affected by atmospheric CO2 levels

The response ofPlantago major ssp,pleiosperma plants, grown on nutrient solution in a climate chamber, to a doubling of the ambient atmospheric CO₂ concentration was investigated. Total dry matter production was increased by 30% after 3 weeks of exposure, due to a transient stimulation of the relative growth rate (RGR) during the first 10 days. Thereafter RGR returned to the level of control plants. Photosynthesis, expressed per unit leaf area, was stimulated during the first two weeks of the experiment, thereafter it dropped and nearly reached the level of the control plants. Root respiration was not affected by increased atmospheric CO₂ levels, whereas shoot, dark respiration was stimulated throughout the experimental period. Dry matter allocation over leaves stems and roots was not affected by the CO₂ level. SLA was reduced by 10%, which can partly be explained by an increased dry matter content of the leaves. Both in the early and later stages of the experiment, shoot respiration accounted for a larger part of the carbon budget in plants grown at elevated atmospheric CO₂. Shifts in the total carbon budget were mainly due to the effects on shoot respiration. Leaf growth accounted for nearly 50% of the C budget at all stages of the experiment and in both treatments.Abbreviations LAR leaf area ratio - LWR leaf weight ratio - RGR relative growth rate - R/S root to shoot ratio - RWR root weight ratio - SLA specific leaf area - SWR stem weight ratio     

Interaction between elevated atmospheric concentration of CO2 and humidity on plant growth: comparison between cotton and radish

Cotton plants (Gossypium hirsutum L. var Deltapine 90) and radish plants (Raphanus sativus L var Round Red) were grown under full sunlight using a factorial combination of atmospheric CO₂ concentrations (350 µmol mol^-1 and 700 µmol mol^-1) and humidities (35% and 90% RH at 32 °C during the day). Cotton plants showed large responses to increased humidity and to doubled CO₂. In cotton plants, the enhanced dry matter yield due to doubled CO₂ concentration was 1.6-fold greater at low humidity than at high humidity. Apart from the direct effect of elevated CO₂ level on photosynthesis, the greater effect of doubled CO₂ concentration on dry matter yield at low humidity was probably due to: (1) increased leaf water potential caused by reduction of transpiration resulting from the negative CO₂ response of stomata to increased CO₂ concentration the consequence being greater leaf area expansion; (2) reduction of CO₂ assimilation rate at low humidity and normal CO₂ concentration as a result of humidity response of stomata causing reduction of intercellular CO₂ concentration. In contrast, apart from the very early stage of development, radish plants do not respond to increased humidity but had a relatively large response to doubled CO₂ concentration. Furthermore, due to the determinate growth pattern as well as having a prominent storage root, the extra photoassimilate derived at doubled CO₂ level is allocated to the storage root.Abbreviatios DAE day after emergence - LAD leaf areal density (leaf dry weight/leaf area) - LAR leaf area ratio (leaf area/plant dry weight) - NAR net assimilation rate - c_i internal CO₂ concentration - PPFD photosynthetic photon flux density - RGR relative growth rate - RLAGR relative leaf area growth rate - VPD vapour pressure deficit     

Effects of elevated CO2 and nitrogen on growth ofPoa pratensis (L.)

The growth responses of a grass,Poa pratensis, to elevated CO₂ and nitrogen were investigated. Light-saturated photosynthetic rate per unit leaf area increased with exposure to elevated CO₂, while dry weight did not respond to increased CO₂. Patterns of biomass allocation within plants, including leaf area, leaf area ratio, specific leaf area, and root to shoot ratios, were not altered by elevated CO₂, but changed considerably with N treatment Shoot and whole-plant tissue N concentrations were significantly diluted by elevated CO₂ (Tukey test, P < 0.05). Total N content did not differ significantly among CO₂ treatments. The absence of a concomitant increase in N uptake under elevated CO₂ may have caused a dilution in plant tissue [N], probably negating the positive effects of increased photosynthesis on biomass accumulation.     

Acquisition and within-plant allocation of 13C and 15N in CO2-enriched Quercus robur plants

We assessed the effects of doubling atmospheric CO₂ concentration, [CO₂], on C and N allocation within pedunculate oak plants (Quercus robur L.) grown in containers under optimal water supply. A short-term dual ¹³CO₂ and ¹⁵NO₃^? labelling experiment was carried out when the plants had formed their third growing flush. The 22-week exposure to 700 μl l^?1 [CO₂] stimulated plant growth and biomass accumulation (+53% as compared with the 350 μl l^?1 [CO₂] treatment) but decreased the root/shoot biomass ratio (-23%) and specific leaf area (-18%). Moreover, there was an increase in net CO₂ assimilation rate (+37% on a leaf dry weight basis; +71% on a leaf area basis), and a decrease in both above- and below-ground CO₂ respiration rates (-32 and -26%, respectively, on a dry mass basis) under elevated [CO₂]. ¹³C acquisition, expressed on a plant mass basis or on a plant leaf area basis, was also markedly stimulated under elevated [CO₂] both after the 12-h ¹³CO₂ pulse phase and after the 60-h chase phase. Plant N content was increased under elevated CO₂ (+36%), but not enough to compensate for the increase in plant C content (+53%). Thus, the plant C/N ratio was increased (+13%) and plant N concentration was decreased (-11%). There was no effect of elevated [CO₂] on fine root-specific ¹⁵N uptake (amount of recently assimilated ¹⁵N per unit fine root dry mass), suggesting that modifications of plant N pools were merely linked to root size and not to root function. N concentration was decreased in the leaves of the first and second growing flushes and in the coarse roots, whereas it was unaffected by [CO₂] in the stem and in the actively growing organs (fine roots and leaves of the third growth flush). Furthermore, leaf N content per unit area was unaffected by [CO₂]. These results are consistent with the short-term optimization of N distribution within the plants with respect to growth and photosynthesis. Such an optimization might be achieved at the expense of the N pools in storage compartments (coarse roots, leaves of the first and second growth flushes). After the 60-h ¹³C chase phase, leaves of the first and second growth flushes were almost completely depleted in recent ¹³C under ambient [CO₂], whereas these leaves retained important amounts of recently assimilated ¹³C (carbohydrate reserves?) under elevated [CO₂].     

Growth, gas exchange, leaf nitrogen and carbohydrate concentrations in NAD‐ME and NADP‐ME C4 grasses grown in elevated CO2

Plants with the C₄ photosynthetic pathway have predominantly one of three decarboxylation enzymes in their bundle sheath cells. Within the grass family (Poaceae) bundle sheath leakiness to CO₂ is purported to be lowest in the nicotinamide adenine dinucleotide phosphate-malic enzyme (NADP-ME, EC 1.1.1.40) group, highest in the NAD-ME (EC 1.1.1.39) group and intermediate in the phosphoenolpyruvate carboxykinase (PCK, EC 4.1.1.32) group. We investigated the hypothesis that growth and photosynthesis of NAD-ME C₄ grasses would respond more to elevated CO₂ treatment than NADP-ME grasses. Plants were grown in 8-1 pots in growth chambers with ample water and fertilizer for 39 days at a continuous CO₂ concentration of either 350 or 700 µl l^?1. NAD-ME species included Bouteloua gracilis Lag. ex Steud (Blue grama), Buchloe dactyloides (Nutt.) Engelm. (Buffalo grass) and Panicum virgatum L. (Switchgrass) and the NADP-ME species were Andropogon gerardii Vittman (Big bluestem), Schizachyrium scoparium (Michx.) Nash (Little bluestem), and Sorghastrum nutans (L.) Nash (Indian grass). Contrary to our hypothesis, growth of the NADP-ME grasses was generally greater under elevated CO₂ (significant for A. gerardii and S. nutans), while none of the NAD-ME grasses had a significant growth response. Increased leaf total non-structural carbohydrate (TNC) was associated with greater growth responses of NADP-ME grasses. Decreased leaf nitrogen in NADP-ME species grown at elevated CO₂ was found to be an artifact of TNC dilution. Assimilation (A) vs intercellular CO₂ (C_i) curves revealed that leaf photosynthesis was not saturated at 350 µl l^?1 CO₂ in any of these C₄ grasses. Assimilation of elevated CO₂-grown A. gerardii was higher than in plants grown in ambient CO₂. In contrast, B. gracilis grown in elevated CO₂ displayed lower A, a trait more commonly reported in C₃ plants. Photosynthetic acclimation in B. gracilis was not related to leaf TNC or nitrogen concentrations, but A:C_i curves suggest a reduction in activity of both phosphoenolpyruvate (PEP) carboxylase (EC 4.1.1.31) and ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco, EC 4.1.1.39). Some adaptation of stomatal functioning was also seen in B. gracilis and A. gerardii leaves grown in elevated CO₂. Our study shows that C₄ grasses have the capacity for increased growth and photosynthesis under elevated CO₂ even when water and nutrients are non-limiting. While it was the NADP-ME species which had significant responses in the present study, we have previously reported significant growth increases in elevated CO₂ for B. gracilis.     

Soybean nodulation and N2 fixation response to drought under carbon dioxide enrichment

The combined effects of carbon dioxide (CO₂) enrichment and water deficits on nodulation and N₂ fixation were analysed in soybean [Glycine max (L.) Merr.]. Two short-term experiments were conducted in greenhouses with plants subjected to soil drying, while exposed to CO₂ atmospheres of either 360 or 700 μmol CO₂ mol^–1. Under drought-stressed conditions, elevated [CO₂] resulted in a delay in the decrease in N₂ fixation rates associated with drying of the soil used in these experiments. The elevated [CO₂] also allowed the plants under drought to sustain significant increases in nodule number and mass relative to those under ambient [CO₂]. The total non-structural carbohydrate (TNC) concentration was lower in the shoots of the plants exposed to drought; however, plants under elevated CO₂ had much higher TNC levels than those under ambient CO₂. For both [CO₂] treatments, drought stress induced a substantial accumulation of TNC in the nodules that paralleled N₂ fixation decline, which indicates that nodule activity under drought may not be carbon limited. Under drought stress, ureide concentration increased in all plant tissues. However, exposure to elevated [CO₂] resulted in substantially less drought-induced ureide accumulation in leaf and petiole tissues. A strong negative correlation was found between ureide accumulation and TNC levels in the leaves. This relationship, together with the large effect of elevated [CO₂] on the decrease of ureide accumulation in the leaves, indicated the importance of ureide breakdown in the response of N₂ fixation to drought and of feedback inhibition by ureides on nodule activity. It is concluded that an important effect of CO₂ enrichment on soybean under drought conditions is an enhancement of photoassimilation, an increased partitioning of carbon to nodules and a decrease of leaf ureide levels, which is associated with sustained nodule growth and N₂ rates under soil water deficits. We suggest that future [CO₂] increases are likely to benefit soybean production by increasing the drought tolerance of N₂ fixation.     

Nitrogen and phosphorus requirements of cotton and wheat under changing atmospheric CO2 concentrations

The influence of increasing atmospheric CO₂ on shoot growth, leaf nitrogen and phosphorus concentrations and carbohydrate composition was investigated in cotton and wheat. Shoot dry weight of both species was generally higher at elevated CO₂, especially at high rates of available soil N and P. Critical leaf N concentration was reduced but critical P concentration was increased in both species at high CO₂.     

Quantifying the response of photosynthesis to changes in leaf nitrogen content and leaf mass per area in plants grown under atmospheric CO2 enrichment

Previous modelling exercises and conceptual arguments have predicted that a reduction in biochemical capacity for photosynthesis (A_area) at elevated CO₂ may be compensated by an increase in mesophyll tissue growth if the total amount of photosynthetic machinery per unit leaf area is maintained (i.e. morphological upregulation). The model prediction was based on modelling photosynthesis as a function of leaf N per unit leaf area (N_area), where N_area = N_mass×LMA. Here, N_mass is percentage leaf N and is used to estimate biochemical capacity and LMA is leaf mass per unit leaf area and is an index of leaf morphology. To assess the relative importance of changes in biochemical capacity versus leaf morphology we need to control for multiple correlations that are known, or that are likely to exist between CO₂ concentration, N_area, N_mass, LMA and A_area. Although this is impractical experimentally, we can control for these correlations statistically using systems of linear multiple-regression equations. We developed a linear model to partition the response of A_area to elevated CO₂ into components representing the independent and interactive effects of changes in indexes of biochemical capacity, leaf morphology and CO₂ limitation of photosynthesis. The model was fitted to data from three pine and seven deciduous tree species grown in separate chamber-based field experiments. Photosynthetic enhancement at elevated CO₂ due to morphological upregulation was negligible for most species. The response of A_area in these species was dominated by the reduction in CO₂ limitation occurring at higher CO₂ concentration. However, some species displayed a significant reduction in potential photosynthesis at elevated CO₂ due to an increase in LMA that was independent of any changes in N_area. This morphologically based inhibition of A_area combined additively with a reduction in biochemical capacity to significantly offset the direct enhancement of A_area caused by reduced CO₂ limitation in two species. This offset was 100% for Acer rubrum, resulting in no net effect of elevated CO₂ on A_area for this species, and 44% for Betula pendula. This analysis shows that interactions between biochemical and morphological responses to elevated CO₂ can have important effects on photosynthesis.     

Leaf gas exchange and nitrogen dynamics of N2-fixing, field-grown Alnus glutinosa under elevated atmospheric CO2

Few studies have investigated the effects of elevated CO₂ on the physiology of symbiotic N₂-fixing trees. Tree species grown in low N soils at elevated CO₂ generally show a decline in photosynthetic capacity over time relative to ambient CO₂ controls. This negative adjustment may be due to a reallocation of leaf N away from the photosynthetic apparatus, allowing for more efficient use of limiting N. We investigated the effect of twice ambient CO₂ on net CO₂ assimilation (A), photosynthetic capacity, leaf dark respiration, and leaf N content of N2-fixing Alnus glutinosa (black alder) grown in field open top chambers in a low N soil for 160 d. At growth CO₂, A was always greater in elevated compared to ambient CO₂ plants. Late season A vs. internal leaf p(CO₂) response curves indicated no negative adjustment of photosynthesis in elevated CO₂ plants. Rather, elevated CO₂ plants had 16% greater maximum rate of CO₂ fixation by Rubisco. Leaf dark respiration was greater at elevated CO₂ on an area basis, but unaffected by CO₂ on a mass or N basis. In elevated CO₂ plants, leaf N content (μg N cm^?2) increased 50% between Julian Date 208 and 264. Leaf N content showed little seasonal change in ambient CO₂ plants. A single point acetylene reduction assay of detached, nodulated root segments indicated a 46% increase in specific nitrogenase activity in elevated compared to ambient CO₂ plants. Our results suggest that N₂-fixing trees will be able to maintain high A with minimal negative adjustment of photosynthetic capacity following prolonged exposure to elevated CO₂ on N-poor soils.     

Photosynthesis of Helianthus annuus does not acclimate toelevated CO2 regardless of N supply

A number of studies have shown that relatively long-term exposure to elevated levels of CO₂ can lead to the downward acclimation of photosynthesis. Although the exact mechanisms are not clearly understood, it has been suggested that such a downward adjustment may be more common under limited N availability. Here we examined the effect of N supply on the photosynthetic acclimation response of Helianthus annuus L. cv. Teddy Bear plants to elevated CO₂ at three growth stages – 18, 38 and 56 d after emergence corresponding to vegetative, pre-flowering and flowering stages. Plants were grown at CO₂ partial pressures of 37 or 70 Pa, and supplied with 0.5, 2.5 or 5 mol·m^–3 N. After 18 d of treatment, photosynthetic capacity of H. annuus as evaluated by parameters derived from the A-C_i data (Rubisco carboxylation capacity, V_c,max; electron transport capacity, J_max; and capacity for triose phosphate utilization, TPU) showed no acclimation to elevated CO₂. The leaf nitrogen concentration, [N], and total non-structural carbohydrates, [TNC], were also comparable between ambient- and elevated-CO₂-grown plants. However, all these photosynthetic parameters as well as leaf [N], but not [TNC], significantly increased in response to N supply. Similarly, after 38 and 56 d of exposure to CO₂ treatments, photosynthetic capacities, foliar [N] and [TNC] did not significantly differ between ambient- and elevated-CO₂ plants. These results suggest that H. annuus plants maintained their photosynthetic capacity during long-term exposure to elevated CO₂ because of their capacity to maintain leaf N-status. It is further suggested that plant capacity to maintain the balance between C and N acquisition rather than simply N-supply level, may determine whether photosynthetic acclimation in response to elevated CO₂ occurs or not.     

Barley Growth and Its Underlying Components are Affected by Elevated CO2 and Salt Concentration

The future environment will exhibit increases in soil salt concentrations and atmospheric CO₂. In general, plant growth is inhibited by salt stress and stimulated by elevated CO₂. This study investigated whether elevated CO₂ could improve plant growth under salt stress and the mechanisms involved. We measured functional and morphological components of growth in barley (cv. Iranis) subjected to 0, 80, 160, or 240 mM NaCl and grown at either 350 (ambient) or 700 (elevated) μmol mol^?1 CO₂. Under nonsaline conditions, elevated CO₂ stimulated growth by increasing the relative growth rate (RGR). Maximum CO₂ stimulation was observed within the first 10 days of development, before the start of the salt treatment. Afterwards, salt stress caused reductions in biomass production and RGR by decreasing the photosynthetic rate and increasing the respiration rate; this resulted in a reduced net assimilation rate (functional component). In addition, salt stress caused nutritional imbalances, which reduced the leaf expansion capacity, and changed the root-to-shoot ratio. This resulted in reductions in the specific leaf area and leaf weight ratio (morphological components). However, the functional component became more relevant with increasing salt stress. Under elevated CO₂ conditions, salt stress inhibited growth less than that observed at ambient CO₂. This occurred because (1) more dry biomass was synthesized for a given leaf area due to higher photosynthetic rates, and (2) greater leaf area and root biomass were maintained for photosynthesis and water and mineral uptake, respectively.     

Long-term response of Sitka spruce (Picea sitchensis (Bong.) Carr.) to CO2 enrichment and nitrogen supply. I. Growth,biomass allocation and physiology

ABSTRACT

After a 3-year exposure to elevated CO₂, young trees of Sitka spruce (Picea sitchensis (Bong.) Carr.) were planted in native, nutrient-deficient forest soil and grown for two more years with three CO₂ treatments in open-top chambers, and with two nutrient treatments (with and without supplied N). Elevated CO₂ resulted in larger fresh mass, dry mass, leaf area and leaf thickness in two-year old needles, but had no effect on one-year old and current needles. Tree height, basal diameter and biomass production also increased, regardless of N supply. In trees without added N, elevated CO₂ resulted in higher root-to-shoot and absorbing roots-to-stump ratios. Regardless of N supply, trees grown in elevated CO₂ had lower photosynthetic rates on a leaf area basis. Photosynthesis reduction was accompanied by a decline in Rubisco activity and leaf N concentration. Under elevated CO₂, added N elevated photosynthesis and Rubisco activity, suggesting a dependence on N availability of the photosynthetic response to elevated CO₂. Stomatal conductance of trees grown with added N decreased in response to elevated CO₂. This may account for the larger reduction in intercellular CO₂ concentration, and hence photosynthesis, in the trees supplied with N than in those without N supply.     

A hierarchical analysis of the interactive effects of elevated CO<Subscript>2</Subscript> and water availability on the nitrogen and transpiration productivities of velvet mesquite seedlings

In this study we apply new extensions of classical growth analysis to assess the interactive effects of elevated CO₂ and differences in water availability on the leaf-nitrogen and transpiration productivities of velvet mesquite (Prosopis velutina Woot.) seedlings. The models relate transpiration productivity (biomass gained per mass of water transpired per day) and leaf-nitrogen productivity (biomass gain per unit leaf N per day) to whole-plant relative growth rate (RGR) and to each other, allowing a comprehensive hierarchical analysis of how physiological and morphological responses to the treatments interact with each other to affect plant growth. Elevated CO₂ led to highly significant increases in N and transpiration productivities but reduced leaf N per unit leaf area and transpiration per unit leaf area, resulting in no net effect of CO₂ on the RGR of seedlings. In contrast, higher water availability led to an increase in leaf-tissue thickness or density without affecting leaf N concentration, resulting in a higher leaf N per unit leaf area and consequently a higher assimilatory capacity per unit leaf area. The net effect was a marginal increase in seedling RGR. Perhaps most important from an ecological perspective was a 41% reduction in whole-plant water use due to elevated CO₂. These results demonstrate that even in the absence of CO₂ effects on integrative measures of plant growth such as RGR, highly significant effects may be observed at the physiological and morphological level that effectively cancel each other out. The quantitative framework presented here enables some of these tradeoffs to be identified and related directly to each other and to plant growth.     

Photosynthetic acclimation to elevated CO2 in Phaseolus vulgaris L. is altered by growth response to nitrogen supply

Abstract Long‐term exposure of plants to elevated CO₂ often leads to downward photosynthetic acclimation. Nitrogen (N) deficiency could potentially exacerbate this response by reducing growth rate and the sink for photosynthates, but this has not always been observed. Experimentally, the interpretation of N effects on CO₂ responses can be confounded by increasing severity of tissue N deficiency over time when N supply is not adjusted as demand increases. In this study, N supply ranged from sub‐ to supra‐optimal (20–540 kgN ha^–l equivalent), and relatively stable levels of tissue N concentration were obtained in all treatments by varying twice‐weekly application rates in proportion to plant growth. The effects of N on photosynthesis and growth of beans (Phaseolus vulgaris L.) raised at ambient (35 Pa) and three elevated (70, 105, 140 Pa) CO₂ partial pressures (pCO₂) were evaluated. Averaging across N treatments, leaf total non‐structural carbohydrates (TNC) were 2.5‐ to 3‐fold higher and leaf N concentrations were 31–35% lower at elevated compared to ambient pCO₂. Light‐saturated net CO₂ assimilation rates measured at growth pCO₂ (A_satg) were significantly higher (26–40% depending on N supply) in plants grown at elevated compared to ambient pCO₂. When measured at a common pCO₂ of 35 Pa, the A_sat of plants grown at elevated CO₂ was 15–29% less than that of plants grown at 35 Pa, indicative of downward photosynthetic acclimation. The magnitude of downward photosynthetic acclimation to elevated CO₂ was greater in plants grown at high (180 and 540 kgN ha^–l) compared to low (20 and 60 kgN ha^–l) N supply, and this was associated with a higher A_sat at growth pCO₂, higher leaf area ratio (leaf area/total biomass), and higher TNC in leaves of high‐N plants. Our results indicate that the effect of N on acclimation to CO₂ will depend on the balance between supply and demand for N during the growing period, and the effect this has on biomass allocation and source‐sink C balance at the whole‐plant level.     

Photosynthetic down-regulation in N2-fixing alfalfa under elevated CO2 alters rubisco content and decreases nodule metabolism via nitrogenase and tricarboxylic acid cycle

Although responsiveness of N₂-fixing plants to elevated CO₂ conditions have been analyzed in previous studies, important uncertainties remain in relation to the effect enhanced CO₂ in nodule proteomic profile and its implication in leaf responsiveness. The aim of our study was to deepen our understanding of the relationship between leaf and nodule metabolism of N₂-fixing alfalfa plants after long-term exposure to elevated CO₂. After 30-day exposure to elevated CO₂, plants showed photosynthetic down-regulation with reductions in the light-saturated rate of CO₂ assimilation (A _sat) and the maximum rate of rubisco carboxylation (Vc_max). Under elevated CO₂ conditions, the rubisco availability limited potential photosynthesis by around 12 %, which represented the majority of the observed fall in Vc_max. Photosynthetic down-regulation has been associated with decreased N availability even if those plants are capable to assimilate N₂. Diminishment in shoot N demand (as reflected by the lower rubisco and leaf N content) suggests that the lower aboveground N requirements affected negatively nodule performance. In this condition, specific nodule activity was reduced due to an effect on nodule metabolism that manifested as a lower amount of nitrogenase reductase. Moreover, the nodule proteomic approach also revealed that nodule functioning was altered simultaneously in various enzyme quantity apart from nitrogenase. At elevated CO₂, the tricarboxylic acid cycle was also altered with a reduced amount of isocitrate synthase protein. The nodule proteome analysis also revealed the relaxation of the antioxidant system as shown by a decline in the amount of catalase and isoflavone reductase protein.     

Effects of the interaction between vapor-pressure deficit and salinity on growth and photosynthesis of <Emphasis Type="Italic">Cucumis sativus</Emphasis> seedlings under different CO<Subscript>2</Subscript> concentrations

We studied growth and photosynthesis of cucumber (Cucumis sativus) seedlings under two vapor-pressure deficit levels (VPD; 0.4 and 3.0 kPa), two salinity levels (0 mM and 34 mM NaCl), and two CO₂ concentrations ([CO₂]; 400 and 1,000 μmol mol^–1). Relative growth rate (RGR) decreased with increasing VPD, but the causal factor differed between salinity levels and CO₂ concentrations. Under ambient [CO₂], RGR decreased with increasing VPD at low salinity mainly due to decreased leaf area ratio (LAR), and decreased net assimilation rate (NAR) at high salinity. The decrease in intercellular [CO₂] (C_i) with decreasing stomatal conductance caused by high VPD did not significantly limit net photosynthetic rate (P_N) at low salinity, but P_N was potentially limited by C_i at high salinity. At high [CO₂], high VPD reduced LAR, but did not affect NAR. This is because the decrease in C_i occurred where slope of P_N–C_i curve was almost flat.     

The influence of plants grown under elevated CO2 and N fertilization on soil nitrogen dynamics

We investigated the effects of spring barley growth on nitrogen (N) transformations and rhizosphere microbial processes in a controlled system under elevated carbon dioxide (CO₂) at two levels of N fertilization (applied with ¹⁵N labelling). After 25 d, elevated CO₂ (twice ambient) increased plant growth (dry weight, DW) by 141% at low‐N fertilization and by 60% at high‐N fertilization, but its positive effect on the root‐to‐shoot ratio was only significant at low‐N input. As a result of this plant response, elevated CO₂ caused a greater soil CO₂ efflux, rhizosphere soil DW, and soil microbial biomass under N‐limiting conditions than under high N availability. Elevated CO₂ also caused a significant (P < 0.001) increase in the N recovered by the plant from both the labelled (N_f) and unlabelled (N_s + N_uf) N pools. The dynamics of N in the system as affected by elevated CO₂ were driven principally by mineralization–immobilization turnover, with little loss by denitrification. Under N‐limiting conditions, there is evidence to suggest enhanced nutrient release from soil organic matter (SOM) pools—a process which could be defined as priming. The results of our experiment did not indicate a direct plant‐mediated effect of elevated CO₂ on nitrous oxide (N₂O) fluxes or denitrification activity.     

A rhizolysimeter facility for studying the dynamics of crop and soil processes: description and evaluation

Increased atmospheric carbon dioxide (CO₂) concentration will likely cause changes in plant productivity and composition that might affect soil decomposition processes. The objective of this study was to test to what extent elevated CO₂ and N fertility-induced changes in residue quality controlled decomposition rates. Cotton (Gossypium hirsutum L.) was grown in 8-l pots and exposed to two concentrations of CO₂ (390 or 722 μmol mol^-1) and two levels of N fertilization (1.0 or 0.25 g l^-1 soil) within greenhouse chambers for 8 wks. Plants were then chemically defoliated and air-dried. Leaf, stem and root residues were assayed for total non-structural carbohydrates (TNC), lignin (LTGA), proanthocyanidins (PA), C and N. Respiration rates of an unsterilized sandy soil (Lakeland Sand) mixed with residues from the various treatments were determined using a soda lime trap to measure CO₂ release. At harvest, TNC and PA concentrations were 17 to 45% higher in residues previously treated with elevated CO₂ compared with controls. Leaf and stem residue LTGA concentrations were not significantly affected by either the elevated CO₂ or N fertilization treatments, although root residue LTGA concentration was 30% greater in plants treated with elevated CO₂. The concentration of TNC in leaf residues from the low N fertilization treatment was 2.3 times greater than that in the high N fertilization treatment, although TNC concentration in root and stem residues was suppressed 13 to 23% by the low soil N treatment. PA and LTGA concentrations in leaf, root and stem residues were affected by less than 10% by the low N fertilization treatment. N concentration was 14 to 44% lower in residues obtained from the elevated CO₂ and low N fertilization treatments. In the soil microbial respiration assay, cumulative CO₂ release was 10 to 14% lower in soils amended with residues from the elevated CO₂ and low N fertility treatments, although treatment differences diminished as the experiment progressed. Treatment effects on residue N concentration and C:N ratios appeared to be the most important factors affecting soil microbial respiration. The results of our study strongly suggest that, although elevated CO₂ and N fertility may have significant impact on post-harvest plant residue quality of cotton, neither factor is likely to substantially affect decomposition. Thus, C cycling might not be affected in this way, but via simple increases in plant biomass production. This revised version was published online in June 2006 with corrections to the Cover Date.     

The response of fast- and slow-growing Acacia species to elevated atmospheric CO2: an analysis of the underlying components of relative growth rate

In this study we assessed the impact of elevated CO₂ with unlimited water and complete nutrient on the growth and nitrogen economy of ten woody Acacia species that differ in relative growth rate (RGR). Specifically, we asked whether fast- and slow-growing species systematically differ in their response to elevated CO₂. Four slow-growing species from semi-arid environments (Acacia aneura, A. colei, A. coriacea and A. tetragonophylla) and six fast-growing species from mesic environments (Acacia dealbata, A. implexa, A. mearnsii, A. melanoxylon, A. irrorata and A. saligna) were grown in glasshouses with either ambient (˜350 ppm) or elevated (˜700 ppm) atmospheric CO₂. All species reached greater final plant mass with the exception of A. aneura, and RGR, averaged across all species, increased by 10% over a 12-week period when plants were exposed to elevated CO₂. The stimulation of RGR was evident throughout the 12-week growth period. Elevated CO₂ resulted in less foliage area per unit foliage dry mass, which was mainly the result of an increase in foliage thickness with a smaller contribution from greater dry matter content per unit fresh mass. The net assimilation rate (NAR, increase in plant mass per unit foliage area and time) of the plants grown at elevated CO₂ was higher in all species (on average 30% higher than plants in ambient CO₂) and was responsible for the increase in RGR. The higher NAR was associated with a substantial increase in foliar nitrogen productivity in all ten Acacia species. Plant nitrogen concentration was unaltered by growth at elevated CO₂ for the slow-growing Acacia species, but declined by 10% for faster-growing species. The rate of nitrogen uptake per unit root mass was higher in seven of the species when grown under elevated CO₂, and leaf area per unit root mass was reduced by elevated CO₂ in seven of the species. The absolute increase in RGR due to growth under elevated CO₂ was greater for fast- than for slow-growing Acacia species. Received: 21 December 1998 / Accepted: 31 May 1999