Nitrogen dynamics during O3-induced accelerated senescence in hybrid poplar

Experiments were conducted to determine the fate of nitrogen (N) remobilized as a result of ozone (O₃)‐induced accelerated senescence in hybrid poplar subjected to declining N availability concurrent with O₃ stress. Cuttings were grown in sand culture where the supply of N to the plant could be controlled on a daily basis and reduced in half of the plants when desired. Plants all initially received 3·57 mm N daily until approximately the 20 leaf stage after which daily supply of N was reduced to 0·71 mm . Plants were grown in open‐top chambers in the field (Rock Springs, PA, USA) and received charcoal‐filtered air, half also received supplemental O₃ to a level of 0·08 µL L^?1. Allocation of newly acquired N was determined with ¹⁵N. The specific allocation (mg labelled N mg^?1 total N) of labelled N to upper, expanding leaf N was not affected by O₃, but was strongly affected by N treatment. However, O₃ increased the relative partitioning of labelled N to the expanding leaves and the roots. The balance between partitioning of newly acquired N to the upper leaves and roots was not affected by O₃, but was reduced by N withdrawal. Calculated net N flux was strongly negative in the lower leaves of O₃‐exposed, N withdrawal plants. Nitrogen uptake was not reduced by O₃. The allometric relationships between the roots and shoots were not affected by O₃ or N availability. The relative contribution of newly acquired versus remobilized N to new growth appears to be determined by N supply. Ozone exposure alters the allocation of newly acquired N via alterations in plant size, whereas N availability exerts a strong effect upon both plant size and N allocation.     

Increased cytokinin levels in transgenic PSAG12–IPT tobacco plants have large direct and indirect effects on leaf senescence, photosynthesis and N partitioning

We studied the impact of delayed leaf senescence on the functioning of plants growing under conditions of nitrogen remobilization. Interactions between cytokinin metabolism, Rubisco and protein levels, photosynthesis and plant nitrogen partitioning were studied in transgenic tobacco (Nicotiana tabacum L.) plants showing delayed leaf senescence through a novel type of enhanced cytokinin syn‐thesis, i.e. targeted to senescing leaves and negatively auto‐regulated (P_SAG12–IPT), thus preventing developmental abnormalities. Plants were grown with growth‐limiting nitrogen supply. Compared to the wild‐type, endogenous levels of free zeatin (Z)‐ and Z riboside (ZR)‐type cytokinins were increased up to 15‐fold (total ZR up to 100‐fold) in senescing leaves, and twofold in younger leaves of P_SAG12–IPT. In these plants, the senescence‐associated declines in N, protein and Rubisco levels and photosynthesis rates were delayed. Senescing leaves accumulated more (¹⁵N‐labelled) N than younger leaves, associated with reduced shoot N accumulation (–60%) and a partially inverted canopy N profile in P_SAG12–IPT plants. While root N accumulation was not affected, N translocation to non‐senescing leaves was progressively reduced. We discuss potential consequences of these modified sink–source relations, associated with delayed leaf senescence, for plant productivity and the efficiency of utilization of light and minerals.     

Carbon dioxide enrichment does not reduce leaf longevity or alter accumulation of carbon reserves in the woodland spring ephemeral Erythronium americanum

Background and Aims

Woodland spring ephemerals exhibit a relatively short epigeous growth period prior to canopy closure. However, it has been suggested that leaf senescence is induced by a reduction in the carbohydrate sink demand, rather than by changes in light availability. To ascertain whether a potentially higher net carbon (C) assimilation rate could shorten leaf lifespan due to an accelerated rate of storage, Erythronium americanum plants were grown under ambient (400 ppm) and elevated (1100 ppm) CO₂ concentrations.

Methods

During this growth-chamber experiment, plant biomass, bulb starch concentration and cell size, leaf phenology, gas exchange rates and nutrient concentrations were monitored.

Key Results

Plants grown at 1100 ppm CO₂ had greater net C assimilation rates than those grown at 400 ppm CO₂. However, plant size, final bulb mass, bulb filling rate and timing of leaf senescence did not differ.

Conclusions

Erythronium americanum fixed more C under elevated than under ambient CO₂ conditions, but produced plants of similar size. The similar bulb growth rates under both CO₂ concentrations suggest that the bulb filling rate is dependant on bulb cell elongation rate, rather than on C availability. Elevated CO₂ stimulated leaf and bulb respiratory rates; this might reduce feed-back inhibition of photosynthesis and avoid inducing premature leaf senescence.Key words: Source–sink relations, assimilation rates, growth rates, CO₂ enrichment, respiration, spring ephemeral, leaf senescence, bulbous plant, carbohydrate storage, Erythronium americanum     

Quantifying the effects of ozone on plant reproductive growth and development

Tropospheric ozone (O₃) is a harmful air pollutant that can negatively impact plant growth and development. Current O₃ concentrations ([O₃]) decrease forest productivity and crop yields and future [O₃] will likely increase if current emission rates continue. However, the specific effects of elevated [O₃] on reproductive development, a critical stage in the plant's lifecycle, have not been quantitatively reviewed. Data from 128 peer‐reviewed articles published from 1968 to 2010 describing the effects of O₃ on reproductive growth and development were analysed using meta‐analytic techniques. Studies were categorized based on experimental conditions, photosynthetic type, lifecycle, growth habit and flowering class. Current ambient [O₃] significantly decreased seed number (?16%), fruit number (?9%) and fruit weight (?22%) compared to charcoal‐filtered air. In addition, pollen germination and tube growth were decreased by elevated [O₃] compared to charcoal‐filtered air. Relative to ambient air, fumigation with [O₃] between 70 and 100 ppb decreased yield by 27% and individual seed weight by 18%. Reproductive development of both C₃ and C₄ plants was sensitive to elevated [O₃], and lifecycle, flowering class and reproductive growth habit did not significantly affect a plant's response to elevated [O₃] for many components of reproductive development. However, elevated [O₃] decreased fruit weight and fruit number significantly in indeterminate plants, and had no effect on these parameters in determinate plants. While gaps in knowledge remain about the effects of O₃ on plants with different growth habits, reproductive strategies and photosynthetic types, the evidence strongly suggests that detrimental effects of O₃ on reproductive growth and development are compromising current crop yields and the fitness of native plant species.     

Influence of carbon dioxide enrichment, ozone and nitrogen fertilization on cotton (Gossypium hirsutum L.) leaf and root composition

The objective of this study was to test whether elevated [CO₂], [O₃] and nitrogen (N) fertility altered leaf mass per area (LMPA), non‐structural carbohydrate (TNC), N, lignin (LTGA) and proanthocyanidin (PA) concentrations in cotton (Gossypium hirsutum L.) leaves and roots. Cotton was grown in 14 dm³ pots with either sufficient (0·8 g N dm ^{? 3}) or deficient (0·4 and 0·2 g N dm ^{? 3}) N fertilization, and treated in open‐top chambers with either ambient or elevated ( + 175 and + 350 μ mol mol ^{? 1}) [CO₂] in combination with either charcoal‐filtered air (CF) or non‐filtered air plus 1·5 times ambient [O₃]. At about 50 d after planting, LMPA, starch and PA concentrations in canopy leaves were as much as 51–72% higher in plants treated with elevated [CO₂] compared with plants treated with ambient [CO₂], whereas leaf N concentration was 29% lower in elevated [CO₂]‐treated plants compared with controls. None of the treatments had a major effect on LTGA concentrations on a TNC‐free mass basis. LMPA and starch levels were up to 48% lower in plants treated with elevated [O₃] and ambient [CO₂] compared with CF controls, although the elevated [O₃] effect was diminished when plants were treated concurrently with elevated [CO₂]. On a total mass basis, leaf N and PA concentrations were higher in samples treated with elevated [O₃] in ambient [CO₂], but the difference was much reduced by elevated [CO₂]. On a TNC‐free basis, however, elevated [O₃] had little effect on tissue N and PA concentrations. Fertilization treatments resulted in higher PA and lower N concentrations in tissues from the deficient N fertility treatments. The experiment showed that suppression by elevated [O₃] of LMPA and starch was largely prevented by elevated [CO₂], and that interpretation of [CO₂] and [O₃] effects should include comparisons on a TNC‐free basis. Overall, the experiment indicated that allocation to starch and PA may be related to how environmental factors affect source–sink relationships in plants, although the effects of elevated [O₃] on secondary metabolites differed in this respect.     

Competition increasingly dominates the responsiveness of juvenile beech and spruce to elevated CO2 and/or O3 concentrations throughout two subsequent growing seasons

Saplings of Fagus sylvatica and Picea abies were grown in mono‐ and mixed cultures in a 2‐year phytotron study under all four combinations of ambient and elevated ozone (O₃) and carbon dioxide (CO₂) concentrations. The hypotheses tested were (1) that the competitiveness of beech rather than spruce is negatively affected by the exposure to enhanced O₃ concentrations, (2) spruce benefits from the increase of resource availability (elevated CO₂) in the mixed culture and (3) that the responsiveness of plants to CO₂ and O₃ depends on the type of competition (i.e. intra vs. interspecific). Beech displayed a competitive disadvantage when growing in mixture with spruce: after two growing seasons under interspecific competition, beech showed significant reductions in leaf gas exchange, biomass development and crown volume as compared with beech plants growing in monoculture. In competition with spruce, beech appeared to be nitrogen (N)‐limited, whereas spruce tended to benefit in terms of its plant N status. The responsiveness of the juvenile trees to the atmospheric treatments differed between species and was dominated by the type of competition: spruce growth benefited from elevated CO₂ concentrations, while beech growth suffered from the enhanced O₃ regime. In general, interspecific competition enhanced these atmospheric treatment effects, supporting our hypotheses. Significant differences in root : shoot biomass ratio between the type of competition under both elevated O₃ and CO₂ were not caused by readjustments of biomass partitioning, but were dependent on tree size. Our study stresses that competition is an important factor driving plant development, and suggests that the knowledge about responses of plants to elevated CO₂ and/or O₃, acquired from plants growing in monoculture, may not be transferred to plants grown under interspecific competition as typically found in the field.     

Accumulation of phenolic compounds in birch leaves is changed by elevated carbon dioxide and ozone

Atmospheric change may affect plant phenolic compounds, which play an important part in plant survival. Therefore, we studied the impacts of CO₂ and O₃ on the accumulation of 27 phenolic compounds in the short‐shoot leaves of two European silver birch (Betula pendula Roth) clones (clones 4 and 80). Seven‐year‐old soil‐grown trees were exposed in open‐top chambers over three growing seasons to ambient and twice ambient CO₂ and O₃ concentrations singly and in combination in central Finland. Elevated CO₂ increased the concentration of the phenolic acids (+25%), myricetin glycosides (+18%), catechin derivatives (+13%) and soluble condensed tannins (+19%) by increasing their accumulation in the leaves of the silver birch trees, but decreased the flavone aglycons (?7%) by growth dilution. Elevated O₃ increased the concentration of 3,4′‐dihydroxypropiophenone 3‐β‐d ‐glucoside (+22%), chlorogenic acid (+19%) and flavone aglycons (+4%) by inducing their accumulation possibly as a response to increased oxidative stress in the leaf cells. Nevertheless, this induction of antioxidant phenolic compounds did not seem to protect the birch leaves from detrimental O₃ effects on leaf weight and area, but may have even exacerbated them. On the other hand, elevated CO₂ did seem to protect the leaves from elevated O₃ because all the O₃‐derived effects on the leaf phenolics and traits were prevented by elevated CO₂. The effects of the chamber and elevated CO₂ on some compounds changed over time in response to the changes in the leaf traits, which implies that the trees were acclimatizing to the altered environmental conditions. Although the two clones used possessed different composition and concentrations of phenolic compounds, which could be related to their different latitudinal origin and physiological characteristics, they responded similarly to the treatments. However, in some cases the variation in phenolic concentrations caused by genotype or chamber environment was much larger than the changes caused by either elevated CO₂ or O₃.     

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.     

Profiles of isoprene emission and photosynthetic parameters in hybrid poplars exposed to free‐air CO2 enrichment†

Poplar (Populus × euroamericana) saplings were grown in the field to study the changes of photosynthesis and isoprene emission with leaf ontogeny in response to free air carbon dioxide enrichment (FACE) and soil nutrient availability. Plants growing in elevated [CO₂] produced more leaves than those in ambient [CO₂]. The rate of leaf expansion was measured by comparing leaves along the plant profile. Leaf expansion and nitrogen concentration per unit of leaf area was similar between nutrient treatment, and this led to similar source–sink functional balance. Consequently, soil nutrient availability did not cause downward acclimation of photosynthetic capacity in elevated [CO₂] and did not affect isoprene synthesis. Photosynthesis assessed in growth [CO₂] was higher in plants growing in elevated than in ambient [CO₂]. After normalizing for the different number of leaves over the profile, maximal photosynthesis was reached and started to decline earlier in elevated than in ambient [CO₂]. This may indicate a [CO₂]‐driven acceleration of leaf maturity and senescence. Isoprene emission was adversely affected by elevated [CO₂]. When measured on the different leaves of the profile, isoprene peak emission was higher and was reached earlier in ambient than in elevated [CO₂]. However, a larger number of leaves was emitting isoprene in plant growing in elevated [CO₂]. When integrating over the plant profile, emissions in the two [CO₂] levels were not different. Normalization as for photosynthesis showed that profiles of isoprene emission were remarkably similar in the two [CO₂] levels, with peak emissions at the centre of the profile. Only the rate of increase of the emission of young leaves may have been faster in elevated than in ambient [CO₂]. Our results indicate that elevated [CO₂] may overall have a limited effect on isoprene emission from young seedlings and that plants generally regulate the emission to reach the maximum at the centre of the leaf profile, irrespective of the total leaf number. In comparison with leaf expansion and photosynthesis, isoprene showed marked and repeatable differences among leaves of the profile and may therefore be a useful trait to accurately monitor changes of leaf ontogeny as a consequence of elevated [CO₂].     

Differential responses in two varieties of winter wheat to elevated ozone concentration under fully open‐air field conditions

Two modern cultivars [Yangmai16 (Y16) and Yangfumai 2 (Y2)] of winter wheat (Triticum aestivum L.) with almost identical phenology were investigated to determine the impacts of elevated ozone concentration (E‐O₃) on physiological characters related to photosynthesis under fully open‐air field conditions in China. The plants were exposed from the initiation of tillering to final harvest, with E‐O₃ of 127% of the ambient ozone concentration (A‐O₃). Measurements of pigments, gas exchange rates, chlorophyll a fluorescence and lipid oxidation were made in three replicated plots throughout flag leaf development. In cultivar Y2, E‐O₃ significantly accelerated leaf senescence, as indicated by increased lipid oxidation as well as faster declines in pigment amounts and photosynthetic rates. The lower photosynthetic rates were mainly due to nonstomatal factors, e.g. lower maximum carboxylation capacity, electron transport rates and light energy distribution. In cultivar Y16, by contrast, the effects of E‐O₃ were observed only at the very last stage of flag leaf ageing. Since the two cultivars had almost identical phenology and very similar leaf stomatal conductance before senescence, the greater impacts of E‐O₃ on cultivars Y2 than Y16 cannot be explained by differential ozone uptake. Our findings will be useful for scientists to select O₃‐tolerant wheat cultivars against the rising surface [O₃] in East and South Asia.     

Interacting effects of elevated CO2, nutrient availability and plant species on a generalist invertebrate herbivore

By affecting plant growth and phytochemistry elevated CO₂ may have indirect effects on the performance of herbivores. These effects show considerable variability across studies and may depend on nutrient availability, the carbon/nutrient‐balance in plant tissues and the secondary metabolism of plants. We studied the responses to elevated CO₂ and different nutrient availability of 12 herbaceous plant species differing in their investment into secondary compounds. Caterpillars of the generalist herbivore Spodoptera littoralis were reared on the leaves produced and their consumption and growth rates analysed. Elevated CO₂ resulted in a similar increase of biomass in all plant species, whereas the positive effect of fertilization varied among plant species. Specific leaf weight was influenced by elevated CO₂, but the effect depended on nutrient level and identity of plant species. Elevated CO₂ increased the C/N ratio of the leaves of most species. Caterpillars consumed more leaf material when plants were grown under elevated CO₂ and low nutrients. This indicates compensatory feeding due to lower tissue quality. However, the effects of elevated CO₂, nutrient availability and plant species identity on leaf consumption interacted. Both the effects of CO₂ and nutrient availability on the relative growth rate of the herbivore depended on the plant species. The feeding rate of S. littoralis on plant species that do not produce nitrogen‐containing secondary compounds (NCSC) was higher under low nutrient availability. In contrast, in plants producing NCSC nutrient availability had no effect on the feeding rate. This suggests that compensatory feeding in response to low nutrient contents may not be possible if plants produce NCSC. We conclude that elevated CO₂ causes species‐specific changes in the quality of plant tissues and consequently in changes in the preferences of herbivores for plant species. This could result in changes in plant community composition.     

Above- and belowground response of Populus grandidentata to elevated atmospheric CO2 and soil N availability

Soil N availability may play an important role in regulating the long-term responses of plants to rising atmospheric CO₂ partial pressure. To further examine the linkage between above- and belowground C and N cycles at elevated CO₂, we grew clonally propagated cuttings of Populus grandidentata in the field at ambient and twice ambient CO₂ in open bottom root boxes filled with organic matter poor native soil. Nitrogen was added to all root boxes at a rate equivalent to net N mineralization in local dry oak forests. Nitrogen added during August was enriched with ¹⁵N to trace the flux of N within the plant-soil system. Above-and belowground growth, CO₂ assimilation, and leaf N content were measured non-destructively over 142 d. After final destructive harvest, roots, stems, and leaves were analyzed for total N and ¹⁵N. There was no CO₂ treatment effect on leaf area, root length, or net assimilation prior to the completion of N addition. Following the N addition, leaf N content increased in both CO₂ treatments, but net assimilation showed a sustained increase only in elevated CO₂ grown plants. Root relative extension rate was greater at elevated CO₂, both before and after the N addition. Although final root biomass was greater at elevated CO₂, there was no CO₂ effect on plant N uptake or allocation. While low soil N availability severely inhibited CO₂ responses, high CO₂ grown plants were more responsive to N. This differential behavior must be considered in light of the temporal and spatial heterogeneity of soil resources, particularly N which often limits plant growth in temperate forests.     

Consequences of CO2 and light interactions for leaf phenology, growth, and senescence in Quercus rubra

We investigated how light and CO₂ levels interact to influence growth, phenology, and the physiological processes involved in leaf senescence in red oak (Quercus rubra) seedlings. We grew plants in high and low light and in elevated and ambient CO₂. At the end of three years of growth, shade plants showed greater biomass enhancement under elevated CO₂ than sun plants. We attribute this difference to an increase in leaf area ratio (LAR) in shade plants relative to sun plants, as well as to an ontogenetic effect: as plants increased in size, the LAR declined concomitant with a decline in biomass enhancement under elevated CO₂ Elevated CO₂ prolonged the carbon gain capacity of shade‐grown plants during autumnal senescence, thus increasing their functional leaf lifespan. The prolongation of carbon assimilation, however, did not account for the increased growth enhancement in shade plants under elevated CO₂. Elevated CO₂ did not significantly alter leaf phenology. Nitrogen concentrations in both green and senesced leaves were lower under elevated CO₂ and declined more rapidly in sun leaves than in shade leaves. Similar to nitrogen concentration, the initial slope of A/C_i curves indicated that Rubisco activity declined more rapidly in sun plants than in shade plants, particularly under elevated CO₂. Absolute levels of chlorophyll were affected by the interaction of CO₂ and light, and chlorophyll content declined to a minimal level in sun plants sooner than in shade plants. These declines in N concentration, in the initial slope of A/C_i curves, and in chlorophyll content were consistent with declining photosynthesis, such that elevated CO₂ accelerated senescence in sun plants and prolonged leaf function in shade plants. These results have implications for the carbon economy of seedlings and the regeneration of red oak under global change conditions.     

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.     

Response of nodulated alfalfa to water supply,temperature and elevated CO2: productivity and water relations

Exposing plants to long-term CO₂ enrichment generally leads to increases in plant biomass, total leaf area and alterations on leaf net photosynthetic rates, stomatal conductance and water use efficiency. However, the magnitude of such effects is dependent on the availability of other potentially limiting resources. The aim of our study was to elucidate the effects of elevated CO₂, applied at different temperature and water availability regimes, on nodulated alfalfa plants. Regardless of water supply, elevated CO₂ enhanced plant growth, especially when combined with increased temperature although no differences were detected until 30 days of treatment. Absence of differences in leaf relative growth rate, and gas exchange measurements, suggested that plants grown in a low water regime adjusted their growth to the amount of available water. Elevated CO₂ enhanced water use efficiency because of reduced water consumption and a greater dry mass production. Increased dry matter production of plants grown under elevated CO₂ and temperature was the result of stimulated photosynthetic rates, greater leaf area and water use efficiency. Lack of CO₂ effect on photosynthesis of plants grown at ambient temperature might be consequence of down-regulation phenomena. Plants grown at 700 μmol mol⁻¹ CO₂ maintained control nitrogen levels, discarding enhanced nitrogen availability as the main factor explaining enhanced dry matter.     

Host-specific aphid population responses to elevated CO2 and increased N availability

Sap-feeding insects such as aphids are the only insect herbivores that show positive responses to elevated CO₂. Recent models predict that increased nitrogen will increase aphid population size under elevated CO₂, but few experiments have tested this idea empirically. To determine whether soil nitrogen (N) availability modifies aphid responses to elevated CO₂, we tested the performance of Macrosiphum euphorbiae feeding on two host plants; a C₃ plant (Solanum dulcamara), and a C₄ plant (Amaranthus viridis). We expected aphid population size to increase on plants in elevated CO₂, with the degree of increase depending on the N availability. We found a significant CO₂× N interaction for the response of population size for M. euphorbiae feeding on S. dulcamara: aphids feeding on plants grown in ambient CO₂, low N conditions increased in response to either high N availability or elevated CO₂. No population size responses were observed for aphids infesting A. viridis. Elevated CO₂ increased plant biomass, specific leaf weight, and C : N ratios of the C₃ plant, S. dulcamara but did not affect the C₄ plant, A. viridis. Increased N fertilization significantly increased plant biomass, leaf area, and the weight : height ratio in both experiments. Elevated CO₂ decreased leaf N in S. dulcamara and had no effect on A. viridis, while higher N availability increased leaf N in A. viridis and had no effect in S. dulcamara. Aphid infestation only affected the weight : height ratio of S. dulcamara. We only observed an increase in aphid population size in response to elevated CO₂ or increased N availability for aphids feeding on S. dulcamara grown under low N conditions. There appears to be a maximum population growth rate that M. euphorbiae aphids can attain, and we suggest that this response is because of intrinsic limits on development time and fecundity.     

Growth and nitrogen accretion of dinitrogen-fixing Alnus glutinosa (L.) Gaertn. under elevated carbon dioxide

Short-term studies of tree growth at elevated CO₂ suggest that forest productivity may increase as atmospheric CO₂ concentrations rise, although low soil N availability may limit the magnitude of this response. There have been few studies of growth and N₂ fixation by symbiotic N₂-fixing woody species under elevated CO₂ and the N inputs these plants could provide to forest ecosystems in the future. We investigated the effect of twice ambient CO₂ on growth, tissue N accretion, and N₂ fixation of nodulated Alnus glutinosa (L.) Gaertn. grown under low soil N conditions for 160 d. Root, nodule, stem, and leaf dry weight (DW) and N accretion increased significantly in response to elevated CO₂. Whole-plant biomass and N accretion increased 54% and 40%, respectively. Delta-¹⁵N analysis of leaf tissue indicated that plants from both treatments derived similar proportions of their total N from symbiotic fixation suggesting that elevated CO₂ grown plants fixed approximately 40% more N than did ambient CO₂ grown plants. Leaves from both CO₂ treatments showed similar relative declines in leaf N content prior to autumnal leaf abscission, but total N in leaf litter increased 24% in elevated compared to ambient CO₂ grown plants. These results suggest that with rising atmospheric CO₂ N₂-fixing woody species will accumulate greater amounts of biomass N through N₂ fixation and may enhance soil N levels by increased litter N inputs.     

Elevated atmospheric CO2 and feedback between carbon and nitrogen cycles

We tested a conceptual model describing the influence of elevated atmospheric CO₂ on plant production, soil microorganisms, and the cycling of C and N in the plant-soil system. Our model is based on the observation that in nutrient-poor soils, plants (C₃) grown in an elevated CO₂ atmosphere often increase production and allocation to belowground structures. We predicted that greater belowground C inputs at elevated CO₂ should elicit an increase in soil microbial biomass and increased rates of organic matter turnover and nitrogen availability. We measured photosynthesis, biomass production, and C allocation of Populus grandidentata Michx. grown in nutrient-poor soil for one field season at ambient and twice-ambient (i.e., elevated) atmospheric CO₂ concentrations. Plants were grown in a sandy subsurface soil i) at ambient CO₂ with no open top chamber, ii) at ambient CO₂ in an open top chamber, and iii) at twice-ambient CO₂ in an open top chamber. Plants were fertilized with 4.5 g N m⁻² over a 47 d period midway through the growing season. Following 152 d of growth, we quantified microbial biomass and the availabilities of C and N in rhizosphere and bulk soil. We tested for a significant CO₂ effect on plant growth and soil C and N dynamics by comparing the means of the chambered ambient and chambered elevated CO₂ treatments. Rates of photosynthesis in plants grown at elevated CO₂ were significantly greater than those measured under ambient conditions. The number of roots, root length, and root length increment were also substantially greater at elevated CO₂. Total and belowground biomass were significantly greater at elevated CO₂. Under N-limited conditions, plants allocated 50–70% of their biomass to roots. Labile C in the rhizosphere of elevated-grown plants was significantly greater than that measured in the ambient treatments; there were no significant differences between labile C pools in the bulk soil of ambient and elevated-grown plants. Microbial biomass C was significantly greater in the rhizosphere and bulk soil of plants grown at elevated CO₂ compared to that in the ambient treatment. Moreover, a short-term laboratory assay of N mineralization indicated that N availability was significantly greater in the bulk soil of the elevated-grown plants. Our results suggest that elevated atmospheric CO₂ concentrations can have a positive feedback effect on soil C and N dynamics producing greater N availability. Experiments conducted for longer periods of time will be necessary to test the potential for negative feedback due to altered leaf litter chemistry. ei]{gnH}{fnLambers} ei]{gnA C}{fnBorstlap}     

Wheat genotypes differing in aluminum tolerance differ in their growth response to CO2 enrichment in acid soils

Aluminum (Al) toxicity is a major factor limiting plant growth in acid soils. Elevated atmospheric CO₂ [CO₂] enhances plant growth. However, there is no report on the effect of elevated [CO₂] on growth of plant genotypes differing in Al tolerance grown in acid soils. We investigated the effect of short‐term elevated [CO₂] on growth of Al‐tolerant (ET8) and Al‐sensitive (ES8) wheat plants and malate exudation from root apices by growing them in acid soils under ambient [CO₂] and elevated [CO₂] using open‐top chambers. Exposure of ET8 plants to elevated [CO₂] enhanced root biomass only. In contrast, shoot biomass of ES8 was enhanced by elevated [CO₂]. Given that exudation of malate to detoxify apoplastic Al is a mechanism for Al tolerance in wheat plants, ET8 plants exuded greater amounts of malate from root apices than ES8 plants under both ambient and elevated [CO₂]. These results indicate that elevated [CO₂] has no effect on malate exudation in both ET8 and ES8 plants. These novel findings have important implications for our understanding how plants respond to elevated [CO₂] grown in unfavorable edaphic conditions in general and in acid soils in particular.