The response of photosynthesis and stomatal conductance to rising [CO2]: mechanisms and environmental interactions

Differential anatomical responses to elevated CO2 in saplings of four hardwood species

To determine whether an elevated carbon dioxide concentration ([CO(2)]) can induce changes in the wood structure and stem radial growth in forest trees, we investigated the anatomical features of conduit cells and cambial activity in 4-year-old saplings of four deciduous broadleaved tree species - two ring-porous (Quercus mongolica and Kalopanax septemlobus) and two diffuse-porous species (Betula maximowicziana and Acer mono) - grown for three growing seasons in a free-air CO(2) enrichment system. Elevated [CO(2)] had no effects on vessels, growth and physiological traits of Q. mongolica, whereas tree height, photosynthesis and vessel area tended to increase in K. septemlobus. No effects of [CO(2)] on growth, physiological traits and vessels were seen in the two diffuse-porous woods. Elevated [CO(2)] increased larger vessels in all species, except B. maximowicziana and number of cambial cells in two ring-porous species. Our results showed that the vessel anatomy and radial stem growth of Q. mongolica, B. maximowicziana and A. mono were not affected by elevated [CO(2)], although vessel size frequency and cambial activity in Q. mongolica were altered. In contrast, changes in vessel anatomy and cambial activity were induced by elevated [CO(2)] in K. septemlobus. The different responses to elevated [CO(2)] suggest that the sensitivity of forest trees to CO(2) is species dependent.

Differential responses to changes in growth temperature between trees from different functional groups and biomes: a review and synthesis of data

The response of tree growth to a change in temperature may differ in predictable ways. Trees with conservative growth strategies may have little ability to respond to a changing climate. In addition, high latitude and altitude tree growth may be temperature-limited and thus benefit from some degree of warming, as opposed to warm-adapted species. Using data from 63 studies, we examined whether trees from different functional groups and thermal niches differed in their growth response to a change in growth temperature. We also investigated whether responses predicted for a change in growth temperature (both reduced and elevated) were similar for increased temperatures by repeating the analysis on the subset of raised temperature data to confirm the validity of our results for use in a climate-warming scenario. Using both the temperature-change response and the warming response, we found that elevated temperatures enhanced growth (measured as shoot height, stem diameter and biomass) in deciduous species more than in evergreen trees. Tropical species were indeed more susceptible to warming-induced growth declines than temperate or boreal trees in both analyses. More carbon may be available to allocate to growth at high temperatures because respiration acclimated more strongly than photosynthesis, increasing carbon assimilation but moderating carbon losses. Trees that developed at elevated temperatures did not simply accelerate growth but followed different developmental trajectories than unwarmed trees, allocating more biomass to leaves and less to roots and growing taller for a given stem diameter. While there were insufficient data to analyze trends for particular species, we generated equations to describe general trends in tree growth to temperature changes and to warming for use at large spatial scales or where data are lacking. We discuss the implications of these results in the context of a changing climate and highlight the areas of greatest uncertainty regarding temperature and tree growth where future research is needed.

Synergic effect of salinity and CO2 enrichment on growth and photosynthetic responses of the invasive cordgrass Spartina densiflora

Spartina densiflora is a C(4) halophytic species that has proved to have a high invasive potential which derives from its clonal growth and its physiological plasticity to environmental factors, such as salinity. A greenhouse experiment was designed to investigate the synergic effect of 380 and 700 ppm CO(2) at 0, 171, and 510 mM NaCl on the growth and the photosynthetic apparatus of S. densiflora by measuring chlorophyll fluorescence parameters, gas exchange and photosynthetic pigment concentrations. PEPC activity and total ash, sodium, potassium, calcium, magnesium, and zinc concentrations were determined, as well as the C/N ratio. Elevated CO(2) stimulated growth of S. densiflora at 0 and 171 mM NaCl external salinity after 90 d of treatment. This growth enhancement was associated with a greater leaf area and improved leaf water relations rather than with variations in net photosynthetic rate (A). Despite the fact that stomatal conductance decreased in response to 700 ppm CO(2) after 30 d of treatment, A was not affected. This response of A to elevated CO(2) concentration might be explained by an enhanced PEPC carboxylation capacity. On the whole, plant nutrient concentrations declined under elevated CO(2), which can be ascribed to the dilution effect caused by an increase in biomass and the higher water content found at 700 ppm CO(2). Finally, CO(2) and salinity had a marked overall effect on the photochemical (PSII) apparatus and the synthesis of photosynthetic pigments.

Stomatal uptake of O3 in aspen and aspen-birch forests under free-air CO2 and O3 enrichment

Rising atmospheric carbon dioxide (CO2) may alleviate the toxicological impacts of concurrently rising tropospheric ozone (O3) during the present century if higher CO2 is accompanied by lower stomatal conductance (gs), as assumed by many models. We investigated how elevated concentrations of CO2 and O3, alone and in combination, affected the accumulated stomatal flux of O3 (AFst) by canopies and sun leaves in closed aspen and aspen-birch forests in the free-air CO2-O3 enrichment experiment near Rhinelander, Wisconsin. Stomatal conductance for O3 was derived from sap flux data and AFst was estimated either neglecting or accounting for the potential influence of non-stomatal leaf surface O3 deposition. Leaf-level AFst (AFst(l)) was not reduced by elevated CO2. Instead, there was a significant CO2 x O(3) interaction on AFst(l), as a consequence of lower values of gs in control plots and the combination treatment than in the two single-gas treatments. In addition, aspen leaves had higher AFst(l) than birch leaves, and estimates of AFst(l) were not very sensitive to non-stomatal leaf surface O3 deposition. Our results suggest that model projections of large CO2-induced reductions in gs alleviating the adverse effect of rising tropospheric O3 may not be reasonable for northern hardwood forests.

Will photosynthetic capacity of aspen trees acclimate after long-term exposure to elevated CO2 and O3?

Photosynthetic acclimation under elevated carbon dioxide (CO(2)) and/or ozone (O(3)) has been the topic of discussion in many papers recently. We examined whether or not aspen plants grown under elevated CO(2) and/or O(3) will acclimate after 11 years of exposure at the Aspen Face site in Rhinelander, WI, USA. We studied diurnal patterns of instantaneous photosynthetic measurements as well as A/C(i) measurements monthly during the 2004-2008 growing seasons. Our results suggest that the responses of two aspen clones differing in O(3) sensitivity showed no evidence of photosynthetic and stomatal acclimation under either elevated CO(2), O(3) or CO(2) + O(3). Both clones 42E and 271 did not show photosynthetic nor stomatal acclimation under elevated CO(2) and O(3) after a decade of exposure. We found that the degree of increase or decrease in the photosynthesis and stomatal conductance varied significantly from day to day and from one season to another.

Photorespiration

Photorespiration is initiated by the oxygenase activity of ribulose-1,5-bisphosphate-carboxylase/oxygenase (RUBISCO), the same enzyme that is also responsible for CO(2) fixation in almost all photosynthetic organisms. Phosphoglycolate formed by oxygen fixation is recycled to the Calvin cycle intermediate phosphoglycerate in the photorespiratory pathway. This reaction cascade consumes energy and reducing equivalents and part of the afore fixed carbon is again released as CO(2). Because of this, photorespiration was often viewed as a wasteful process. Here, we review the current knowledge on the components of the photorespiratory pathway that has been mainly achieved through genetic and biochemical studies in Arabidopsis. Based on this knowledge, the energy costs of photorespiration are calculated, but the numerous positive aspects that challenge the traditional view of photorespiration as a wasteful pathway are also discussed. An outline of possible alternative pathways beside the major pathway is provided. We summarize recent results about photorespiration in photosynthetic organisms expressing a carbon concentrating mechanism and the implications of these results for understanding Arabidopsis photorespiration. Finally, metabolic engineering approaches aiming to improve plant productivity by reducing photorespiratory losses are evaluated.

Functional prokaryotic RubisCO from an oceanic metagenomic library

Culture-independent studies have indicated that there is significant diversity in the ribulose 1,5-bisphosphate carboxylase/oxygenase (RubisCO) enzymes used by marine, freshwater, and terrestrial autotrophic bacteria. Surprisingly, little is known about the catalytic properties of many environmentally significant RubisCO enzymes. Because one of the goals of RubisCO research is to somehow modify or select for RubisCO molecules with improved kinetic properties, a facile means to isolate functional and novel RubisCO molecules directly from the environment was developed. In this report, we describe the first example of functional RubisCO proteins obtained from genes cloned and characterized from metagenomic libraries derived from DNA isolated from environmental samples. Two form IA marine RubisCO genes were cloned, and each gene supported both photoheterotrophic and photoautotrophic growth of a RubisCO deletion strain of Rhodobacter capsulatus, strain SBI/II(-), indicating that catalytically active recombinant RubisCO was synthesized. The catalytic properties of the metagenomic RubisCO molecules were further characterized. These experiments demonstrated the feasibility of studying the functional diversity and enzymatic properties of RubisCO enzymes without first cultivating the host organisms. Further, this "proof of concept" experiment opens the way for development of a simple functional screen to examine the properties of diverse RubisCO genes isolated from any environment, and subsequent further bioselection may be possible if the growth conditions of complemented R. capsulatus strain SBI/II(-) are varied.

The evolution of water transport in plants: an integrated approach

This review examines the evolution of the plant vascular system from its beginnings in the green algae to modern arborescent plants, highlighting the recent advances in developmental, organismal, geochemical and climatological research that have contributed to our understanding of the evolution of xylem. Hydraulic trade-offs in vascular structure-function are discussed in the context of canopy support and drought and freeze-thaw stress resistance. This qualitative and quantitative neontological approach to palaeobotany may be useful for interpreting the water-transport efficiencies and hydraulic limits in fossil plants. Large variations in atmospheric carbon dioxide levels are recorded in leaf stomatal densities, and may have had profound impacts on the water conservation strategies of ancient plants. A hypothesis that links vascular function with stomatal density is presented and examined in the context of the evolution of wood and/or vessels. A discussion of the broader impacts of plant transport on hydrology and climate concludes this review.

Diurnal and seasonal variations in stomatal conductance of rice at elevated atmospheric CO(2) under fully open-air conditions

Understanding of leaf stomatal responses to the atmospheric CO(2) concentration, [CO(2)], is essential for accurate prediction of plant water use under future climates. However, limited information is available for the diurnal and seasonal changes in stomatal conductance (g(s)) under elevated [CO(2)]. We examined the factors responsible for variations in g(s) under elevated [CO(2)] with three rice cultivars grown in an open-field environment under flooded conditions during two growing seasons (a total of 2140 individual measurements). Conductance of all cultivars was generally higher in the morning and around noon than in the afternoon, and elevated [CO(2)] decreased g(s) by up to 64% over the 2 years (significantly on 26 out of 38 measurement days), with a mean g(s) decrease of 23%. We plotted the g(s) variations against three parameters from the Ball-Berry model and two revised versions of the model, and all parameters explained the g(s) variations well at each [CO(2)] in the morning and around noon (R(2) > 0.68), but could not explain these variations in the afternoon (R(2) < 0.33). The present results provide an important basis for modelling future water use in rice production.

A stomatal optimization theory to describe the effects of atmospheric CO2 on leaf photosynthesis and transpiration

BACKGROUND AND AIMS

Global climate models predict decreases in leaf stomatal conductance and transpiration due to increases in atmospheric CO2. The consequences of these reductions are increases in soil moisture availability and continental scale run-off at decadal time-scales. Thus, a theory explaining the differential sensitivity of stomata to changing atmospheric CO2 and other environmental conditions must be identified. Here, these responses are investigated using optimality theory applied to stomatal conductance.

METHODS

An analytical model for stomatal conductance is proposed based on: (a) Fickian mass transfer of CO2 and H2O through stomata; (b) a biochemical photosynthesis model that relates intercellular CO2 to net photosynthesis; and (c) a stomatal model based on optimization for maximizing carbon gains when water losses represent a cost. Comparisons between the optimization-based model and empirical relationships widely used in climate models were made using an extensive gas exchange dataset collected in a maturing pine (Pinus taeda) forest under ambient and enriched atmospheric CO2. Key Results and Conclusion In this interpretation, it is proposed that an individual leaf optimally and autonomously regulates stomatal opening on short-term (approx. 10-min time-scale) rather than on daily or longer time-scales. The derived equations are analytical with explicit expressions for conductance, photosynthesis and intercellular CO2, thereby making the approach useful for climate models. Using a gas exchange dataset collected in a pine forest, it is shown that (a) the cost of unit water loss lambda (a measure of marginal water-use efficiency) increases with atmospheric CO2; (b) the new formulation correctly predicts the condition under which CO2-enriched atmosphere will cause increasing assimilation and decreasing stomatal conductance.

Elevated atmospheric CO2 concentration enhances salinity tolerance in Aster tripolium L

Our study aimed at investigating the influence of elevated atmospheric CO(2) concentration on the salinity tolerance of the cash crop halophyte Aster tripolium L., thereby focussing on protein expression and enzyme activities. The plants were grown in hydroponics using a nutrient solution with or without addition of NaCl (75% seawater salinity), under ambient (380 ppm) and elevated (520 ppm) CO(2). Under ambient CO(2) concentration enhanced expressions and activities of the antioxidant enzymes superoxide dismutase, ascorbate peroxidase, and glutathione-S-transferase in the salt-treatments were recorded as a reaction to oxidative stress. Elevated CO(2) led to significantly higher enzyme expressions and activities in the salt-treatments, so that reactive oxygen species could be detoxified more effectively. Furthermore, the expression of a protective heat shock protein (class 20) increased under salinity and was even further enhanced under elevated CO(2) concentration. Additional energy had to be provided for the mechanisms mentioned above, which was indicated by the increased expression of a beta ATPase subunit and higher v-, p- and f-ATPase activities under salinity. The higher ATPase expression and activities also enable a more efficient ion transport and compartmentation for the maintenance of ion homeostasis. We conclude that elevated CO(2) concentration is able to improve the survival of A. tripolium under salinity because more energy is provided for the synthesis and enhanced activity of enzymes and proteins which enable a more efficient ROS detoxification and ion compartmentation/transport.

Leaf and canopy conductance in aspen and aspen-birch forests under free-air enrichment of carbon dioxide and ozone

Increasing concentrations of atmospheric carbon dioxide (CO2) and tropospheric ozone (O3) have the potential to affect tree physiology and structure, and hence forest feedbacks on climate. Here, we investigated how elevated concentrations of CO2 (+45%) and O3 (+35%), alone and in combination, affected conductance for mass transfer at the leaf and canopy levels in pure aspen (Populus tremuloides Michx.) and in mixed aspen and birch (Betula papyrifera Marsh.) forests in the free-air CO2-O3 enrichment experiment near Rhinelander, Wisconsin (Aspen FACE). The study was conducted during two growing seasons, when steady-state leaf area index (L) had been reached after > 6 years of exposure to CO2- and O3-enrichment treatments. Canopy conductance (g(c)) was estimated from stand sap flux, while leaf-level conductance of sun leaves in the upper canopy was derived by three different and independent methods: sap flux and L in combination with vertical canopy modelling, leaf 13C discrimination methodology in combination with photosynthesis modelling and leaf-level gas exchange. Regardless of the method used, the mean values of leaf-level conductance were higher in trees growing under elevated CO2 and/or O3 than in trees growing in control plots, causing a CO2 x O3 interaction that was statistically significant (P < or = 0.10) for sap flux- and (for birch) 13C-derived leaf conductance. Canopy conductance was significantly increased by elevated CO2 but not significantly affected by elevated O3. Investigation of a short-term gap in CO2 enrichment demonstrated a +10% effect of transient exposure of elevated CO2-grown trees to ambient CO2 on g(c). All treatment effects were similar in pure aspen and mixed aspen-birch communities. These results demonstrate that short-term primary stomatal closure responses to elevated CO2 and O3 were completely offset by long-term cumulative effects of these trace gases on tree and stand structure in determining canopy- and leaf-level conductance in pure aspen and mixed aspen-birch forests. Our results, together with the findings from other long-term FACE experiments with trees, suggest that model assumptions of large reductions in stomatal conductance under rising atmospheric CO2 are very uncertain for forests.

Acclimation of leaf hydraulic conductance and stomatal conductance of Pinus taeda (loblolly pine) to long-term growth in elevated CO(2) (free-air CO(2) enrichment) and N-fertilization

We investigated how leaf hydraulic conductance (K(leaf)) of loblolly pine trees is influenced by soil nitrogen amendment (N) in stands subjected to ambient or elevated CO(2) concentrations (CO(2)(a) and CO(2)(e), respectively). We also examined how K(leaf) varies with changes in reference leaf water potential (Psi(leaf-ref)) and stomatal conductance (g(s-ref)) calculated at vapour pressure deficit, D of 1 kPa. We detected significant reductions in K(leaf) caused by N and CO(2)(e), but neither treatment affected pre-dawn or midday Psi(leaf). We also detected a significant CO(2)(e)-induced reduction in g(s-ref) and Psi(leaf-ref). Among treatments, the sensitivity of K(leaf) to Psi(leaf) was directly related to a reference K(leaf) (K(leaf-ref) computed at Psi(leaf-ref)). This liquid-phase response was reflected in a similar gas-phase response, with g(s) sensitivity to D proportional to g(s-ref). Because leaves represented a substantial component of the whole-tree conductance, reduction in K(leaf) under CO(2)(e) affected whole-tree water use by inducing a decline in g(s-ref). The consequences of the acclimation of leaves to the treatments were: (1) trees growing under CO(2)(e) controlled morning leaf water status less than CO(2)(a) trees resulting in a higher diurnal loss of K(leaf); (2) the effect of CO(2)(e) on g(s-ref) was manifested only during times of high soil moisture.

Long-term relationships among atmospheric CO2, stomata, and intrinsic water use efficiency in individual trees

Leaf-level responses to increases in atmospheric carbon dioxide (CO(2)) concentrations could have large implications for water and carbon cycles. We investigated whether stomatal density, guard cell length, and intrinsic water use efficiency (iWUE) of 27 individual trees growing at the Arnold Arboretum in Boston, Massachusetts have responded to changing environmental conditions over the last 100 years. We examined leaves from 74 herbarium specimens collected from three genera-Acer (maples), Quercus (oaks), and Carpinus (hornbeams)-from 1893 to 2006. During this period, global average atmospheric CO(2) concentrations increased by approximately 29% (86 ppm), and temperatures in Boston increased by 1.8°C. Stomatal density and guard cell length were negatively correlated in oaks and hornbeams. Although stomatal density declined and guard cell length increased over time, the changes were not dependent on the magnitude of changes in CO(2) concentrations. Intrinsic WUE did not change significantly over time. Our findings suggest that iWUE may not respond to changes in CO(2) concentrations over the lifetimes of individual trees, possibly because of compensating changes in stomatal density and guard cell size. We provide an example of a method that can enable researchers to differentiate between genetic and plastic responses to global change in long-lived trees.

A focus on natural variation for abiotic constraints response in the model species Arabidopsis thaliana

Plants are particularly subject to environmental stress, as they cannot move from unfavourable surroundings. As a consequence they have to react in situ. In any case, plants have to sense the stress, then the signal has to be transduced to engage the appropriate response. Stress response is effected by regulating genes, by turning on molecular mechanisms to protect the whole organism and its components and/or to repair damage. Reactions vary depending on the type of stress and its intensity, but some are commonly turned on because some responses to different abiotic stresses are shared. In addition, there are multiple ways for plants to respond to environmental stress, depending on the species and life strategy, but also multiple ways within a species depending on plant variety or ecotype. It is regularly accepted that populations of a single species originating from diverse geographic origins and/or that have been subjected to different selective pressure, have evolved retaining the best alleles for completing their life cycle. Therefore, the study of natural variation in response to abiotic stress, can help unravel key genes and alleles for plants to cope with their unfavourable physical and chemical surroundings. This review is focusing on Arabidopsis thaliana which has been largely adopted by the global scientific community as a model organism. Also, tools and data that facilitate investigation of natural variation and abiotic stress encountered in the wild are set out. Characterization of accessions, QTLs detection and cloning of alleles responsible for variation are presented.

Evolution of stomatal responsiveness to CO(2) and optimization of water-use efficiency among land plants

The stomata of angiosperms respond to changes in ambient atmospheric concentrations of CO(2) (C(a)) in ways that appear to optimize water-use efficiency. It is unknown where in the history of land plants this important stomatal control mechanism evolved. Here, we test the hypothesis that major clades of plants have distinct stomatal sensitivities to C(a) reflecting a relatively recent evolution of water-use optimization in derived angiosperms. Responses of stomatal conductance (g(s)) to step changes between elevated, ambient and low C(a) (600, 380 and 100 micromol mol(-1), respectively) were compared in a phylogenetically and ecologically diverse range of higher angiosperms, conifers, ferns and lycopods. All species responded to low C(a) by increasing g(s) but only angiosperm stomata demonstrated a significant closing response when C(a) was elevated to 600 micromol mol(-1). As a result, angiosperms showed significantly greater increases in water-use efficiency under elevated C(a) than the other lineages. The data suggest that the angiosperms have mechanisms for detecting and responding to increases in C(a) that are absent from earlier diverging lineages, and these mechanisms impart a greater capacity to optimize water-use efficiency.

Water-use responses of 'living fossil' conifers to CO2 enrichment in a simulated Cretaceous polar environment

BACKGROUND AND AIMS

During the Mesozoic, the polar regions supported coniferous forests that experienced warm climates, a CO(2)-rich atmosphere and extreme seasonal variations in daylight. How the interaction between the last two factors might have influenced water use of these conifers was investigated. An experimental approach was used to test the following hypotheses: (1) the expected beneficial effects of elevated [CO(2)] on water-use efficiency (WUE) are reduced or lost during the 24-h light of the high-latitude summer; and (2) elevated [CO(2)] reduces plant water use over the growing season.

METHODS

Measurements of leaf and whole-plant gas exchange, and leaf-stable carbon isotope composition were made on one evergreen (Sequoia sempervirens) and two deciduous (Metasequoia glyptostroboides and Taxodium distichum) 'living fossil' coniferous species after 3 years' growth in controlled-environment simulated Cretaceous Arctic (69 degrees N) conditions at either ambient (400 micromol mol(-1)) or elevated (800 micromol mol(-1)) [CO(2)].

KEY RESULTS

Stimulation of whole-plant WUE (WUE(P)) by CO(2) enrichment was maintained over the growing season for the three studied species but this pattern was not reflected in patterns of WUE inferred from leaf-scale gas exchange measurements (iWUE(L)) and delta(13)C of foliage (tWUE(L)). This response was driven largely by increased rates of carbon uptake, because there was no overall CO(2) effect on daily whole-plant transpiration or whole-plant water loss integrated over the study period. Seasonal patterns of tWUE(L) differed from those measured for iWUE(L). The results suggest caution against over simplistic interpretations of WUE(P) based on leaf isotopic composition.

CONCLUSIONS

The data suggest that the efficiency of whole-tree water use may be improved by CO(2) enrichment in a simulated high-latitude environment, but that transpiration is relatively insensitive to atmospheric CO(2) in the living fossil species investigated.

Super-elevated CO2 interferes with stomatal response to ABA and night closure in soybean (Glycine max)

Studies have shown stomatal conductance (g(s)) of plants exposed to super-elevated CO2 (>5000micromol mol(-1)) increases in several species, in contrast to a decrease of g(s) caused by moderate CO2 enrichment. We conducted a series of experiments to determine whether super-elevated CO2 alters stomatal development and/or interferes with stomatal closure in soybean (Glycine max). Plants were grown at nominal ambient (400), elevated (1200) and super-elevated (10,000micromol mol(-1)) CO2 in controlled environmental chambers. Stomatal density of the plant leaf was examined by a scanning electron microscope (SEM), while the stomatal response to the application of exogenous abscisic acid (ABA), a phytohormone associated with water stress and stomatal control, was investigated in intact growing plants by measuring the g(s) of abaxial leaf surfaces using a steady-state porometer. Relative to the control (400micromol mol(-1) CO2) plants, daytime stomatal conductance (g(s,day)) of the plants grown under 1200 and 10,000micromol mol(-1) CO2 was reduced by 38% and 15%, respectively. Dark period stomatal conductance (g(s,night)) was unaffected by growing under 1200mumol mol(-1) CO2) but dramatically increased under 10,000micromol mol(-1) CO2. Stomatal density increased by 10% in the leaves of 10,000micromol mol(-1) CO2-grown plants, which in part contributed to the higher g(s,night) values. Elevating [CO2] to 1200micromol mol(-1) enhanced ABA-induced stomatal closure, but further increasing CO2 to 10,000micromol mol(-1) significantly reduced ABA-induced stomatal closure. These results demonstrated that stomatal response to ABA is CO2 dependent. Hence, a stomatal failure to effectively respond to an ABA signal and to close at night under extremely high CO2 may increase plants susceptibility to other abiotic stresses.

Transcriptomic comparison in the leaves of two aspen genotypes having similar carbon assimilation rates but different partitioning patterns under elevated [CO2]

This study compared the leaf transcription profiles, physiological characteristics and primary metabolites of two Populus tremuloides genotypes (clones 216 and 271) known to differ in their responses to long-term elevated [CO2] (e[CO2]) at the Aspen free-air CO2 enrichment site near Rhinelander, WI, USA. The physiological responses of these clones were similar in terms of photosynthesis, stomatal conductance and leaf area index under e[CO2], yet very different in terms of growth enhancement (0-10% in clone 216; 40-50% in clone 271). Although few genes responded to long-term exposure to e[CO2], the transcriptional activity of leaf e[CO2]-responsive genes was distinctly different between the clones, differentially impacting multiple pathways during both early and late growing seasons. An analysis of transcript abundance and carbon/nitrogen biochemistry suggested that the CO2-responsive clone (271) partitions carbon into pathways associated with active defense/response to stress, carbohydrate/starch biosynthesis and subsequent growth. The CO2-unresponsive clone (216) partitions carbon into pathways associated with passive defense (e.g. lignin, phenylpropanoid) and cell wall thickening. This study indicates that there is significant variation in expression patterns between different tree genotypes in response to long-term exposure to e[CO2]. Consequently, future efforts to improve productivity or other advantageous traits for carbon sequestration should include an examination of genetic variability in CO2 responsiveness.

Seasonal response of photosynthetic electron transport and energy dissipation in the eighth year of exposure to elevated atmospheric CO2 (FACE) in Pinus taeda (loblolly pine)

To determine the effect of growth under elevated CO(2) partial pressures (pCO(2)) on photosynthetic electron transport and photoprotective energy dissipation, we examined light-saturated net photosynthetic CO(2) assimilation (A(sat)), the capacity for photosynthetic O(2) evolution, chlorophyll fluorescence emission and the pigment composition of upper-canopy loblolly pine needles in the eighth year of exposure to elevated pCO(2) (20 Pa above ambient) at the free-air CO(2) enrichment facility in the Duke Forest. During the summer growing season, A(sat) was 50% higher in current-year needles and 24% higher in year-old needles in elevated pCO(2) in comparison with needles of the same age cohort in ambient pCO(2). Thus, photosynthetic down-regulation at elevated pCO(2) was observed in the summer in year-old needles. In the winter, A(sat) was not significantly affected by growth pCO(2). Reductions in A(sat), the capacity for photosynthetic O(2) evolution and photosystem II (PSII) efficiency in the light-acclimated and fully-oxidized states were observed in the winter when compared to summer. Growth at elevated pCO(2) had no significant effect on the capacity for photosynthetic O(2) evolution, PSII efficiencies in the light-acclimated and fully-oxidized states, chlorophyll content or the size and conversion state of the xanthophyll cycle, regardless of season or needle age cohort. Therefore, we observed no evidence that photosynthetic electron transport or photoprotective energy dissipation responded to compensate for the effects of elevated pCO(2) on Calvin cycle activity.

Plant-pathogen interactions and elevated CO2: morphological changes in favour of pathogens

Crop losses caused by pests and weeds have been estimated at 42% worldwide, with plant pathogens responsible for almost $10 billion worth of damage in the USA in 1994 alone. Elevated carbon dioxide [ECO(2)] and associated climate change have the potential to accelerate plant pathogen evolution, which may, in turn, affect virulence. Plant-pathogen interactions under increasing CO(2) concentrations have the potential to disrupt both agricultural and natural systems severely, yet the lack of experimental data and the subsequent ability to predict future outcomes constitutes a fundamental knowledge gap. Furthermore, nothing is known about the mechanistic bases of increasing pathogen agressiveness. In the absence of information on crop species, it is shown here that plant pathogen (Erysiphe cichoracearum) aggressiveness is increased under ECO(2), together with changes in the leaf epidermal characteristics of the model plant Arabidopsis thaliana L. Stomatal density, guard cell length, and trichome numbers on leaves developing post-infection are increased under ECO(2) in direct contrast to non-infected responses. As many plant pathogens utilize epidermal features for successful infection, these responses provide a positive feedback mechanism facilitating an enhanced susceptibility of newly developed leaves to further pathogen attack. Furthermore, a screen of resistant and susceptible ecotypes suggest inherent differences in epidermal responses to ECO(2).

The importance of nutritional regulation of plant water flux

Transpiration is generally considered a wasteful but unavoidable consequence of photosynthesis, occurring because water is lost when stomata open for CO(2) uptake. Additionally, transpiration has been ascribed the functions of cooling leaves, driving root to shoot xylem transport and mass flow of nutrients through the soil to the rhizosphere. As a consequence of the link between nutrient mass flow and transpiration, nutrient availability, particularly that of NO(3)(-), partially regulates plant water flux. Nutrient regulation of transpiration may function through the concerted regulation of: (1) root hydraulic conductance through control of aquaporins by NO(3)(-), (2) shoot stomatal conductance (g(s)) through NO production, and (3) pH and phytohormone regulation of g(s). These mechanisms result in biphasic responses of water flux to NO(3)(-) availability. The consequent trade-off between water and nutrient flux has important implications for understanding plant distributions, for production of water use-efficient crops and for understanding the consequences of global-change-linked CO(2) suppression of transpiration for plant nutrient acquisition.

Long-term drought results in a reversible decline in photosynthetic capacity in mango leaves, not just a decrease in stomatal conductance

The negative effects of drought on plant growth, development of natural plant communities and crop productivity are well established, but some of the responses remain poorly characterized, particularly the effect of long-term drought on photosynthetic capacity. We hypothesized that long-term drought results in a decline in leaf photosynthetic capacity, and not just a decrease in diffusive conductance. To test this hypothesis, we studied the effect of drought, slowly developed over 3.5 months, in leaves of eight potted mango (Mangifera indica L.) trees. We found that photosynthesis was not only limited by stomatal closure, but was also downregulated as a consequence of a strong decrease in photosynthetic capacity assessed by the measurements of maximal net photosynthesis (A(max)) and the light-saturated rate of photosynthetic electron transport (J(max)). The rapid recovery of A(max) and J(max), after only 1 week of rewatering, the maintenance of a stable pool of leaf nitrogen throughout the trial, and the decrease in quantum efficiency of open centers of photosystem II, indicate that the photosynthetic machinery escaped photodamage in the drought-treated trees and was simply downregulated during drought. The hexose-to-sucrose ratio was higher in leaves from drought-treated trees than in control leaves, suggesting that photosynthetic capacity decreased as a consequence of sink limitation.

Gas exchange and photosynthetic performance of the tropical tree Acacia nigrescens when grown in different CO(2) concentrations

The photosynthetic responses of the tropical tree species Acacia nigrescens Oliv. grown at different atmospheric CO(2) concentrations-from sub-ambient to super-ambient-have been studied. Light-saturated rates of net photosynthesis (A (sat)) in A. nigrescens, measured after 120 days exposure, increased significantly from sub-ambient (196 microL L(-1)) to current ambient (386 microL L(-1)) CO(2) growth conditions but did not increase any further as [CO(2)] became super-ambient (597 microL L(-1)). Examination of photosynthetic CO(2) response curves, leaf nitrogen content, and leaf thickness showed that this acclimation was most likely caused by reduction in Rubisco activity and a shift towards ribulose-1,5-bisphosphate regeneration-limited photosynthesis, but not a consequence of changes in mesophyll conductance. Also, measurements of the maximum efficiency of PSII and the carotenoid to chlorophyll ratio of leaves indicated that it was unlikely that the pattern of A (sat) seen was a consequence of growth [CO(2)] induced stress. Many of the photosynthetic responses examined were not linear with respect to the concentration of CO(2) but could be explained by current models of photosynthesis.

Coppicing shifts CO2 stimulation of poplar productivity to above-ground pools: a synthesis of leaf to stand level results from the POP/EUROFACE experiment

A poplar short rotation coppice (SRC) grown for the production of bioenergy can combine carbon (C) storage with fossil fuel substitution. Here, we summarize the responses of a poplar (Populus) plantation to 6 yr of free air CO(2) enrichment (POP/EUROFACE consisting of two rotation cycles). We show that a poplar plantation growing in nonlimiting light, nutrient and water conditions will significantly increase its productivity in elevated CO(2) concentrations ([CO(2)]). Increased biomass yield resulted from an early growth enhancement and photosynthesis did not acclimate to elevated [CO(2)]. Sufficient nutrient availability, increased nitrogen use efficiency (NUE) and the large sink capacity of poplars contributed to the sustained increase in C uptake over 6 yr. Additional C taken up in high [CO(2)] was mainly invested into woody biomass pools. Coppicing increased yield by 66% and partly shifted the extra C uptake in elevated [CO(2)] to above-ground pools, as fine root biomass declined and its [CO(2)] stimulation disappeared. Mineral soil C increased equally in ambient and elevated [CO(2)] during the 6 yr experiment. However, elevated [CO(2)] increased the stabilization of C in the mineral soil. Increased productivity of a poplar SRC in elevated [CO(2)] may allow shorter rotation cycles, enhancing the viability of SRC for biofuel production.

Growth at elevated CO(2) delays the adverse effects of drought stress on leaf photosynthesis of the C(4) sugarcane

Sugarcane (Saccharum officinarum L. cv. CP72-2086) was grown in sunlit greenhouses at daytime [CO(2)] of 360 (ambient) and 720 (elevated)mumolmol(-1). Drought stress was imposed for 13d when plants were 4 months old, and various photosynthetic parameters and levels of nonstructural carbohydrates were determined for uppermost fully expanded leaves of well-watered (control) and drought stress plants. Control plants at elevated [CO(2)] were 34% and 25% lower in leaf stomatal conductance (g(s)) and transpiration rate (E) and 35% greater in leaf water-use efficiency (WUE) than their counterparts at ambient [CO(2)]. Leaf CO(2) exchange rate (CER) and activities of Rubisco, NADP-malate dehydrogenase, NADP-malic enzyme and pyruvate P(i) dikinase were marginally affected by elevated [CO(2)], but were reduced by drought, whereas activity of PEP carboxylase was reduced by elevated [CO(2)], but not by drought. At severe drought developed at day 12, leaf g(s) and WUE of ambient-[CO(2)] stress plants declined to 5% and 7%, while elevated-[CO(2)] stress plants still maintained g(s) and WUE at 20% and 74% of their controls. In control plants, elevated [CO(2)] did not enhance the midday levels of starch, sucrose, or reducing sugars. For both ambient- and elevated-[CO(2)] stress plants, severe drought did not affect the midday level of sucrose but substantially reduced that of starch. Nighttime starch decomposition in control plants was 55% for ambient [CO(2)] and 59% for elevated [CO(2)], but was negligible for stress plants of both [CO(2)] treatments. For both ambient-[CO(2)] control and stress plants, midday sucrose level at day 12 was similar to the predawn value at day 13. In contrast, sucrose levels of elevated-[CO(2)] control and stress plants at predawn of day 13 were 61-65% of the midday values of day 12. Levels of reducing sugars were much greater for both ambient- and elevated-[CO(2)] stress plants, implying an adaptation to drought stress. Sugarcane grown at elevated [CO(2)] had lower leaf g(s) and E and greater leaf WUE, which helped to delay the adverse effects of drought and, thus, allowed the stress plants to continue photosynthesis for at least an extra day during episodic drought cycles.

Effects of long-term exposure to elevated CO(2) conditions in slow-growing plants using a (12)C-enriched CO(2)-labelling technique

Despite their relevancy, long-term studies analyzing elevated CO(2) effect in plant production and carbon (C) management on slow-growing plants are scarce. A special chamber was designed to perform whole-plant above-ground gas-exchange measurements in two slow-growing plants (Chamaerops humilis and Cycas revoluta) exposed to ambient (ca. 400 micromol mol(-1)) and elevated (ca. 800 micromol mol(-1)) CO(2) conditions over a long-term period (20 months). The ambient isotopic (13)C/(12)C composition (delta(13)C) of plants exposed to elevated CO(2) conditions was modified (from ca. -12.8 per thousand to ca. -19.2 per thousand) in order to study carbon allocation in leaf, shoot and root tissues. Elevated CO(2) increased plant growth by ca. 45% and 60% in Chamaerops and Cycas, respectively. The whole-plant above-ground gas-exchange determinations revealed that, in the case of Chamaerops, elevated CO(2) decreased the photosynthetic activity (determined on leaf area basis) as a consequence of the limited ability to increase C sink strength. On the other hand, the larger C sink strength (reflected by their larger CO(2) stimulatory effect on dry mass) in Cycas plants exposed to elevated CO(2) enabled the enhancement of their photosynthetic capacity. The delta(13)C values determined in the different plant tissues (leaf, shoot and root) suggest that Cycas plants grown under elevated CO(2) had a larger ability to export the excess leaf C, probably to the main root. The results obtained highlighted the different C management strategies of both plants and offered relevant information about the potential response of two slow-growing plants under global climate change conditions.

The role of botanical gardens in climate change research

Botanical gardens have a unique set of resources that allows them to host important climate change research projects not easily undertaken elsewhere. These resources include controlled growing conditions, living collections with broad taxonomic representation, meticulous record-keeping, networks spanning wide geographic areas, and knowledgeable staff. Indeed, botanical gardens have already contributed significantly to our understanding of biological responses to climate change, particularly the effects of temperature on the timing of flowering and leaf-out. They have also made significant contributions to the understanding of the relationships among climate, physiology, and anatomy. Gardens are finding new uses for traditional research tools such as herbarium specimens and historical photographs, which are increasingly being used to obtain information on past plant behavior. Additional work on invasive species and comparative studies of responses to climatic variation are providing insights on important ecological, evolutionary, and management questions. With their large collections of plant species from throughout the world and excellent herbaria, botanical gardens are well positioned to expand their current activities to continue to provide leadership in climate change research and education.

Elevated CO2 effects on plant carbon, nitrogen, and water relations: six important lessons from FACE

Plant responses to the projected future levels of CO(2) were first characterized in short-term experiments lasting days to weeks. However, longer term acclimation responses to elevated CO(2) were subsequently discovered to be very important in determining plant and ecosystem function. Free-Air CO(2) Enrichment (FACE) experiments are the culmination of efforts to assess the impact of elevated CO(2) on plants over multiple seasons and, in the case of crops, over their entire lifetime. FACE has been used to expose vegetation to elevated concentrations of atmospheric CO(2) under completely open-air conditions for nearly two decades. This review describes some of the lessons learned from the long-term investment in these experiments. First, elevated CO(2) stimulates photosynthetic carbon gain and net primary production over the long term despite down-regulation of Rubisco activity. Second, elevated CO(2) improves nitrogen use efficiency and, third, decreases water use at both the leaf and canopy scale. Fourth, elevated CO(2) stimulates dark respiration via a transcriptional reprogramming of metabolism. Fifth, elevated CO(2) does not directly stimulate C(4) photosynthesis, but can indirectly stimulate carbon gain in times and places of drought. Finally, the stimulation of yield by elevated CO(2) in crop species is much smaller than expected. While many of these lessons have been most clearly demonstrated in crop systems, all of the lessons have important implications for natural systems.

Elevated atmospheric CO2 concentration ameliorates effects of NaCl salinity on photosynthesis and leaf structure of Aster tripolium L

This study investigated the interaction of NaCl-salinity and elevated atmospheric CO(2) concentration on gas exchange, leaf pigment composition, and leaf ultrastructure of the potential cash crop halophyte Aster tripolium. The plants were irrigated with five different salinity levels (0, 25, 50, 75, 100% seawater salinity) under ambient and elevated (520 ppm) CO(2). Under saline conditions (ambient CO(2)) stomatal and mesophyll resistance increased, leading to a significant decrease in photosynthesis and water use efficiency (WUE) and to an increase in oxidative stress. The latter was indicated by dilations of the thylakoid membranes and an increase in superoxide dismutase (SOD) activity. Oxidative stress could be counteracted by thicker epidermal cell walls of the leaves, a thicker cuticle, a reduced chlorophyll content, an increase in the chlorophyll a/b ratio and a transient decline of the photosynthetic efficiency. Elevated CO(2) led to a significant increase in photosynthesis and WUE. The improved water and energy supply was used to increase the investment in mechanisms reducing water loss and oxidative stress (thicker cell walls and cuticles, a higher chlorophyll and carotenoid content, higher SOD activity), resulting in more intact thylakoids. As these mechanisms can improve survival under salinity, A. tripolium seems to be a promising cash crop halophyte which can help in desalinizing and reclaiming degraded land.

Why are nitrogen concentrations in plant tissues lower under elevated CO2? A critical examination of the hypotheses

Plants grown under elevated atmospheric [CO2] typically have decreased tissue concentrations of N compared with plants grown under current ambient [CO2]. The physiological mechanisms responsible for this phenomenon have not been definitely established, although a considerable number of hypotheses have been advanced to account for it. In this review we discuss and critically evaluate these hypotheses. One contributing factor to the decreases in tissue N concentrations clearly is dilution of N by increased photosynthetic assimilation of C. In addition, studies on intact plants show strong evidence for a general decrease in the specific uptake rates (uptake per unit mass or length of root) of N by roots under elevated CO2. This decreased root uptake appears likely to be the result both of decreased N demand by shoots and of decreased ability of the soil-root system to supply N. The best-supported mechanism for decreased N supply is a decrease in transpiration-driven mass flow of N in soils due to decreased stomatal conductance at elevated CO2, although some evidence suggests that altered root system architecture may also play a role. There is also limited evidence suggesting that under elevated CO2, plants may exhibit increased rates of N loss through volatilization and/or root exudation, further contributing to lowering tissue N concentrations.

Stomatal characteristics and infection biology of Pyrenopeziza betulicola in Betula pendula trees grown under elevated CO2 and O3

Two silver birch clones were exposed to ambient and elevated concentrations of CO(2) and O(3), and their combination for 3 years, using open-top chambers. We evaluated the effects of elevated CO(2) and O(3) on stomatal conductance (g(s)), density (SD) and index (SI), length of the guard cells, and epidermal cell size and number, with respect to crown position and leaf type. The relationship between the infection biology of the fungus (Pyrenopeziza betulicola) causing leaf spot disease and stomatal characteristics was also studied. Leaf type was an important determinant of O(3) response in silver birch, while crown position and clone played only a minor role. Elevated CO(2) reduced the g(s), but had otherwise no significant effect on the parameters studied. No significant interactions between elevated CO(2) and O(3) were found. The infection biology of P. betulicola was not correlated with SD or g(s), but it did occasionally correlate positively with the length of the guard cells.

Arabidopsis transcript and metabolite profiles: ecotype-specific responses to open-air elevated [CO2]

A Free-Air CO(2) Enrichment (FACE) experiment compared the physiological parameters, transcript and metabolite profiles of Arabidopsis thaliana Columbia-0 (Col-0) and Cape Verde Island (Cvi-0) at ambient (approximately 0.375 mg g(-1)) and elevated (approximately 0.550 mg g(-1)) CO(2) ([CO(2)]). Photoassimilate pool sizes were enhanced in high [CO(2)] in an ecotype-specific manner. Short-term growth at elevated [CO(2)] stimulated carbon gain irrespective of down-regulation of plastid functions and altered expression of genes involved in nitrogen metabolism resembling patterns observed under N-deficiency. The study confirmed well-known characteristics, but the use of a time course, ecotypic genetic differences, metabolite analysis and the focus on clusters of functional categories provided new aspects about responses to elevated [CO(2)]. Longer-term Cvi-0 responded by down-regulating functions favouring carbon accumulation, and both ecotypes showed altered expression of genes for defence, redox control, transport, signalling, transcription and chromatin remodelling. Overall, carbon fixation with a smaller commitment of resources in elevated [CO(2)] appeared beneficial, with the extra C only partially utilized possibly due to disturbance of the C : N ratio. To different degrees, both ecotypes perceived elevated [CO(2)] as a metabolic perturbation that necessitated increased functions consuming or storing photoassimilate, with Cvi-0 emerging as more capable of acclimating. Elevated [CO(2)] in Arabidopsis favoured adjustments in reactive oxygen species (ROS) homeostasis and signalling that defined genotypic markers.

Elevated ozone alters soybean-virus interaction

Increasing concentrations of ozone (O(3)) in the troposphere affect many organisms and their interactions with each other. To analyze the changes in a plant-pathogen interaction, soybean plants were infected with Soybean mosaic virus (SMV) while they were fumigated with O(3). In otherwise natural field conditions, elevated O(3) treatment slowed systemic infection and disease development by inducing a nonspecific resistance against SMV for a period of 3 weeks. During this period, the negative effect of virus infection on light-saturated carbon assimilation rate was prevented by elevated O(3) exposure. To identify the molecular basis of a soybean nonspecific defense response, high-throughput gene expression analysis was performed in a controlled environment. Transcripts of fungal, bacterial, and viral defense-related genes, including PR-1, PR-5, PR-10, and EDS1, as well as genes of the flavonoid biosynthesis pathways (and concentrations of their end products, quercetin and kaempherol derivatives) increased in response to elevated O(3). The drastic changes in soybean basal defense response under altered atmospheric conditions suggest that one of the elements of global change may alter the ecological consequences and, eventually, coevolutionary relationship of plant-pathogen interactions in the future.

Elevated CO2 increases photosynthesis, biomass and productivity, and modifies gene expression in sugarcane

Because of the economical relevance of sugarcane and its high potential as a source of biofuel, it is important to understand how this crop will respond to the foreseen increase in atmospheric [CO(2)]. The effects of increased [CO(2)] on photosynthesis, development and carbohydrate metabolism were studied in sugarcane (Saccharum ssp.). Plants were grown at ambient (approximately 370 ppm) and elevated (approximately 720 ppm) [CO(2)] during 50 weeks in open-top chambers. The plants grown under elevated CO(2) showed, at the end of such period, an increase of about 30% in photosynthesis and 17% in height, and accumulated 40% more biomass in comparison with the plants grown at ambient [CO(2)]. These plants also had lower stomatal conductance and transpiration rates (-37 and -32%, respectively), and higher water-use efficiency (c.a. 62%). cDNA microarray analyses revealed a differential expression of 35 genes on the leaves (14 repressed and 22 induced) by elevated CO(2). The latter are mainly related to photosynthesis and development. Industrial productivity analysis showed an increase of about 29% in sucrose content. These data suggest that sugarcane crops increase productivity in higher [CO(2)], and that this might be related, as previously observed for maize and sorghum, to transient drought stress.

Why is plant-growth response to elevated CO2 amplified when water is limiting, but reduced when nitrogen is limiting? A growth-optimisation hypothesis

Experimental evidence indicates that the stomatal conductance and nitrogen concentration ([N]) of foliage decline under CO₂ enrichment, and that the percentage growth response to elevated CO₂ is amplified under water limitation, but reduced under nitrogen limitation. We advance simple explanations for these responses based on an optimisation hypothesis applied to a simple model of the annual carbon-nitrogen-water economy of trees growing at a CO₂-enrichment experiment at Oak Ridge, Tennessee, USA. The model is shown to have an optimum for leaf [N], stomatal conductance and leaf area index (LAI), where annual plant productivity is maximised. The optimisation is represented in terms of a trade-off between LAI and stomatal conductance, constrained by water supply, and between LAI and leaf [N], constrained by N supply. At elevated CO₂ the optimum shifts to reduced stomatal conductance and leaf [N] and enhanced LAI. The model is applied to years with contrasting rainfall and N uptake. The predicted growth response to elevated CO₂ is greatest in a dry, high-N year and is reduced in a wet, low-N year. The underlying physiological explanation for this contrast in the effects of water versus nitrogen limitation is that leaf photosynthesis is more sensitive to CO₂ concentration ([CO₂]) at lower stomatal conductance and is less sensitive to [CO₂] at lower leaf [N].

Carbon partitioning in N2 fixing Medicago sativa plants exposed to different CO2 and temperature conditions

Many of the studies analysing the CO₂ effect on plant development have been conducted in optimal growth conditions. Furthermore, although some of those studies suggest that legumes might show a steady productivity increase with rising CO₂, the role of nodule activity on the plant responsiveness to predicted atmospheric CO₂ enhancement is not well understood. In this study, C (metabolism and allocation) and N (nodule activity) interaction between the plant and the bacterial symbiont during the photosynthetic acclimation of N₂-fixing alfalfa (Medicago sativa L. cv. Aragón) plants exposed to elevated CO₂ and temperature conditions was analysed. The plants were grown in temperature gradient greenhouses (TGG) where, in the case of elevated CO₂ treatments, the isotopic ¹³C/¹²C composition (δ¹³C) inside the TGG was modified. Compared with the corresponding temperature treatment, exposure to 700 μmol mol^-1 CO₂ enhanced dry mass (DM) of plants in elevated temperature treatments (26%), whereas no significant effect was detected in ambient temperature treatments. The δ¹³C data revealed that although all the carbon corresponding to leaf total organic matter (TOM) came from newly assimilated C, plants exposed to elevated CO₂ did not develop strong sink activity (especially in ambient temperature conditions). Leaf carbohydrate build-up induced reduction in the Rubisco (E.C. 4.1.1.39) carboxylation capacity of plants. Despite this reduction in Rubisco content, plants exposed to elevated CO₂ conditions maintained (at ambient temperature) or increased (at elevated temperature) photosynthetic rates (measured at growth conditions) by increasing N use efficiency. The larger C sink strength of nodules in plants grown at elevated CO₂ and temperature conditions did not contribute towards overcoming photosynthetic acclimation. Further, the inhibitory effect of CO₂ on nodule total activity was caused by a large depletion in total soluble protein (TSP) of nodules. Depletion of leaf N demand, together with the reduction in nodule carbohydrate availability (as reflected by the nodule starch concentration), negatively affected the nodule TSP content and enzymatic activity.