Growth In Elevated CO2 Research Articles

Respiratory and Photosynthetic Responses of Antarctic Vascular Plants Are Differentially Affected by CO2 Enrichment and Nocturnal Warming

Projected rises in atmospheric CO2 concentration and minimum night-time temperatures may have important effects on plant carbon metabolism altering the carbon balance of the only two vascular plant species in the Antarctic Peninsula. We assessed the effect of nocturnal warming (8/5 °C vs. 8/8 °C day/night) and CO2 concentrations (400 ppm and 750 ppm) on gas exchange, non-structural carbohydrates, two respiratory-related enzymes, and mitochondrial size and number in two species of vascular plants. In Colobanthus quitensis, light-saturated photosynthesis measured at 400 ppm was reduced when plants were grown in the elevated CO2 or in the nocturnal warming treatments. Growth in elevated CO2 reduced stomatal conductance but nocturnal warming did not. The short-term sensitivity of respiration, relative protein abundance, and mitochondrial traits were not responsive to either treatment in this species. Moreover, some acclimation to nocturnal warming at ambient CO2 was observed. Altogether, these responses in C. quitensis led to an increase in the respiration-assimilation ratio in plants grown in elevated CO2. The response of Deschampsia antarctica to the experimental treatments was quite distinct. Photosynthesis was not affected by either treatment; however, respiration acclimated to temperature in the elevated CO2 treatment. The observed short-term changes in thermal sensitivity indicate type I acclimation of respiration. Growth in elevated CO2 and nocturnal warming resulted in a reduction in mitochondrial numbers and an increase in mitochondrial size in D. antarctica. Overall, our results suggest that with climate change D. antarctica could be more successful than C. quitensis, due to its ability to make metabolic adjustments to maintain its carbon balance.

Acclimation to elevated CO2 is improved by low Rubisco and carbohydrate content, and enhanced Rubisco transcripts in the G132 barley mutant

Variations induced by mutagenesis may identify targets for adapting to rising atmospheric CO2 concentrations. G132 is a barley mutant with strong decreases in photosynthesis, Rubisco and carbohydrate content, but increased Rubisco transcripts. We tested the hypothesis that G132 will record a greater stimulation of photosynthesis and dry mass gain than the wild-type (WT) in response to CO2 enrichment. A growth chamber experiment compared the mutant G132 and the WT developed in ambient (∼390μmolmol−1) and elevated (1200μmolmol−1) CO2. Photosynthesis, protein and carbohydrate contents, Rubisco and nitrate reductase activities, leaf morphology, plant dry matter and leaf area were determined. The transcriptome responses of fully expanded leaves of G132 and WT to elevated CO2 were investigated using microarrays and qRT-PCR. Growth in elevated CO2 stimulated photosynthesis more in G132 than in WT, in association with a lack of Rubisco down-regulation and lower carbohydrate levels in G132. Electron transport-related proteins increased in G132 and decreased in WT. Elevated CO2 induced more changes in gene expression in WT than G132. Clustering of the corresponding transcripts showed that the expression of genes involved in carbohydrate synthesis and nitrogen assimilation was enhanced, while that for stress-related genes was repressed in both genotypes, and gene expression for jasmonate metabolism was activated in G132. Elevated CO2 decreased the transcript levels for a greater number of transcription factors in WT than in G132. CO2 enrichment increased leaf area in G132 but not in WT, and induced greater dry mass increases in the mutant. The low Rubisco and carbohydrate content and enhanced Rubisco transcripts recorded in G132 are traits that may improve adaptation to rising CO2.

Nitrate supply and plant development influence nitrogen uptake and allocation under elevated CO2 in durum wheat grown hydroponically

Growth in elevated CO2 often leads to decreased plant nitrogen contents and down-regulation of photosynthetic capacity. Here, we investigated whether elevated CO2 limits nitrogen uptake when nutrient movement to roots is unrestricted, and the dependence of this limitation on nitrogen supply and plant development in durum wheat (Triticum durum Desf.). Plants were grown hydroponically at two N supplies and ambient and elevated CO2 concentrations. Elevated CO2 decreased nitrate uptake per unit root mass with low N supply at early grain filling, but not at anthesis. This decrease was not associated with higher nitrate or amino acid, or lower non-structural carbohydrate contents in roots. At anthesis, elevated CO2 decreased the nitrogen content of roots with both levels of N and that of aboveground organs with high N. With low N, elevated CO2 increased N allocation to aboveground plant organs and nitrogen concentration per unit flag leaf area at anthesis, and per unit aboveground dry mass at both growth stages. The results from the hydroponic experiment suggest that elevated CO2 restricts nitrate uptake late in development, high N supply overriding this restriction. Increased nitrogen allocation to young leaves at low N supply could alleviate photosynthetic acclimation to elevated CO2.

Involvement of nitrogen and cytokinins in photosynthetic acclimation to elevated CO2 of spring wheat

Acclimation of photosynthetic capacity to elevated CO2 involves a decrease of the leaf Rubisco content. In the present study, it was hypothesized that nitrogen uptake and partitioning within the leaf and among different aboveground organs affects the down-regulation of Rubisco. Given the interdependence of nitrogen and cytokinin signals at the whole plant level, it is also proposed that cytokinins affect the nitrogen economy of plants under elevated CO2, and therefore the acclimatory responses. Spring wheat received varying levels of nitrogen and cytokinin in field chambers with ambient (370μmolmol−1) or elevated (700μmolmol−1) atmospheric CO2. Gas exchange, Rubisco, soluble protein and nitrogen contents were determined in the top three leaves in the canopy, together with total nitrogen contents per shoot. Growth in elevated CO2 induced decreases in photosynthetic capacity only when nitrogen supply was low. However, the leaf contents of Rubisco, soluble protein and total nitrogen on an area basis declined in elevated CO2 regardless of nitrogen supply. Total nitrogen in the shoot was no lower in elevated than ambient CO2, but the fraction of this nitrogen located in flag and penultimate leaves was lower in elevated CO2. Decreased Rubisco: chlorophyll ratios accompanied losses of leaf Rubisco with CO2 enrichment. Cytokinin applications increased nitrogen content in all leaves and nitrogen allocation to senescing leaves, but decreased Rubisco contents in flag leaves at anthesis and in all leaves 20 days later, together with the amount of Rubisco relative to soluble protein in all leaves at both growth stages. The results suggest that down regulation of Rubisco in leaves at elevated CO2 is linked with decreased allocation of nitrogen to the younger leaves and that cytokinins cause a fractional decrease of Rubisco and therefore do not alleviate acclimation to elevated CO2.

The effects of CO2 and nutrient enrichment on photosynthesis and growth of Poa annua in two consecutive generations

AbstractWe studied short‐ and long‐term growth responses of Poa annua L. (Gramineae) at ambient and elevated (ambient +200 μmol mol−1) atmospheric CO2. In experiment 1 we compared plant growth during the early, vegetative and final, reproductive growth phases. Plant growth in elevated CO2 was significantly enhanced during the early phase, but this was reversed in the reproductive phase. Seed mass and percentage germination were significantly reduced in elevated CO2. Experiment 2 tested for the impact of transgenerational and nutrient effects on the response of Poa annua to elevated CO2. Plants were grown at ambient and elevated CO2 for one or two consecutive generations at three soil nutrient levels. Leaf photosynthesis was significantly higher at elevated CO2, but was also affected by both soil nutrient status and plant generation. Plants grown at elevated CO2 and under conditions of low nutrient availability showed photosynthetic acclimation after 12 weeks of growth but not after 6 weeks. First‐generation growth remained unaffected by elevated CO2, while second‐generation plants produced significantly more tillers and flowers when grown in elevated CO2 compared to ambient conditions. This effect was strongest at low nutrient availability. Average above‐ and belowground biomass after 12 weeks of growth was enhanced in elevated CO2 during both generations, but more so during plant generation 2. This study demonstrates the importance of temporal/maternal effects in plant responses to elevated CO2.

Future CO2 concentrations, though not warmer temperatures, enhance wheat photosynthesis temperature responses

The temperature dependence of C3 photosynthesis is known to vary according to the growth environment. Atmospheric CO2 concentration and temperature are predicted to increase with climate change. To test whether long-term growth in elevated CO2 and temperature modifies photosynthesis temperature response, wheat (Triticum aestivum L.) was grown in ambient CO2 (370 micromol mol(-1)) and elevated CO2 (700 micromol mol(-1)) combined with ambient temperatures and 4 degrees C warmer ones, using temperature gradient chambers in the field. Flag leaf photosynthesis was measured at temperatures ranging from 20 to 35 degrees C and varying CO2 concentrations between ear emergence and anthesis. The maximum rate of carboxylation was determined in vitro in the first year of the experiment and from the photosynthesis-intercellular CO2 response in the second year. With measurement CO2 concentrations of 330 micromol mol(-1) or lower, growth temperature had no effect on flag leaf photosynthesis in plants grown in ambient CO2, while it increased photosynthesis in elevated growth CO2. However, warmer growth temperatures did not modify the response of photosynthesis to measurement temperatures from 20 to 35 degrees C. A central finding of this study was that the increase with temperature in photosynthesis and the photosynthesis temperature optimum were significantly higher in plants grown in elevated rather than ambient CO2. In association with this, growth in elevated CO2 increased the temperature response (activation energy) of the maximum rate of carboxylation. The results provide field evidence that growth under CO2 enrichment enhances the response of Rubisco activity to temperature in wheat.

Transcriptional Analysis of the Bacillus anthracis Capsule Regulators

The poly-d-glutamic acid capsule of Bacillus anthracis is essential for virulence. Control of capsule synthesis occurs at the level of transcription and involves positive regulation of the capsule biosynthetic operon capBCAD by a CO2/bicarbonate signal and three plasmid-borne regulators: atxA, acpA, and acpB. Although the molecular mechanism for control of cap transcription is unknown, atxA affects cap expression via positive control of acpA and acpB, two genes with partial functional similarity. Transcriptional analyses of a genetically complete strain indicate that capB expression is several hundred-fold higher during growth in 5% CO2 compared to growth in air. atxA was expressed appreciably during growth in air and induced only 2.5-fold by CO2. In contrast, expression of acpA and acpB was induced up to 23-fold and 59-fold, respectively, by CO2. The 5'-end mapping of gene transcripts revealed atxA-regulated and atxA-independent apparent transcription start sites for capB, acpA, and acpB. Transcripts mapping to all atxA-regulated start sites were increased during growth in elevated CO2. The acpA gene has one atxA-regulated and one atxA-independent start site. acpB lies downstream of capBCAD. A single atxA-independent start site maps immediately upstream of acpB. atxA-mediated control of acpB appears to occur via transcriptional read-through from atxA-dependent start sites 5' of capB. One atxA-independent and two atxA-regulated start sites map upstream of capB. Transcription from the atxA-regulated start sites of capBCAD was reduced significantly in an acpA acpB double mutant but unaffected in mutants with deletion of only acpA or acpB, in agreement with the current model for epistatic relationships between the regulators.

No Down‐Regulation of Leaf Photosynthesis in Mature Forest Trees after Three Years of Exposure to Elevated CO2

The photosynthetic responses of six species of mature forest trees to long-term exposure to elevated CO2 (ca. 530 ppm) were determined at the Swiss Canopy Crane (SCC) site near Basel, Switzerland. In the third year of growth in elevated CO2, using web-FACE technology, net photosynthesis (As) in fully sunlit, upper canopy foliage was stimulated by ca. 40% compared to ambient controls. This enhancement did not differ from the instantaneous increase in As found in ambient-grown leaves that were temporarily measured at elevated CO2. A complete lack of down-regulation of photosynthesis was found in all species and in both the early and the late growing season. Neither was leaf nitrogen content significantly affected by long-term exposure to elevated CO2. Our results document a persistent enhancement in leaf level photosynthesis in response to elevated CO2 in mature forest trees over a period of three years. Circumstantial evidence suggests that the additional assimilates feed into large sinks other than stem and shoot growth.

Physiological ecology of Mesozoic polar forests in a high CO2 environment.

Fossils show that coniferous forests extended into polar regions during the Mesozoic, a time when models and independent paleo-CO2 indicators suggest that the atmospheric CO2 concentration was at least double that of the present day. Consequently, such polar forests would have experienced high CO2 interacting with an extreme variation in light. Here we describe an experiment investigating this plant-environment interaction for extant tree species that were important components of polar forests, and give results from the first year of treatment. Specifically, we tested the hypotheses that growth in elevated CO2 (1) stimulates photosynthesis; (2) reduces photoinhibition during the polar summer; and (3) reduces respiration of above- and below-ground plant organs. Our results indicate that CO2 fertilization generally does not affect photosynthesis under continuous daylight characteristic of the polar summer but does increase it when the period of illumination is shorter. Growth in elevated CO2 did not alter the potential for photoinhibition. CO2 enrichment significantly reduced leaf and root respiration rates by 50 and 25 %, respectively, in a range of evergreen taxa. Incorporating these observed CO2 effects into numerical simulations using a process-based model of coniferous forest growth indicates that a high paleo-CO2 concentration would have increased the productivity of Cretaceous conifer forests in northern Alaska. This results from decreased respiratory costs that more than compensate for the absence of high CO2-high temperature interactions during the polar summer. The longer-term effects of CO2 enrichment on seasonal changes in the above- and below-ground carbon balance of trees are discussed.

Growth in Elevated CO2 Can Both Increase and Decrease Photochemistry and Photoinhibition of Photosynthesis in a Predictable Manner. Dactylis glomerata Grown in Two Levels of Nitrogen Nutrition

Biochemically based models of C(3) photosynthesis can be used to predict that when photosynthesis is limited by the amount of Rubisco, increasing atmospheric CO(2) partial pressure (pCO(2)) will increase light-saturated linear electron flow through photosystem II (J(t)). This is because the stimulation of electron flow to the photosynthetic carbon reduction cycle (J(c)) will be greater than the competitive suppression of electron flow to the photorespiratory carbon oxidation cycle (J(o)). Where elevated pCO(2) increases J(t), then the ratio of absorbed energy dissipated photochemically to that dissipated non-photochemically will rise. These predictions were tested on Dactylis glomerata grown in fully controlled environments, at either ambient (35 Pa) or elevated (65 Pa) pCO(2), and at two levels of nitrogen nutrition. As was predicted, for D. glomerata grown in high nitrogen, J(t) was significantly higher in plants grown and measured at elevated pCO(2) than for plants grown and measured at ambient pCO(2). This was due to a significant increase in J(c) exceeding any suppression of J(o). This increase in photochemistry at elevated pCO(2) protected against photoinhibition at high light. For plants grown at low nitrogen, J(t) was significantly lower in plants grown and measured at elevated pCO(2) than for plants grown and measured at ambient pCO(2). Elevated pCO(2) again suppressed J(o); however growth in elevated pCO(2) resulted in an acclimatory decrease in leaf Rubisco content that removed any stimulation of J(c). Consistent with decreased photochemistry, for leaves grown at low nitrogen, the recovery from a 3-h photoinhibitory treatment was slower at elevated pCO(2).

Growth in Elevated CO2 Can Both Increase and Decrease Photochemistry and Photoinhibition of Photosynthesis in a Predictable Manner. Dactylis glomerata Grown in Two Levels of Nitrogen Nutrition

Growth in Elevated CO2 Can Both Increase and Decrease Photochemistry and Photoinhibition of Photosynthesis in a Predictable Manner. Dactylis glomerata Grown in Two Levels of Nitrogen Nutrition

The onset of photosynthetic acclimation to elevated CO2 partial pressure in field‐grown Pinus radiata D. Don. after 4 years

ABSTRACTThe effects of CO2 enrichment on photosynthesis and ribulose‐1,5‐bisphosphate carboxylase/oxygenase (rubisco) were studied in current year and 1‐year‐old needles of the same branch of field‐grown Pinus radiata D. Don trees. All measurements were made in the fourth year of growth in large, open‐top chambers continuously maintained at ambient (36 Pa) or elevated (65 Pa) CO2 partial pressures. Photosynthetic rates of the 1‐year‐old needles made at the growth CO2 partial pressure averaged 10·5 ± 0·5 μmol m−2 s−1 in the 36 Pa grown trees and 11·8 ± 0·4 μmol m−2 s−1 in the 65 Pa grown trees, and were not significantly different from each other. The photosynthetic capacity of 1‐year‐old needles was reduced by 25% from 23·0 ± 1·8 μmol m−2 s−1 in the 36 Pa CO2 grown trees to 17·3 ± 0·7 μmol m−2 s−1 in the 65 Pa grown trees. Growth in elevated CO2 also resulted in a 25% reduction in Vcmax (maximum carboxylation rate), a 23% reduction in Jmax (RuBP regeneration capacity mediated by maximum electron transport rate) and a 30% reduction in Rubisco activity and content. Total non‐structural carbohydrates (TNC) as a fraction of total dry mass increased from 12·8 ± 0·4% in 1‐year‐old needles from the 36 Pa grown trees to 14·2 ± 0·7% in 1‐year‐old needles from the 65 Pa grown trees and leaf nitrogen content decreased from 1·30 ± 0·02 to 1·09 ± 0·10 g m−2. The current‐year needles were not of sufficient size for gas exchange measurements, but none of the biochemical parameters measured (Rubisco, leaf chlorophyll, TNC and N), were effected by growth in elevated CO2. These results demonstrate that photosynthetic acclimation, which was not found in the first 2 years of this experiment, can develop over time in field‐grown trees and may be regulated by source‐sink balance, sugar feedback mechanisms and nitrogen allocation.

Long-term growth of Ginkgo with CO(2) enrichment increases leaf ice nucleation temperatures and limits recovery of the photosynthetic system from freezing.

The importance of subzero temperature interactions with elevated CO(2) on plant carbon metabolism has received rather little attention, despite their likely role in influencing future vegetation productivity and dynamics. Here we focused on the critical issues of CO(2)-enrichment effects on leaf-freezing temperatures, subsequent membrane damage, and recovery of the photosynthetic system. We show that growth in elevated CO(2) (70 Pa) results in a substantial and significant (P<0.01) increase (up to 4 degrees C) in the ice nucleation temperature of leaves of Maidenhair tree (Ginkgo biloba), which was observed consistently throughout the 1999 growing season relative to their ambient CO(2) (35 Pa) counterparts. We suggest that increased sensitivity of leaves to ice damage after growth in elevated CO(2) provides an explanation for increased photoinhibition observed in the field early and late in the growing season when low nighttime temperatures are experienced. This new mechanism is proposed in addition to the earlier postulated explanation for this phenomenon involving a reduction in the rate of triose-P utilization owing to a decrease in the rate of carbohydrate export from the leaf.

Elevated CO2 stimulates associative N2 fixation in a C3 plant of the Chesapeake Bay wetland

ABSTRACTIn this study, the response of N2 fixation to elevated CO2 was measured in Scirpus olneyi, a C3 sedge, and Spartina patens, a C4 grass, using acetylene reduction assay and 15N2 gas feeding. Field plants grown in PVC tubes (25 cm long, 10 cm internal diameter) were used. Exposure to elevated CO2 significantly (P&lt; 0·05) caused a 35% increase in nitrogenase activity and 73% increase in 15N incorporated by Scirpus olneyi. In Spartina patens, elevated CO2 (660 ± 1 μmol mol−1) increased nitrogenase activity and 15N incorporation by 13 and 23%, respectively. Estimates showed that the rate of N2 fixation in Scirpus olneyi under elevated CO2 was 611 ± 75 ng 15N fixed plant−1 h−1 compared with 367 ± 46 ng 15N fixed plant−1 h−1 in ambient CO2 plants. In Spartina patens, however, the rate of N2 fixation was 12·5 ± 1·1 versus 9·8 ± 1·3 ng 15N fixed plant−1 h−1 for elevated and ambient CO2, respectively. Heterotrophic non‐symbiotic N2 fixation in plant‐free marsh sediment also increased significantly (P&lt; 0·05) with elevated CO2. The proportional increase in 15N2 fixation correlated with the relative stimulation of photosynthesis, in that N2 fixation was high in the C3 plant in which photosynthesis was also high, and lower in the C4 plant in which photosynthesis was relatively less stimulated by growth in elevated CO2. These results are consistent with the hypothesis that carbon fixation in C3 species, stimulated by rising CO2, is likely to provide additional carbon to endophytic and below‐ground microbial processes.

Photosynthetic sunfleck utilization potential of understory saplings growing under elevated CO2 in FACE.

Few studies have evaluated elevated CO2 responses of trees in variable light despite its prevalence in forest understories and its potential importance for sapling survival. We studied two shade-tolerant species (Acer rubrum, Cornus florida) and two shade-intolerant species (Liquidambar styraciflua, Liriodendron tulipifera) growing in the understory of a Pinus taeda plantation under ambient and ambient+200 ppm CO2 in a free air carbon enrichment (FACE) experiment. Photosynthetic and stomatal responses to artificial changes in light intensity were measured on saplings to determine rates of induction gain under saturating light and induction loss under shade. We expected that growth in elevated CO2 would alter photosynthetic responses to variable light in these understory saplings. The results showed that elevated CO2 caused the expected enhancement in steady-state photosynthesis in both high and low light, but did not affect overall stomatal conductance or rates of induction gain in the four species. Induction loss after relatively short shade periods (<6 min) was slower in trees grown in elevated CO2 than in trees grown in ambient CO2 despite similar decreases in stomatal conductance. As a result leaves grown in elevated CO2 that maintained induction well in shade had higher carbon gain during subsequent light flecks than was expected from steady-state light response measurements. Thus, when frequent sunflecks maintain stomatal conductance and photosynthetic induction during the day, enhancements of long-term carbon gain by elevated CO2 could be underestimated by steady-state photosynthetic measures. With respect to species differences, both a tolerant, A. rubrum, and an intolerant species, L. tulipifera, showed rapid induction gain, but A. rubrum also lost induction rapidly (c. 12 min) in shade. These results, as well as those from independent studies in the literature, show that induction dynamics are not closely related to species shade tolerance. Therefore, it cannot be concluded that shade-tolerant species necessarily induce faster in the variable light conditions common in understories. Although our study is the first to examine dynamic photosynthetic responses to variable light in contrasting species in elevated CO2, studies on ecologically diverse species will be required to establish whether shade-tolerant and -intolerant species show different photosynthetic responses in elevated CO2 during sunflecks. We conclude that elevated CO2 affects dynamic gas exchange most strongly via photosynthetic enhancement during induction as well as in the steady state.

Photosynthetic responses to CO2 enrichment of four hardwood species in a forest understory.

We compared the CO2- and light-dependence of photosynthesis of four tree species (Acer rubrum, Carya glabra, Cercis canadensis, Liquidambar styraciflua) growing in the understory of a loblolly pine plantation under ambient or ambient plus 200 µl l-1 CO2. Naturally-established saplings were fumigated with a free-air CO2 enrichment system. Light-saturated photosynthetic rates were 159-190% greater for Ce. canadensis saplings grown and measured under elevated CO2. This species had the greatest CO2 stimulation of photosynthesis. Photosynthetic rates were only 59% greater for A. rubrum saplings under CO2 enrichment and Ca. glabra and L. styraciflua had intermediate responses. Elevated CO2 stimulated light-saturated photosynthesis more than the apparent quantum yield. The maximum rate of carboxylation of ribulose-1,5-bisphosphate carboxylase, estimated from gas-exchange measurements, was not consistently affected by growth in elevated CO2. However, the maximum electron transport rate estimated from gas- exchange measurements and from chlorophyll fluorescence, when averaged across species and dates, was approximately 10% higher for saplings in elevated CO2. The proportionately greater stimulation of light-saturated photosynthesis than the apparent quantum yield and elevated rates of maximum electron transport suggests that saplings growing under elevated CO2 make more efficient use of sunflecks. The stimulation of light-saturated photosynthesis by CO2 did not appear to correlate with shade-tolerance ranking of the individual species. However, the species with the greatest enhancement of photosynthesis, Ce. canadensis and L. styraciflua, also invested the greatest proportion of soluble protein in Rubisco. Environmental and endogenous factors affecting N partitioning may partially explain interspecific variation in the photosynthetic response to elevated CO2.

Photosynthetic characteristics of 10 Acacia species grown under ambient and elevated atmospheric CO2

Ten contrasting Acacia species were grown in glasshouses with normal ambient CO2 or ele-vated to 700 µL L–1. Plants were grown in sand with a complete nutrient solution, including 5 mМ nitrate. Our objective was to determine the degree to which photosynthesis, and the efficiency of nitrogen and water use, were affected by growth under elevated CO2 in contrasting plant species that differ in specific foliage area (foliage area per unit foliage dry mass). Photosynthetic characteristics were measured at several stages. Growth and measurement of gas exchange under 700 mL L–1 CO2 resulted in enhanced rates of CO2 assimilation per unit foliage area in nine of the species. The degree of enhancement was independent of specific foliage area. The exception was the slow-growing A. aneura, which had lower rates of CO2 assimilation when grown and measured at 700 µL L–1 CO2 compared to plants grown and measured at 350 µL L–1 CO2, at 50, 78 and 93 d after transplanting. Leaf conductance was reduced by growth in elevated CO2 in only six of the species. Overall, elevated CO2 improved the ratio of CO2 assimi-lation to conductance by 78% and increased CO2 assimilation per unit of foliage nitrogen by 30% at a given specific foliage area. Detailed study of A. saligna and A. aneura revealed that the effects of the CO2 treatment were similarly evident on all fully expanded phyllodes, regardless of their age. Intercellular CO2 response curves were analysed on four species and revealed no change in the ratio of electron transport to Rubisco activities. However, for A. aneura and A. melanoxylon, both electron transport and Rubisco activities were reduced per unit foliage nitrogen, by growth under elevated CO2 . For A. saligna and A. implexa, these activities per unit nitrogen, were not altered by the elevated CO2 treatment. To relate CO2 assimilation rates to net assimilation rates (dry matter increment per unit foliage area per day) derived from growth analysis, between 30 and 50% of daily photosynthesis appeared to be consumed in respiration. This proportion was not altered by CO2 treatment for seven of the Acacia species, but appeared to be reduced in the other three. The increase in CO2 assimilation rate by growth under 700 com-pared to 350 µL L–1 CO2 that was measured (26%, mean of all species from two surveys), matched the increase in net assimilation rate that had been derived from destructive sampling (30%). We conclude that the increase in CO2 assimilation rate in the selected Acacia species was independent of species, growth rate and foliage area per unit foliage dry mass.

Elevated CO2and plant structure: a review

SummaryConsequences of increasing atmospheric CO2concentration on plant structure, an important determinant of physiological and competitive success, have not received sufficient attention in the literature. Understanding how increasing carbon input will influence plant developmental processes, and resultant form, will help bridge the gap between physiological response and ecosystem level phenomena. Growth in elevated CO2alters plant structure through its effects on both primary and secondary meristems of shoots and roots. Although not well established, a review of the literature suggests that cell division, cell expansion, and cell patterning may be affected, driven mainly by increased substrate (sucrose) availability and perhaps also by differential expression of genes involved in cell cycling (e.g. cyclins) or cell expansion (e.g. xyloglucan endotransglycosylase). Few studies, however, have attempted to elucidate the mechanistic basis for increased growth at the cellular level. Regardless of specific mechanisms involved, plant leaf size and anatomy are often altered by growth in elevated CO2, but the magnitude of these changes, which often decreases as leaves mature, hinges upon plant genetic plasticity, nutrient availability, temperature, and phenology. Increased leaf growth results more often from increased cell expansion rather than increased division. Leaves of crop species exhibit greater increases in leaf thickness than do leaves of wild species. Increased mesophyll and vascular tissue cross‐sectional areas, important determinates of photosynthetic rates and assimilate transport capacity, are often reported. Few studies, however, have quantified characteristics more reflective of leaf function such as spatial relationships among chlorenchyma cells (size, orientation, and surface area), intercellular spaces, and conductive tissue. Greater leaf size and/or more leaves per plant are often noted; plants grown in elevated CO2exhibited increased leaf area per plant in 66% of studies, compared to 28% of observations reporting no change, and 6% reported a decrease in whole plant leaf area. This resulted in an average net increase in leaf area per plant of 24%. Crop species showed the greatest average increase in whole plant leaf area (+ 37%) compared to tree species (+ 14%) and wild, nonwoody species (+ 15%). Conversely, tree species and wild, nontrees showed the greatest reduction in specific leaf area (– 14% and – 20%) compared to crop plants (– 6%). Alterations in developmental processes at the shoot apex and within the vascular cambium contributed to increased plant height, altered branching characteristics, and increased stem diameters. The ratio of internode length to node number often increased, but the length and sometimes the number of branches per node was greater, suggesting reduced apical dominance. Data concerning effects of elevated CO2on stem/branch anatomy, vital for understanding potential shifts in functional relationships of leaves with stems, roots with stems, and leaves with roots, are too few tomake generalizations. Growth in elevated CO2typically leads to increased root length, diameter, and altered branching patterns. Altered branching characteristics in both shoots and roots may impact competitive relationships above and below the ground. Understanding how increased carbon assimilation affects growth processes (cell division, cell expansion, and cell patterning) will facilitate a better understanding of how plant form will change as atmospheric CO2increases. Knowing how basic growth processes respond to increased carbon inputs may also provide a mechanistic basis for the differential phenotypic plasticity exhibited by different plant species/functional types to elevated CO2.

Photosystem-II Activity Is Decreased by Yellowing of Barley Primary Leaves during Growth in Elevated Carbon Dioxide.

Leaf yellowing was studied in 10-18-d-old barley seedlings (Hordeum vulgare L. cv. Brant) grown at ambient (38 Pa) and at elevated (68, 100, and 140 Pa) CO2 partial pressures in controlled-environment chambers. Maximal total chlorophyll (Chl) concentrations of primary leaves from all four CO2 growth treatments were 0.36+/-0.01 g m-2, and these concentrations were observed 10-12 d after sowing (DAS). Total Chl levels in primary leaves were 35%, 64%, and 78% below maximal levels in the 38, 68, and 100 Pa CO2 growth treatments, respectively, when measured 18 DAS. Losses of Chl in 18-d-old primary leaves were similar in the 100 and 140 Pa CO2 treatments. Decreases of Chl a and Chl b in response to CO2 enrichment were comparable in isolated chloroplast preparations and in intact 18-d-old barley primary leaves of plants grown at 38 and 68 Pa CO2. Total thylakoid membrane proteins, the Chl a/b binding protein (LHC-II), and D1 protein levels were also lower in chloroplast preparations from plants grown in the elevated compared to the ambient CO2 treatment. Both ferricyanide reduction and whole-chain electron-transport rates (H2O --> methylviologen) were significantly lower for chloroplasts from plants grown at 68 Pa CO2 compared with those grown at 38 Pa CO2. However, photosystem-I-dependent chloroplast photoreductions did not differ between CO2 treatments. The results indicated that the CO2-dependent yellowing of barley primary leaves adversely affected photosystem-II activity. Growth in elevated CO2 may have increased the susceptibility of photosystem-II to light damage.

Effects of long‐term exposure to elevated CO2 and increased nutrient supply on bracken (Pteridium aquilinum)

1. Bracken (Pteridium aquilinum) is an important fern with a global distribution. Little is known of the response of this species to elevated CO2. We investigated the effects of high CO2 (570 compared with 370 μmol mol–1) with and without an increased nutrient supply (a combined N, P, K application) on the growth and physiology of bracken, growing in containers in controlled‐environment glasshouses, over two full growing seasons. Results of growth and physiology determinations are reported for the second season. 2. Elevated CO2 had little impact on the growth or allocation of dry mass in bracken. No significant changes were detected in dry mass of the total plant or any of the organs: rhizomes, roots and fronds. In contrast to the small effects of high CO2, the high nutrient treatment caused a three‐fold stimulation of total plant dry mass and an increase in the allocation of dry mass to above ground when compared with low nutrient controls. 3. Net photosynthetic rates in saturating light were increased by both high CO2 and nutrient treatments, particularly in spring months (May and June). Growth in elevated CO2 did not cause a down‐regulation in light‐saturated rates of photosynthesis. The increased carbon gain in the high CO2 treatments was accompanied, in the low‐nutrient plants, by higher concentrations of carbohydrates. However, in high‐nutrient plants the CO2 treatment did not cause an accumulation of carbohydrates. The absence of a growth response to elevated CO2 in bracken despite significant increases in photosynthesis requires further investigation.