Respiratory oxygen uptake is not decreased by an instantaneous elevation of [CO2], but is increased with long-term growth in the field at elevated [CO2]

Measurement of Photorespiratory Oxygen Exchange by Membrane Inlet Mass Spectrometry

Photosynthesis and metabolism in plants involve oxygen as both a product and substrate. Oxygen is taken up during photorespiration and respiration and produced through water splitting during photosynthesis. To distinguish between processes that produce or consume O2 in leaves, isotope mass separation and detection by mass spectrometry allows measurement of evolution and uptake of O2 as well as CO2 uptake. This chapter describes how to calculate the rate of Rubisco oxygenation and carboxylation from in vivo gas exchange of stable isotopes of 16O2 and 18O2 with a closed cuvette system for leaf discs and membrane inlet mass spectrometry.

Measurement of O2 Uptake and Evolution in Leaves In Vivo Using Stable Isotopes and Membrane Inlet Mass Spectrometry

Oxygen is both product and substrate of photosynthesis and metabolism in plants, by oxygen evolution through water splitting and uptake by photorespiration and respiration. It is important to investigate these processes simultaneously in leaves, especially in response to environmental variables, such as light and temperature. To distinguish between processes that evolve or take up O2 in leaves in the light, in vivo gas exchange of stable isotopes of oxygen and membrane inlet mass spectrometry is used. A closed-cuvette system for gas exchange of leaf discs is described, using the stable isotopes 16O2 and 18O2, with a semi-permeable membrane gas inlet and isotope mass separation and detection by mass spectrometry. Measurement of evolution and uptake, as well as CO2 uptake, at a range of light levels allows composition of a light response curve, here described for French bean (Phaseolus vulgaris) and maize (Zea mays) leaf discs.

Meta-analysis of the responses of tree and herb to elevated CO2 in Brazil

The CO2 concentration has increased in the atmosphere due to fossil fuel consumption, deforestation, and land-use changes. Brazil represents one of the primary sources of food on the planet and is also the world's largest tropical rainforest, one of the hot spots of biodiversity in the world. In this work, a meta-analysis was conducted to compare several CO2 Brazilian experiments displaying the diversity of plant responses according to life habits, such as trees (79% natives and 21% cultivated) and herbs (33% natives and 67% cultivated). We found that trees and herbs display different responses. The young trees tend to allocate carbon from increased photosynthetic rates and lower respiration in the dark-to organ development, increasing leaves, roots, and stem biomasses. In addition, more starch is accumulated in the young trees, denoting a fine control of carbon metabolism through carbohydrate storage. Herbs increased drastically in water use efficiency, controlled by stomatal conductance, with more soluble sugars, probably with a transient accumulation of carbon primarily stored in seeds as a response to elevated CO2.

Rethinking the potential productivity of crassulacean acid metabolism by integrating metabolic dynamics with shoot architecture, using the example of Agave tequilana

Terrestrial CAM plants typically occur in hot semiarid regions, yet can show high crop productivity under favorable conditions. To achieve a more mechanistic understanding of CAM plant productivity, a biochemical model of diel metabolism was developed and integrated with 3-D shoot morphology to predict the energetics of light interception and photosynthetic carbon assimilation. Using Agave tequilana as an example, this biochemical model faithfully simulated the four diel phases of CO2 and metabolite dynamics during the CAM rhythm. After capturing the 3-D form over an 8-yr production cycle, a ray-tracing method allowed the prediction of the light microclimate across all photosynthetic surfaces. Integration with the biochemical model thereby enabled the simulation of plant and stand carbon uptake over daily and annual courses. The theoretical maximum energy conversion efficiency of Agave spp. is calculated at 0.045-0.049, up to 7% higher than for C3 photosynthesis. Actual light interception, and biochemical and anatomical limitations, reduced this to 0.0069, or 15.6 Mg ha-1  yr-1 dry mass annualized over an 8-yr cropping cycle, consistent with observation. This is comparable to the productivity of many C3 crops, demonstrating the potential of CAM plants in climates where little else may be grown while indicating strategies that could raise their productivity.

Climate change challenges, plant science solutions

Climate change is a defining challenge of the 21st century, and this decade is a critical time for action to mitigate the worst effects on human populations and ecosystems. Plant science can play an important role in developing crops with enhanced resilience to harsh conditions (e.g. heat, drought, salt stress, flooding, disease outbreaks) and engineering efficient carbon-capturing and carbon-sequestering plants. Here, we present examples of research being conducted in these areas and discuss challenges and open questions as a call to action for the plant science community.

Evidence that mitochondrial alternative oxidase respiration supports carbon balance in source leaves of Nicotiana tabacum

Alternative oxidase (AOX) represents a non-energy conserving pathway within the mitochondrial electron transport chain. One potential physiological role of AOX could be to manage leaf carbohydrate amounts by supporting respiratory carbon oxidation reactions. In this study, several approaches tested the hypothesis that AOX1a gene expression in Nicotiana tabacum leaf is enhanced in conditions expected to promote an increased leaf carbohydrate status. These approaches included supplying leaves with exogenous carbohydrates, comparing plants grown at different atmospheric CO₂ concentrations, comparing sink leaves with source leaves, comparing plants with different ratios of source to sink activity, and examining gene expression over the diel cycle. In each case, the pattern of AOX1a gene expression was compared with that of other genes known to respond to carbohydrates and/or other factors related to source:sink activity. These included GPT1 and GPT3 (that encode chloroplast glucose 6-phosphate/phosphate translocators), SPS (that encodes sucrose phosphate synthase), SUT1 (that encodes a sucrose/H⁺ symporter involved in phloem loading) and UCP1 (that encodes a mitochondrial uncoupling protein). The AOX1a transcript amount was higher following the leaf sink-to-source transition, and in plants with higher source relative to sink activity due to increasing plant age. Further, these effects were amplified in plants grown at elevated CO₂ to stimulate source activity, particularly at end-of-day time periods. The AOX1a transcript amount was also higher following treatment of leaves with carbohydrate, in particular sucrose. Overall, the results provide evidence that, while source leaf sucrose accumulation may signal for a down-regulation of sucrose synthesis and transport, it also signals for means to manage the excess cytosolic carbohydrate pools. This includes increased AOX respiration to support carbon oxidation pathways even if energy charge is high, in combination perhaps with some return flux of carbohydrate from cytosol to stroma through the GPT3 translocator. As discussed, these activities could contribute to maintaining plant source:sink balance, as well as photosynthetic and phloem loading capacity.

A meta-analysis of responses of C3 plants to atmospheric CO2 : dose-response curves for 85 traits ranging from the molecular to the whole-plant level

Generalised dose-response curves are essential to understand how plants acclimate to atmospheric CO₂ . We carried out a meta-analysis of 630 experiments in which C₃ plants were experimentally grown at different [CO₂ ] under relatively benign conditions, and derived dose-response curves for 85 phenotypic traits. These curves were characterised by form, plasticity, consistency and reliability. Considered over a range of 200-1200 µmol mol^-1 CO₂ , some traits more than doubled (e.g. area-based photosynthesis; intrinsic water-use efficiency), whereas others more than halved (area-based transpiration). At current atmospheric [CO₂ ], 64% of the total stimulation in biomass over the 200-1200 µmol mol^-1 range has already been realised. We also mapped the trait responses of plants to [CO₂ ] against those we have quantified before for light intensity. For most traits, CO₂ and light responses were of similar direction. However, some traits (such as reproductive effort) only responded to light, others (such as plant height) only to [CO₂ ], and some traits (such as area-based transpiration) responded in opposite directions. This synthesis provides a comprehensive picture of plant responses to [CO₂ ] at different integration levels and offers the quantitative dose-response curves that can be used to improve global change simulation models.

A hybrid kinetic and constraint-based model of leaf metabolism allows predictions of metabolic fluxes in different environments

While flux balance analysis (FBA) provides a framework for predicting steady-state leaf metabolic network fluxes, it does not readily capture the response to environmental variables without being coupled to other modelling formulations. To address this, we coupled an FBA model of 903 reactions of soybean (Glycine max) leaf metabolism with e-photosynthesis, a dynamic model that captures the kinetics of 126 reactions of photosynthesis and associated chloroplast carbon metabolism. Successful coupling was achieved in an iterative formulation in which fluxes from e-photosynthesis were used to constrain the FBA model and then, in turn, fluxes computed from the FBA model used to update parameters in e-photosynthesis. This process was repeated until common fluxes in the two models converged. Coupling did not hamper the ability of the kinetic module to accurately predict the carbon assimilation rate, photosystem II electron flux, and starch accumulation of field-grown soybean at two CO2 concentrations. The coupled model also allowed accurate predictions of additional parameters such as nocturnal respiration, as well as analysis of the effect of light intensity and elevated CO2 on leaf metabolism. Predictions included an unexpected decrease in the rate of export of sucrose from the leaf at high light, due to altered starch-sucrose partitioning, and altered daytime flux modes in the tricarboxylic acid cycle at elevated CO2 . Mitochondrial fluxes were notably different between growing and mature leaves, with greater anaplerotic, tricarboxylic acid cycle and mitochondrial ATP synthase fluxes predicted in the former, primarily to provide carbon skeletons and energy for protein synthesis.

Warming and elevated CO2 alter tamarack C fluxes, growth and mortality: evidence for heat stress-related C starvation in the absence of water stress

Climate warming is increasing the frequency of climate-induced tree mortality events. While drought combined with heat is considered the primary cause of this mortality, little is known about whether moderately high temperatures alone can induce mortality, or whether rising CO2 would prevent mortality at high growth temperatures. We grew tamarack (Larix laricina) under ambient (400 p.p.m.) and elevated (750 p.p.m.) CO2 concentrations combined with ambient, ambient +4 °C and ambient +8 °C growth temperatures to investigate whether high growth temperatures lead to carbon (C) limitations and mortality. Growth at +8 °C led to 40% mortality in the ambient CO2 (8TAC) treatment, but no mortality in the elevated CO2 treatment. Thermal acclimation of respiration led to similar leaf C balances across the warming treatments, despite a lack of photosynthetic acclimation. Photosynthesis was stimulated under elevated CO2, increasing seedling growth, but not leaf C concentrations. However, growth and foliar C concentrations were lowest in the +8 °C treatments, even with elevated CO2. Dying 8TAC seedlings had lower needle C concentrations and lower ratios of photosynthesis to respiration than healthy 8TAC seedlings, indicating that C limitations were likely the cause of seedling mortality under high growth temperatures.

Different Metabolic Roles for Alternative Oxidase in Leaves of Palustrine and Terrestrial Species

The alternative oxidase pathway (AOP) is associated with excess energy dissipation in leaves of terrestrial plants. To address whether this association is less important in palustrine plants, we compared the role of AOP in balancing energy and carbon metabolism in palustrine and terrestrial environments by identifying metabolic relationships between primary carbon metabolites and AOP in each habitat. We measured oxygen isotope discrimination during respiration, gas exchange, and metabolite profiles in aerial leaves of ten fern and angiosperm species belonging to five families organized as pairs of palustrine and terrestrial species. We performed a partial least square model combined with variable importance for projection to reveal relationships between the electron partitioning to the AOP (τa) and metabolite levels. Terrestrial plants showed higher values of net photosynthesis (AN) and τa, together with stronger metabolic relationships between τa and sugars, important for water conservation. Palustrine plants showed relationships between τa and metabolites related to the shikimate pathway and the GABA shunt, to be important for heterophylly. Excess energy dissipation via AOX is less crucial in palustrine environments than on land. The basis of this difference resides in the contrasting photosynthetic performance observed in each environment, thus reinforcing the importance of AOP for photosynthesis.

Three new methods indicate that CO2 concentration affects plant respiration in the range relevant to global change

Short-term responses of plant dark respiration to carbon dioxide concentration ([CO₂]) in the range anticipated in the atmosphere with global change remain controversial, primarily because it is difficult to convincingly eliminate the many possible sources of experimental error in measurements of carbon dioxide or oxygen exchange rates. Plant dark respiration is a major component of the carbon balance of many ecosystems. In seedlings without senescent tissue, the rate of loss of dry mass during darkness indicates the rate of respiration. This method of measuring respiration was used to test for [CO₂] effects on respiration in seedlings of three species with relatively large seeds. The time it took respiration to exhaust substrates and cause seedling death in darkness was used as an indicator of respiration rate in four other species with smaller seeds. The third method was measuring rates of CO₂ exchange in excised petioles sealed in a cuvette submerged in water to prevent leaks. Petioles were utilized as the plant tissue type with the most reliable rates of respiration, for excised tissue. The rate of loss of dry mass in the dark decreased with increasing [CO₂] in the range of 200-800 μmol mol^-1 in all three large-seeded species. The seedling survival time in the dark increased with [CO₂] in the same concentration range in all four of the smaller-seeded species. Respiration rates of excised petioles of several species also decreased over this [CO₂] range. The data provide new evidence that the rate of dark respiration in plant tissue often decreases with increasing [CO₂] in the 200-800 μmol mol^-1 range.

ABA and BAP improve the accumulation of carbohydrates and alter carbon allocation in potato plants at elevated CO2

Elevated CO2 interactions with other factors affects the plant performance. Regarding the differences between cultivars in response to CO2 concentrations, identifying the cultivars that better respond to such conditions would maximize their potential benefits. Increasing the ability of plants to benefit more from elevated CO2 levels alleviates the adverse effects of photoassimilate accumulation on photosynthesis and increases the productivity of plants. Despite its agronomic importance, there is no information about the interactive effects of elevated CO2 concentration and plant growth regulators (PGRs) on potato (Solanum tuberosum L.) plants. Hence, the physiological response and source-sink relationship of potato plants (cvs. Agria and Fontane) to combined application of CO2 levels (400 vs. 800 µmol mol-1) and plant growth regulators (PGR) [6-benzylaminopurine (BAP) + Abscisic acid (ABA)] were evaluated under a controlled environment. The results revealed a variation between the potato cultivars in response to a combination of PGRs and CO2 levels. Cultivars were different in leaf chlorophyll content; Agria had higher chlorophyll a, b, and total chlorophyll content by 23, 43, and 23%, respectively, compared with Fontane. The net photosynthetic rate was doubled at the elevated compared with the ambient CO2. In Agria, the ratio of leaf intercellular to ambient air CO2 concentrations [Ci:Ca] was declined in elevated-CO2-grown plants, which indicated the stomata would become more conservative at higher CO2 levels. On the other hand, the increased Ci:Ca in Fontane showed a stomatal acclimation to higher CO2 concentration. The higher leaf dark respiration of the elevated CO2-grown and BAP + ABA-treated plants was associated with a higher leaf soluble carbohydrates and starch content. Elevated CO2 and BAP + ABA shifted the dry matter partitioning to the belowground more than the above-media organs. The lower leaf soluble carbohydrate content and greater tuber yield in Fontane might indicate a more efficient photoassimilate translocation than Agria. The results highlighted positive synergic effects of the combined BAP + ABA and elevated CO2 on tuber yield and productivity of the potato plants.

Assessing the potential functions of nocturnal stomatal conductance in C3 and C4 plants

Nocturnal stomatal conductance contributes to water loss at night without carbon gain in C₃ or C₄ plants because photosynthesis does not occur in the dark. The functional relevance of nocturnal conductance thus remains an unresolved conundrum. Here, we review and re-analyse previously published datasets on nocturnal conductance (g_n ) globally (176 species) to synthesize our current understanding on its potential biological function and to identify remaining research gaps. We found that g_n was positively correlated with relative growth rate, which is compatible with the postulate that circadian-driven nocturnal conductance enhances predawn stomatal conductance, thereby priming stomata for photosynthesis in early daylight. The variation in g_n across plant species and functional types was not consistent with the hypotheses that the main function of g_n is to: remove excess CO_2, which might limit growth; enhance oxygen delivery to the functional sapwood; enhance nutrient supply; or that g_n is due to stomatal leakiness. We suggest further study regarding the potential of g_n to be an important functional and ecological trait influencing competitive outcomes and we outline a research programme to achieve that objective.

Growth at Elevated CO2 Requires Acclimation of the Respiratory Chain to Support Photosynthesis

Plants will experience an elevated atmospheric concentration of CO2 (ECO2) in the future. Growth of tobacco (Nicotiana tabacum) at ECO2 more than doubled the leaf protein amount of alternative oxidase (AOX), a non-energy-conserving component of mitochondrial respiration. To test the functional significance of this AOX increase, wild-type tobacco was compared with AOX knockdown and overexpression lines, following growth at ambient CO2 or ECO2 The ECO2-grown AOX knockdowns had a reduced capacity for triose phosphate use (TPU) during photosynthesis compared with the other plant lines. This TPU limitation of CO2 assimilation was associated with an increased accumulation of glucose-6-phosphate, sucrose, and starch in the leaves of the knockdowns. Under TPU-limiting conditions, the size of the proton gradient and proton motive force across the thylakoid membrane was enhanced in the knockdowns relative to the other plant lines, suggesting a restriction of chloroplast ATP synthase activity. This restriction was not due to a decline in ATP synthase (AtpB) protein amount. The knockdowns also displayed a photosystem stoichiometry adjustment at ECO2, which was absent in the other plant lines. Additional experiments showed that the way in which AOX supports photosynthesis at ECO2 is distinct from its previously described role in supporting photosynthesis during water deficit. The results are discussed in terms of how AOX contributes to TPU capacity and the maintenance of chloroplast ATP synthase activity at ECO2 Overall, the evidence suggests that AOX respiration is needed to maintain both the carbon and energy balance in photosynthetic tissues during growth at ECO2.

Measurement of O2 Uptake and Evolution in Leaves In Vivo Using Stable Isotopes and Membrane Inlet Mass Spectrometry

Oxygen is both product and substrate of photosynthesis and metabolism in plants, by oxygen evolution through water splitting and uptake by photorespiration and respiration. It is important to investigate these processes simultaneously in leaves, especially in response to environmental variables, such as light and temperature. To distinguish between processes that evolve or take up O₂ in leaves in the light, in vivo gas exchange of stable isotopes of oxygen and membrane inlet mass spectrometry is used. A closed-cuvette system for gas exchange of leaf disks is described, using the stable isotopes ¹⁶O₂ and ¹⁸O₂, with a semipermeable membrane gas inlet and isotope mass separation and detection by mass spectrometry. Measurement of evolution and uptake, as well as CO₂ uptake, at a range of light levels allows composition of a light-response curve, here described for French bean and maize leaf disks.

The Kok effect in Vicia faba cannot be explained solely by changes in chloroplastic CO2 concentration

The Kok effect - an abrupt decline in quantum yield (QY) of net CO₂ assimilation at low photosynthetic photon flux density (PPFD) - is widely used to estimate respiration in the light (R), which assumes the effect is caused by light suppression of R. A recent report suggested much of the Kok effect can be explained by declining chloroplastic CO₂ concentration (c_c ) at low PPFD. Several predictions arise from the hypothesis that the Kok effect is caused by declining c_c , and we tested these predictions in Vicia faba. We measured CO₂ exchange at low PPFD, in 2% and 21% oxygen, in developing and mature leaves, which differed greatly in R in darkness. Our results contradicted each of the predictions based on the c_c effect: QY exceeded the theoretical maximum value for photosynthetic CO₂ uptake; QY was larger in 21% than 2% oxygen; and the change in QY at the Kok effect breakpoint was unaffected by oxygen. Our results strongly suggest the Kok effect arises largely from a progressive decline in R with PPFD that includes both oxygen-sensitive and -insensitive components. We suggest an improved Kok method that accounts for high c_c at low PPFD.

A common thermal niche among geographically diverse populations of the widely distributed tree species Eucalyptus tereticornis: No evidence for adaptation to climate-of-origin

Impacts of climate warming depend on the degree to which plants are constrained by adaptation to their climate-of-origin or exhibit broad climatic suitability. We grew cool-origin, central and warm-origin provenances of Eucalyptus tereticornis in an array of common temperature environments from 18 to 35.5°C to determine if this widely distributed tree species consists of geographically contrasting provenances with differentiated and narrow thermal niches, or if provenances share a common thermal niche. The temperature responses of photosynthesis, respiration, and growth were equivalent across the three provenances, reflecting a common thermal niche despite a 2,200 km geographic distance and 13°C difference in mean annual temperature at seed origin. The temperature dependence of growth was primarily mediated by changes in leaf area per unit plant mass, photosynthesis, and whole-plant respiration. Thermal acclimation of leaf, stem, and root respiration moderated the increase in respiration with temperature, but acclimation was constrained at high temperatures. We conclude that this species consists of provenances that are not differentiated in their thermal responses, thus rejecting our hypothesis of adaptation to climate-of-origin and suggesting a shared thermal niche. In addition, growth declines with warming above the temperature optima were driven by reductions in whole-plant leaf area and increased respiratory carbon losses. The impacts of climate warming will nonetheless vary across the geographic range of this and other such species, depending primarily on each provenance's climate position on the temperature response curves for photosynthesis, respiration, and growth.

Leaf and canopy scale drivers of genotypic variation in soybean response to elevated carbon dioxide concentration

The atmospheric [CO2 ] in which crops grow today is greater than at any point in their domestication history and represents an opportunity for positive effects on seed yield that can counteract the negative effects of greater heat and drought this century. In order to maximize yields under future atmospheric [CO2 ], we need to identify and study crop cultivars that respond most favorably to elevated [CO2 ] and understand the mechanisms contributing to their responsiveness. Soybean (Glycine max Merr.) is a widely grown oilseed crop and shows genetic variation in response to elevated [CO2 ]. However, few studies have studied the physiological basis for this variation. Here, we examined canopy light interception, photosynthesis, respiration and radiation use efficiency along with yield and yield parameters in two cultivars of soybean (Loda and HS93-4118) previously reported to have similar seed yield at ambient [CO2 ], but contrasting responses to elevated [CO2 ]. Seed yield increased by 26% at elevated [CO2 ] (600 μmol/mol) in the responsive cultivar Loda, but only by 11% in HS93-4118. Canopy light interception and leaf area index were greater in HS93-4118 in ambient [CO2 ], but increased more in response to elevated [CO2 ] in Loda. Radiation use efficiency and harvest index were also greater in Loda than HS93-4118 at both ambient and elevated [CO2 ]. Daily C assimilation was greater at elevated [CO2 ] in both cultivars, while stomatal conductance was lower. Electron transport capacity was also greater in Loda than HS93-4118, but there was no difference in the response of photosynthetic traits to elevated [CO2 ] in the two cultivars. Overall, this greater understanding of leaf- and canopy-level photosynthetic traits provides a strong conceptual basis for modeling genotypic variation in response to elevated [CO2 ].

Genomewide transcriptional reprogramming in the seagrass Cymodocea nodosa under experimental ocean acidification

Here, we report the first use of massive-scale RNA-sequencing to explore seagrass response to CO₂ -driven ocean acidification (OA). Large-scale gene expression changes in the seagrass Cymodocea nodosa occurred at CO₂ levels projected by the end of the century. C. nodosa transcriptome was obtained using Illumina RNA-Seq technology and de novo assembly, and differential gene expression was explored in plants exposed to short-term high CO₂ /low pH conditions. At high pCO₂ , there was a significant increased expression of transcripts associated with photosynthesis, including light reaction functions and CO₂ fixation, and also to respiratory pathways, specifically for enzymes involved in glycolysis, in the tricarboxylic acid cycle and in the energy metabolism of the mitochondrial electron transport. The upregulation of respiratory metabolism is probably supported by the increased availability of photosynthates and increased energy demand for biosynthesis and stress-related processes under elevated CO₂ and low pH. The upregulation of several chaperones resembling heat stress-induced changes in gene expression highlighted the positive role these proteins play in tolerance to intracellular acid stress in seagrasses. OA further modifies C. nodosa secondary metabolism inducing the transcription of enzymes related to biosynthesis of carbon-based secondary compounds, in particular the synthesis of polyphenols and isoprenoid compounds that have a variety of biological functions including plant defence. By demonstrating which physiological processes are most sensitive to OA, this research provides a major advance in the understanding of seagrass metabolism in the context of altered seawater chemistry from global climate change.

Atmospheric CO2 mole fraction affects stand-scale carbon use efficiency of sunflower by stimulating respiration in light

Plant carbon-use-efficiency (CUE), a key parameter in carbon cycle and plant growth models, quantifies the fraction of fixed carbon that is converted into net primary production rather than respired. CUE has not been directly measured, partly because of the difficulty of measuring respiration in light. Here, we explore if CUE is affected by atmospheric CO₂ . Sunflower stands were grown at low (200 μmol mol^-1 ) or high CO₂ (1000 μmol mol^-1 ) in controlled environment mesocosms. CUE of stands was measured by dynamic stand-scale ¹³ C labelling and partitioning of photosynthesis and respiration. At the same plant age, growth at high CO₂ (compared with low CO₂ ) led to 91% higher rates of apparent photosynthesis, 97% higher respiration in the dark, yet 143% higher respiration in light. Thus, CUE was significantly lower at high (0.65) than at low CO₂ (0.71). Compartmental analysis of isotopic tracer kinetics demonstrated a greater commitment of carbon reserves in stand-scale respiratory metabolism at high CO₂ . Two main processes contributed to the reduction of CUE at high CO₂ : a reduced inhibition of leaf respiration by light and a diminished leaf mass ratio. This work highlights the relevance of measuring respiration in light and assessment of the CUE response to environment conditions.

Measurement of Mitochondrial Respiration in Isolated Protoplasts: Cytochrome and Alternative Pathways

The electron partitioning between COX and AOX pathways of mitochondria and their coordination is necessary to meet the energy demands as well as to maintain optimized redox status in plants under varying environmental conditions. The relative contribution of these two pathways to total respiration is an important measure during a given stress condition. We describe in detail the procedure that allows the measurement of the parameters of COX and AOX pathway of respiration in mesophyll protoplasts using Clark-type O2 electrode. This chapter also lists the steps for rapid isolation procedure for mesophyll protoplasts from pea leaves. The advantages and limitations of the use of metabolic inhibitors and the protoplasts for measuring the respiration are also briefly discussed.

Metabolic and Transcriptional Analysis of Durum Wheat Responses to Elevated CO2 at Low and High Nitrate Supply

Elevated [CO₂] (eCO₂) can lead to photosynthetic acclimation and this is often intensified by low nitrogen (N). Despite intensive studies of plant responses to eCO₂, the regulation mechanism of primary metabolism at the whole-plant level in interaction with [Formula: see text] supply remains unclear. We examined the metabolic and transcriptional responses triggered by eCO₂ in association with physiological-biochemical traits in flag leaves and roots of durum wheat grown hydroponically in ambient and elevated [CO₂] with low (LN) and high (HN) [Formula: see text] supply. Multivariate analysis revealed a strong interaction between eCO₂ and [Formula: see text] supply. Photosynthetic acclimation induced by eCO₂ in LN plants was accompanied by an increase in biomass and carbohydrates, and decreases of leaf organic N per unit area, organic acids, inorganic ions, Calvin-Benson cycle intermediates, Rubisco, nitrate reductase activity, amino acids and transcripts for N metabolism, particularly in leaves, whereas [Formula: see text] uptake was unaffected. In HN plants, eCO₂ did not decrease photosynthetic capacity or leaf organic N per unit area, but induced transcripts for N metabolism, especially in roots. In conclusion, the photosynthetic acclimation in LN plants was associated with an inhibition of leaf [Formula: see text] assimilation, whereas up-regulation of N metabolism in roots could have mitigated the acclimatory effect of eCO₂ in HN plants.

Is there potential to adapt soybean (Glycine max Merr.) to future [CO₂]? An analysis of the yield response of 18 genotypes in free-air CO₂ enrichment

Rising atmospheric [CO2] is a uniform, global change that increases C3 photosynthesis and could offset some of the negative effects of global climate change on crop yields. Genetic variation in yield responsiveness to rising [CO2] would provide an opportunity to breed more responsive crop genotypes. A multi-year study of 18 soybean (Glycine max Merr.) genotypes was carried out to identify variation in responsiveness to season-long elevated [CO2] (550 ppm) under fully open-air replicated field conditions. On average across 18 genotypes, elevated [CO2] stimulated total above-ground biomass by 22%, but seed yield by only 9%, in part because most genotypes showed a reduction in partitioning of energy to seeds. Over four years of study, there was consistency from year to year in the genotypes that were most and least responsive to elevated [CO2], suggesting heritability of CO2 response. Further analysis of six genotypes did not reveal a photosynthetic basis for the variation in yield response. Although partitioning to seed was decreased, cultivars with the highest partitioning coefficient in current [CO2 ] also had the highest partitioning coefficient in elevated [CO2]. The results show the existence of genetic variation in soybean response to elevated [CO2], which is needed to breed soybean to the future atmospheric environment.

Genome-wide transcriptomic analysis of the effects of sub-ambient atmospheric oxygen and elevated atmospheric carbon dioxide levels on gametophytes of the moss, Physcomitrella patens

It is widely accepted that atmospheric O2 has played a key role in the development of life on Earth, as evident from the coincidence between the rise of atmospheric O2 concentrations in the Precambrian and biological evolution. Additionally, it has also been suggested that low atmospheric O2 is one of the major drivers for at least two of the five mass-extinction events in the Phanerozoic. At the molecular level, our understanding of the responses of plants to sub-ambient O2 concentrations is largely confined to studies of the responses of underground organs, e.g. roots to hypoxic conditions. Oxygen deprivation often results in elevated CO2 levels, particularly under waterlogged conditions, due to slower gas diffusion in water compared to air. In this study, changes in the transcriptome of gametophytes of the moss Physcomitrella patens arising from exposure to sub-ambient O2 of 13% (oxygen deprivation) and elevated CO2 (1500 ppmV) were examined to further our understanding of the responses of lower plants to changes in atmospheric gaseous composition. Microarray analyses revealed that the expression of a large number of genes was affected under elevated CO2 (814 genes) and sub-ambient O2 conditions (576 genes). Intriguingly, the expression of comparatively fewer numbers of genes (411 genes) was affected under a combination of both sub-ambient O2 and elevated CO2 condition (low O2-high CO2). Overall, the results point towards the effects of atmospheric changes in CO2 and O2 on transcriptional reprogramming, photosynthetic regulation, carbon metabolism, and stress responses.

The space-time continuum: the effects of elevated CO2 and temperature on trees and the importance of scaling

To predict how forests will respond to rising temperatures and atmospheric CO₂ concentrations, we need to understand how trees respond to both of these environmental factors. In this review, we discuss the importance of scaling, moving from leaf-level responses to those of the canopy, and from short-term to long-term responses of vegetation to climate change. While our knowledge of leaf-level, instantaneous responses of photosynthesis, respiration, stomatal conductance, transpiration and water-use efficiency to elevated CO₂ and temperature is quite good, our ability to scale these responses up to larger spatial and temporal scales is less developed. We highlight which physiological processes are least understood at various levels of study, and discuss how ignoring differences in the spatial or temporal scale of a physiological process impedes our ability to predict how forest carbon and water fluxes forests will be altered in the future. We also synthesize data from the literature to show that light respiration follows a generalized temperature response across studies, and that the light compensation point of photosynthesis is reduced by elevated growth CO₂. Lastly, we emphasize the need to move beyond single factorial experiments whenever possible, and to combine both CO₂ and temperature treatments in studies of tree performance.

A photorespiratory bypass increases plant growth and seed yield in biofuel crop Camelina sativa

BACKGROUND

Camelina sativa is an oilseed crop with great potential for biofuel production on marginal land. The seed oil from camelina has been converted to jet fuel and improved fuel efficiency in commercial and military test flights. Hydrogenation-derived renewable diesel from camelina is environmentally superior to that from canola due to lower agricultural inputs, and the seed meal is FDA approved for animal consumption. However, relatively low yield makes its farming less profitable. Our study is aimed at increasing camelina seed yield by reducing carbon loss from photorespiration via a photorespiratory bypass. Genes encoding three enzymes of the Escherichia coli glycolate catabolic pathway were introduced: glycolate dehydrogenase (GDH), glyoxylate carboxyligase (GCL) and tartronic semialdehyde reductase (TSR). These enzymes compete for the photorespiratory substrate, glycolate, convert it to glycerate within the chloroplasts, and reduce photorespiration. As a by-product of the reaction, CO2 is released in the chloroplast, which increases photosynthesis. Camelina plants were transformed with either partial bypass (GDH), or full bypass (GDH, GCL and TSR) genes. Transgenic plants were evaluated for physiological and metabolic traits.

RESULTS

Expressing the photorespiratory bypass genes in camelina reduced photorespiration and increased photosynthesis in both partial and full bypass expressing lines. Expression of partial bypass increased seed yield by 50-57 %, while expression of full bypass increased seed yield by 57-73 %, with no loss in seed quality. The transgenic plants also showed increased vegetative biomass and faster development; they flowered, set seed and reached seed maturity about 1 week earlier than WT. At the transcriptional level, transgenic plants showed differential expression in categories such as respiration, amino acid biosynthesis and fatty acid metabolism. The increased growth of the bypass transgenics compared to WT was only observed in ambient or low CO2 conditions, but not in elevated CO2 conditions.

CONCLUSIONS

The photorespiratory bypass is an effective approach to increase photosynthetic productivity in camelina. By reducing photorespiratory losses and increasing photosynthetic CO2 fixation rates, transgenic plants show dramatic increases in seed yield. Because photorespiration causes losses in productivity of most C3 plants, the bypass approach may have significant impact on increasing agricultural productivity for C3 crops.

A photorespiratory bypass increases plant growth and seed yield in biofuel crop Camelina sativa

BACKGROUND

Camelina sativa is an oilseed crop with great potential for biofuel production on marginal land. The seed oil from camelina has been converted to jet fuel and improved fuel efficiency in commercial and military test flights. Hydrogenation-derived renewable diesel from camelina is environmentally superior to that from canola due to lower agricultural inputs, and the seed meal is FDA approved for animal consumption. However, relatively low yield makes its farming less profitable. Our study is aimed at increasing camelina seed yield by reducing carbon loss from photorespiration via a photorespiratory bypass. Genes encoding three enzymes of the Escherichia coli glycolate catabolic pathway were introduced: glycolate dehydrogenase (GDH), glyoxylate carboxyligase (GCL) and tartronic semialdehyde reductase (TSR). These enzymes compete for the photorespiratory substrate, glycolate, convert it to glycerate within the chloroplasts, and reduce photorespiration. As a by-product of the reaction, CO2 is released in the chloroplast, which increases photosynthesis. Camelina plants were transformed with either partial bypass (GDH), or full bypass (GDH, GCL and TSR) genes. Transgenic plants were evaluated for physiological and metabolic traits.

RESULTS

Expressing the photorespiratory bypass genes in camelina reduced photorespiration and increased photosynthesis in both partial and full bypass expressing lines. Expression of partial bypass increased seed yield by 50-57 %, while expression of full bypass increased seed yield by 57-73 %, with no loss in seed quality. The transgenic plants also showed increased vegetative biomass and faster development; they flowered, set seed and reached seed maturity about 1 week earlier than WT. At the transcriptional level, transgenic plants showed differential expression in categories such as respiration, amino acid biosynthesis and fatty acid metabolism. The increased growth of the bypass transgenics compared to WT was only observed in ambient or low CO2 conditions, but not in elevated CO2 conditions.

CONCLUSIONS

The photorespiratory bypass is an effective approach to increase photosynthetic productivity in camelina. By reducing photorespiratory losses and increasing photosynthetic CO2 fixation rates, transgenic plants show dramatic increases in seed yield. Because photorespiration causes losses in productivity of most C3 plants, the bypass approach may have significant impact on increasing agricultural productivity for C3 crops.

Developmental stage specificity of transcriptional, biochemical and CO2 efflux responses of leaf dark respiration to growth of Arabidopsis thaliana at elevated [CO2]

Plant respiration responses to elevated growth [CO(2)] are key uncertainties in predicting future crop and ecosystem function. In particular, the effects of elevated growth [CO(2)] on respiration over leaf development are poorly understood. This study tested the prediction that, due to greater whole plant photoassimilate availability and growth, elevated [CO(2)] induces transcriptional reprogramming and a stimulation of nighttime respiration in leaf primordia, expanding leaves and mature leaves of Arabidopsis thaliana. In primordia, elevated [CO(2)] altered transcript abundance, but not for genes encoding respiratory proteins. In expanding leaves, elevated [CO(2)] induced greater glucose content and transcript abundance for some respiratory genes, but did not alter respiratory CO(2) efflux. In mature leaves, elevated [CO(2)] led to greater glucose, sucrose and starch content, plus greater transcript abundance for many components of the respiratory pathway, and greater respiratory CO(2) efflux. Therefore, growth at elevated [CO(2)] stimulated dark respiration only after leaves transitioned from carbon sinks into carbon sources. This coincided with greater photoassimilate production by mature leaves under elevated [CO(2)] and peak respiratory transcriptional responses. It remains to be determined if biochemical and transcriptional responses to elevated [CO(2)] in primordial and expanding leaves are essential prerequisites for subsequent alterations of respiratory metabolism in mature leaves.

Transcriptional reprogramming and stimulation of leaf respiration by elevated CO2 concentration is diminished, but not eliminated, under limiting nitrogen supply

Plant respiration responses to elevated CO2 concentration ( [CO2 ] ) have been studied for three decades without consensus about the mechanism of response. Positive effects of elevated [CO2 ] on leaf respiration have been attributed to greater substrate supply resulting from stimulated photosynthesis. Negative effects of elevated [CO2 ] on leaf respiration have been attributed to reduced demand for energy for protein turnover assumed to result from lower leaf N content. Arabidopsis thaliana was grown in ambient (370 ppm) and elevated (750 ppm) [CO2 ] with limiting and ample N availabilities. The stimulation of leaf dark respiration was attenuated in limiting N (+12%) compared with ample N supply (+30%). This response was associated with smaller stimulation of photosynthetic CO2 uptake, but not interactive effects of elevated CO2 and N supply on leaf protein, amino acids or specific leaf area. Elevated [CO2 ] also resulted in greater abundance of transcripts for many components of the respiratory pathway. A greater transcriptional response to elevated [CO2 ] was observed in ample N supply at midday versus midnight, consistent with reports that protein synthesis is greatest during the day. Greater foliar expression of respiratory genes under elevated [CO2 ] has now been observed in diverse herbaceous species, suggesting a widely conserved response.

Stimulated leaf dark respiration in tomato in an elevated carbon dioxide atmosphere

It is widely accepted that leaf dark respiration is a determining factor for the growth and maintenance of plant tissues and the carbon cycle. However, the underlying effect and mechanism of elevated CO2 concentrations ([CO2]) on dark respiration remain unclear. In this study, tomato plants grown at elevated [CO2] showed consistently higher leaf dark respiratory rate, as compared with ambient control plants. The increased respiratory capacity was driven by a greater abundance of proteins, carbohydrates, and transcripts involved in pathways of glycolysis carbohydrate metabolism, the tricarboxylic acid cycle, and mitochondrial electron transport energy metabolism. This study provides substantial evidence in support of the concept that leaf dark respiration is increased by elevated [CO2] in tomato plants and suggests that the increased availability of carbohydrates and the increased energy status are involved in the increased rate of dark respiration in response to elevated [CO2].

Direct and indirect effects of elevated atmospheric CO2 on net ecosystem production in a Chesapeake Bay tidal wetland

The rapid increase in atmospheric CO2 concentrations (Ca ) has resulted in extensive research efforts to understand its impact on terrestrial ecosystems, especially carbon balance. Despite these efforts, there are relatively few data comparing net ecosystem exchange of CO2 between the atmosphere and the biosphere (NEE), under both ambient and elevated Ca . Here we report data on annual sums of CO2 (NEE(net) ) for 19 years on a Chesapeake Bay tidal wetland for Scirpus olneyi (C3 photosynthetic pathway)- and Spartina patens (C4 photosynthetic pathway)-dominated high marsh communities exposed to ambient and elevated Ca (ambient + 340 ppm). Our objectives were to (i) quantify effects of elevated Ca on seasonally integrated CO2 assimilation (NEE(net) = NEE(day) + NEE(night) , kg C m(-2) y(-1) ) for the two communities; and (ii) quantify effects of altered canopy N content on ecosystem photosynthesis and respiration. Across all years, NEE(net) averaged 1.9 kg m(-2) y(-1) in ambient Ca and 2.5 kg m(-2) y(-1) in elevated Ca , for the C3 -dominated community. Similarly, elevated Ca significantly (P < 0.01) increased carbon uptake in the C4 -dominated community, as NEE(net) averaged 1.5 kg m(-2) y(-1) in ambient Ca and 1.7 kg m(-2) y(-1) in elevated Ca . This resulted in an average CO2 stimulation of 32% and 13% of seasonally integrated NEE(net) for the C3 - and C4 -dominated communities, respectively. Increased NEE(day) was correlated with increased efficiencies of light and nitrogen use for net carbon assimilation under elevated Ca , while decreased NEE(night) was associated with lower canopy nitrogen content. These results suggest that rising Ca may increase carbon assimilation in both C3 - and C4 -dominated wetland communities. The challenge remains to identify the fate of the assimilated carbon.

Optimal crop canopy architecture to maximise canopy photosynthetic CO2 uptake under elevated CO2 - a theoretical study using a mechanistic model of canopy photosynthesis

Canopy architecture has been a major target in crop breeding for improved yields. Whether crop architectures in current elite crop cultivars can be modified for increased canopy CO2 uptake rate (Ac) under elevated atmospheric CO2 concentrations (Ca) is currently unknown. To study this question, we developed a new model of canopy photosynthesis, which includes three components: (i) a canopy architectural model; (ii) a forward ray tracing algorithm; and (iii) a steady-state biochemical model of C3 photosynthesis. With this model, we demonstrated that the Ac estimated from 'average' canopy light conditions is ~25% higher than that from light conditions at individual points in the canopy. We also evaluated theoretically the influence of canopy architectural on Ac under current and future Ca in rice. Simulation results suggest that to gain an optimal Ac for the examined rice cultivar, the stem height, leaf width and leaf angles can be manipulated to enhance canopy photosynthesis. This model provides a framework for designing ideal crop architectures to gain optimal Ac under future changing climate conditions. A close linkage between canopy photosynthesis modelling and canopy photosynthesis measurements is required to fully realise the potential of such modelling approaches in guiding crop improvements.

Light inhibition of leaf respiration in field-grown Eucalyptus saligna in whole-tree chambers under elevated atmospheric CO2 and summer drought

We investigated whether the degree of light inhibition of leaf respiration (R) differs among large Eucalyptus saligna grown in whole-tree chambers and exposed to present and future atmospheric [CO(2) ] and summer drought. Associated with month-to-month changes in temperature were concomitant changes in R in the light (R(light) ) and darkness (R(dark) ), with both processes being more temperature dependent in well-watered trees than under drought. Overall rates of R(light) and R(dark) were not significantly affected by [CO(2) ]. By contrast, overall rates of R(dark) (averaged across both [CO(2) ]) were ca. 25% lower under drought than in well-watered trees. During summer, the degree of light inhibition of leaf R was greater in droughted (ca. 80% inhibition) than well-watered trees (ca. 50% inhibition). Notwithstanding these treatment differences, an overall positive relationship was observed between R(light) and R(dark) when data from all months/treatments were combined (R(2)  = 0.8). Variations in R(light) were also positively correlated with rates of Rubisco activity and nitrogen concentration. Light inhibition resulted in a marked decrease in the proportion of light-saturated photosynthesis respired (i.e. reduced R/A(sat) ). Collectively, these results highlight the need to account for light inhibition when assessing impacts of global change drivers on the carbon economy of tree canopies.

Accelerating yield potential in soybean: potential targets for biotechnological improvement

Soybean (Glycine max Merr.) is the world's most widely grown legume and provides an important source of protein and oil. Global soybean production and yield per hectare increased steadily over the past century with improved agronomy and development of cultivars suited to a wide range of latitudes. In order to meet the needs of a growing world population without unsustainable expansion of the land area devoted to this crop, yield must increase at a faster rate than at present. Here, the historical basis for the yield gains realized in the past 90 years are examined together with potential metabolic targets for achieving further improvements in yield potential. These targets include improving photosynthetic efficiency, optimizing delivery and utilization of carbon, more efficient nitrogen fixation and altering flower initiation and abortion. Optimization of investment in photosynthetic enzymes, bypassing photorespiratory metabolism, engineering the electron transport chain and engineering a faster recovery from the photoprotected state are different strategies to improve photosynthesis in soybean. These potential improvements in photosynthetic carbon gain will need to be matched by increased carbon and nitrogen transport to developing soybean pods and seeds in order to maximize the benefit. Better understanding of control of carbon and nitrogen transport along with improved knowledge of the regulation of flower initiation and abortion will be needed to optimize sink capacity in soybean. Although few single targets are likely to deliver a quantum leap in yields, biotechnological advances in molecular breeding techniques that allow for alteration of the soybean genome and transcriptome promise significant yield gains.

Over-expressing the C(3) photosynthesis cycle enzyme Sedoheptulose-1-7 Bisphosphatase improves photosynthetic carbon gain and yield under fully open air CO(2) fumigation (FACE)

BACKGROUND

Biochemical models predict that photosynthesis in C(3) plants is most frequently limited by the slower of two processes, the maximum capacity of the enzyme Rubisco to carboxylate RuBP (V(c,max)), or the regeneration of RuBP via electron transport (J). At current atmospheric [CO(2)] levels Rubisco is not saturated; consequently, elevating [CO(2)] increases the velocity of carboxylation and inhibits the competing oxygenation reaction which is also catalyzed by Rubisco. In the future, leaf photosynthesis (A) should be increasingly limited by RuBP regeneration, as [CO(2)] is predicted to exceed 550 ppm by 2050. The C(3) cycle enzyme sedoheptulose-1,7 bisphosphatase (SBPase, EC 3.1.3.17) has been shown to exert strong metabolic control over RuBP regeneration at light saturation.

RESULTS

We tested the hypothesis that tobacco transformed to overexpressing SBPase will exhibit greater stimulation of A than wild type (WT) tobacco when grown under field conditions at elevated [CO(2)] (585 ppm) under fully open air fumigation. Growth under elevated [CO(2)] stimulated instantaneous A and the diurnal photosynthetic integral (A') more in transformants than WT. There was evidence of photosynthetic acclimation to elevated [CO(2)] via downregulation of V(c,max) in both WT and transformants. Nevertheless, greater carbon assimilation and electron transport rates (J and J(max)) for transformants led to greater yield increases than WT at elevated [CO(2)] compared to ambient grown plants.

CONCLUSION

These results provide proof of concept that increasing content and activity of a single photosynthesis enzyme can enhance carbon assimilation and yield of C(3) crops grown at [CO(2)] expected by the middle of the 21st century.

Does ear C sink strength contribute to overcoming photosynthetic acclimation of wheat plants exposed to elevated CO2?

Wheat plants (Triticum durum Desf., cv. Regallo) were grown in the field to study the effects of contrasting [CO(2)] conditions (700 versus 370 μmol mol(-1)) on growth, photosynthetic performance, and C management during the post-anthesis period. The aim was to test whether a restricted capacity of sink organs to utilize photosynthates drives a loss of photosynthetic capacity in elevated CO(2). The ambient (13)C/(12)C isotopic composition (δ(13)C) of air CO(2) was changed from -10.2‰ in ambient [CO(2)] to -23.6‰ under elevated [CO(2)] between the 7th and the 14th days after anthesis in order to study C assimilation and partitioning between leaves and ears. Elevated [CO(2)] had no significant effect on biomass production and grain filling, and caused an accumulation of C compounds in leaves. This was accompanied by up-regulation of phosphoglycerate mutase and ATP synthase protein content, together with down-regulation of adenosine diphosphate glucose pyrophosphatase protein. Growth in elevated [CO(2)] negatively affected Rubisco and Rubisco activase protein content and induced photosynthetic down-regulation. CO(2) enrichment caused a specific decrease in Rubisco content, together with decreases in the amino acid and total N content of leaves. The C labelling revealed that in flag leaves, part of the C fixed during grain filling was stored as starch and structural C compounds whereas the rest of the labelled C (mainly in the form of soluble sugars) was completely respired 48 h after the end of labelling. Although labelled C was not detected in the δ(13)C of ear total organic matter and respired CO(2), soluble sugar δ(13)C revealed that a small amount of labelled C reached the ear. The (12)CO(2) labelling suggests that during the beginning of post-anthesis the ear did not contribute towards overcoming flag leaf carbohydrate accumulation, and this had a consequent effect on protein expression and photosynthetic acclimation.

Analysis of the rice mitochondrial carrier family reveals anaerobic accumulation of a basic amino acid carrier involved in arginine metabolism during seed germination

Given the substantial changes in mitochondrial gene expression, the mitochondrial proteome, and respiratory function during rice (Oryza sativa) germination under anaerobic and aerobic conditions, we have attempted to identify changes in mitochondrial membrane transport capacity during these processes. We have assembled a preliminary rice mitochondrial carrier gene family of 50 members, defined its orthology to carriers of known function, and observed significant changes in microarray expression data for these rice genes during germination under aerobic and anaerobic conditions and across rice development. To determine if these transcript changes reflect alteration of the carrier profile itself and to determine which members of the family encode the major mitochondrial carrier proteins, we analyzed mitochondrial integral membrane protein preparations using sodium dodecyl sulfate-polyacrylamide gel electrophoresis and peptide mass spectrometry, identifying seven distinct carrier proteins. We have used mass spectrometry-based quantitative approaches to compare the abundance of these carriers between mitochondria from dry seeds and those from aerobic- or anaerobic-germinated seeds. We highlight an anaerobic-enhanced basic amino acid carrier and show concomitant increases in mitochondrial arginase and the abundance of arginine and ornithine in anaerobic-germinated seeds, consistent with an anaerobic role of this mitochondria carrier. The potential role of this carrier in facilitating mitochondrial involvement in arginine metabolism and the plant urea cycle during the growth of rice coleoptiles and early seed nitrate assimilation under anaerobic conditions are discussed.

Analysis of a Farquhar-von Caemmerer-Berry leaf-level photosynthetic rate model for Populus tremuloides in the context of modeling and measurement limitations

The balance of mechanistic detail with mathematical simplicity contributes to the broad use of the Farquhar, von Caemmerer and Berry (FvCB) photosynthetic rate model. Here the FvCB model was coupled with a stomatal conductance model to form an [A,g(s)] model, and parameterized for mature Populus tremuloides leaves under varying CO(2) and temperature levels. Data were selected to be within typical forest light, CO(2) and temperature ranges, reducing artifacts associated with data collected at extreme values. The error between model-predicted photosynthetic rate (A) and A data was measured in three ways and found to be up to three times greater for each of two independent data sets than for a base-line evaluation using parameterization data. The evaluation methods used here apply to comparisons of model validation results among data sets varying in number and distribution of data, as well as to performance comparisons of [A,g(s)] models differing in internal-process components.

Acclimation to future atmospheric CO2 levels increases photochemical efficiency and mitigates photochemistry inhibition by warm temperatures in wheat under field chambers

A study was conducted over 2 years to determine whether growth under elevated CO(2) (700 μmol mol(-1) ) and temperature (ambient + 4 °C) conditions modifies photochemical efficiency or only the use of electron transport products in spring wheat grown in field chambers. Elevated atmospheric CO(2) concentrations increased crop dry matter at maturity by 12-17%, while above-ambient temperatures did not significantly affect dry matter yield. In measurements with ambient CO(2) at ear emergence and after anthesis, growth at elevated CO(2) concentrations decreased flag leaf light-saturated carbon assimilation. The quantum yield of electron transport (Φ(PSII) ) measured at ambient CO(2) and higher irradiances increased at ear emergence and decreased after anthesis in plants grown at elevated CO(2) . At higher light intensities, but not in low light, photochemical quenching (qP) decreased after growth in elevated CO(2) conditions. Growth under CO(2) enrichment increased dark- (Fv:Fm) and light-adapted (Fv':Fm') photochemical efficiencies, and decreased the chlorophyll a:b ratio, suggesting an increase in light-harvesting complexes relative to PSII reaction centres. A relatively higher decrease in carbon assimilation than the decrease in Φ(PSII) pointed to a sink other than CO(2) assimilation for electron transport products at defined growth stages. With higher light intensities, warmer temperatures increased Φ(PSII) and Fv':Fm' at ear emergence and decreased Φ(PSII) after anthesis; in ambient-but not elevated-CO(2) , warmer temperatures also decreased qP after anthesis. CO(2) fixation increased or did not change with temperature, depending on the growth stage and year. We conclude that elevated CO(2) decreases the carbon assimilation capacity, but increases photochemistry and resource allocation to light harvesting, and that elevated levels of CO(2) can mitigate photochemistry inhibition as a result of warm temperatures.

Acclimation to future atmospheric CO2 levels increases photochemical efficiency and mitigates photochemistry inhibition by warm temperatures in wheat under field chambers

A study was conducted over 2 years to determine whether growth under elevated CO(2) (700 μmol mol(-1) ) and temperature (ambient + 4 °C) conditions modifies photochemical efficiency or only the use of electron transport products in spring wheat grown in field chambers. Elevated atmospheric CO(2) concentrations increased crop dry matter at maturity by 12-17%, while above-ambient temperatures did not significantly affect dry matter yield. In measurements with ambient CO(2) at ear emergence and after anthesis, growth at elevated CO(2) concentrations decreased flag leaf light-saturated carbon assimilation. The quantum yield of electron transport (Φ(PSII) ) measured at ambient CO(2) and higher irradiances increased at ear emergence and decreased after anthesis in plants grown at elevated CO(2) . At higher light intensities, but not in low light, photochemical quenching (qP) decreased after growth in elevated CO(2) conditions. Growth under CO(2) enrichment increased dark- (Fv:Fm) and light-adapted (Fv':Fm') photochemical efficiencies, and decreased the chlorophyll a:b ratio, suggesting an increase in light-harvesting complexes relative to PSII reaction centres. A relatively higher decrease in carbon assimilation than the decrease in Φ(PSII) pointed to a sink other than CO(2) assimilation for electron transport products at defined growth stages. With higher light intensities, warmer temperatures increased Φ(PSII) and Fv':Fm' at ear emergence and decreased Φ(PSII) after anthesis; in ambient-but not elevated-CO(2) , warmer temperatures also decreased qP after anthesis. CO(2) fixation increased or did not change with temperature, depending on the growth stage and year. We conclude that elevated CO(2) decreases the carbon assimilation capacity, but increases photochemistry and resource allocation to light harvesting, and that elevated levels of CO(2) can mitigate photochemistry inhibition as a result of warm temperatures.

Genomic basis for stimulated respiration by plants growing under elevated carbon dioxide

Photosynthetic and respiratory exchanges of CO(2) by plants with the atmosphere are significantly larger than anthropogenic CO(2) emissions, and these fluxes will change as growing conditions are altered by climate change. Understanding feedbacks in CO(2) exchange is important to predicting future atmospheric [CO(2)] and climate change. At the tissue and plant scale, respiration is a key determinant of growth and yield. Although the stimulation of C(3) photosynthesis by growth at elevated [CO(2)] can be predicted with confidence, the nature of changes in respiration is less certain. This is largely because the mechanism of the respiratory response is insufficiently understood. Molecular, biochemical and physiological changes in the carbon metabolism of soybean in a free-air CO(2) enrichment experiment were investigated over 2 growing seasons. Growth of soybean at elevated [CO(2)] (550 micromol x mol(-1)) under field conditions stimulated the rate of nighttime respiration by 37%. Greater respiratory capacity was driven by greater abundance of transcripts encoding enzymes throughout the respiratory pathway, which would be needed for the greater number of mitochondria that have been observed in the leaves of plants grown at elevated [CO(2)]. Greater respiratory quotient and leaf carbohydrate content at elevated [CO(2)] indicate that stimulated respiration was supported by the additional carbohydrate available from enhanced photosynthesis at elevated [CO(2)]. If this response is consistent across many species, the future stimulation of net primary productivity could be reduced significantly. Greater foliar respiration at elevated [CO(2)] will reduce plant carbon balance, but could facilitate greater yields through enhanced photoassimilate export to sink tissues.

Growth in elevated CO2 enhances temperature response of photosynthesis in wheat

The temperature dependence of C(3) photosynthesis may be altered by the growth environment. The effects of long-term growth in elevated CO(2) on photosynthesis temperature response have been investigated in wheat (Triticum aestivum L.) grown in controlled chambers with 370 or 700 mumol mol(-1) CO(2) from sowing through to anthesis. Gas exchange was measured in flag leaves at ear emergence, and the parameters of a biochemical photosynthesis model were determined along with their temperature responses. Elevated CO(2) slightly decreased the CO(2) compensation point and increased the rate of respiration in the light and ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) V(cmax), although the latter effect was reversed at 15 degrees C. With elevated CO(2), J(max) decreased in the 15-25 degrees C temperature range and increased at 30 and 35 degrees C. The temperature response (activation energy) of V(cmax) and J(max) increased with growth in elevated CO(2). CO(2) enrichment decreased the ribulose 1,5-bisphosphate (RuBP)-limited photosynthesis rates at lower temperatures and increased Rubisco- and RuBP-limited rates at higher temperatures. The results show that the photosynthesis temperature response is enhanced by growth in elevated CO(2). We conclude that if temperature acclimation and factors such as nutrients or water availability do not modify or negate this enhancement, the effects of future increases in air CO(2) on photosynthetic electron transport and Rubisco kinetics may improve the photosynthetic response of wheat to global warming.

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.

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.

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.