Plant growth in elevated CO2 alters mitochondrial number and chloroplast fine structure

Elevated CO2 concentration enhance carbon and nitrogen metabolism and biomass accumulation of Ormosiahosiei

Elevated CO2 concentrations may inhibit photosynthesis due to nitrogen deficiency, but legumes may be able to overcome this limitation and continue to grow. Our study confirms this conjecture well. First, we placed the two-year-old potted saplings of Ormosia hosiei (O. hosiei) (a leguminous tree species) in the open-top chamber (OTC) with three CO2 concentrations of 400 (CK), 600 (E1), and 800 μmol·mol-1 (E2) to simulate the elevated CO2 concentration environment. After 146 days, the light saturation point (LSP), light compensation point (LCP), apparent quantum efficiency (AQE), and dark respiration rate (Rd) of O. hosiei were increased under increasing CO2 concentration and obtain the maximum ribulose diphosphate (RuBP) carboxylation rate (Vc max) and RuBP regenerated photosynthetic electron transfer rate (Jmax) were also significantly increased under E2 treatment (P < 0.05). This results in a significant increase of the maximum assimilation rate (Amax) under elevated CO2 concentrations. Sucrose phosphate synthase (SPS) activity in sucrose metabolism increased in the leaves, more soluble sugars, starches, and sucrose was produced, but sucrose content only in leaves increased at E2, and more carbon flows to the roots. The activity of the NH4+ assimilating enzymes glutamine synthetase (GS), glutamate synthetase (GOGAT), and glutamate dehydrogenase (GDH) in the leaves of O. hosiei increases under elevated CO2 concentrations to promote nitrogen synthesis that reduces the content of ammonium nitrogen and increases the content of nitrate nitrogen. In addition, under E1 conditions, sucrose synthase (SS), direction of synthesis activity was highest and sucrose invertase (INV) activity was lowest, this means that the balance of C and N metabolism is maintained. While under E2 conditions SS activity decreased and INV activity increased, this increased C/N and nitrogen use efficiency. So, the elevated CO2 concentration promotes the accumulation of O. hosiei biomass, especially in the aboveground part, but did not have a significant effect on the accumulation of root biomass. This means that O. hosiei is able to cope under the elevated CO2 concentration without showing photosynthetic adaptation during the experimental period.

Elevated CO2 concentration enhances drought resistance of soybean by regulating cell structure, cuticular wax synthesis, photosynthesis, and oxidative stress response

The atmospheric [CO2] and the frequency and intensity of extreme weather events such as drought are increased, leading to uncertainty to soybean production. Elevated [CO2] (eCO2) partially mitigates the adverse effects of drought stress on crop growth and photosynthetic performance, but the mitigative mechanism is not well understood. In this study, soybean seedlings under drought stress simulated by PEG-6000 were grown in climate chambers with different [CO2] (400 μmol mol-1 and 700 μmol mol-1). The changes in anatomical structure, wax content, photosynthesis, and antioxidant enzyme were investigated by the analysis of physiology and transcriptome sequencing (RNA-seq). The results showed that eCO2 increased the thickness of mesophyll cells and decreased the thickness of epidermal cells accompanied by reduced stomatal conductance, thus reducing water loss in soybean grown under drought stress. Meanwhile, eCO2 up-regulated genes related to wax anabolism, thus producing more epidermal wax. Under drought stress, eCO2 increased net photosynthetic rate (PN), ribulose-1,5-bisphosphate carboxylase/oxygenase activity, and alerted the gene expressions in photosynthesis. The increased sucrose synthesis and decreased sucrose decomposition contributed to the progressive increase in the soluble saccharide contents under drought stress with or without eCO2. In addition, eCO2 increased the expressions of genes associated with peroxidase (POD) and proline (Pro), thus enhancing POD activity and Pro content and improving the drought resistance in soybean. Taken together, these findings deepen our understanding of the effects of eCO2 on alleviating drought stress in soybean and provide potential target genes for the genetic improvement of drought tolerance in soybean.

Short- and long-term responses of leaf day respiration to elevated atmospheric CO2

Evaluating leaf day respiration rate (RL), which is believed to differ from that in the dark (RDk), is essential for predicting global carbon cycles under climate change. Several studies have suggested that atmospheric CO2 impacts RL. However, the magnitude of such an impact and associated mechanisms remain uncertain. To explore the CO2 effect on RL, wheat (Triticum aestivum) and sunflower (Helianthus annuus) plants were grown under ambient (410 ppm) and elevated (820 ppm) CO2 mole fraction ([CO2]). RL was estimated from combined gas exchange and chlorophyll fluorescence measurements using the Kok method, the Kok-Phi method, and a revised Kok method (Kok-Cc method). We found that elevated growth [CO2] led to an 8.4% reduction in RL and a 16.2% reduction in RDk in both species, in parallel to decreased leaf N and chlorophyll contents at elevated growth [CO2]. We also looked at short-term CO2 effects during gas exchange experiments. Increased RL or RL/RDk at elevated measurement [CO2] were found using the Kok and Kok-Phi methods, but not with the Kok-Cc method. This discrepancy was attributed to the unaccounted changes in Cc in the former methods. We found that the Kok and Kok-Phi methods underestimate RL and overestimate the inhibition of respiration under low irradiance conditions of the Kok curve, and the inhibition of RL was only 6%, representing 26% of the apparent Kok effect. We found no significant long-term CO2 effect on RL/RDk, originating from a concurrent reduction in RL and RDk at elevated growth [CO2], and likely mediated by acclimation of nitrogen metabolism.

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.

The Influence of Elevated CO2 on Volatile Emissions, Photosynthetic Characteristics, and Pigment Content in Brassicaceae Plants Species and Varieties

Climate change will determine a sharp increase in carbon dioxide in the following years. To study the influence of elevated carbon dioxide on plants, we grew 13 different species and varieties from the Brassicaceae family at three carbon dioxide concentrations: 400, 800, and 1200 ppmv. The photosynthetic parameters (assimilation rate and stomatal conductance to water vapor) increase for all species. The emission of monoterpenes increases for plants grown at elevated carbon dioxide while the total polyphenols and flavonoids content decrease. The chlorophyll content is affected only for some species (such as Lipidium sativum), while the β-carotene concentrations in the leaves were not affected by carbon dioxide.

Dek504 Encodes a Mitochondrion-Targeted E+-Type Pentatricopeptide Repeat Protein Essential for RNA Editing and Seed Development in Maize

In flowering plants, RNA editing is a post-transcriptional process that selectively deaminates cytidines (C) to uridines (U) in organellar transcripts. Pentatricopeptide repeat (PPR) proteins have been identified as site-specific recognition factors for RNA editing. Here, we report the map-based cloning and molecular characterization of the defective kernel mutant dek504 in maize. Loss of Dek504 function leads to delayed embryogenesis and endosperm development, which produce small and collapsed kernels. Dek504 encodes an E+-type PPR protein targeted to the mitochondria, which is required for RNA editing of mitochondrial NADH dehydrogenase 3 at the nad3-317 and nad3-44 sites. Biochemical analysis of mitochondrial protein complexes revealed a significant reduction in the mitochondrial NADH dehydrogenase complex I activity, indicating that the alteration of the amino acid sequence at nad3-44 and nad3-317 through RNA editing is essential for NAD3 function. Moreover, the amino acids are highly conserved in monocots and eudicots, whereas the events of C-to-U editing are not conserved in flowering plants. Thus, our results indicate that Dek504 is essential for RNA editing of nad3, which is critical for NAD3 function, mitochondrial complex I stability, and seed development in maize.

Acclimation and adaptation to elevated pCO2 increase arsenic resilience in marine diatoms

Arsenic pollution is a widespread threat to marine life, but the ongoing rise pCO₂ levels is predicted to decrease bio-toxicity of arsenic. However, the effects of arsenic toxicity on marine primary producers under elevated pCO₂ are not well characterized. Here, we studied the effects of arsenic toxicity in three globally distributed diatom species (Phaeodactylum tricornutum, Thalassiosira pseudonana, and Chaetoceros mulleri) after short-term acclimation (ST, 30 days), medium-term exposure (MT, 750 days), and long-term (LT, 1460 days) selection under ambient (400 µatm) and elevated (1000 and 2000 µatm) pCO₂. We found that elevated pCO₂ alleviated arsenic toxicity even after short acclimation times but the magnitude of the response decreased after mid and long-term adaptation. When fed with these elevated pCO₂ selected diatoms, the scallop Patinopecten yessoensis had significantly lower arsenic content (3.26-52.83%). Transcriptomic and biochemical analysis indicated that the diatoms rapidly developed arsenic detoxification strategies, which included upregulation of transporters associated with shuttling harmful compounds out of the cell to reduce arsenic accumulation, and upregulation of proteins involved in synthesizing glutathione (GSH) to chelate intracellular arsenic to reduce arsenic toxicity. Thus, our results will expand our knowledge to fully understand the ecological risk of trace metal pollution under increasing human activity induced ocean acidification.

Short photoperiod attenuates CO2 fertilization effect on shoot biomass in Arabidopsis thaliana

The level of carbon dioxide (CO2) in the air can affect several traits in plants. Elevated atmospheric CO2 (eCO2) can enhance photosynthesis and increase plant productivity, including biomass, although there are inconsistencies regarding the effects of eCO2 on the plant growth response. The compounding effects of ambient environmental conditions such as light intensity, photoperiod, water availability, and soil nutrient composition can affect the extent to which eCO2 enhances plant productivity. This study aimed to investigate the growth response of Arabidopsis thaliana to eCO2 (800 ppm) under short photoperiod (8/16 h, light/dark cycle). Here, we report an attenuated fertilization effect of eCO2 on the shoot biomass of Arabidopsis plants grown under short photoperiod. The biomass of two-, three-, and four-week-old Arabidopsis plants was increased by 10%, 15%, and 28%, respectively, under eCO2 compared to the ambient CO2 (aCO2, 400 ppm) i.e. control. However, the number of rosette leaves, rosette area, and shoot biomass were similar in mature plants under both CO2 conditions, despite 40% higher photosynthesis in eCO2 exposed plants. The levels of chlorophylls and carotenoids were similar in the fully expanded rosette leaves regardless of the level of CO2. In conclusion, CO2 enrichment moderately increased Arabidopsis shoot biomass at the juvenile stage, whereas the eCO2-induced increment in shoot biomass was not apparent in mature plants. A shorter day-length can limit the source-to-sink resource allocation in a plant in age-dependent manner, hence diminishing the eCO2 fertilization effect on the shoot biomass in Arabidopsis plants grown under short photoperiod.

The phyto-impact of fluazinam fungicide on cellular structure, agro-physiological, and yield traits of pepper and eggplant crops

Fluazinam is a widely used fungicide; most of the available information associated with its impact predominately on birds, invertebrates, mammals, and algae and scarce works studied its impact on crop plants. A two years-field experiments were conducted to study the response of pepper and eggplant to fluazinam at 0, 1, 2, and 3 times of the fluazinam-recommended dose (0, 0.5, 1, and 1.5 mL/L). The results revealed that fluazinam did not cause toxic effect on the tested plants except for temporary decline of shoot weights and lengths after 3 days of fluazinam application. However, fluazinam improved the physiological status of leaves via promoting metabolites, antioxidants, better membrane integrity, and adjustment of the redox status of fluazinam-sprayed plants. The ultrastructure changes of fluazinam-treated leaves associated with increment of chloroplasts' starch granules, giant nucleus, and elevated number of mitochondria. After 35 days of treatments, plant length of fungicide-treated plants was found to be higher than control and flowering time showed significant earliness. Furthermore, the yield traits were increased significantly in response to fluazinam. Our findings suggested that fluazinam-treated plants could initiate an early defense mechanism to mitigate the permanent growth retardation. This study could serve as a matrix for further studies to seek elucidation of plants' response to other doses of fluazinam. Graphical abstract .

Elevated CO2 concentration induces photosynthetic down-regulation with changes in leaf structure, non-structural carbohydrates and nitrogen content of soybean

BACKGROUND

Understanding the mechanisms of crops in response to elevated CO2 concentrations is pivotal to estimating the impacts of climate change on the global agricultural production. Based on earlier results of the "doubling-CO2 concentration" experiments, many current climate models may overestimate the CO2 fertilization effect on crops, and meanwhile, underestimate the potential impacts of future climate change on global agriculture ecosystem when the atmospheric CO2 concentration goes beyond the optimal levels for crop growth.

RESULTS

This study examined the photosynthetic response of soybean (Glycine max (L.) Merr.) to elevated CO2 concentration associated with changes in leaf structure, non-structural carbohydrates and nitrogen content with environmental growth chambers where the CO2 concentration was controlled at 400, 600, 800, 1000, 1200, 1400, 1600 ppm. We found CO2-induced down-regulation of leaf photosynthesis as evidenced by the consistently declined leaf net photosynthetic rate (An) with elevated CO2 concentrations. This down-regulation of leaf photosynthesis was evident in biochemical and photochemical processes since the maximum carboxylation rate (Vcmax) and the maximum electron transport rate (Jmax) were dramatically decreased at higher CO2 concentrations exceeding their optimal values of about 600 ppm and 400 ppm, respectively. Moreover, the down-regulation of leaf photosynthesis at high CO2 concentration was partially attributed to the reduced stomatal conductance (Gs) as demonstrated by the declines in stomatal density and stomatal area as well as the changes in the spatial distribution pattern of stomata. In addition, the smaller total mesophyll size (palisade and spongy tissues) and the lower nitrogen availability may also contribute to the down-regulation of leaf photosynthesis when soybean subjected to high CO2 concentration environment.

CONCLUSIONS

Down-regulation of leaf photosynthesis associated with the changes in stomatal traits, mesophyll tissue size, non-structural carbohydrates, and nitrogen availability of soybean in response to future high atmospheric CO2 concentration and climate change.

Alternative Oxidase Capacity of Mitochondria in Microsporophylls May Function in Cycad Thermogenesis

Cone thermogenesis is a widespread phenomenon in cycads and may function to promote volatile emissions that affect pollinator behavior. Given their large population size and intense and durable heat-producing effects, cycads are important organisms for comprehensive studies of plant thermogenesis. However, knowledge of mitochondrial morphology and function in cone thermogenesis is limited. Therefore, we investigated these mitochondrial properties in the thermogenic cycad species Cycas revoluta Male cones generated heat even in cool weather conditions. Female cones produced heat, but to a lesser extent than male cones. Ultrastructural analyses of the two major tissues of male cones, microsporophylls and microsporangia, revealed the existence of a population of mitochondria with a distinct morphology in the microsporophylls. In these cells, we observed large mitochondria (cross-sectional area of 2 μm² or more) with a uniform matrix density that occupied >10% of the total mitochondrial volume. Despite the size difference, many nonlarge mitochondria (cross-sectional area <2 μm²) also exhibited a shape and a matrix density similar to those of large mitochondria. Alternative oxidase (AOX) capacity and expression levels in microsporophylls were much higher than those in microsporangia. The AOX genes expressed in male cones revealed two different AOX complementary DNA sequences: CrAOX1 and CrAOX2 The expression level of CrAOX1 mRNA in the microsporophylls was 100 times greater than that of CrAOX2 mRNA. Collectively, these results suggest that distinctive mitochondrial morphology and CrAOX1-mediated respiration in microsporophylls might play a role in cycad cone thermogenesis.

Plant carbon metabolism and climate change: elevated CO2 and temperature impacts on photosynthesis, photorespiration and respiration

Contents Summary 32 I. The importance of plant carbon metabolism for climate change 32 II. Rising atmospheric CO2 and carbon metabolism 33 III. Rising temperatures and carbon metabolism 37 IV. Thermal acclimation responses of carbon metabolic processes can be best understood when studied together 38 V. Will elevated CO2 offset warming-induced changes in carbon metabolism? 40 VI. No plant is an island: water and nutrient limitations define plant responses to climate drivers 41 VII. Conclusions 42 Acknowledgements 42 References 42 Appendix A1 48 SUMMARY: Plant carbon metabolism is impacted by rising CO₂ concentrations and temperatures, but also feeds back onto the climate system to help determine the trajectory of future climate change. Here we review how photosynthesis, photorespiration and respiration are affected by increasing atmospheric CO₂ concentrations and climate warming, both separately and in combination. We also compile data from the literature on plants grown at multiple temperatures, focusing on net CO₂ assimilation rates and leaf dark respiration rates measured at the growth temperature (A_growth and R_growth , respectively). Our analyses show that the ratio of A_growth to R_growth is generally homeostatic across a wide range of species and growth temperatures, and that species that have reduced A_growth at higher growth temperatures also tend to have reduced R_growth , while species that show stimulations in A_growth under warming tend to have higher R_growth in the hotter environment. These results highlight the need to study these physiological processes together to better predict how vegetation carbon metabolism will respond to climate change.

Canopy warming accelerates development in soybean and maize, offsetting the delay in soybean reproductive development by elevated CO2 concentrations

Increases in atmospheric CO2 concentrations ([CO2 ]) and surface temperature are known to individually have effects on crop development and yield, but their interactive effects have not been adequately investigated under field conditions. We evaluated the impacts of elevated [CO2 ] with and without canopy warming as a function of development in soybean and maize using infrared heating arrays nested within free air CO2 enrichment plots over three growing seasons. Vegetative development accelerated in soybean with temperature plus elevated [CO2 ] resulting in higher node number. Reproductive development was delayed in soybean under elevated [CO2 ], but warming mitigated this delay. In maize, both vegetative and reproductive developments were accelerated by warming, whereas elevated [CO2 ] had no apparent effect on development. Treatment-induced changes in the leaf carbohydrates, dark respiration rate, morphological parameters, and environmental conditions accompanied the changes in plant development. We used two thermal models to investigate their ability to predict the observed development under warming and elevated [CO2 ]. Whereas the growing degree day model underestimated the thermal threshold to reach each developmental stage, the alternative process-based model used (β function) was able to predict crop development under climate change conditions.

Variations in assimilation rate, photoassimilate translocation, and cellular fine structure of potato cultivars (Solanum Tuberosum L.) exposed to elevated CO2

Rising atmospheric CO₂ concentrations are expected to impact the productivity of plants. Cultivars demonstrate different responses to CO₂ levels, hence, screening and recognizing the cultivars with a higher capacity for translocation of photoassimilates would certainly be beneficiary. To investigate the interactive impact of enhancing CO₂ on physiology, cellular fine structure and photoassimilate translocation of micro-propagated potato plantlets, plantlets (cvs. Agria and Fontane) were grown under ambient (400 ppm) or elevated (800 ppm) CO₂ concentrations in controlled environments. These high-yielding cultivars are widely cultivated in Iran and have a wide range of consumption as fresh marketing, French fries, and chips industry. Transmission electron micrographs showed an increase in the length, width, and area of chloroplasts. The number of chloroplasts per cell area was significantly increased in Agria at elevated CO₂. Also, there was an increase in mitochondria number in Agria and Fontane. Chloroplast number and Np were increased by a similar magnitude at doubled CO₂, while, mitochondria number was increased greater than the leaf Rd enhancement at elevated CO₂. Elevated CO₂ increased net photosynthesis, dark respiration (Rd), and starch concentration in leaves. However, there was no dramatic change in the leaf soluble carbohydrate content in the plants grown at elevated CO₂, apart from at 75 days after transplant (DAT) in Agria. Net photosynthesis remained relatively unchanged for each cultivar throughout the growing season at elevated CO₂, which demonstrated more efficient CO₂ assimilation to ambient CO₂. The greatest starch content was measured at 55 DAT that was accompanied by lower Np and higher Rd. The diminished starch content of leaves was contributed to a lower leaf dry matter as well as a greater tuber dry matter in Fontane. Our results highlighted a variation in photoassimilate translocation between these cultivars, in which Fontane demonstrated a more efficient photoassimilate translocation system at the elevated CO₂.

DcC4H and DcPER Are Important in Dynamic Changes of Lignin Content in Carrot Roots under Elevated Carbon Dioxide Stress

In our study, isobaric tags for relative and absolute quantification (iTRAQ) was conducted to determine the significantly changed proteins in the fleshy roots of carrots under different carbon dioxide (CO₂) treatments. A total of 1523 proteins were identified, of which 257 were differentially expressed proteins (DEPs). On the basis of annotation analysis, the DEPs were identified to be involved in energy metabolism, carbohydrate metabolism, and some other metabolic processes. DcC4H and DcPER, two lignin-related proteins, were identified from the DEPs. Under elevated CO₂ stress, both carrot lignin content and the expression profiles of lignin biosynthesis genes changed significantly. The protein-protein interactions among lignin-related enzymes proved the importance of DcC4H and DcPER. The results of our study provided potential new insights into the molecular mechanism of lignin content changes in carrot roots under elevated CO₂ stress.

Leaf-age dependent response of carotenoid accumulation to elevated CO2 in Arabidopsis

Carotenoids contribute to photosynthesis, photoprotection, phytohormone and apocarotenoid biosynthesis in plants. Carotenoid-derived metabolites control plant growth, development and signalling processes and their accumulation can depend upon changes in the environment. Elevated carbon dioxide (eCO₂) often enhances carbon assimilation, early growth patterns and overall plant biomass, and may increase carotenoid accumulation due to higher levels of precursors from isoprenoid biosynthesis. Variable effects of eCO₂ on carotenoid accumulation in leaves have been observed for different plant species. Here, we determined whether the variable response of carotenoids to eCO₂ was potentially a function of leaf age and the impact of eCO₂ on leaf development by growing Arabidopsis in ambient CO₂ (400 ppm) and eCO₂ (800 ppm). eCO₂ increased plant leaf number, rosette area, biomass, seed yield and net photosynthesis. In addition, eCO₂ increased carotenoid content by 10-20% in younger emerging leaves, but not in older mature leaves. Older leaves contained approximately 60% less total carotenoids compared to younger leaves. The age-dependent effect on carotenoid content was observed for cotyledon, juvenile and adult phase leaves. We conclude that younger leaves utilize additional carbon from enhanced photosynthesis in eCO₂ to increase carotenoid content, yet older leaves have less capacity to store additional carbon into carotenoids.

Leaf day respiration: low CO2 flux but high significance for metabolism and carbon balance

Contents 986 I. 987 II. 987 III. 988 IV. 991 V. 992 VI. 995 VII. 997 VIII. 998 References 998 SUMMARY: It has been 75 yr since leaf respiratory metabolism in the light (day respiration) was identified as a low-flux metabolic pathway that accompanies photosynthesis. In principle, it provides carbon backbones for nitrogen assimilation and evolves CO₂ and thus impacts on plant carbon and nitrogen balances. However, for a long time, uncertainties have remained as to whether techniques used to measure day respiratory efflux were valid and whether day respiration responded to environmental gaseous conditions. In the past few years, significant advances have been made using carbon isotopes, 'omics' analyses and surveys of respiration rates in mesocosms or ecosystems. There is substantial evidence that day respiration should be viewed as a highly dynamic metabolic pathway that interacts with photosynthesis and photorespiration and responds to atmospheric CO₂ mole fraction. The view of leaf day respiration as a constant and/or negligible parameter of net carbon exchange is now outdated and it should now be regarded as a central actor of plant carbon-use efficiency.

The temperature response of leaf dark respiration in 15 provenances of Eucalyptus grandis grown in ambient and elevated CO2

The effects of elevated CO2 on the short-term temperature response of leaf dark respiration (R) remain uncertain for many forest tree species. Likewise, variation in leaf R among populations within tree species and potential interactive effects of elevated CO2 are poorly understood. We addressed these uncertainties by measuring the short-term temperature response of leaf R in 15 provenances of Eucalyptus grandis W. Hill ex Maiden from contrasting thermal environments grown under ambient [CO2] (aCO2; 400µmolmol-1) and elevated [CO2] (640µmolmol-1; eCO2). Leaf R per unit area (Rarea) measured across a range of temperatures was higher in trees grown in eCO2 and varied up to 104% among provenances. However, eCO2 increased leaf dry mass per unit area (LMA) by 21%, and when R was expressed on a mass basis (i.e. Rmass), it did not differ between CO2 treatments. Likewise, accounting for differences in LMA among provenances, Rmass did not differ among provenances. The temperature sensitivity of R (i.e. Q10) did not differ between CO2 treatments or among provenances. We conclude that eCO2 had no direct effect on the temperature response of R in E. grandis, and respiratory physiology was similar among provenances of E. grandis regardless of home-climate temperature conditions.

Acclimation of light and dark respiration to experimental and seasonal warming are mediated by changes in leaf nitrogen in Eucalyptus globulus

Quantifying the adjustments of leaf respiration in response to seasonal temperature variation and climate warming is crucial because carbon loss from vegetation is a large but uncertain part of the global carbon cycle. We grew fast-growing Eucalyptus globulus Labill. trees exposed to +3 °C warming and elevated CO2 in 10-m tall whole-tree chambers and measured the temperature responses of leaf mitochondrial respiration, both in light (RLight) and in darkness (RDark), over a 20-40 °C temperature range and during two different seasons. RLight was assessed using the Laisk method. Respiration rates measured at a standard temperature (25 °C - R25) were higher in warm-grown trees and in the warm season, related to higher total leaf nitrogen (N) investment with higher temperatures (both experimental and seasonal), indicating that leaf N concentrations modulated the respiratory capacity to changes in temperature. Once differences in leaf N were accounted for, there were no differences in R25 but the Q10 (i.e., short-term temperature sensitivity) was higher in late summer compared with early spring. The variation in RLight between experimental treatments and seasons was positively correlated with carboxylation capacity and photorespiration. RLight was less responsive to short-term changes in temperature than RDark, as shown by a lower Q10 in RLight compared with RDark. The overall light inhibition of R was ∼40%. Our results highlight the dynamic nature of leaf respiration to temperature variation and that the responses of RLight do not simply mirror those of RDark. Therefore, it is important not to assume that RLight is the same as RDark in ecosystem models, as doing so may lead to large errors in predicting plant CO2 release and productivity.

Major evolutionary transitions of life, metabolic scaling and the number and size of mitochondria and chloroplasts

We investigate the effects of trophic lifestyle and two types of major evolutionary transitions in individuality-the endosymbiotic acquisition of organelles and development of multicellularity-on organellar and cellular metabolism and allometry. We develop a quantitative framework linking the size and metabolic scaling of eukaryotic cells to the abundance, size and metabolic scaling of mitochondria and chloroplasts and analyse a newly compiled, unprecedented database representing unicellular and multicellular cells covering diverse phyla and tissues. Irrespective of cellularity, numbers and total volumes of mitochondria scale linearly with cell volume, whereas chloroplasts scale sublinearly and sizes of both organelles remain largely invariant with cell size. Our framework allows us to estimate the metabolic scaling exponents of organelles and cells. Photoautotrophic cells and organelles exhibit photosynthetic scaling exponents always less than one, whereas chemoheterotrophic cells and organelles have steeper respiratory scaling exponents close to one. Multicellularity has no discernible effect on the metabolic scaling of organelles and cells. In contrast, trophic lifestyle has a profound and uniform effect, and our results suggest that endosymbiosis fundamentally altered the metabolic scaling of free-living bacterial ancestors of mitochondria and chloroplasts, from steep ancestral scaling to a shallower scaling in their endosymbiotic descendants.

Qualitative and quantitative modifications of root mitochondria during senescence of above-ground parts of Arabidopis thaliana

This work studied modifications experienced by root mitochondria during whole plant senescence or under light deprivation, using Arabidopsis thaliana plants with YFP tagged to mitochondria. During post-bolting development, root respiratory activity started to decline after aboveground organs (i.e., rosette leaves) had senesced. This suggests that carbohydrate starvation may induce root senescence. Similarly, darkening the whole plant induced a decrease in respiration of roots. This was partially due to a decrease in the number of total mitochondria (YFP-labelled mitochondria) and most probably to a decrease in the quantity of mitochondria with a developed inner membrane potential (ΔΨm, i.e., Mitotracker red- labelled mitochondria). Also, the lower amount of mitochondria with ΔΨm compared to YFP-labelled mitochondria at 10d of whole darkened plant, suggests the presence of mitochondria in a "standby state". The experiments also suggest that small mitochondria made the main contribution to the respiratory activity that was lost during root senescence. Sugar supplementation partially restored the respiration of mitochondria after 10d of whole plant dark treatment. These results suggest that root senescence is triggered by carbohydrate starvation, with loss of ΔΨm mitochondria and changes in mitochondrial size distribution.

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.

The effects of CO2 and nutrient fertilisation on the growth and temperature response of the mangrove Avicennia germinans

In order to understand plant responses to both the widespread phenomenon of increased nutrient inputs to coastal zones and the concurrent rise in atmospheric CO2 concentrations, CO2-nutrient interactions need to be considered. In addition to its potential stimulating effect on photosynthesis and growth, elevated CO2 affects the temperature response of photosynthesis. The scarcity of experiments testing how elevated CO2 affects the temperature response of tropical trees hinders our ability to model future primary productivity. In a glasshouse study, we examined the effects of elevated CO2 (800 ppm) and nutrient availability on seedlings of the widespread mangrove Avicennia germinans. We assessed photosynthetic performance, the temperature response of photosynthesis, seedling growth and biomass allocation. We found large synergistic gains in both growth (42 %) and photosynthesis (115 %) when seedlings grown under elevated CO2 were supplied with elevated nutrient concentrations relative to their ambient growing conditions. Growth was significantly enhanced under elevated CO2 only under high-nutrient conditions, mainly in above-ground tissues. Under low-nutrient conditions and elevated CO2, root volume was more than double that of seedlings grown under ambient CO2 levels. Elevated CO2 significantly increased the temperature optimum for photosynthesis by ca. 4 °C. Rising CO2 concentrations are likely to have a significant positive effect on the growth rate of A. germinans over the next century, especially in areas where nutrient availability is high.

Elevated CO2 can modify the response to a water status gradient in a steppe grass: from cell organelles to photosynthetic capacity to plant growth

BACKGROUND

The atmospheric CO2 concentration is rising continuously, and abnormal precipitation may occur more frequently in the future. Although the effects of elevated CO2 and drought on plants have been well reported individually, little is known about their interaction, particularly over a water status gradient. Here, we aimed to characterize the effects of elevated CO2 and a water status gradient on the growth, photosynthetic capacity, and mesophyll cell ultrastructure of a dominant grass from a degraded grassland.

RESULTS

Elevated CO2 stimulated plant biomass to a greater extent under moderate changes in water status than under either extreme drought or over-watering conditions. Photosynthetic capacity and stomatal conductance were also enhanced by elevated CO2 under moderate drought, but inhibited with over-watering. Severe drought distorted mesophyll cell organelles, but CO2 enrichment partly alleviated this effect. Intrinsic water use efficiency (WUEi) and total biomass water use efficiency (WUEt) were increased by elevated CO2, regardless of water status. Plant structural traits were also found to be tightly associated with photosynthetic potentials.

CONCLUSION

The results indicated that CO2 enrichment alleviated severe and moderate drought stress, and highlighted that CO2 fertilization's dependency on water status should be considered when projecting key species' responses to climate change in dry ecosystems.

Contributions of photosynthetic and non-photosynthetic cell types to leaf respiration in Vicia faba L. and their responses to growth temperature

In intact leaves, mitochondrial populations are highly heterogeneous among contrasting cell types; how such contrasting populations respond to sustained changes in the environment remains, however, unclear. Here, we examined respiratory rates, mitochondrial protein composition and response to growth temperature in photosynthetic (mesophyll) and non-photosynthetic (epidermal) cells from fully expanded leaves of warm-developed (WD) and cold-developed (CD) broad bean (Vicia faba L.). Rates of respiration were significantly higher in mesophyll cell protoplasts (MCPs) than epidermal cell protoplasts (ECPs), with both protoplast types exhibiting capacity for cytochrome and alternative oxidase activity. Compared with ECPs, MCPs contained greater relative quantities of porin, suggesting higher mitochondrial surface area in mesophyll cells. Nevertheless, the relative quantities of respiratory proteins (normalized to porin) were similar in MCPs and ECPs, suggesting that ECPs have lower numbers of mitochondria yet similar protein complement to MCP mitochondria (albeit with lower abundance serine hydroxymethyltransferase). Several mitochondrial proteins (both non-photorespiratory and photorespiratory) exhibited an increased abundance in response to cold in both protoplast types. Based on estimates of individual protoplast respiration rates, combined with leaf cell abundance data, epidermal cells make a small but significant (2%) contribution to overall leaf respiration which increases twofold in the cold. Taken together, our data highlight the heterogeneous nature of mitochondrial populations in leaves, both among contrasting cell types and in how those populations respond to growth temperature.

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.

Physiological and molecular alterations in plants exposed to high [CO2] under phosphorus stress

Atmospheric [CO2] has increased substantially in recent decades and will continue to do so, whereas the availability of phosphorus (P) is limited and unlikely to increase in the future. P is a non-renewable resource, and it is essential to every form of life. P is a key plant nutrient controlling the responsiveness of photosynthesis to [CO2]. Increases in [CO2] typically results in increased biomass through stimulation of net photosynthesis, and hence enhance the demand for P uptake. However, most soils contain low concentrations of available P. Therefore, low P is one of the major growth-limiting factors for plants in many agricultural and natural ecosystems. The adaptive responses of plants to [CO2] and P availability encompass alterations at morphological, physiological, biochemical and molecular levels. In general low P reduces growth, whereas high [CO2] enhances it particularly in C3 plants. Photosynthetic capacity is often enhanced under high [CO2] with sufficient P supply through modulation of enzyme activities involved in carbon fixation such as ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco). However, high [CO2] with low P availability results in enhanced dry matter partitioning towards roots. Alterations in below-ground processes including root morphology, exudation and mycorrhizal association are influenced by [CO2] and P availability. Under high P availability, elevated [CO2] improves the uptake of P from soil. In contrast, under low P availability, high [CO2] mainly improves the efficiency with which plants produce biomass per unit P. At molecular level, the spatio-temporal regulation of genes involved in plant adaptation to low P and high [CO2] has been studied individually in various plant species. Genome-wide expression profiling of high [CO2] grown plants revealed hormonal regulation of biomass accumulation through complex transcriptional networks. Similarly, differential transcriptional regulatory networks are involved in P-limitation responses in plants. Analysis of expression patterns of some typical P-limitation induced genes under high [CO2] suggests that long-term exposure of plants to high [CO2] would have a tendency to stimulate similar transcriptional responses as observed under P-limitation. However, studies on the combined effect of high [CO2] and low P on gene expression are scarce. Such studies would provide insights into the development of P efficient crops in the context of anticipated increases in atmospheric [CO2].

How well do you know your growth chambers? Testing for chamber effect using plant traits

BACKGROUND

Plant growth chambers provide a controlled environment to analyse the effects of environmental parameters (light, temperature, atmospheric gas composition etc.) on plant function. However, it has been shown that a 'chamber effect' may exist whereby results observed are not due to an experimental treatment but to inconspicuous differences in supposedly identical chambers. In this study, Vicia faba L. 'Aquadulce Claudia' (broad bean) plants were grown in eight walk-in chambers to establish if a chamber effect existed, and if so, what plant traits are best for detecting such an effect. A range of techniques were used to measure differences between chamber plants, including chlorophyll fluorescence measurements, gas exchange analysis, biomass, reproductive yield, anatomical traits and leaf stable carbon isotopes.

RESULTS AND DISCUSSION

Four of the eight chambers exhibited a chamber effect. In particular, we identified two types of chamber effect which we term 'resolvable' or 'unresolved'; a resolvable chamber effect is caused by malfunctioning components of a chamber and an unresolved chamber effect is caused by unknown factors that can only be mitigated by appropriate experimental design and sufficient replication. Not all measured plant traits were able to detect a chamber effect and no single trait was capable of detecting all chamber effects. Fresh weight and flower count detected a chamber effect in three chambers, stable carbon isotopes (δ(13)C) and net rate CO2 assimilation (An) identified a chamber effect in two chambers, stomatal conductance (gs) and total performance index detected an effect only in one chamber.

CONCLUSION

(1) Chamber effects can be adequately detected by fresh weight measurements and flower counts on Vicia faba plants. These methods were the most effective in terms of detection and most efficient in terms of time. (2) δ(13)C, gs and An measurements help distinguish between resolvable and unresolved chamber effects. (3) Unresolved chamber effects require experimental unit replication while resolvable chamber effects require investigation, repair and retesting in advance of initiating further experiments.

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.

Influence of microwave frequency electromagnetic radiation on terpene emission and content in aromatic plants

Influence of environmental stress factors on both crop and wild plants of nutritional value is an important research topic. The past research has focused on rising temperatures, drought, soil salinity and toxicity, but the potential effects of increased environmental contamination by human-generated electromagnetic radiation on plants have little been studied. Here we studied the influence of microwave irradiation at bands corresponding to wireless router (WLAN) and mobile devices (GSM) on leaf anatomy, essential oil content and volatile emissions in Petroselinum crispum, Apium graveolens and Anethum graveolens. Microwave irradiation resulted in thinner cell walls, smaller chloroplasts and mitochondria, and enhanced emissions of volatile compounds, in particular, monoterpenes and green leaf volatiles (GLV). These effects were stronger for WLAN-frequency microwaves. Essential oil content was enhanced by GSM-frequency microwaves, but the effect of WLAN-frequency microwaves was inhibitory. There was a direct relationship between microwave-induced structural and chemical modifications of the three plant species studied. These data collectively demonstrate that human-generated microwave pollution can potentially constitute a stress to the plants.

Effects of elevated CO2 on levels of primary metabolites and transcripts of genes encoding respiratory enzymes and their diurnal patterns in Arabidopsis thaliana: possible relationships with respiratory rates

Elevated CO2 affects plant growth and photosynthesis, which results in changes in plant respiration. However, the mechanisms underlying the responses of plant respiration to elevated CO2 are poorly understood. In this study, we measured diurnal changes in the transcript levels of genes encoding respiratory enzymes, the maximal activities of the enzymes and primary metabolite levels in shoots of Arabidopsis thaliana grown under moderate or elevated CO2 conditions (390 or 780 parts per million by volume CO2, respectively). We examined the relationships between these changes and respiratory rates. Under elevated CO2, the transcript levels of several genes encoding respiratory enzymes increased at the end of the light period, but these increases did not result in changes in the maximal activities of the corresponding enzymes. The levels of some primary metabolites such as starch and sugar phosphates increased under elevated CO2, particularly at the end of the light period. The O2 uptake rate at the end of the dark period was higher under elevated CO2 than under moderate CO2, but higher under moderate CO2 than under elevated CO2 at the end of the light period. These results indicate that the changes in O2 uptake rates are not directly related to changes in maximal enzyme activities and primary metabolite levels. Instead, elevated CO2 may affect anabolic processes that consume respiratory ATP, thereby affecting O2 uptake rates.

Our eclectic adventures in the slower eras of photosynthesis: from New England Down Under to biosphere 2 and beyond

This is a tale of a career in plant physiological ecology that enjoyed the freedom to address photosynthetic physiology and biochemistry in leaves of plants from diverse environments. It was supported by block funding (now sadly a thing of the past) for research at the Australian National University, by grants during appointments in the United States and in Germany, and by Columbia University. It became a "career experiment" in which long-term, high-trust support for curiosity-driven plant biology in Australia, and at times in the United States, led to surprisingly innovative results. Although the rich diversity of short-term competitive grant opportunities in the United States sustained ongoing research, it proved difficult to mobilize support for more risky long-term projects. A decade after the closure of the Biosphere 2 Laboratory, this article highlights the achievements of colleagues in experimental climate change research from 1998 to 2003.

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].

Light saturated RuBP oxygenation by Rubisco is a robust predictor of light inhibition of respiration in Triticum aestivum L

Plant respiratory metabolism is complicated by the fact that the rate of non-photorespiratory mitochondrial CO2 release in the light (R light) may be lower than the rate of leaf respiration in the dark (R dark). A body of work on this topic implies a linkage between light inhibition of respiration and photorespiration, although the direction of effect and underlying mechanisms remain uncertain. In this study we used a variety of short- and long-term environmental manipulations to explicitly manipulate the rate of photorespiration (νo) and quantify the effect on the inhibition of mitochondrial respiration in the light (R light:R dark). We address the following three questions: (i) will the R light:R dark ratio increase or decrease with high CO2 or low O2 and at low temperatures; (ii) does νo correlate with R light:R dark, and if so, in what way; (iii) will suppression of respiration by light (the 'Kok effect') be seen to the same extent in Zea mays, a C4 plant, and in Triticum aestivum, a C3 plant? We found that Rlight :Rdark decreased under conditions that suppressed νo in wheat, and this resulted in a positive relationship between R light:R dark and νo. Inhibition of respiration by light in C4 maize did not respond to environmental treatment, and the fixed R light:R dark (0.46-0.72) was consistent with the wheat response, assuming a νo approaching zero. The most likely mechanism to explain this finding is that R light increases (or the inhibition of respiration by light decreases) when there is an increase in photorespiration and thus an increase in the demand for TCA cycle substrates associated with the recovery of photorespiratory cycle intermediates in the peroxisome. This work is significant because it combines a comparison of C3 and C4 metabolism with a range of environmental treatments to independently suppress νo.

Differential induction of mitochondrial machinery by light intensity correlates with changes in respiratory metabolism and photorespiration in rice leaves

The light responsiveness of mitochondrial function was investigated through changes in mitochondrial composition and metabolism in rice (Oryza sativa) shoots. The mitochondrial proteome and metabolite abundances under low light, (LL, 100 μmol m(-2) s(-1) ), and high light (HL, 700 μmol m(-2) s(-1) ) were measured along with information on shoot photosynthetic, respiratory and photorespiratory activity. Specific steps in mitochondrial tricarboxylic acid (TCA) cycle metabolism were decreased under HL, correlating with lower respiration rate under HL. The abundance of mitochondrial enzymes in branch chain metabolism was reduced under HL/LL, and correlated with a decrease in the abundance of a range of amino acids in the HL/LL. Mitochondrial nucleoside diphosphate kinase was increased under LL/HL treatments. Significant accumulation of glycine decarboxylase P, T subunits and serine hydroxymethyltransferase occurred in response to light. The abundance of the glycine decarboxylase (GDC) H subunit proteins was not changed by HL/LL treatments, and the abundance of GDC L subunit protein was halved under HL, indicating a change in the stoichiometry of GDC subunits, while photorespiration was fourfold higher in LL- than in HL-treated plants. Insights into these light-dependent phenomena and their importance for understanding the initiation of photorespiration in rice and adaptation of mitochondria to function in photosynthetic cells are discussed.

Differential induction of mitochondrial machinery by light intensity correlates with changes in respiratory metabolism and photorespiration in rice leaves

The light responsiveness of mitochondrial function was investigated through changes in mitochondrial composition and metabolism in rice (Oryza sativa) shoots. The mitochondrial proteome and metabolite abundances under low light, (LL, 100 μmol m(-2) s(-1) ), and high light (HL, 700 μmol m(-2) s(-1) ) were measured along with information on shoot photosynthetic, respiratory and photorespiratory activity. Specific steps in mitochondrial tricarboxylic acid (TCA) cycle metabolism were decreased under HL, correlating with lower respiration rate under HL. The abundance of mitochondrial enzymes in branch chain metabolism was reduced under HL/LL, and correlated with a decrease in the abundance of a range of amino acids in the HL/LL. Mitochondrial nucleoside diphosphate kinase was increased under LL/HL treatments. Significant accumulation of glycine decarboxylase P, T subunits and serine hydroxymethyltransferase occurred in response to light. The abundance of the glycine decarboxylase (GDC) H subunit proteins was not changed by HL/LL treatments, and the abundance of GDC L subunit protein was halved under HL, indicating a change in the stoichiometry of GDC subunits, while photorespiration was fourfold higher in LL- than in HL-treated plants. Insights into these light-dependent phenomena and their importance for understanding the initiation of photorespiration in rice and adaptation of mitochondria to function in photosynthetic cells are discussed.

Respiratory flexibility and efficiency are affected by simulated global change in Arctic plants

Laboratory studies indicate that, in response to environmental conditions, plants modulate respiratory electron partitioning between the 'energy-wasteful' alternative pathway (AP) and the 'energy-conserving' cytochrome pathway (CP). Field data, however, are scarce. Here we investigate how 20-yr field manipulations simulating global change affected electron partitioning in Alaskan Arctic tundra species. We sampled leaves from three dominant tundra species - Betula nana, Eriophorum vaginatum and Rubus chamaemorus - that had been strongly affected by manipulations of soil nutrients, light availability, and warming. We measured foliar dark respiration, in-vivo electron partitioning and alternative oxidase/cytochrome c oxidase concentrations in addition to leaf traits and mitochondrial ultrastructure. Changes in leaf traits and ultrastructure were similar across species. Respiration at 20°C (R(20)) was reduced 15% in all three species grown at elevated temperature, suggesting thermal acclimation of respiration. In Betula, the species with the largest growth response to added nutrients, CP activity increased from 9.4 ± 0.8 to 16.6 ± 1.6 nmol O(2) g(-1) DM s(-1) whereas AP activity was unchanged. The ability of Betula to selectively increase CP activity in response to the environment may contribute to its overall ecological success by increasing respiratory energy efficiency, and thus retaining more carbon for growth.

Leaf- and cell-level carbon cycling responses to a nitrogen and phosphorus gradient in two Arctic tundra species

PREMISE OF THE STUDY

Consequences of global climate change are detectable in the historically nitrogen- and phosphorus-limited Arctic tundra landscape and have implications for the terrestrial carbon cycle. Warmer temperatures and elevated soil nutrient availability associated with increased microbial activity may influence rates of photosynthesis and respiration. •

METHODS

This study examined leaf-level gas exchange, cellular ultrastructure, and related leaf traits in two dominant tundra species, Betula nana, a woody shrub, and Eriophorum vaginatum, a tussock sedge, under a 3-yr-old treatment gradient of nitrogen (N) and phosphorus (P) fertilization in the North Slope of Alaska. •

KEY RESULTS

Respiration increased with N and P addition-the highest rates corresponding to the highest concentrations of leaf N in both species. The inhibition of respiration by light ("Kok effect") significantly reduced respiration rates in both species (P < 0.001), ranged from 12-63% (mean 34%), and generally decreased with fertilization for both species. However, in both species, observed rates of photosynthesis did not increase, and photosynthetic nitrogen use efficiency generally decreased under increasing fertilization. Chloroplast and mitochondrial size and density were highly sensitive to N and P fertilization (P < 0.001), though species interactions indicated divergent cellular organizational strategies. •

CONCLUSIONS

Results from this study demonstrate a species-specific decoupling of respiration and photosynthesis under N and P fertilization, implying an alteration of the carbon balance of the tundra ecosystem under future conditions.

Leaf structural characteristics are less important than leaf chemical properties in determining the response of leaf mass per area and photosynthesis of Eucalyptus saligna to industrial-age changes in [CO2] and temperature

The rise in atmospheric [CO(2)] is associated with increasing air temperature. However, studies on plant responses to interactive effects of [CO(2)] and temperature are limited, particularly for leaf structural attributes. In this study, Eucalyptus saligna plants were grown in sun-lit glasshouses differing in [CO(2)] (290, 400, and 650 µmol mol(-1)) and temperature (26 °C and 30 °C). Leaf anatomy and chloroplast parameters were assessed with three-dimensional confocal microscopy, and the interactive effects of [CO(2)] and temperature were quantified. The relative influence of leaf structural attributes and chemical properties on the variation of leaf mass per area (LMA) and photosynthesis within these climate regimes was also determined. Leaf thickness and mesophyll size increased in higher [CO(2)] but decreased at the warmer temperature; no treatment interaction was observed. In pre-industrial [CO(2)], warming reduced chloroplast diameter without altering chloroplast number per cell, but the opposite pattern (reduced chloroplast number per cell and unchanged chloroplast diameter) was observed in both current and projected [CO(2)]. The variation of LMA was primarily explained by total non-structural carbohydrate (TNC) concentration rather than leaf thickness. Leaf photosynthetic capacity (light- and [CO(2)]-saturated rate at 28 °C) and light-saturated photosynthesis (under growth [CO(2)] and temperature) were primarily determined by leaf nitrogen contents, while secondarily affected by chloroplast gas exchange surface area and chloroplast number per cell, respectively. In conclusion, leaf structural attributes are less important than TNC and nitrogen in affecting LMA and photosynthesis responses to the studied climate regimes, indicating that leaf structural attributes have limited capacity to adjust these functional traits in a changing climate.

SLO2, a mitochondrial pentatricopeptide repeat protein affecting several RNA editing sites, is required for energy metabolism

Pentatricopeptide repeat (PPR) proteins belong to a family of approximately 450 members in Arabidopsis, of which few have been characterized. We identified loss of function alleles of SLO2, defective in a PPR protein belonging to the E+ subclass of the P-L-S subfamily. slo2 mutants are characterized by retarded leaf emergence, restricted root growth, and late flowering. This phenotype is enhanced in the absence of sucrose, suggesting a defect in energy metabolism. The slo2 growth retardation phenotypes are largely suppressed by supplying sugars or increasing light dosage or the concentration of CO₂. The SLO2 protein is localized in mitochondria. We identified four RNA editing defects and reduced editing at three sites in slo2 mutants. The resulting amino acid changes occur in four mitochondrial proteins belonging to complex I of the electron transport chain. Both the abundance and activity of complex I are highly reduced in the slo2 mutants, as well as the abundance of complexes III and IV. Moreover, ATP, NAD+, and sugar contents were much lower in the mutants. In contrast, the abundance of alternative oxidase was significantly enhanced. We propose that SLO2 is required for carbon energy balance in Arabidopsis by maintaining the abundance and/or activity of complexes I, III, and IV of the mitochondrial electron transport chain.

Leaf structural responses to pre-industrial, current and elevated atmospheric [CO2] and temperature affect leaf function in Eucalyptus sideroxylon

Leaf structure and chemistry both play critical roles in regulating photosynthesis. Yet, a key unresolved issue in climate change research is the role of changes in leaf structure in photosynthetic responses to temperature and atmospheric CO2 concentration ([CO2]), ranging from pre-industrial to future levels. We examined the interactive effects of [CO2] (290, 400 and 650μLL-1) and temperature (ambient, ambient +4°C) on leaf structural and chemical traits that regulate photosynthesis in Eucalyptus sideroxylon A.Cunn. ex Woolls. Rising [CO2] from pre-industrial to elevated levels increased light-saturated net photosynthetic rates (Asat), but reduced photosynthetic capacity (Amax). Changes in leaf N per unit area (Narea) and the number of palisade layers accounted for 56 and 14% of the variation in Amax, respectively, associated with changes in leaf mass per area. Elevated temperature increased stomatal frequency, but did not affect Amax. Further, rising [CO2] and temperature generally did not interactively affect leaf structure or function. These results suggest that leaf Narea and the number of palisade layers are the key chemical and structural factors regulating photosynthetic capacity of E. sideroxylon under rising [CO2], whereas the lack of photosynthetic responses to elevated temperature may reflect the limited effect of temperature on leaf structure and chemistry.

Effects of CO(2) enrichment on growth and development of Impatiens hawkeri

The effects of CO(2) enrichment on growth and development of Impatiens hawkeri, an important greenhouse flower, were investigated for the purpose of providing scientific basis for CO(2) enrichment to this species in greenhouse. The plants were grown in CO(2)-controlled growth chambers with 380 (the control) and 760 (CO(2) enrichment) μmol · mol(-1), respectively. The changes in morphology, physiology, biochemistry, and leaf ultrastructure of Impatiens were examined. Results showed that CO(2) enrichment increased flower number and relative leaf area compared with the control. In addition, CO(2) enrichment significantly enhanced photosynthetic rate, contents of soluble sugars and starch, activities of peroxidase (POD), superoxide dismutase (SOD), and ascorbate peroxidase (APX), but reduced chlorophyll content and malondialdehyde (MDA) content. Furthermore, significant changes in chloroplast ultrastructure were observed at CO(2) enrichment: an increased number of starch grains with an expanded size, and an increased ratio of stroma thylakoid to grana thylakoid. These results suggest that CO(2) enrichment had positive effects on Impatiens, that is, it can improve the visual value, promote growth and development, and enhance antioxidant capacity.

The effect of experimental warming on leaf functional traits, leaf structure and leaf biochemistry in Arabidopsis thaliana

BACKGROUND

The leaf is an important plant organ, and how it will respond to future global warming is a question that remains unanswered. The effects of experimental warming on leaf photosynthesis and respiration acclimation has been well studied so far, but relatively little information exists on the structural and biochemical responses to warming. However, such information is very important to better understand the plant responses to global warming. Therefore, we grew Arabidopsis thaliana at the three day/night temperatures of 23/18°C (ambient temperature), 25.5/20.5°C (elevated by 2.5°C) and 28/23°C (elevated by 5°C) to simulate the middle and the upper projected warming expected within the 21st century for this purpose.

RESULTS

The 28/23°C treatment significantly reduced the life span, total biomass and total weight of seeds compared with the other two temperatures. Among the three temperature regimes, the concentrations of starch, chlorophyll, and proline were the lowest at 28/23°C, whereas the total weight of seeds, concentrations of chlorophyll and proline, stomatal density (SD), stomatal conductance (gs), net CO2 assimilation rate (A) and transpiration rate (E) were the highest at 25.5/20.5°C. Furthermore, the number of chloroplasts per cell and mitochondrial size were highest at 25.5/20.5°C and lowest at 28/23°C.

CONCLUSIONS

The conditions whereby the temperature was increased by 2.5°C were advantageous for Arabidopsis. However, a rise of 5°C produced negative effects, suggesting that lower levels of warming may benefit plants, especially those which belong to the same functional group as Arabidopsis, whereas higher levels of warming may produce negative affects. In addition, the increase in A under moderately warm conditions may be attributed to the increase in SD, chlorophyll content, and number of chloroplasts. Furthermore, starch accumulation in chloroplasts may be the main factor influencing chloroplast ultrastructure, and elevated temperature regulates plant respiration by probably affecting mitochondrial size. Finally, high SOD and CAT activities may enable plants grown at elevated temperatures to exhibit relatively high tolerance to temperature stress, thus alleviating the harmful effects of superoxide anion radicals and hydrogen peroxide.

Diurnal changes in mitochondrial function reveal daily optimization of light and dark respiratory metabolism in Arabidopsis

Biomass production by plants is often negatively correlated with respiratory rate, but the value of this rate changes dramatically during diurnal cycles, and hence, biomass is the cumulative result of complex environment-dependent metabolic processes. Mitochondria in photosynthetic plant tissues undertake substantially different metabolic roles during light and dark periods that are dictated by substrate availability and the functional capacity of mitochondria defined by their protein composition. We surveyed the heterogeneity of the mitochondrial proteome and its function during a typical night and day cycle in Arabidopsis shoots. This used a staged, quantitative analysis of the proteome across 10 time points covering 24 h of the life of 3-week-old Arabidopsis shoots grown under 12-h dark and 12-h light conditions. Detailed analysis of enzyme capacities and substrate-dependent respiratory processes of isolated mitochondria were also undertaken during the same time course. Together these data reveal a range of dynamic changes in mitochondrial capacity and uncover day- and night-enhanced protein components. Clear diurnal changes were evident in mitochondrial capacities to drive the TCA cycle and to undertake functions associated with nitrogen and sulfur metabolism, redox poise, and mitochondrial antioxidant defense. These data quantify the nature and nuances of a daily rhythm in Arabidopsis mitochondrial respiratory capacity.

BVOC emissions, photosynthetic characteristics and changes in chloroplast ultrastructure of Platanus orientalis L. exposed to elevated CO2 and high temperature

To investigate the interactive effects of increasing [CO(2)] and heat wave occurrence on isoprene (IE) and methanol (ME) emissions, Platanus orientalis was grown for one month in ambient (380 micromol mol(-1)) or elevated (800 micromol mol(-1)) [CO(2)] and exposed to high temperature (HT) (38 degrees C/4 h). In pre-existing leaves, IE emissions were always higher but ME emissions lower as compared to newly-emerged leaves. They were both stimulated by HT. Elevated [CO(2)] significantly reduced IE in both leaf types, whereas it increased ME in newly-emerged leaves only. In newly-emerged leaves, elevated [CO(2)] decreased photosynthesis and altered the chloroplast ultrastructure and membrane integrity. These harmful effects were amplified by HT. HT did not cause any unfavorable effects in pre-existing leaves, which were characterized by inherently higher IE rates. We conclude that: (1) these results further prove the isoprene's putative thermo-protective role of membranes; (2) HT may likely outweigh the inhibitory effects of elevated [CO(2)] on IE in the future.

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.

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.