CO(2) Inhibits Respiration in Leaves of Rumex crispus L

Instantaneous Q10 of night-time leaf respiratory CO2 efflux - measurement and analytical protocol considerations

The temperature sensitivity (e.g. Q10) of night-time leaf respiratory CO2 efflux (RCO2) is a fundamental aspect of leaf physiology. The Q10 typically exhibits a dependence on measurement temperature, and it is speculated that this is due to temperature-dependent shifts in the relative control of leaf RCO2. Two decades ago, a review hypothesized that this mechanistically caused change in values of Q10 is predictable across plant taxa and biomes. Here, we discuss the most appropriate measuring protocol among existing data and for future data collection, to form the foundation of a future mechanistic understanding of Q10 of leaf RCO2 at different temperature ranges. We do this primarily via a review of existing literature on Q10 of night-time RCO2 and only supplement this to a lesser degree with our own original data. Based on mechanistic considerations, we encourage that instantaneous Q10 of leaf RCO2 to represent night-time should be measured: only at night-time; only in response to short-term narrow temperature variation (e.g. max. 10°C) to represent a given midpoint temperature at a time; in response to as many temperatures as possible within the chosen temperature range; and on still attached leaves.

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.

Leaf temperature and CO2 effects on photosynthetic CO2 assimilation and chlorophyll a fluorescence light responses during mid-ripening of Vitis vinifera cv. Shiraz grapevines grown in outdoor conditions

Responses of CO2 assimilation and chlorophyll a fluorescence to light intensity for Shiraz leaves on vines grown outdoors were examined in relation to leaf temperature. The study aimed to assess whether perturbing the carbon source, by manipulating short-term CO2 concentrations, would affect photosynthetic responses to temperature. Strong interactions occurred between leaf temperature and CO2 on photosynthetic and electron transport light responses. Most responses to temperature occurred at low to moderate CO2 and little response to temperature occurred at high CO2 . While assimilation responses accorded with increasing substrate CO2 , electron transport was inhibited by elevated CO2 . By contrast, chlorophyll a fluorescence was not affected by a temperature×CO2 interaction and CO2 had no effect on PSII quantum efficiency or photochemical quenching; whereas there was a moderate effect of temperature. Quantum efficiency of PSII was most severely reduced at low temperatures. Most photochemical quenching also occurred at low temperatures and the least at 40°C, in keeping with the warm to hot growth climate and the apparent assimilation bias towards the higher temperatures of the growing season. No changes in temperature dependency of assimilation were detected at the different CO2 concentrations, confirming sinks have a greater effect on assimilation than does the source.

Potential metabolic mechanisms for inhibited chloroplast nitrogen assimilation under high CO2

Improving photosynthesis is considered a major and feasible option to dramatically increase crop yield potential. Increased atmospheric CO2 concentration often stimulates both photosynthesis and crop yield, but decreases protein content in the main C3 cereal crops. This decreased protein content in crops constrains the benefits of elevated CO2 on crop yield and affects their nutritional value for humans. To support studies of photosynthetic nitrogen assimilation and its complex interaction with photosynthetic carbon metabolism for crop improvement, we developed a dynamic systems model of plant primary metabolism, which includes the Calvin-Benson cycle, the photorespiration pathway, starch synthesis, glycolysis-gluconeogenesis, the tricarboxylic acid cycle, and chloroplastic nitrogen assimilation. This model successfully captures responses of net photosynthetic CO2 uptake rate (A), respiration rate, and nitrogen assimilation rate to different irradiance and CO2 levels. We then used this model to predict inhibition of nitrogen assimilation under elevated CO2. The potential mechanisms underlying inhibited nitrogen assimilation under elevated CO2 were further explored with this model. Simulations suggest that enhancing the supply of α-ketoglutarate is a potential strategy to maintain high rates of nitrogen assimilation under elevated CO2. This model can be used as a heuristic tool to support research on interactions between photosynthesis, respiration, and nitrogen assimilation. It also provides a basic framework to support the design and engineering of C3 plant primary metabolism for enhanced photosynthetic efficiency and nitrogen assimilation in the coming high-CO2 world.

Funneliformis mosseae Improves Growth and Nutrient Accumulation in Wheat by Facilitating Soil Nutrient Uptake under Elevated CO2 at Daytime, Not Nighttime

The concurrent effect of elevated CO₂ (eCO₂) concentrations and arbuscular mycorrhizal fungi (AMF) on plant growth, carbon (C), nitrogen (N), phosphorus (P) and potassium (K) accumulations in plant and soil is largely unknown. To understand the mechanisms of eCO₂ and mycorrhization on wheat (Triticum aestivum) performance and soil fertility, wheat seedlings were grown under four different CO₂ environments for 12 weeks, including (1) ambient CO₂ (ACO₂, 410/460 ppm, daytime/nighttime), (2) sole daytime eCO₂ (DeCO₂, 550/460 ppm), (3) sole nighttime eCO₂ (NeCO₂, 410/610 ppm), and (4) dual or continuous daytime/nighttime eCO₂ ((D + N)eCO₂, 550/610 ppm), and with or without AMF (Funneliformis mosseae) colonization. DeCO₂, NeCO₂ and (D + N)eCO₂ generally significantly increased shoot and root biomass, plant C, N, P and K accumulation, soil invertase and urease activity, but decreased shoot and root N, P and K concentrations, and soil available N, P and K. Compared with non-AMF, AMF effects on above-mentioned characteristics were significantly positive under ACO₂, DeCO₂ and (D + N)eCO₂, but negative on plant biomass, C, N, P and K accumulation under NeCO₂. Overall, AMF colonization alleviated soil nutrient constraints on plant responses to DeCO₂, while NeCO₂ decreased AMF's beneficial effects on plants. These results demonstrated that an integration of AMF's benefits to plants under factual field DeCO₂ and/or NeCO₂ will be critical for managing the long-term consequence of future CO₂ rising on global cropping systems.

Daytime, Not Nighttime, Elevated Atmospheric Carbon Dioxide Exposure Improves Plant Growth and Leaf Quality of Mulberry (Morus alba L.) Seedlings

Almost all elevated atmospheric CO2 concentrations (eCO2) studies have not addressed the potential responses of plant growth to different CO2 in daytime and nighttime. The present study was to determine the impact of daytime and/or nighttime eCO2 on growth and quality of mulberry (Morus alba L.), a perennial multipurpose cash plant. Six-month-old mulberry seedlings were hence grown in environmentally auto-controlled growth chambers under four CO2 concentrations: (1) ambient CO2 (ACO2, 410 μmol mol-1 daytime/460 μmol mol-1 nighttime), (2) sole daytime elevated CO2 (DeCO2, 710 μmol mol-1/460 μmol mol-1), (3) sole nighttime elevated CO2 (NeCO2, 410 μmol mol-1/760 μmol mol-1), and (4) continuous daytime and nighttime elevated CO2 (D + NeCO2, 710 μmol mol-1/760 μmol mol-1). Plant growth characteristics, nutrient uptake, and leaf quality were then examined after 120 days of CO2 exposure. Compared to control, DeCO2 and (D + N)eCO2 increased plant biomass production and thus the harvest of nutrients and accumulation of leaf carbohydrates (starch, soluble sugar, and fatty acid) and N-containing compounds (free amino acid and protein), though there were some decreases in the concentration of leaf N, P, Mg, Fe, and Zn. NeCO2 had no significant effects on leaf yield but an extent positive effect on leaf nutritional quality due to their concentration increase in leaf B, Cu, starch, and soluble sugar. Meanwhile, (D + N)eCO2 decreased mulberry leaf yield and harvest of nutritious compounds for silkworm when compared with DeCO2. The reason may be associated to N, P, Mg, Fe, and Zn that are closely related to leaf pigment and N metabolism. Therefore, the rational application of mineral nutrient (especially N, P, Fe, Mg, and Zn) fertilizers is important for a sustainable mulberry production under future atmosphere CO2 concentrations.

Adaptive physiological response, carbon partitioning, and biomass production of Withania somnifera (L.) Dunal grown under elevated CO2 regimes

Winter cherry or Ashwagandha (Withania somnifera) is an important medicinal plant used in traditional and herbal medicine system. Yet, there is no information available on response of this plant to changing climatic conditions particularly elevated atmospheric CO2 concentrations. Therefore, we conducted an experiment to examine the effect of elevated CO2 concentrations (ECs) on Withania somnifera. The variations in traits of physiological adaptation, net primary productivity, carbon partitioning, morphology, and biomass in response to elevated CO2 concentrations (ambient, 600 and 800 µmol mol-1) during one growth cycle were investigated within the open top chamber (OTC) facility in the foothill of the Himalayas, Dehardun, India. ECs significantly increased photosynthetic rate, transpiration rate, stomatal conductance, water use efficiency, soil respiration, net primary productivity and the carbon content of plant tissues (leaf, stem, and root), and soil carbon. Furthermore, ECs significantly enhanced biomass production (root and shoot), although declined night leaf respiration. Overall, it was summarized that photosynthesis, stomatal conductance, water use efficiency, leaf, and soil carbon and biomass increased under ECs rendering the physiological adaptation to the plant. Increased net primary productivity might facilitate mitigation effects by sequestering elevated levels of carbon dioxide. We advocate further studies to investigate the effects of ECs on the accumulation of secondary metabolites and health-promoting substances of this as well as other medicinal plants.

The combination of gas-phase fluorophore technology and automation to enable high-throughput analysis of plant respiration

BACKGROUND

Mitochondrial respiration in the dark (Rdark) is a critical plant physiological process, and hence a reliable, efficient and high-throughput method of measuring variation in rates of Rdark is essential for agronomic and ecological studies. However, currently methods used to measure Rdark in plant tissues are typically low throughput. We assessed a high-throughput automated fluorophore system of detecting multiple O2 consumption rates. The fluorophore technique was compared with O2-electrodes, infrared gas analysers (IRGA), and membrane inlet mass spectrometry, to determine accuracy and speed of detecting respiratory fluxes.

RESULTS

The high-throughput fluorophore system provided stable measurements of Rdark in detached leaf and root tissues over many hours. High-throughput potential was evident in that the fluorophore system was 10 to 26-fold faster per sample measurement than other conventional methods. The versatility of the technique was evident in its enabling: (1) rapid screening of Rdark in 138 genotypes of wheat; and, (2) quantification of rarely-assessed whole-plant Rdark through dissection and simultaneous measurements of above- and below-ground organs.

DISCUSSION

Variation in absolute Rdark was observed between techniques, likely due to variation in sample conditions (i.e. liquid vs. gas-phase, open vs. closed systems), indicating that comparisons between studies using different measuring apparatus may not be feasible. However, the high-throughput protocol we present provided similar values of Rdark to the most commonly used IRGA instrument currently employed by plant scientists. Together with the greater than tenfold increase in sample processing speed, we conclude that the high-throughput protocol enables reliable, stable and reproducible measurements of Rdark on multiple samples simultaneously, irrespective of plant or tissue type.

The combination of gas-phase fluorophore technology and automation to enable high-throughput analysis of plant respiration

BACKGROUND

Mitochondrial respiration in the dark (R_dark) is a critical plant physiological process, and hence a reliable, efficient and high-throughput method of measuring variation in rates of R_dark is essential for agronomic and ecological studies. However, currently methods used to measure R_dark in plant tissues are typically low throughput. We assessed a high-throughput automated fluorophore system of detecting multiple O₂ consumption rates. The fluorophore technique was compared with O₂-electrodes, infrared gas analysers (IRGA), and membrane inlet mass spectrometry, to determine accuracy and speed of detecting respiratory fluxes.

RESULTS

The high-throughput fluorophore system provided stable measurements of R_dark in detached leaf and root tissues over many hours. High-throughput potential was evident in that the fluorophore system was 10 to 26-fold faster per sample measurement than other conventional methods. The versatility of the technique was evident in its enabling: (1) rapid screening of R_dark in 138 genotypes of wheat; and, (2) quantification of rarely-assessed whole-plant R_dark through dissection and simultaneous measurements of above- and below-ground organs.

DISCUSSION

Variation in absolute R_dark was observed between techniques, likely due to variation in sample conditions (i.e. liquid vs. gas-phase, open vs. closed systems), indicating that comparisons between studies using different measuring apparatus may not be feasible. However, the high-throughput protocol we present provided similar values of R_dark to the most commonly used IRGA instrument currently employed by plant scientists. Together with the greater than tenfold increase in sample processing speed, we conclude that the high-throughput protocol enables reliable, stable and reproducible measurements of R_dark on multiple samples simultaneously, irrespective of plant or tissue type.

Current methods for estimating the rate of photorespiration in leaves

Photorespiration is a process that competes with photosynthesis, in which Rubisco oxygenates, instead of carboxylates, its substrate ribulose 1,5-bisphosphate. The photorespiratory metabolism associated with the recovery of 3-phosphoglycerate is energetically costly and results in the release of previously fixed CO2. The ability to quantify photorespiration is gaining importance as a tool to help improve plant productivity in order to meet the increasing global food demand. In recent years, substantial progress has been made in the methods used to measure photorespiration. Current techniques are able to measure multiple aspects of photorespiration at different points along the photorespiratory C2 cycle. Six different methods used to estimate photorespiration are reviewed, and their advantages and disadvantages discussed.

C3 plants enhance rates of photosynthesis by reassimilating photorespired and respired CO2

Photosynthetic carbon gain in plants using the C(3) photosynthetic pathway is substantially inhibited by photorespiration in warm environments, particularly in atmospheres with low CO(2) concentrations. Unlike C(4) plants, C(3) plants are thought to lack any mechanism to compensate for the loss of photosynthetic productivity caused by photorespiration. Here, for the first time, we demonstrate that the C(3) plants rice and wheat employ a specific mechanism to trap and reassimilate photorespired CO(2) . A continuous layer of chloroplasts covering the portion of the mesophyll cell periphery that is exposed to the intercellular air space creates a diffusion barrier for CO(2) exiting the cell. This facilitates the capture and reassimilation of photorespired CO(2) in the chloroplast stroma. In both species, 24-38% of photorespired and respired CO(2) were reassimilated within the cell, thereby boosting photosynthesis by 8-11% at ambient atmospheric CO(2) concentration and 17-33% at a CO(2) concentration of 200 µmol mol(-1) . Widespread use of this mechanism in tropical and subtropical C(3) plants could explain why the diversity of the world's C(3) flora, and dominance of terrestrial net primary productivity, was maintained during the Pleistocene, when atmospheric CO(2) concentrations fell below 200 µmol mol(-1) .

Excess CO2 supply inhibits mixotrophic growth of Chlorella protothecoides and Nannochloropsis salina

Mixotrophy can be exploited to support algal growth over night or in dark-zones of a photobioreactor. In order to achieve the maximal productivity, however, it is fundamental also to provide CO(2) in excess to maximize photosynthetic activity and phototropic biomass production. The aim of this paper is to verify the possibility of exploiting mixotrophy in combination with excess CO(2). Two species with high biomass productivity were selected, Nannochloropsis salina and Chlorella protothecoides. Different organic substrates available at industrial scale were tested, and glycerol chosen for its ability to support growth of both species. In mixotrophic conditions, excess CO(2) stimulated photosynthesis but blocked the metabolization of the organic substrate, thus canceling the advantages of mixotrophy. By cultivating microalgae under day-night cycle, organic substrate supported growth during the night, but only if CO(2) supply was not provided. This represents thus a possible method to reconcile CO(2) stimulation of photosynthesis with mixotrophy.

Characteristics of CO2 exchange between peach stems and the atmosphere

Gas exchange by stems is dominated by respiratory CO₂ emission, but photosynthetic CO₂ uptake might also occur in stem bark. We show that light-dependent CO₂ uptake was present and often exceeded CO₂ release by respiration in illuminated current-year peach (Prunus persica L.) stems. Respiration of peach stems, as detected by ¹²CO₂ release into air in which the natural concentration of ¹²CO₂ was replaced with ¹³CO₂, was lower in the light than in the dark, but this accounted for only a fraction of the observed total CO₂ uptake by illuminated stems. Stem photosynthesis was saturated at low light and was negatively affected by elevated assay temperatures (30°C), especially when combined with light intensities above saturation. An inefficient mechanism of heat dissipation by transpiration in stomata-free stems might help explain this effect. Photosynthesis was rapidly stimulated and the electron transport rate was reduced when photorespiration was suppressed by exposure to low (2 kPa) oxygen. The time-course of these changes was closely associated with a transient burst of CO₂ uptake concurrent with a reduced inhibition of fluorescence yield. Photosynthesis was also stimulated by exposure to elevated (twice ambient) CO₂ concentration. These combined measurements of gas exchange and fluorescence suggested that (a) photorespiration may also be active in the bark of peach stems, (b) O₂ and CO₂ concentrations in the bark of peach stems may be similar to ambient concentrations, (c) a large amount of electron transport unrelated to photosynthesis and photorespiration may also be present in peach stems, and (d) stem photosynthesis may be enhanced under future atmospheric conditions.

Plant respiration and elevated atmospheric CO2 concentration: cellular responses and global significance

BACKGROUND

Elevated levels of atmospheric [CO2] are likely to enhance photosynthesis and plant growth, which, in turn, should result in increased specific and whole-plant respiration rates. However, a large body of literature has shown that specific respiration rates of plant tissues are often reduced when plants are exposed to, or grown at, high [CO2] due to direct effects on enzymes and indirect effects derived from changes in the plant's chemical composition.

SCOPE

Although measurement artefacts may have affected some of the previously reported effects of CO2 on respiration rates, the direction and magnitude for the effects of elevated [CO2] on plant respiration may largely depend on the vertical scale (from enzymes to ecosystems) at which measurements are taken. In this review, the effects of elevated [CO2] from cells to ecosystems are presented within the context of the enzymatic and physiological controls of plant respiration, the role(s) of non-phosphorylating pathways, and possible effects associated with plant size.

CONCLUSIONS

Contrary to what was previously thought, specific respiration rates are generally not reduced when plants are grown at elevated [CO2]. However, whole ecosystem studies show that canopy respiration does not increase proportionally to increases in biomass in response to elevated [CO2], although a larger proportion of respiration takes place in the root system. Fundamental information is still lacking on how respiration and the processes supported by it are physiologically controlled, thereby preventing sound interpretations of what seem to be species-specific responses of respiration to elevated [CO2]. Therefore the role of plant respiration in augmenting the sink capacity of terrestrial ecosystems is still uncertain.

Night temperature has a minimal effect on respiration and growth in rapidly growing plants

BACKGROUND AND AIMS

Carbon gain depends on efficient photosynthesis and adequate respiration. The effect of temperature on photosynthetic efficiency is well understood. In contrast, the temperature response of respiration is based almost entirely on short-term (hours) measurements in mature organisms to develop Q(10) values for maintenance and whole-plant respiration. These Q(10) values are then used to extrapolate across whole life cycles to predict the influence of temperature on plant growth.

METHODS

In this study, night temperature in young, rapidly growing plant communities was altered from 17 to 34 degrees C for up to 20 d. Day temperature was maintained at 25 degrees C. CO(2) gas-exchange was continuously monitored in ten separate chambers to quantify the effect of night-temperature on respiration, photosynthesis and the efficiency of carbon gain (carbon use efficiency).

KEY RESULTS

Respiration increased only 20-46 % for each 10 degrees C rise in temperature (total respiratory Q(10) of between 1.2 to about 1.5). This change resulted in only a 2-12 % change in carbon use efficiency, and there was no effect on cumulative carbon gain or dry mass. No acclimation of respiration was observed after 20 d of treatment.

CONCLUSIONS

These findings indicate that whole-plant respiration of rapidly growing plants has a small sensitivity to temperature, and that the sensitivity does not change among the species tested, even after 20 d of treatment. Finally, the results support respiration models that separate respiration into growth and maintenance components.

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]

Averaged across many previous investigations, doubling the CO2 concentration ([CO2]) has frequently been reported to cause an instantaneous reduction of leaf dark respiration measured as CO2 efflux. No known mechanism accounts for this effect, and four recent studies have shown that the measurement of respiratory CO2 efflux is prone to experimental artifacts that could account for the reported response. Here, these artifacts are avoided by use of a high-resolution dual channel oxygen analyzer within an open gas exchange system to measure respiratory O2 uptake in normal air. Leaf O2 uptake was determined in response to instantaneous elevation of [CO2] in nine contrasting species and to long-term elevation in seven species from four field experiments. Over six hundred separate measurements of respiration failed to reveal any decrease in respiratory O2 uptake with an instantaneous increase in [CO2]. Respiration was found insensitive not only to doubling [CO2], but also to a 5-fold increase and to decrease to zero. Using a wide range of species and conditions, we confirm earlier reports that inhibition of respiration by instantaneous elevation of [CO2] is likely an experimental artifact. Instead of the expected decrease in respiration per unit leaf area in response to long-term growth in the field at elevated [CO2], there was a significant increase of 11% and 7% on an area and mass basis, respectively, averaged across all experiments. The findings suggest that leaf dark respiration will increase not decrease as atmospheric [CO2] rises.

Seasonal variation in respiration of 1-year-old shoots of scots pine exposed to elevated carbon dioxide and temperature for 4 years

Sixteen 20-year-old Scots pine (Pinus sylvestris L.) trees growing in the field were enclosed for 4 years in environment-controlled chambers that maintained: (1) ambient conditions (CON); (2) elevated atmospheric CO2 concentration (ambient + 350 micro mol mol-1; EC); (3) elevated temperature (ambient +2-6 degrees C; ET); or (4) elevated CO2 and elevated temperature (ECT). The dark respiration rates of 1-year-old shoots, from which needles had been partly removed, were measured over the growing season in the fourth year. In all treatments, the temperature coefficient of respiration, Q10, changed with season, being smaller during the growing season than at other times. Respiration rate varied diurnally and seasonally with temperature, being highest around mid-summer and declining gradually thereafter. When measurements were made at the temperature of the chamber, respiration rates were reduced by the EC treatment relative to CON, but were increased by ET and ECT treatments. However, respiration rates at a reference temperature of 15 degrees C were reduced by ET and ECT treatments, reflecting a decreased capacity for respiration at warmer temperatures (negative acclimation). The interaction between season and treatment was not significant. Growth respiration did not differ between treatments, but maintenance respiration did, and the differences in mean daily respiration rate between the treatments were attributable to the maintenance component. We conclude that maintenance respiration should be considered when modelling respiratory responses to elevated CO2 and elevated temperature, and that increased atmospheric temperature is more important than increasing CO2 when assessing the carbon budget of pine forests under conditions of climate change.

Measuring and simulating crown respiration of Scots pine with increased temperature and carbon dioxide enrichment

Acclimation to elevated atmospheric carbon dioxide concentration and temperature of respiration by the foliage in the crown of Scots pine (Pinus sylvestris) trees is measured and modelled. Starting in 1996, individual 20-year-old trees were enclosed in chambers and exposed to either normal ambient conditions (CON), elevated CO2 concentration (EC), elevated temperature (ET) or a combination of EC and ET (ECT). Respiration of individual leaves within the crown was measured in 2000. To extrapolate the response of respiration of individual leaves to the whole crown, a multi-layer model was developed and used to predict daily and annual crown respiration, in which the crown structure and corresponding microclimate data were used as input. Respiration measurements showed that EC led to higher Q10 values (4.6%) relative to CON, but lower basal respiration rates at 20 degrees C [R1.d(20)] (-7.1%) during the main growth season (days 120-240), whereas ET and ECT both reduced Q10 (-12.0 and -9.8%, respectively) throughout the year but increased R1.d(20) (27.2 and 21.6%, respectively) during the period of no-growth, and slightly reduced R1.d(20) (-1.7 and -2.8%, respectively) during the main growth season. Model computations showed that annual crown respiration increased: (1) by 16% in EC, with 92% of this increase attributable to the increase in foliage area; (2) by 35% in ET, with 66% related to the increase in foliage area and 17% to the rise in ambient temperature; and (3) by 27% in the case of ECT, with 43% attributable to the increase in foliage area and 29% to the rise in ambient temperature. Changed respiration parameters for individual leaves, induced by treatments, made only a small contribution to the annual crown respiration compared with the increased foliage area. The effects of changes in crown architecture and nitrogen distribution, caused by treatments, on the daily and annual course of crown respiration are discussed.

Leaf O(2) uptake in the dark is independent of coincident CO(2) partial pressure

Elevated CO(2), in the dark, is sometimes reported to inhibit leaf respiration, with respiration usually measured as CO(2) efflux. Oxygen uptake may be a better gauge of respiration because non-respiratory processes can affect dark CO(2) efflux in elevated CO(2). Two methods of quantifying O(2) uptake indicated that leaf respiration was unaffected by coincident CO(2) level in the dark.

Interactive effects of ozone and elevated carbon dioxide on the growth and physiology of black cherry, green ash, and yellow-poplar seedlings

Potted seedlings of black cherry (Prunus serotina Ehrh.) (BC), green ash (Fraxinus pennsylvanica Marsh.) (GA), and yellow-poplar (Liriodendron tulipifera L.) (YP) were exposed to one of the four treatments: (1) charcoal-filtered air (CF) at ambient CO(2) (control); (2) twice ambient O(3) (2 x O(3)); (3) twice ambient CO(2) (650 microl l(-1)) plus CF air (2 x CO(2)); or (4) twice ambient CO(2) (650 microl l(-1)) plus twice ambient O(3) (2 x CO(2) + 2 x O(3)). The treatments were duplicated in eight continuously stirred tank reactors for 10 weeks. Gas exchange was measured during the last 3 weeks of treatment and all seedlings were destructively harvested after 10 weeks. Significant interactive effects of O(3) and CO(2) on the gas exchange of all three species were limited. The effects of elevated CO(2) and O(3), singly and combined, on light-saturated net photosynthesis (A(max)) and stomatal conductance (g(s)) were inconsistent across species. In all three species, elevated O(3) had no effect on g(s). Elevated CO(2) significantly increased A(max) in GA and YP foliage, and decreased g(s) in YP foliage. Maximum carbon exchange rates and quantum efficiencies derived from light-response curves increased, while compensation irradiance and dark respiration decreased in all three species when exposed to 2 x CO(2). Elevated O(3) affected few of these parameters but any change that was observed was opposite to that from exposure to 2 x CO(2)-air. Interactive effects of CO(2) and O(3) on light-response parameters were limited. Carboxylation efficiencies, derived from CO(2)-response curves (A/C(i) curves) decreased only in YP foliage exposed to 2 x CO(2)-air. In general, growth was significantly stimulated by 2 x CO(2) in all three species; though there were few significant growth responses following exposure to 2 x O(3) or the combination of 2 x CO(2) plus 2 x O(3). Results indicate that responses to interacting stressors such as O(3) and CO(2) are species specific.

Regulating plant/insect interactions using CO2 enrichment in model ecosystems

The greenhouse environment is a challenging artificial ecosystem in which it is possible to study selected plant/insect interaction in a controlled environment. Due to a combination of "direct" and "indirect" effects of CO2 enrichment on plant photosynthesis and plant development, canopy productivity is generally increased. In this paper, we discuss the effects of daytime and nighttime CO2 enrichment protocols on gas exchange of pepper plants (Capsicum annuum L, cv Cubico) grown in controlled environments. In addition, we present the effects of thrips, a common Insect pest, on the photosynthetic and respiratory activity of these plant canopies. Carbon dioxide has diverse effects on the physiology and mortality of insects. However, our data indicate that thrips and whiteflies, at least, are not killed "directly" by CO2 levels used to enhance photosynthesis and plant growth. Together the data suggest that the insect population is affected "indirectly" by CO2 and that the primary effect of CO2 is via its effects on plant metabolism.

Impacts of CO2 Enrichment on Productivity and Light Requirements of Eelgrass

Seagrasses, although well adapted for submerged existence, are CO2-limited and photosynthetically inefficient in seawater. This leads to high light requirements for growth and survival and makes seagrasses vulnerable to light limitation. We explored the long-term impact of increased CO2 availability on light requirements, productivity, and C allocation in eelgrass (Zostera marina L.). Enrichment of seawater CO2 increased photosynthesis 3-fold, but had no long-term impact on respiration. By tripling the rate of light-saturated photosynthesis, CO2 enrichment reduced the daily period of irradiance-saturated photosynthesis (Hsat) that is required for the maintenance of positive whole-plant C balance from 7 to 2.7 h, allowing plants maintained under 4 h of Hsat to perform like plants growing in unenriched seawater with 12 h of Hsat. Eelgrass grown under 4 h of Hsat without added CO2 consumed internal C reserves as photosynthesis rates and chlorophyll levels dropped. Growth ceased after 30 d. Leaf photosynthesis, respiration, chlorophyll, and sucrose-phosphate synthase activity of CO2-enriched plants showed no acclimation to prolonged enrichment. Thus, the CO2-stimulated improvement in photosynthesis reduced light requirements in the long term, suggesting that globally increasing CO2 may enhance seagrass survival in eutrophic coastal waters, where populations have been devastated by algal proliferation and reduced water-column light transparency.

MORE EFFICIENT PLANTS: A Consequence of Rising Atmospheric CO2?

The primary effect of the response of plants to rising atmospheric CO2 (Ca) is to increase resource use efficiency. Elevated Ca reduces stomatal conductance and transpiration and improves water use efficiency, and at the same time it stimulates higher rates of photosynthesis and increases light-use efficiency. Acclimation of photosynthesis during long-term exposure to elevated Ca reduces key enzymes of the photosynthetic carbon reduction cycle, and this increases nutrient use efficiency. Improved soil-water balance, increased carbon uptake in the shade, greater carbon to nitrogen ratio, and reduced nutrient quality for insect and animal grazers are all possibilities that have been observed in field studies of the effects of elevated Ca. These effects have major consequences for agriculture and native ecosystems in a world of rising atmospheric Ca and climate change.

Direct Inhibition of Plant Mitochondrial Respiration by Elevated CO2

Doubling the concentration of atmospheric CO2 often inhibits plant respiration, but the mechanistic basis of this effect is unknown. We investigated the direct effects of increasing the concentration of CO2 by 360 [mu]L L-1 above ambient on O2 uptake in isolated mitochondria from soybean (Glycine max L. cv Ransom) cotyledons. Increasing the CO2 concentration inhibited the oxidation of succinate, external NADH, and succinate and external NADH combined. The inhibition was greater when mitochondria were preincubated for 10 min in the presence of the elevated CO2 concentration prior to the measurement of O2 uptake. Elevated CO2 concentration inhibited the salicylhydroxamic acid-resistant cytochrome pathway, but had no direct effect on the cyanide-resistant alternative pathway. We also investigated the direct effects of elevated CO2 concentration on the activities of cytochrome c oxidase and succinate dehydrogenase (SDH) and found that the activity of both enzymes was inhibited. The kinetics of inhibition of cytochrome c oxidase were time-dependent. The level of SDH inhibition depended on the concentration of succinate in the reaction mixture. Direct inhibition of respiration by elevated CO2 in plants and intact tissues may be due at least in part to the inhibition of cytochrome c oxidase and SDH.

Dark Leaf Respiration in Light and Darkness of an Evergreen and a Deciduous Plant Species

Dark respiration in light as well as in dark was estimated for attached leaves of an evergreen (Heteromeles arbutifolia Ait.) and a deciduous (Lepechinia fragans Greene) shrub species using an open gas-exchange system. Dark respiration in light was estimated by the Laisk method. Respiration rates in the dark were always higher than in the light, indicating that light inhibited respiration in both species. The rates of respiration in the dark were higher in the leaves of the deciduous species than in the evergreen species. However, there were no significant differences in respiration rates in light between the species. Thus, the degree of inhibition of respiration by light was greater in the deciduous species (62%) than in the evergreen species (51%). Respiration in both the light and darkness decreased with increasing leaf age. However, because respiration in the light decreased faster with leaf age than respiration in darkness, the degree of inhibition of respiration by light increased with leaf age (from 36% in the youngest leaves to 81% in the mature leaves). This suggests that the rate of dark respiration in the light is related to the rate of biosynthetic processes. Dark respiration in the light decreased with increasing light intensity. Respiration both in the light and in the dark was dependent on leaf temperature. We concluded that respiration in light and respiration in darkness are tightly coupled, with variation in respiration in darkness accounting for more than 60% of the variation in respiration in light. Care must be taken when the relation between respiration in light and respiration in darkness is studied, because the relation varies with species, leaf age, and light intensity.

Energy content, construction cost and phytomass accumulation of Glycine max (L.) Merr. and Sorghum bicolor (L.) Moench grown in elevated CO2 in the field

Grain sorghum [Sorghum bicolor (L.) Moench, a C₄ crop] and soybean [Glycine max (L.) Merr. cv. Stonewall, a C₃ crop] plants were grown in ambient (c. 360μl 1^-1 ) and twice-ambient (c. 720 μl 1^-1 ) CO₂ levels in open-top chambers in soil without root constriction. Plant dry mass, energy content, composition and construction cost (i.e. amount of carbohydrate required to synthesize a unit of plant dry mass) were assessed at the end of the growing season. Elevated CO₂ (a) increased phytomass accumulation (kg per plant) in both species, (b) had little affect on energy concentration (MJ kg^-1 plant) but caused large increases in the amount of plant energy per ground area (MJ m^-2 ground), and (c) did not alter specific growth cost (kg carbohydrate kg^-1 plant growth) but greatly increased growth cost per ground area (kg carbohydrate m^-2 ground) because growth was enhanced. For soybean, twice-ambient CO₂ resulted in a 50 % increase in the amount of nitrogen and energy in grain (seed plus pod) per ground area. This response to elevated CO₂ has important implications for agricultural productivity during the next century because the rate of human population growth is exceeding the rate of increase of land used for agriculture so that future food demands can only be met by greater production per ground area.

Acclimation of Respiratory O2 Uptake in Green Tissues of Field-Grown Native Species after Long-Term Exposure to Elevated Atmospheric CO2

C3 and C4 plants were grown in open-top chambers in the field at two CO2 concentrations, normal ambient (ambient) and normal ambient + 340 [mu]LL-1 (elevated). Dark oxygen uptake was measured in leaves and stems using a liquid-phase Clark-type oxygen electrode. High CO2 treatment decreased dark oxygen uptake in stems of Scirpus olneyi (C3) and leaves of Lindera benzoin (C3) expressed on either a dry weight or area basis. Respiration of Spartina patens (C4) leaves was unaffected by CO2 treatment. Leaf dry weight per unit area was unchanged by CO2, but respiration per unit of carbon or per unit of nitrogen was decreased in the C3 species grown at high CO2. The component of respiration in stems of S. olneyi and leaves of L. benzoin primarily affected by long-term exposure to the elevated CO2 treatment was the activity of the cytochrome pathway. Elevated CO2 had no effect on activity and capacity of the alternative pathway in S. olneyi. The cytochrome c oxidase activity, assayed in a cell-free extract, was strongly decreased by growth at high CO2 in stems of S. olneyi but it was unaffected in S. patens leaves. The activity of cytochrome c oxidase and complex III extracted from mature leaves of L. benzoin was also decreased after one growing season of plant exposure to elevated CO2 concentration. These results show that in some C3 species respiration will be reduced when plants are grown in elevated atmospheric CO2. The possible physiological causes and implications of these effects are discussed.

Aerenchyma Carbon Dioxide Can Be Assimilated in Typha Iatifolia L. Leaves

Leaf structural characteristics and gas-exchange measurements were used to determine whether photosynthetic tissue of Typha Iatifolia L. (cattail) utilized CO2 from the aerenchyma gas spaces, part of an internal pathway for gas transport in this wetland species. The partial pressure of CO2 (pCO2) in these aerenchyma gas spaces can be more than 10 times atmospheric pCO2. The photosynthetic tissue occurred in structurally similar adaxial and abaxial palisades, which were distinctly separated from each other by the aerenchyma gas spaces. In each palisade there were three to four layers of tightly packed, nonchlorophyllous cells separating the photosynthetic tissue from the aerenchyma gas space. Different lines of evidence indicated that CO2 conductance in the light was significantly greater across the epidermal surface than across the internal surface of both palisades. However, at an epidermal pCO2 of 350 [mu]bars and an internal pCO2 of 820 [mu]bars, the net rates of CO2 uptake (PN) across the epidermal and internal surfaces were about equal. PN across the internal surface was greater than across the epidermal surface at higher internal pCO2. Gas space pCO2 can be greater than 820 [mu]bars in the field, and therefore, PN across the internal surface could be a significant proportion of epidermal surface PN.

High soil carbon dioxide concentrations inhibit root respiration of Douglas fir

Total and basal respiration (R_t and R_b , respectively) of intact and undisturbed roots of one-year-old Douglas fir seedlings, Pseudotsuga menziesii var. glauca [Beissn] Franco, were measured at experimentally varied soil carbon dioxide concentrations ([CO₂ ]). Use of specially designed root boxes and a CO₂ gas-flow compensating system designed around an infrared gas analyzer (IRGA) allowed controlled delivery of CO₂ to roots and simultaneous measurements of CO₂ released by roots. Root respiration rate responded to each inlet [CO₂ ], independent of whether the previous concentration had been higher or lower, within two to three hours (paired t test = 0.041, P= 0.622, and n= 13). Total and basal respiration rates decreased exponentially as soil [CO₂ ] rose from 130 ppm, well below atmospheric [CO₂ ], to 7015 ppm, a concentration not uncommon in field soils. Analyses of variance (ANOVA) showed that the effects of soil [CO₂ ] on rates of total and basal root respiration were statistically significant. Root respiration rates decreased by 4 to 5 nmol CO₂ g^-1 dry weight of roots s^-1 for every doubling of [CO₂ ] according to the following equations: ln(R₁ ) (nmol CO₂ g^-1 s^-1 ) = 5.24-0.30*ln[CO₂ ] with r= 0.78, P < 0.0001, and n= 70; and ln(R_b ) (nmol CO₂ g^-1 s^-1 ) = 6.29-0.52 * ln[CO₂ ] with r= 0.82, P < 0.0001, and n= 35. The sensitivity of root respiration to [CO₂ ] suggests that some previous laboratory measurements of root respiration at atmospheric [CO₂ ], which is 3 to 10-fold lower than [CO₂ ] in field soils, overestimated root respiration in the field. Further, the potential importance of soil [CO₂ ] indicates that it should be accounted for in models of below-ground carbon budgets.

Comparison of Methods to Estimate Dark Respiration in the Light in Leaves of Two Woody Species

Dark respiration in the light was estimated in leaves of two woody species (Heteromeles arbutifolia Ait. and Lepechinia fragans Greene) using two different approaches based on gas-exchange techniques: the Kok method and the Laisk method. In all cases, dark respiration in the light was lower (P < 0.05) than respiration in darkness, indicating that dark respiration was inhibited in the light. Rates of dark respiration in the light estimated by the Laisk method were 52% higher (P < 0.05) than those estimated by the Kok method. Differences between the methods could be explained by the low ambient CO2 concentrations required by the Laisk approach. The mean value of the inhibition of respiration by light for the two species, corrected for the ambient CO2 concentration effect, was 55%. Despite the differences in leaf characteristics between the species, values of the CO2 photocompensation point, at which the rate of photosynthetic CO2 uptake equaled that of photorespiratory CO2 evolution, were very constant, suggesting an excellent consistency in the results obtained with the Laisk approach.

Scaling CO2-photosynthesis relationships from the leaf to the canopy

Responses of individual leaves to short-term changes in CO2 partial pressure have been relatively well studied. Whole-plant and plant community responses to elevated CO2 are less well understood and scaling up from leaves to canopies will be complicated if feedbacks at the small scale differ from feedbacks at the large scale. Mathematical models of leaf, canopy, and ecosystem processes are important tools in the study of effects on plants and ecosystems of global environmental change, and in particular increasing atmospheric CO2, and might be used to scale from leaves to canopies. Models are also important in assessing effects of the biosphere on the atmosphere. Presently, multilayer and big leaf models of canopy photosynthesis and energy exchange exist. Big leaf models - which are advocated here as being applicable to the evaluation of impacts of 'global change' on the biosphere - simplify much of the underlying leaf-level physics, physiology, and biochemistry, yet can retain the important features of plant-environment interactions with respect to leaf CO2 exchange processes and are able to make useful, quantitative predictions of canopy and community responses to environmental change. The basis of some big leaf models of photosynthesis, including a new model described herein, is that photosynthetic capacity and activity are scaled vertically within a canopy (by plants themselves) to match approximately the vertical profile of PPFD. The new big leaf model combines physically based models of leaf and canopy level transport processes with a biochemically based model of CO2 assimilation. Predictions made by the model are consistent with canopy CO2 exchange measurements, although a need exists for further testing of this and other canopy physiology models with independent measurements of canopy mass and energy exchange at the time scale of 1 h or less.

Direct and Indirect Effects of Atmospheric Carbon Dioxide Enrichment on Leaf Respiration of Glycine max (L.) Merr

Long-term and short-term effects of CO2 enrichment on dark respiration were investigated using soybean (Glycine max [L.] Merr.) plants grown at either 35.5 or 71.0 Pa CO2. Indirect effects, or effects of growth in elevated CO2, were examined using a functional model that partitioned respiration into growth and maintenance components. Direct effects, or immediate effects of a short-term change in CO2, were examined by measuring dark respiration, first, at the CO2 partial pressure at which plants were grown, and second, after equilibration in the reciprocal CO2 partial pressure. The functional component model indicated that the maintenance coefficient of respiration increased 34% with elevated CO2, whereas the growth coefficient was not significantly affected. Changes in maintenance respiration were correlated with a 33% increase in leaf total nonstructural carbohydrate concentration, but leaf nitrogen content of soybean leaves was not affected by CO2 enrichment. Thus, increased maintenance respiration may be a consequence of increased nonstructural carbohydrate accumulation. When whole soybean plants were switched from low CO2 to high CO2 for a brief period, leaf respiration was always reduced. However, this direct effect of CO2 partial pressure was approximately 50% less in plants grown in elevated CO2. We conclude from this study that there are potentially important effects of CO2 enrichment on plant respiration but that the effects are different for plants given a short-term increase in CO2 partial pressure versus plants grown in elevated CO2.