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

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.

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.

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

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

An ATP synthase harboring an atypical γ-subunit is involved in ATP synthesis in tomato fruit chromoplasts

Chromoplasts are non-photosynthetic plastids specialized in the synthesis and accumulation of carotenoids. During fruit ripening, chloroplasts differentiate into photosynthetically inactive chromoplasts in a process characterized by the degradation of the thylakoid membranes, and by the active synthesis and accumulation of carotenoids. This transition renders chromoplasts unable to photochemically synthesize ATP, and therefore these organelles need to obtain the ATP required for anabolic processes through alternative sources. It is widely accepted that the ATP used for biosynthetic processes in non-photosynthetic plastids is imported from the cytosol or is obtained through glycolysis. In this work, however, we show that isolated tomato (Solanum lycopersicum) fruit chromoplasts are able to synthesize ATP de novo through a respiratory pathway using NADPH as an electron donor. We also report the involvement of a plastidial ATP synthase harboring an atypical γ-subunit induced during ripening, which lacks the regulatory dithiol domain present in plant and algae chloroplast γ-subunits. Silencing of this atypical γ-subunit during fruit ripening impairs the capacity of isolated chromoplast to synthesize ATP de novo. We propose that the replacement of the γ-subunit present in tomato leaf and green fruit chloroplasts by the atypical γ-subunit lacking the dithiol domain during fruit ripening reflects evolutionary changes, which allow the operation of chromoplast ATP synthase under the particular physiological conditions found in this organelle.

Plastic and adaptive responses of plant respiration to changes in atmospheric CO(2) concentration

The concentration of atmospheric CO2 has increased from below 200 microl l(-1) during last glacial maximum in the late Pleistocene to near 280 microl l(-1) at the beginning of the Holocene and has continuously increased since the onset of the industrial revolution. Most responses of plants to increasing atmospheric CO2 levels result in increases in photosynthesis, water use efficiency and biomass. Less known is the role that respiration may play during adaptive responses of plants to changes in atmospheric CO2. Although plant respiration does not increase proportionally with CO2-enhanced photosynthesis or growth rates, a reduction in respiratory costs in plants grown at subambient CO2 can aid in maintaining a positive plant C-balance (i.e. enhancing the photosynthesis-to-respiration ratio). The understanding of plant respiration is further complicated by the presence of the alternative pathway that consumes photosynthate without producing chemical energy [adenosine triphosphate (ATP)] as effectively as respiration through the normal cytochrome pathway. Here, we present the respiratory responses of Arabidopsis thaliana plants selected at Pleistocene (200 microl l(-1)), current Holocene (370 microl l(-1)), and elevated (700 microl l(-1)) concentrations of CO2 and grown at current CO2 levels. We found that respiration rates were lower in Pleistocene-adapted plants when compared with Holocene ones, and that a substantial reduction in respiration was because of reduced activity of the alternative pathway. In a survey of the literature, we found that changes in respiration across plant growth forms and CO2 levels can be explained in part by differences in the respiratory energy demand for maintenance of biomass. This trend was substantiated in the Arabidopsis experiment in which Pleistocene-adapted plants exhibited decreases in respiration without concurrent reductions in tissue N content. Interestingly, N-based respiration rates of plants adapted to elevated CO2 also decreased. As a result, ATP yields per unit of N increased in Pleistocene-adapted plants compared with current CO2 adapted ones. Our results suggest that mitochondrial energy coupling and alternative pathway-mediated responses of respiration to changes in atmospheric CO2 may enhance survival of plants at low CO2 levels to help overcome a low carbon balance. Therefore, increases in the basal activity of the alternative pathway are not necessarily associated to metabolic plant stress in all cases.

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

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

Transient shade and drought have divergent impacts on the temperature sensitivity of dark respiration in leaves of Geum urbanum

The respiratory response of plants to temperature is a critical biotic feedback in the study of global climate change. Few studies, however, have investigated the effects of environmental stresses on the short-term temperature response of dark respiration (R_dark) at the leaf level. We investigated the effect of shade and transient drought on the temperature sensitivity (Q₁₀; the proportional increase in respiration per 10°C increase in temperature) of R_dark of Geum urbanum L. in controlled experiments. Shade effects were most pronounced following sustained, near-darkness, when rates of leaf R_dark at a set measuring temperature (25°C) and the Q₁₀ of R_dark were both reduced. By contrast, rates of leaf R_dark and the Q₁₀ of R_dark both increased in response to the onset of severe water stress. Water stress was associated with a rapid (but reversible) decline in rates of light-saturated photosynthesis (P_sat), stomatal closure (g_s) and progressive wilting. Re-watering resulted in a rapid recovery of P_sat, g_s and a decline in the Q₁₀ of R_dark (due to larger proportional reductions in the rate of R_dark measured at 25°C compared with those measured at 14°C). The concentration of soluble sugars in leaves did not decline during drought (5-7 day cycles) or shading, but during drought the starch concentration dropped, suggesting starch to sugar conversion helped to maintain homeostatic concentrations of soluble sugars. Thus, the drought and shade induced changes in R_dark were unlikely to be due to stress-induced changes in substrate supply. Collectively, the data highlight the dynamic responses of respiratory Q₁₀ values to changes in water supply and sustained reductions in growth irradiance. If widespread, such changes in the Q₁₀ of leaf respiration could have important implications for predicted rates of ecosystem carbon exchange in the future, particularly in areas that experience more frequent droughts.

Changes in respiratory mitochondrial machinery and cytochrome and alternative pathway activities in response to energy demand underlie the acclimation of respiration to elevated CO2 in the invasive Opuntia ficus-indica

Studies on long-term effects of plants grown at elevated CO(2) are scarce and mechanisms of such responses are largely unknown. To gain mechanistic understanding on respiratory acclimation to elevated CO(2), the Crassulacean acid metabolism Mediterranean invasive Opuntia ficus-indica Miller was grown at various CO(2) concentrations. Respiration rates, maximum activity of cytochrome c oxidase, and active mitochondrial number consistently decreased in plants grown at elevated CO(2) during the 9 months of the study when compared to ambient plants. Plant growth at elevated CO(2) also reduced cytochrome pathway activity, but increased the activity of the alternative pathway. Despite all these effects seen in plants grown at high CO(2), the specific oxygen uptake rate per unit of active mitochondria was the same for plants grown at ambient and elevated CO(2). Although decreases in photorespiration activity have been pointed out as a factor contributing to the long-term acclimation of plant respiration to growth at elevated CO(2), the homeostatic maintenance of specific respiratory rate per unit of mitochondria in response to high CO(2) suggests that photorespiratory activity may play a small role on the long-term acclimation of respiration to elevated CO(2). However, despite growth enhancement and as a result of the inhibition in cytochrome pathway activity by elevated CO(2), total mitochondrial ATP production was decreased by plant growth at elevated CO(2) when compared to ambient-grown plants. Because plant growth at elevated CO(2) increased biomass but reduced respiratory machinery, activity, and ATP yields while maintaining O(2) consumption rates per unit of mitochondria, we suggest that acclimation to elevated CO(2) results from physiological adjustment of respiration to tissue ATP demand, which may not be entirely driven by nitrogen metabolism as previously suggested.

Sap flow rates and sapwood density are critical factors in within- and between-tree variation in CO2 efflux from stems of mature Dacrydium cupressinum trees

Measurements of CO2 efflux from stems and branches, sap velocity, and respiratory activity of excised wood cores were conducted in Dacrydium cupressinum trees that differed in diameter, age, and canopy emergence. The objective of this study was to determine if consistent linkages exist among respiratory production of CO2 within stems, xylem transport of CO2, and the rate of CO2 diffusing from stem surfaces. Stem CO2 efflux was depressed during periods of sap flow compared with the efflux rate expected for a given stem temperature and was positively correlated with sapwood density. By contrast, no significant relationships were observed between CO2 efflux and the respiratory activity of wood tissues. Between 86 and 91% of woody tissue respiration diffused to the atmosphere over a 24-h period. However, at certain times of the day, xylem transport and internal storage of CO2 may account for up to 13-38% and 12-18%, respectively, of woody tissue respiration. These results demonstrate that differences in sap flow rates and xylem anatomy are critically important for explaining within- and between-tree variation in CO2 efflux from stems.

The hot and the cold: unravelling the variable response of plant respiration to temperature

When predicting the effects of climate change, global carbon circulation models that include a positive feedback effect of climate warming on the carbon cycle often assume that (1) plant respiration increases exponentially with temperature (with a constant Q₁₀) and (2) that there is no acclimation of respiration to long-term changes in temperature. In this review, we show that these two assumptions are incorrect. While Q₁₀ does not respond systematically to elevated atmospheric CO₂ concentrations, other factors such as temperature, light, and water availability all have the potential to influence the temperature sensitivity of respiratory CO₂ efflux. Roots and leaves can also differ in their Q₁₀ values, as can upper and lower canopy leaves. The consequences of such variable Q₁₀ values need to be fully explored in carbon modelling. Here, we consider the extent of variability in the degree of thermal acclimation of respiration, and discuss in detail the biochemical mechanisms underpinning this variability; the response of respiration to long-term changes in temperature is highly dependent on the effect of temperature on plant development, and on interactive effects of temperature and other abiotic factors (e.g. irradiance, drought and nutrient availability). Rather than acclimating to the daily mean temperature, recent studies suggest that other components of the daily temperature regime can be important (e.g. daily minimum and / or night temperature). In some cases, acclimation may simply reflect a passive response to changes in respiratory substrate availability, whereas in others acclimation may be critical in helping plants grow and survive at contrasting temperatures. We also consider the impact of acclimation on the balance between respiration and photosynthesis; although environmental factors such as water availability can alter the balance between these two processes, the available data suggests that temperature-mediated differences in dark leaf respiration are closely linked to concomitant differences in leaf photosynthesis. We conclude by highlighting the need for a greater process-based understanding of thermal acclimation of respiration if we are to successfully predict future ecosystem CO₂ fluxes and potential feedbacks on atmospheric CO₂ concentrations.

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.

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.