Respiration

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From Climate Change Reconsidered, a work of the Nongovernmental International Panel on Climate Change

Nearly all of earth’s plants respond favorably to increases in the air’s CO2 concentration by exhibiting enhanced rates of net photosynthesis and biomass production during the light part of each day. In many cases, observed increases in these parameters (especially biomass production) are believed to be due, in part, to CO2-induced reductions in carbon losses via respiration during the day and especially at night (called “dark respiration”). In this summary, we examine what has been learned about this subject from experiments conducted on various herbaceous and woody plants.

Additional information on this topic, including reviews on respiration not discussed here, can be found at http://www.co2science.org/subject/r/subject _r.php under the heading Respiration.


Herbaceous Plants

Crops


Baker et al. (2000) grew rice in Soil-Plant-Atmosphere Research (SPAR) units at atmospheric CO2 concentrations of 350 and 700 ppm during daylight hours. Under these conditions, rates of dark respiration decreased in both CO2 treatments with short-term increases in the air’s CO2 concentration at night. However, when dark respiration rates were measured at the CO2 growth concentrations of the plants, they were not significantly different from each other.

Cousins et al. (2001) grew sorghum at atmospheric CO2 concentrations of 370 and 570 ppm within a free-air CO2 enrichment (FACE) facility near Phoenix, Arizona, USA. Within six days of planting, the photosynthetic rates of the second leaves of the CO2-enriched plants were 37 percent greater than those of the second leaves of the ambiently grown plants. However, this CO2-induced photosynthetic enhancement slowly declined with time, stabilizing at approximately 15 percent between 23 and 60 days after planting. In addition, when measuring photosynthetic rates at a reduced oxygen concentration of 2 percent, they observed 16 and 9 percent increases in photosynthesis for the ambient and CO2-enriched plants, respectively. These observations suggest that the extra 200 ppm of CO2 was reducing photorespiratory carbon losses, although this phenomenon did not account for all of the CO2-induced stimulation of photosynthesis.

Das et al. (2002) grew tropical nitrogen-fixing mungbean plants in open-top chambers maintained at atmospheric CO2 concentrations of either 350 or 600 ppm for two growing seasons, with the extra CO2 being provided between either days 0 and 20 or days 21 and 40 after germination. This work revealed that the elevated CO2 decreased rates of respiration by 54-62 percent, with the greatest declines occurring during the first 20 days after germination.

Wang et al. (2004) grew well-watered and fertilized South American tobacco plants from seed in 8.4-liter pots (one plant per pot) filled with sand and housed in controlled-environment growth chambers maintained at atmospheric CO2 concentrations of either 365 or 730 ppm for a total of nine weeks. Over this period they found that the ratio of net photosynthesis per unit leaf area (A) to dark respiration per unit leaf area (Rd) “changed dramatically.” Whereas A/Rd was the same in both treatments at the beginning of the measurement period, a month later it had doubled in the CO2-enriched environment but had risen by only 58 percent in the ambient treatment. Speaking of this finding, the three researchers say that “if the dynamic relationship between A and Rd observed in N. sylvestris is applicable to other species, it will have important implications for carbon cycling in terrestrial ecosystems, since plants will assimilate CO2 more efficiently as they mature.”

Bunce (2005) grew soybeans in the field in open-top chambers maintained at atmospheric CO2 concentrations of ambient and ambient +350 ppm at the Beltsville Agricultural Research Center in Maryland (USA), where net carbon dioxide exchange rate measurements were performed on a total of 16 days between 18 July and 11 September of 2000 and 2003, during the flowering to early pod-filling stages of the growing season. Averaged over the course of the study, he found that daytime net photosynthesis per unit leaf area was 48 percent greater in the plants growing in the CO2-enriched air, while nighttime respiration per unit leaf area was unaffected by elevated CO2. However, because the extra 350 ppm of CO2 increased leaf dry mass per unit area by an average of 23 percent, respiration per unit of mass was significantly lower for the leaves of the soybeans growing in the CO2-enriched air.

Wang and Curtis (2002) conducted a meta-analysis of the results of 45 area-based dark respiration (Rda) and 44 mass-based dark respiration (Rdm) assessments of the effects of a doubling of the air’s CO2 concentration on 33 species of plants derived from 37 scientific studies. This work revealed that the mean leaf Rda of the suite of herbaceous plants studied was significantly higher (+29 percent, P < 0.01) at elevated CO2 than at ambient CO2. However, when the herbaceous plants were separated into groups that had experienced durations of CO2 enrichment that were either shorter or longer than 60 days, it was found that the short-term studies exhibited a mean Rda increase of 51 percent (P < 0.05), while the long-term studies exhibited no effect. Hence, for conditions of continuous atmospheric CO2 enrichment, herbaceous plants would likely experience no change in leaf Rda. In addition, the two researchers found that plants exposed to elevated CO2 for < 100 days “showed significantly less of a reduction in leaf Rdm due to CO2 enrichment (-12%) than did plants exposed for longer periods (-35%, P < 0.01).” Hence, for long-term conditions of continuous atmospheric CO2 enrichment, herbaceous crops would likely experience an approximate 35 percent decrease in leaf Rdm.

Bunce (2004) grew six different 16-plant batches of soybeans within a single controlled-environment chamber, one to a pot filled with 1.8 liters of vermiculite that was flushed daily with a complete nutrient solution. In three experiments conducted at day/night atmospheric CO2 concentrations of 370/390 ppm, air temperatures were either 20, 25 or 30°C, while in three other experiments conducted at an air temperature of 25°C, atmospheric CO2 concentrations were either 40, 370 or 1400 ppm. At the end of the normal 16 hours of light on the 17th day after planting, half of the plants were harvested and used for the measurement of a number of physical parameters, while measurements of the plant physiological processes of respiration, translocation and nitrate reduction were made on the other half of the plants over the following 8-hour dark period.

Plotting translocation and nitrate reduction as functions of respiration, Bunce found that “a given change in the rate of respiration was accompanied by the same change in the rate of translocation or nitrate reduction, regardless of whether the altered respiration was caused by a change in temperature or by a change in atmospheric CO2 concentration.” As a result, and irrespective of whatever mechanisms may have been involved in eliciting the responses observed, Bunce concluded that “the parallel responses of translocation and nitrate reduction for both the temperature and CO2 treatments make it unlikely that the response of respiration to one variable [CO2] was an artifact while the response to the other [temperature] was real.” Hence, there is reason to believe that the oft-observed decreases in dark respiration experienced by plants exposed to elevated levels of atmospheric CO2, as per the review and analysis studies of Drake et al. (1999) and Wang and Curtis (2002), are indeed real and not the result of measurement system defects.

In light of these several findings, it can be concluded that the balance of evidence suggests that the growth of herbaceous crops is generally enhanced by CO2-induced decreases in respiration during the dark period.

Additional information on this topic, including reviews of newer publications as they become available, can be found at http://www.co2science.org/ subject/r/respirationcrops.php.


Other Herbaceous Plants


In this section we review the results of studies of non-crop herbaceous plants to determine if atmospheric CO2 enrichment tends to increase or decrease (or leave unaltered) their respiration rates.

Rabha and Uprety (1998) grew India mustard plants for an entire season in open-top chambers with either ambient or enriched (600 ppm) atmospheric CO2 concentrations and adequate or inadequate soil moisture levels. Their work revealed that the elevated CO2 concentration reduced leaf dark respiration rates by about 25 percent in both soil moisture treatments, which suggests that a greater proportion of the increased carbohydrate pool in the CO2-enriched plants remained within them to facilitate increases in growth and development.

Ziska and Bunce (1999) grew four C4 plants in controlled environment chambers maintained at either full-day (24-hour) atmospheric CO2 concentrations of 350 and 700 ppm or a nocturnal-only CO2 concentration of 700 ppm (with 350 ppm CO2 during the day) for about three weeks. In this particular study, 24-hour CO2 enrichment caused a significant increase in the photosynthesis (+13 percent) and total dry mass (+21 percent) of only one of the four C4 species (Amaranthus retroflexus). However, there was no significant effect of nocturnal-only CO2 enrichment on this species, indicating that the observed increase in biomass, resulting from 24-hour atmospheric CO2 enrichment, was not facilitated by greater carbon conservation stemming from a CO2-induced reduction in dark respiration.

In an experiment that produced essentially the same result, Grunzweig and Korner (2001) constructed model grasslands representative of the Negev of Israel and placed them in growth chambers maintained at atmospheric CO2 concentrations of 280, 440 and 600 ppm for five months. This study also revealed that atmospheric CO2 enrichment had no effect on nighttime respiratory carbon losses.

Moving to the other end of the moisture spectrum, Van der Heijden et al. (2000) grew peat moss hydroponically within controlled environment chambers maintained at atmospheric CO2 concentrations of 350 and 700 ppm for up to six months, while simultaneously subjecting the peat moss to three different levels of nitrogen deposition. In all cases, they found that the elevated CO2 reduced rates of dark respiration consistently throughout the study by 40 to 60 percent.

In a final multi-species study, Gonzalez-Meler et al. (2004) reviewed the scientific literature pertaining to the effects of atmospheric CO2 enrichment on plant respiration from the cellular level to the level of entire ecosystems. They report finding that “contrary to what was previously thought, specific respiration rates are generally not reduced when plants are grown at elevated CO2.” Nevertheless, they note that “whole ecosystem studies show that canopy respiration does not increase proportionally to increases in biomass in response to elevated CO2,” which suggests that respiration per unit biomass is likely somewhat reduced by atmospheric CO2 enrichment. However, they also find that “a larger proportion of respiration takes place in the root system [when plants are grown in CO2-enriched air],” which once again obfuscates the issue.

The three researchers say “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.” They conclude that “the role of plant respiration in augmenting the sink capacity of terrestrial ecosystems is still uncertain.”

Additional information on this topic, including reviews of newer publications as they become available, can be found at http://www.co2science.org/ subject/r/respirationherbaceous.php.


Woody Plants

Coniferous Trees


Jach and Ceulemans (2000) grew three-year old Scots pine seedlings out-of-doors and rooted in the ground in open-top chambers maintained at atmospheric CO2 concentrations of either 350 or 750 ppm for two years. To make the experiment more representative of the natural world, they applied no nutrients or irrigation water to the soils in which the trees grew for the duration of the study. After two years of growth under these conditions, dark respiration on a needle mass basis in the CO2-enriched seedlings was 27 percent and 33 percent lower in current-year and one-year-old needles, respectively, with the greater reduction in the older needles being thought to arise from the greater duration of elevated CO2 exposure experienced by those needles.

Hamilton et al. (2001) studied the short- and long-term respiratory responses of loblolly pines in a free-air CO2-enrichment (FACE) study that was established in 1996 on 13-year-old trees in a North Carolina (USA) plantation, where the CO2-enriched trees were exposed to an extra 200 ppm of CO2. This modest increase in the atmosphere’s CO2 concentration produced no significant short-term suppression of dark respiration rates in the trees’ needles. Neither did long-term exposure to elevated CO2 alter maintenance respiration, which is the amount of CO2 respired to maintain existing plant tissues. However, growth respiration, which is the amount of CO2 respired when constructing new tissues, was reduced by 21 percent.

McDowell et al. (1999) grew five-month-old seedlings of western hemlock in root boxes subjected to various root-space CO2 concentrations (ranging from 90 to 7000 ppm) for periods of several hours to determine the effects of soil CO2 concentration on growth, maintenance and total root respiration. In doing so, they found that although elevated CO2 had no effect on growth respiration, it significantly impacted maintenance and total respiration. At a soil CO2 concentration of 1585 ppm, for example, total and maintenance respiration rates of roots were 55 percent and 60 percent lower, respectively, than they were at 395 ppm. The impact of elevated CO2 on maintenance respiration was so strong that it exhibited an exponential decline of about 37 percent for every doubling of soil CO2 concentration. The implications of this observation are especially important because maintenance respiration comprised 85 percent of total root respiration in this study.

The results of these experiments suggest that both above and below the soil surface, coniferous trees may exhibit reductions in total respiration in a high-CO2 world of the future. Three studies of three species, however, is insufficent evidence to reach a firm conclusion.

Additional information on this topic, including reviews of newer publications as they become available, can be found at http://www.co2science.org/ subject/r/respirationconifers.php.


Deciduous Trees


Wang and Curtis (2001) grew cuttings of two male and two female trembling aspen trees for about five months on soils containing low and high nitrogen contents in open-top chambers maintained at atmospheric CO2 concentrations of 380 and 765 ppm, finding that gender had little effect on dark respiration rates, but that elevated CO2 increased them, by 6 percent and 32 percent in the low and high soil nitrogen treatments, respectively. On the other hand, Karnosky et al. (1999) grew both O3-sensitive and O3-tolerant aspen clones for one full year in free-air CO2-enrichment (FACE) plots maintained at atmospheric CO2 concentrations of 360 and 560 ppm, finding that the extra CO2 decreased dark respiration rates by 24 percent.

Gielen et al. (2003) measured stem respiration rates of white, black and robusta poplar trees in a high-density forest plantation in the third year of a FACE experiment in which the CO2 concentration of the air surrounding the trees was increased to a value of approximately 550 ppm. This study revealed, in their words, that “stem respiration rates were not affected by the FACE treatment,” and that “FACE did not influence the relationships between respiration rate and both stem temperature and relative growth rate.” In addition, they say they could find “no effect of the FACE treatment on Rm [maintenance respiration, which is related to the sustaining of existing cells] and Rg [growth respiration, which is related to the synthesis of new tissues].”

Hamilton et al. (2001) studied respiratory responses of sweetgum trees growing in the understory of a loblolly pine plantation (but occasionally reaching the top of the canopy) to an extra 200 ppm of CO2 in a FACE study conducted in North Carolina, USA. As a result of their measurement program, they determined that the modest increase in atmospheric CO2 concentration did not appear to alter maintenance respiration to any significant degree, but that it reduced dark respiration by an average of 10 percent and growth respiration of leaves at the top of the canopy by nearly 40 percent.

In reviewing the results of these several deciduous tree studies, we see cases of both increases and decreases in respiration rates in response to atmospheric CO2 enrichment, as well as cases of no change in respiration. More data are needed before any general conclusions may safely be drawn.

Additional information on this topic, including reviews of newer publications as they become available, can be found at http://www.co2science.org/ subject/r/respirationdeciduous.php.


Multiple Tree Studies


Amthor (2000) measured dark respiration rates of intact leaves of nine different tree species growing naturally in an American deciduous forest. Within a specially designed leaf chamber, the CO2 concentration surrounding individual leaves was stabilized at 400 ppm for 15 minutes, whereupon their respiration rates were measured for 30 minutes, after which the CO2 concentration in the leaf chamber was raised to 800 ppm for 15 minutes and respiration data were again recorded for the same leaves. This protocol revealed that elevated CO2 had little effect on leaf dark respiration rates. The extra 400 ppm of CO2 within the measurement cuvette decreased the median respiration rate by only 1.5 percent across the nine tree species. This observation led Amthor to state that the “rising atmospheric CO2 concentration has only a small direct effect on tree leaf respiration in deciduous forests;” and he calculated that it can be “more than eliminated by a 0.22°C temperature increase.” Upon this premise, he concluded that “future direct effects of increasing CO2 in combination with warming could stimulate tree leaf respiration in their sum,” and that this consequence “would translate into only slight, if any, effects on the carbon balance of temperate deciduous forests in a future atmosphere containing as much as [800 ppm] CO2.”

Amthor’s conclusion is debatable, for it is based upon the extrapolation of the short-term respiratory responses of individual leaves, exposed to elevated CO2 for only an hour or two, to that of entire trees, many of which will experience rising CO2 levels for a century or more during their lifetimes. Trees are long-lived organisms that should not be expected to reveal the nature of their long-term responses to elevated atmospheric CO2 concentrations on as short a time scale as 15 minutes. Indeed, their respiratory responses may change significantly with the passage of time as they acclimate and optimize their physiology and growth patterns to the gradually rising CO2 content of earth’s atmosphere, as evidenced by the findings of the following two studies.

Wang and Curtis (2002) conducted a meta-analysis of the results of 45 area-based dark respiration (Rda) and 44 mass-based dark respiration (Rdm) assessments of the effects of an approximate doubling of the air’s CO2 concentration on 33 species of plants (both herbaceous and woody) derived from 37 scientific publications. This effort revealed that the mean leaf Rda of the woody plants they analyzed was unaffected by elevated CO2. However, there was an effect on mean leaf Rdm, and it was determined to be time-dependent. The woody plants exposed to elevated CO2 for < 100 days, in the reviewing scientists’ words, “showed significantly less of a reduction in leaf Rdm due to CO2 enrichment (-12%) than did plants exposed for longer periods (-35%, P < 0.01).” Hence, for conditions of continuous long-term atmospheric CO2 enrichment, the results of Wang and Curtis’ analysis suggest woody plants may experience an approximate 35 percent decrease in leaf Rdm.

Drake et al. (1999) also conducted a comprehensive analysis of the peer-reviewed scientific literature to determine the effects of elevated atmospheric CO2 concentrations on plant respiration rates. They found that atmospheric CO2 enrichment typically decreased respiration rates in mature foliage, stems, and roots of CO2-enriched plants relative to rates measured in plants grown in ambient air; and when normalized on a biomass basis, they determined that a doubling of the atmosphere’s CO2 concentration would likely reduce plant respiration rates by an average of 18 percent. To determine the potential effects of this phenomenon on annual global carbon cycling, which the twelve researchers say “will enhance the quantity of carbon stored by forests,” they input a 15 percent CO2-induced respiration reduction into a carbon sequestration model, finding that an additional 6 to 7 Gt of carbon would remain sequestered within the terrestrial biosphere each year, thus substantially strengthening the terrestrial carbon sink.

Davey et al. (2004) reached a different conclusion. “Averaged across many previous investigations, doubling the CO2 concentration has frequently been reported to cause an instantaneous reduction of leaf dark respiration measured as CO2 efflux.” However, as they continue, “no known mechanism accounts for this effect, and four recent studies [Amthor (2000); Anthor et al. (2001); Jahnke (2001); Jahnke and Krewitt (2002)] have shown that the measurement of respiratory CO2 efflux is prone to experimental artifacts that could account for the reported response.”

Using a technique that avoids the potential artifacts of prior attempts to resolve the issue, Davey et al. employed a high-resolution dual channel oxygen analyzer in an open gas exchange system to measure the respiratory O2 uptake of nine different species of plants in response to a short-term increase in atmospheric CO2 concentration, as well as the response of seven species to long-term elevation of the air’s CO2 content in four different field experiments. In doing so, they found that “over six hundred separate measurements of respiration failed to reveal any decrease in respiratory O2 uptake with an instantaneous increase in CO2.” Neither could they detect any response to a five-fold increase in the air’s CO2 concentration nor to the total removal of CO2 from the air. They also note that “this lack of response of respiration to elevated CO2 was independent of treatment method, developmental stage, beginning or end of night, and the CO2 concentration at which the plants had been grown.” In the long-term field studies, however, there was a respiratory response; but it was small (7 percent on a leaf mass basis), and it was positive, not negative.

In light of these contradicgtory results, the most reasonable conclusion is that atmospheric CO2 enrichment may either increase or decrease woody-plant respiration, but not to any great degree, and that in the mean, the net result for the conglomerate of earth’s trees would likely be something of little impact.

Additional information on this topic, including reviews of newer publications as they become available, can be found at http://www.co2science.org/ subject/r/respirationtrees.php.


References

Baker, J.T., Allen, L.H., Jr., Boote, K.J. and Pickering, N.B. 2000. Direct effects of atmospheric carbon dioxide concentration on whole canopy dark respiration of rice. Global Change Biology 6: 275-286.

Bunce, J.A. 2004. A comparison of the effects of carbon dioxide concentration and temperature on respiration, translocation and nitrate reduction in darkened soybean leaves. Annals of Botany 93: 665-669.

Bunce, J.A. 2005. Response of respiration of soybean leaves grown at ambient and elevated carbon dioxide concentrations to day-to-day variation in light and temperature under field conditions. Annals of Botany 95: 1059-1066.

Climate Change Reconsidered: Website of the Nongovernmental International Panel on Climate Change. http://www.nipccreport.org/archive/archive.html

Cousins, A.B., Adam, N.R., Wall, G.W., Kimball, B.A., Pinter Jr., P.J., Leavitt, S.W., LaMorte, R.L., Matthias, A.D., Ottman, M.J., Thompson, T.L. and Webber, A.N. 2001. Reduced photorespiration and increased energy-use efficiency in young CO2-enriched sorghum leaves. New Phytologist 150: 275-284.

Das, M., Zaidi, P.H., Pal, M. and Sengupta, U.K. 2002. Stage sensitivity of mungbean (Vigna radiata L. Wilczek) to an elevated level of carbon dioxide. Journal of Agronomy and Crop Science 188: 219-224.

Drake, B.G., Azcon-Bieto, J., Berry, J., Bunce, J., Dijkstra, P., Farrar, J., Gifford, R.M., Gonzalez-Meler, M.A., Koch, G., Lambers, H., Siedow, J. and Wullschleger, S. 1999. Does elevated atmospheric CO2 inhibit mitochondrial respiration in green plants? Plant, Cell and Environment 22: 649-657.

Wang, X., Anderson, O.R. and Griffin, K.L. 2004. Chloroplast numbers, mitochondrion numbers and carbon assimilation physiology of Nicotiana sylvestris as affected by CO2 concentration. Environmental and Experimental Botany 51: 21-31.

Wang, X. and Curtis, P. 2002. A meta-analytical test of elevated CO2 effects on plant respiration. Plant Ecology 161: 251-261. Gonzalez-Meler, M.A., Taneva, L. and Trueman, R.J. 2004. Plant respiration and elevated atmospheric CO2 concentration: Cellular responses and global significance. Annals of Botany 94: 647-656.

Grunzweig, J.M. and Korner, C. 2001. Growth, water and nitrogen relations in grassland model ecosystems of the semi-arid Negev of Israel exposed to elevated CO2. Oecologia 128: 251-262.

Rabha, B.K. and Uprety, D.C. 1998. Effects of elevated CO2 and moisture stress on Brassica juncea. Photosynthetica 35: 597-602.

Van der Heijden, E., Verbeek, S.K. and Kuiper, P.J.C. 2000. Elevated atmospheric CO2 and increased nitrogen deposition: effects on C and N metabolism and growth of the peat moss Sphagnum recurvum P. Beauv. Var. mucronatum (Russ.) Warnst. Global Change Biology 6: 201-212.

Ziska, L.H. and Bunce, J.A. 1999. Effect of elevated carbon dioxide concentration at night on the growth and gas exchange of selected C4 species. Australian Journal of Plant Physiology 26: 71-77. Hamilton, J.G., Thomas, R.B. and DeLucia, E.H. 2001. Direct and indirect effects of elevated CO2 on leaf respiration in a forest ecosystem. Plant, Cell and Environment 24: 975-982.

Jach, M.E. and Ceulemans, R. 2000. Short- versus long-term effects of elevated CO2 on night-time respiration of needles of Scots pine (Pinus sylvestris L.). Photosynthetica 38: 57-67.

McDowell, N.G., Marshall, J.D., Qi, J. and Mattson, K. 1999. Direct inhibition of maintenance respiration in western hemlock roots exposed to ambient soil carbon dioxide concentrations. Tree Physiology 19: 599-605. Gielen, B., Scarascia-Mugnozza, G. and Ceulemans, R. 2003. Stem respiration of Populus species in the third year of free-air CO2 enrichment. Physiologia Plantarum 117: 500-507.

Hamilton, J.G., Thomas, R.B. and DeLucia, E.H. 2001. Direct and indirect effects of elevated CO2 on leaf respiration in a forest ecosystem. Plant, Cell and Environment 24: 975-982.

Karnosky, D.F., Mankovska, B., Percy, K., Dickson, R.E., Podila, G.K., Sober, J., Noormets, A., Hendrey, G., Coleman, M.D., Kubiske, M., Pregitzer, K.S. and Isebrands, J.G. 1999. Effects of tropospheric O3 on trembling aspen and interaction with CO2: results from an O3-gradient and a FACE experiment. Water, Air, and Soil Pollution 116: 311-322.

Wang, X. and Curtis, P.S. 2001. Gender-specific responses of Populus tremuloides to atmospheric CO2 enrichment. New Phytologist 150: 675-684.


Amthor, J.S. 2000. Direct effect of elevated CO2 on nocturnal in situ leaf respiration in nine temperate deciduous tree species is small. Tree Physiology 20: 139-144.

Amthor, J.S., Koch, G.W., Willms, J.R. and Layzell, D.B. 2001. Leaf O2 uptake in the dark is independent of coincident CO2 partial pressure. Journal of Experimental Botany 52: 2235-2238.

Davey, P.A., Hunt, S., Hymus, G.J., DeLucia, E.H., Drake, B.G., Karnosky, D.F. and Long, S.P. 2004. 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]. Plant Physiology 134: 520-527.

Drake, B.G., Azcon-Bieto, J., Berry, J., Bunce, J., Dijkstra, P., Farrar, J., Gifford, R.M., Gonzalez-Meler, M.A., Koch, G., Lambers, H., Siedow, J. and Wullschleger, S. 1999. Does elevated atmospheric CO2 inhibit mitochondrial respiration in green plants? Plant, Cell and Environment 22: 649-657.

Jahnke, S. 2001. Atmospheric CO2 concentration does not directly affect leaf respiration in bean or poplar. Plant, Cell and Environment 24: 1139-1151.

Jahnke, S. and Krewitt, M. 2002. Atmospheric CO2 concentration may directly affect leaf respiration measurement in tobacco, but not respiration itself. Plant, Cell and Environment 25: 641-651.

Wang, X. and Curtis, P. 2002. A meta-analytical test of elevated CO2 effects on plant respiration. Plant Ecology 161: 251-261.

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