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% 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% between 23 and 60 days after planting. In addition, when measuring photosynthetic rates at a reduced oxygen concentration of 2%, they observed 16 and 9% 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%, 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% 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% 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%, 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%, 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% (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% 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 logically 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 probably be validly concluded that the balance of evidence suggests that the growth of herbaceous crops is generally not only enhanced by CO2-induced increases in net photosynthesis during the light period of the day, it is also enhanced by CO2-induced decreases in respiration during the dark period.

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.

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.