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% 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%) and total dry mass (+21%) 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%.

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 ultimately conclude that "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." Hence, they refreshingly state that "the role of plant respiration in augmenting the sink capacity of terrestrial ecosystems is still uncertain," setting an example of wise restraint that many climate alarmists and politicians would do well to follow with respect to questions about many other aspects of CO2 effects (and non-effects!) on climate and biology.

References
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.