Reference
Aranjuelo, I., Irigoyen, J.J., Perez, P., Martinez-Carrasco, R. and Sanchez-Diaz, M. 2005. The use of temperature gradient tunnels for studying the combined effect of CO2, temperature and water availability in N2 fixing alfalfa plants. Annals of Applied Biology 146: 51-60.
What was done
The authors grew alfalfa (Medicago sativa L.) in 13-L pots for three consecutive June-July periods (2001-2003) out-of-doors in polyethylene-covered temperature gradient tunnels maintained at atmospheric CO2 concentrations that averaged 405 and 730 ppm at ambient (AT) and elevated (ET) temperatures (ET = AT + 4°C) and at high (HW) and low (LW) soil water contents (LW = 0.5HW). All plants were fed adequate nutrients except for nitrogen, in order to insure that "the only source of nitrogen for the plant was the one fixed by the nodule," which was induced to form in response to inoculation with Sinorhizobium meliloti strain 102F78.
What was learned
Aranjuelo et al. report that "the effect of elevated CO2 on plant growth interacted positively with temperature," and that "higher dry mass production of plants grown under elevated CO2 and temperature was a consequence of enhanced photosynthetic rates." This conclusion derives directly from their data, where mean CO2-induced increases in leaf net photosynthesis over the entire experiment were found to be: +5% (HW, AT), +50% (HW, ET), +17% (LW, AT) and +42% (LW, ET), as best we can determine from the bar graphs in their publication. Likewise, mean CO2-induced increases in leaf biomass were +4% (HW, AT), +54% (HW, ET), +23% (LW, AT) and +58% (LW, ET), with the same caveat. For both leaf net photosynthesis and biomass production, these results indicate that the stimulatory effect of the elevated CO2 of this study was about 2.5 times greater in the warmer of the two temperature treatments in the low soil water regime, and that it was ten times greater in the warmer of the two temperature treatments in the high soil water regime. In addition, we note that plant water loss via transpiration was also benefited by the extra CO2 of this study, declining by 25% (HW, AT), 41% (HW, ET), 31% (LW, AT) and 31% (LW, ET).
What it means
Under both well-watered and droughty conditions, atmospheric CO2 enrichment tends to enhance both photosynthesis and biomass production in alfalfa, according to the results of this study, while simultaneously decreasing transpirational water losses. In addition, if the planet were to warm as predicted by most climate models, the CO2-induced stimulation of photosynthesis and biomass production would likely be even greater.