What was done
The authors grew well watered and fertilized alfalfa (Medicago sativa L.) plants that they inoculated with one of two strains (either A2 or NRG34) of the nitrogen-fixing symbiont Sinorhizobium meliloti from seed in 12.5-cm-diameter pots filled with non-sterile topsoil within controlled-environment chambers maintained at atmospheric CO2 concentrations of either 400 or 800 ppm for a period of two months under optimal light (600 µmol/m²/s for 16 hours per day) and day/night temperatures of 22/17°C and for two final weeks at a reduced light level of 200 µmol/m²/s for 8 hours per day and a cold day/night temperature regime of 5/2°C, over which time course they periodically measured a number of plant physiological functions and characteristics.

What was learned
At the end of the experiment, it was determined that the total biomass of the plants in the elevated CO2 treatment was approximately 33% greater than that of the plants in the control treatment when infected with the A2 strain of S. meliloti, but about 36% greater when infected with the NRG34 strain. However, plants in the 800-ppm CO2 treatment were found to be less freezing tolerant than those in the 400-ppm treatment, while the plants inoculated with the NRG34 strain were determined to be less freezing tolerant than those inoculated with the A2 strain.

What it means
In providing some comparative background for their freezing tolerance results, Bertrand et al. write that "CO2 enrichment led to more severe frost damage in leaves of Eucalyptus pauciflora (Barker et al., 2005) and Ginko biloba (Terry et al., 2000), and in a native temperate grassland (Obrist et al., 2001), whereas it increased frost resistance of Betula allaghanensis (Wayne et al., 1998) and Picea mariana (Bigras and Bertrand, 2006) but had no effect on freezing tolerance of Picea abies (Dalen et al., 2001)," which suggests there may not be a single freezing tolerance response to atmospheric CO2 enrichment that is typical of plants in general. However, because their results suggest that "it is possible to select or identify rhizobial strains to improve alfalfa performance under high CO2," Bertrand et al. conclude that the "freezing tolerance as well as the expression of key over-wintering genes of alfalfa can be altered by the strain of rhizobium," which should enable farmers to obtain the best of both worlds, as it were, by benefiting from the significant growth stimulation produced by the ongoing rise in the air's CO2 content, while selecting a strain of rhizobium capable of compensating for a possible CO2-induced reduction in freezing tolerance. But in a world where air temperature is in a rising mode - and where minimum temperatures in the winter appear to be rising fastest of all - this "fine tuning" of rhizobium-strain-to-plant may not be needed, as the environmental transformation that is currently underway automatically insures that there will be a "greening of the earth" throughout natural and agro-ecosystems alike.

References
Barker, D.H., Loveys, B.R., Egerton, J.J.G., Gorton, H., Williams, W.E. and Ball, M.C. 2005. CO2 enrichment predisposes foliage of a eucalypt to freezing injury and reduces spring growth. Plant, Cell and Environment 28: 1506-1515.

Bigras, F.J. and Bertrand, A. 2006. Responses of Picea mariana to elevated CO2 concentration during growth, cold hardening and dehardening: phenology, cold tolerance, photosynthesis and growth. Tree Physiology 26: 875-897.

Dalen, L.S., Johnsen, O. and Ogner, G. 2001. CO2 enrichment and development of freezing tolerance in Norway spruce. Physiologia Plantarum 113: 533-540.

Obrist, D., Arnone III, J.A. and Korner, C. 2001. In situ effects of elevated atmospheric CO2 on leaf freezing resistance and carbohydrates in a native temperate grassland. Annals of Botany 87: 839-844.

Terry, A.C., Quick, W.P. and Beerling, D.J. 2000. Long-term growth of Ginkgo with CO2 enrichment increases leaf ice nucleation temperatures and limits recovery of the photosynthetic system from freezing. Plant Physiology 124: 183-190.

Wayne, P.M., Reekie, E.G. and Bazzaz, F.A. 1998. Elevated CO2 ameliorates birch response to high temperature and frost stress: implications for modeling climate-induced geographic range shifts. Oecologia 114: 335-342.