What was done
Red pines (Pinus densiflora Sieb. et Zucc.) were grown from seed for 18 weeks in a sunlit phytotron at either ambient CO2 (AC = 360 ppm) or elevated CO2 (EC =720 ppm), with or without inoculation of their roots with the ectomycorrhizal fungus Pisolithus tinctorius (Pers.) Coker et Couch (Pt), while a variety of measurements were made of both the seedlings and the fungus.

What was learned
The authors report that "the infection rate of Pt in P. densiflora seedlings grown at EC was significantly higher than at AC," noting that "previous studies have also found that ecotmycorrhizal development in seedlings of several tree species at EC was greater than at AC (Seegmuller and Rennenberg, 1994; Ineichen et al., 1995; Rey and Jarvis, 1997; Runion et al., 1997, Rouhier and Read, 1998)."

What it means
The CO2-induced enhancement of Pt infection rate in P. densiflora and Pt's subsequent more robust development is very significant, for Choi et al. write that "ectomycorrhizal development enlarges the absorptive surface of the root, with widely ramified hyphae allowing the release of phosphatases, which enhance the availability of organic phosphate and exude organic acids," which interactions between host plant and ectomycorrhiza "increase the use efficiency of limited soluble phosphate and organic N in soil (Smith and Read, 1997; Lambers et al., 1998)."  Consequently, they suggest that seedlings with better developed Pt, such as occurs in response to atmospheric CO2 enrichment, "have increased nutrient and water uptake, leading to improved plant nutritional status and giving rise to more vigorous physiological response, in particular photosynthetic activity, and that these responses delay down-regulation at EC."

References
Ineichen, K., Wiemken, V. and Wiemken, A.  1995.  Shoots, roots and ectomycorrhizal formation of pine seedlings at elevated atmospheric carbon dioxide.  Plant, Cell and Environment 18: 703-707.

Lambers, H., Chapin III, F.S. and Pons, T.L.  1998.  Plant Physiological Ecology.  Springer-Verlag, New York, New York, USA.

Rey, A. and Jarvis, P.G.  1997.  Growth response of young birch trees (Betula pendula Roth.) after four and a half years of CO2 exposure.  Annals of Botany 80: 809-816.

Rouhier, H. and Read, D.J.  1998.  Plant and fungal responses to elevated atmospheric carbon dioxide in mycorrhizal seedlings of Pinus sylvestris.  Environmental and Experimental Botany 40: 237-246.

Runion, G.B., Mitchell, R.J., Rogers, H.H., Prior, S.A. and Counts, T.K.  1997.  Effects of nitrogen and water limitation and elevated atmospheric CO2 on ectomycorrhiza of longleaf pine.  New Phytologist 137: 681-689.

Seegmuller, S. and Rennenberg, H.  1994.  Interactive effects of mycorrhization and elevated carbon dioxide on growth of young pedunculate oak (Quercus robur L.) trees.  Plant and Soil 167: 325-329.

Smith, S.E. and Read, D.J.  1997.  Mycorrhizal Symbiosis.  Academic Press, San Diego, California, USA.