What was done
Seedlings of sour orange (Citrus aurantium L.) and sweet orange (Citrus sinensis L. Osbeck) were grown for nearly three months in glasshouses receiving atmospheric CO2 concentrations of 360 and 700 ppm.  In addition, seedlings were either inoculated with arbuscular mycorrhizal fungi or left non-inoculated as control plants at each CO2 concentration.  Thus, the authors studied the effects of elevated CO2 and fungal presence on photosynthesis and growth in these two citrus species of contrasting fungal acceptance: sour orange, which displays strong associations with mycorrhizal symbionts, and sweet orange, which exhibits relatively weaker relationships with mycorrhizal fungi.

What was learned
Elevated CO2 increased photosynthetic rates in non-mycorrhizal and mycorrhizal sour orange seedlings by 18 and 118%, respectively.  Similarly, elevated CO2 enhanced photosynthetic rates in non-mycorrhizal and mycorrhizal sweet orange seedlings by 50 and 67%, respectively.  With respect to sour orange biomass, mycorrhizal seedlings exposed to the ambient CO2 concentration displayed 18% less growth than non-mycorrhizal control seedlings.  However, at elevated CO2, mycorrhizal seedlings displayed 15% more growth than non-mycorrhizal seedlings subjected to the same enriched CO2 concentration.  Thus, atmospheric CO2 enrichment more than compensated for the carbon costs associated with mycorrhizal fungal symbiosis in this citrus species.  In contrast, sweet orange seedlings exposed to elevated CO2 exhibited an approximate 33% increase in biomass, regardless of fungal inoculation, indicating that this species is less dependent upon fungal symbiosis in eliciting CO2-induced growth responses.

What it means
As the CO2 content of the air rises, it is likely these citrus species will respond by exhibiting enhanced rates of photosynthesis and biomass production, regardless of their associations with symbiotic mycorrhizal fungi.  However, in sour orange, photosynthetic and growth responses to elevated CO2 will likely be greater when seedlings are involved in such symbiotic relationships.  In fact, the present paper reported that the degree of CO2-induced photosynthetic downregulation in sour orange seedlings was significantly reduced by the presence of mycorrhizal fungi, which served as a carbon sink for excess carbohydrates synthesized during photosynthesis.  Thus, it is likely that increasing atmospheric CO2 concentrations may increase growth in nearly all tree species, for most are ubiquitously involved in symbiotic relationships with one or more types of mycorrhizal fungi.