Shoots, roots and ectomycorrhiza formation of pine seedlings at elevated atmospheric carbon dioxide

The effect of elevated atmospheric CO 2 concentration on the growth of shoots, roots, mycorrhizas and extraradical mycorrhizal mycelia of pine ( Pinus silvestris L.) was examined. Two and a half‐month‐old seedlings were inoculated axenically with the mycorrhizal fungus Pisolithus tincto‐rius (Pers.) by a method allowing rapid mycorrhiza formation in Petri dishes. The plants were then cultivated for 3 months in growth chambers with daily concentrations of 350 and 600 μmol mol −1 CO 2 during the day. Whereas plants harvested after 1 and 2 months did not differ appreciably between ambient and increased CO 2 concentrations, after 3 months they developed a considerably higher root biomass (%57%) at elevated CO 2 , but did not increase significantly in root length. The mycorrhizal fungus Pisolithus tinctorius , which depended entirely on the plant assimilates in the model system, grew much faster at increased CO 2 : 3 times more mycorrhizal root clusters were formed and the extraradical mycelium produced had twice the biomass at elevated as at ambient CO 2 . No difference in shoot biomass was found between the two treatments after 91 d. However, since the total water consumption of seedlings was similar in the two treatments, the water use efficiency was appreciably higher for the seedlings at increased CO 2 because of the higher below‐ground biomass.