Plant and fungal responses to elevated atmospheric CO2 in mycorrhizal seedlings of Betula pendula

The effects of elevated CO2 concentrations upon carbon allocation in mycorrhizal (M) and non-mycorrhizal (NM) birch (Betula pendula) seedlings were investigated. M plants, colonised by the fungus Paxillus involutus, and NM plants, were exposed for 3 months to ambient (350 mu l 1(-1)) or elevated (700 mu l 1(-1)) CO2 environments. The assimilation and distribution of carbon within the different compartments of the plant-substrate-fungal system were investigated using radioactive carbon as a tracer. In addition, the impact of elevated CO2 upon extension growth of the ectomycorrhizal mycelium of the fungus was determined in transparent observation chambers. Yields of whole plants and of shoots were significantly decreased under elevated CO2 whether they were grown with or without their fungal symbionts. Neither the dry mass production of roots of mycorrhizal plants, nor the amount of carbon allocated to shoots, roots and mycorrhizal tips were affected by elevated CO2. While the number of mycorrhizal root tips was decreased with CO2 enrichment, their relative importance in the total root system was unchanged. There was a significant increase in the extent of development of the external mycelium under elevated CO2. A greater proportion of the radioactive carbon was allocated to the soil compartment under elevated CO2. This increase, probably arising through increased rhizodeposition, was greater in NM than M plants. The responses are discussed in terms of nutrient availability in the growth media and the possible role of increased carbon allocation to mycorrhizal mycelium in nature. (C) 1999 Elsevier Science B.V. All rights reserved.