Plant and fungal responses to elevated atmospheric carbon dioxide in mycorrhizal seedlings of Pinus sylvestris

The effects of elevated CO2 concentration upon the mycorrhizal relationships of Scots pine (Pinus sylvestris) seedlings were investigated. Plants were grown for 4 months with their shoots exposed to ambient (C-AMB = 360 mu l l(-1)) or elevated (C-ELEV = 700 mu l l(-1)) CO2 environments while their root systems, either colonised by the mycorrhizal fungi Paxillus involutus or Suillus bovinus, or left in the non-mycorrhizal condition, were maintained in sealed dishes. In one series of these plants the effects of C-ELEV upon the extent of mycorrhizal development and upon their growth and nutrition were determined, while another series were transferred from the dishes after 1 month, to transparent observation chambers before being returned to the two CO2 environments. In these chambers, the effects of C-ELEV upon development of the external mycelial systems of the two mycorrhizal fungi was determined by measuring the advance of the hyphal fronts of the mycorrhizal fungi across non-sterile peat from the colonised plants. The dry mass and number of mycorrhizal tips were significantly higher in C-ELEV than in the C-AMB condition in plants colonised by both fungi in the dishes. Yields of whole plants and of shoots were higher in the C-ELEV treatment whether or not they were grown in the mycorrhizal condition, but the greater yields were not associated in these sealed systems with enhanced nutrient gain. The dry mass of non-mycorrhizal plants was greater than that of those colonised by mycorrhizal fungi under elevated CO2. This is thought to be attributable to the energetic cost of production of the larger mycorrhizal systems in this treatment. The extent of development of the mycorrhizal mycelial systems of both fungi was greatly increased in C-ELEV relative to that in C-AMB environments. It is hypothesised that increased allocation of carbon to mycorrhizal root systems and their associated mycelia would provide the potential for enhancement of nutrient acquisition in open systems of greater fertility. (C) 1998 Elsevier Science B.V. All rights reserved.