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

The effects of elevated CO2 concentration upon the mycorrhizal relationship s 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 r oot systems, either colonised by the mycorrhizal fungi Paxillus involutus o r Suillus bovinus, or left in the non-mycorrhizal condition, were maintaine d in sealed dishes. In one series of these plants the effects of C-ELEV upo n the extent of mycorrhizal development and upon their growth and nutrition were determined, while another series were transferred from the dishes aft er 1 month, to transparent observation chambers before being returned to th e two CO2 environments. In these chambers, the effects of C-ELEV upon devel opment of the external mycelial systems of the two mycorrhizal fungi was de termined by measuring the advance of the hyphal fronts of the mycorrhizal f ungi across non-sterile peat from the colonised plants. The dry mass and nu mber 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 wh ole 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 t o the energetic cost of production of the larger mycorrhizal systems in thi s 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 env ironments. It is hypothesised that increased allocation of carbon to mycorr hizal root systems and their associated mycelia would provide the potential for enhancement of nutrient acquisition in open systems of greater fertili ty. (C) 1998 Elsevier Science B.V. All rights reserved.