ICOM1 Abstract

talk session 1 CONJEAUD CORINNE, PASCALE SCHEROMM, PASCAL TILLARD, CLAUDE PLASSARD & DANIEL MOUSAIN Laboratoire de Recherches sur les Symbiotes des Racines, INRA, 2 Place Viala, 34060 Montpellier Cedex 1 France. Effects of ectomycorrhizal infection with Hebeloma cylindrosporum and Rhizopogon rubescens on the carbon balance in Pinus pinaster seedlings. The study of the effects of ectomycorrhizal infection on the carbon balance of the host-plant was carried out by measuring (i) the biomass production and the net rates of photosynthesis, (ii) the rates of root respiration and (iii) the distribution of carbon compounds (soluble sugars, starch) in the host-plant. The results showed that each ectomycorrhizal association constituted a different model of carbon balance in Pinus pinaster seedlings. The association with Hebeloma cylindrosporum was characterized by higher rates of photosynthesis, a very strong increase in root respiration and finally caused a growth depression in mycorrhizal plants compared to the control ones. On the contrary, Rhizopogon rubescens had no effect on photosynthesis nor root respiration rates and finally increased the growth of mycorrhizal plants. The distribution of carbon compounds as a function of ectomycorrhizal association was then studied by supplying 13CO2 to the plants, whether mycorrhizal or not, for 4 or 10 days. The results obtained showed again that the presence of H. cylindrosporum increased significantly the carbon allocation to the roots and to the fungus associated to the roots compared to the figures obtained with R. rubescens. These data indicated that the mycelium of H. cylindrosporum increased the sink of carbon compounds, which in turn increased the photosynthesis rate (source-sink concept). Finally, application of different light intensities did not change the effects of both fungal species. Application of different N-regimes during the culture suggested that the competition between H. cylindrosporum and the host plant for N-compounds might be also responsible for growth decrease of mycorrhizal plants.