SUMMARYThe purpose of this study was to determine how ectomycorrhizal infection and phosphorus nutrition affect biomass, photosynthesis and root respiration in the host plant. Maritime pine {Pinus pinaster Soland. in Ait.) seedlings grown in containers filled with perlite-vermiculite were inoculated with the ectomycorrhizal fungus Hebeloma cylindrosporum (strain D3.25.9) and given 0 or 0-5 mM phosphate in the nutrient solution. Hebetmna cylindrosporum infection increased net photosynthesis and root respiration rates compared with those of nonmycorrhizal plants, but there was an accompanying 35 % depression in growth. The addition of phosphorus to non-mycorrhizal plants induced a rise in tissue phosphorus content which made them similar in that respect to mycorrhizal plants but did not result in increased photosynthesis. The nitrogen content of mycorrhizal plants was, moreover, lower than that of the control group. The data recorded were consistent with the photosynthate sourcesink hypothesis. The increase in fixed carbon in mycorrhizal plants was unable to compensate for the increased carbon cost of the mycorrhizal root system.
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