EFFECTS OF PHOSPHORUS AND ECTOMYCORRHIZA ON MARITIME PINE-SEEDLINGS (PINUS-PINASTER)

The purpose of this study was to determine how ectomycorrhizal infecti on and phosphorus nutrition affect biomass, photosynthesis and root re spiration in the host plant. Maritime pine (Pinus pinaster Soland. in Ait.) seedlings grown in containers filled with perlite-vermiculite we re inoculated with the ectomycorrhizal fungus Hebeloma cylindrosporum (strain D3.25.9) and given 0. or 0.5 mM phosphate in the nutrient solu tion. Hebeloma cylindrosporum infection increased net photosynthesis a nd 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 phos phorus content which made them similar in that respect to mycorrhizal plants but did not result in increased photosynthesis. The nitrogen co ntent of mycorrhizal plants was, moreover, lower than that of the cont rol group. The data recorded were consistent with the photosynthate so urce-sink hypothesis. The increase in fixed carbon in mycorrhizal plan ts was unable to compensate for the increased carbon cost of the mycor rhizal root system.