Corrêa, A.Strasser, R. J.Martins-Loução, M. A.2011-09-082011-09-082008-10Mycorrhiza. 2008 Oct; 18(8): 413-270940-6360http://dx.doi.org/10.1007/s00572-008-0195-0http://hdl.handle.net/10451/4049ACESSO via B-on: http://dx.doi.org/10.1007/s00572-008-0195-0In the present work, the following hypotheses were tested: (1) the negative effects of mycorrhization over host plant productivity in N-limited conditions are due to N retention by the fungal partner and not due to excessive C drainage; (2) If mycorrhization results in decreased N uptake, the host plant decreases its C investment in fungal growth. The effects of mycorrhization over a wide range of combinations between N availability, N concentration in plant tissues, and degree of mycorrhizal colonization were studied in Pinus pinaster L. mycorrhizal with Pisolithus tinctorius. Several plant productivity parameters, the seedlings’ N status, chl a fluorescence (JIP test), and mycorrhizal colonization were measured. N was always limiting. A gradient of mycorrhizal effects over the host plant’s growth and vitality was successfully obtained. The mycorrhizal effects on plant growth and N uptake were very strongly and positively correlated, and no evidence was found of a C limitation to growth, confirming hypothesis 1. Indications were found that the plants continued to provide C to the fungus although the N supplied by it was increasingly lower, denying hypothesis 2. A new index, the mycorrhizal N demand–supply balance, was found to efficiently explain, and to have a curvilinear relation with, the variation in response to mycorrhization. The mycorrhizal effect on host plant growth was not related to a negative effect on its photosynthetic performance and, therefore, reflected changes in resource allocation between host plant and mycorrhizal fungus, not in plant vitality.engAmmoniumChlorophyll a fluorescenceColonization densityCost–benefitDemand–supplyEctomycorrhizaeMycorrhizal responseNitrogenO-J-I-P testpreprint