Are mycorrhiza always beneficial?

In this work we evaluate whether the effect of ectomycorrhiza in the early developmental stages of symbiosis establishment is detrimental or beneficial to plant productivity and whether this effect is dependent on either N nutrition or plant age. Groups of Pinus pinaster L. plants with different ages and nutritional status were inoculated with alive or dead Pisolithus tinctorius. The plants were fed with either 1.9 mM or 3.8 mM ammonium as N source. Ectomycorrhiza establishment was monitored until 1 month after the inoculation through daily chlorophyll a fluorescence measurements and the analysis of fast fluorescence kinetics O-J-I-P, biomass increment and photosynthesis. Our results show that plants react differently to ectomycorrhiza formation depending on their age (stage of development, leaf area), their initial nutritional status, and the amount of nitrogen supplied. Mycorrhiza formation was found to constitute a stress depending on the plants' age. Increased availability of N softened or eliminated the negative impact of mycorrhiza formation. Only younger plants eventually developed a higher net photosynthesis rate when mycorrhizal. It is concluded that ectomycorrhiza formation may have a detrimental rather than a beneficial effect on plants' productivity during their establishment and early developmental stages, and that this depends on the amount of N available to the plant, on the nutritional status and on the age of the plant. Chlorophyll a fluorescence measurements proved to be a non-destructive, non-invasive and reliable tool able to identify the first signals of plant-mycorrhiza fungi interactions.