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Résumé :
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As in 1994, 1995 and 1996, all the experiments, in laboratory conditions or in nursery or field conditions, started or continued as planned.The results of this fourth year of work were scientifically very interesting. We better understand the relationships between micro-organisms, rhizogenesis processes and mycorrhizal infection. The utilisation of AIA transport inhibitors has greatly improved our knowledge on the role of fungal IAA on plant rhizogenese and mycorrhizal infection. In 1994, if it was relatively easy to identify a specific strain by PCR typing in pure culture or in association with the host in nursery conditions, it was much more difficult to use this method in field situation. In 1995, we have solved this problem and we are now able to identify ectomycorrhizal strains in field situations. In 1996 n 1997 we were able to track Laccaria s 238 N in field conditions. The final objective which was to associate clones selected because of their superior form and vigour or wood quality, with superior strains of ectomycorrhizal fungi or bacteria seams still feasible at least for Douglas fir. We have solved several difficulties and now we are able to get a good mycorrhizal infection in in vitro conditions, except for oak, and in non axenic conditions, during or after the rooting phase of the cuttings. We almost always got an improvement of the performances of the cuttings during the nursery phase, with an exception (Douglas fir cuttings in a AFOCEL experiment). In 1995, we had no positive effects of nursery inoculation on the performances of the cuttings after outplanting. For field experiment, the year 1996 was interesting with two examples of effects of nursery inoculation. In 1997, a positive effect, at Sully, confirmed that the association between Laccaria S 238 and Douglas-fir can be profitable in the field as well as in the nursery. Such results were already known on seedlings and their application to cuttings are more and more probable. Or Norway spruce cuttings, if a positive effect of inoculation on growth or survival can happen during the growing phase, it never happened clearly after planting, except a brief positive effect on second-year growth, at Vidaillat. In Great Britain, there is still an indication that inoculating Sitka spruce cuttings with Laccaria bicolor (S 238) can improve height growth after outplanting. In Spain the field trials have been settled since one year and it is too soon to have results. From these experiments, it still appears that several years are necessary to get comprehensible results after transplantation in forest conditions. The molecular characterisation of Paxillus involutus strains was successfully achieved in 1996 by PCR products, RFLPs and by sequencing the IGS region. The results obtained in 1997 confirmed the Paxillus involutus inoculation could be very useful for Oak and Beech seedlings or cuttings. These molecular techniques also allowed the monitoring of Laccaria bicolor S 238 N after field transplantation. We have confirmed the rapid disappearance of Laccaria bicolor S 238 N in a Norway spruce plantation and its survival with Douglas fir cuttings.
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