CONTROLLED MYCORRHIZAL INITIATION AS A MEANS TO IMPROVE ROOT DEVELOPMENT IN SOMATIC EMBRYO PLANTLETS OF HYBRID LARCH (LARIX X EUROLEPIS)

This study represents the first report of in vitro mycorrhization of s omatic embryo-derived plantlets. Effects of fungi on the rooting of pl antlets as well as on their ultrastructure were studied. Embryo-derive d plantlets of a hybrid larch (Larix x eurolepis Henry) were grown ase ptically in the presence of four ectomycorrhizal fungi. One month afte r inoculation with Laccaria laccata, Hebeloma cylindrosporum and Pisol ithus tinctorius, the number of plantlets which had developed a well-g rowing root system was significantly improved compared with controls. Furthermore, root branching was strongly stimulated by L. laccata and H. cylindrosporum, which were more efficient species than P. tinctoriu s and Suillus grevillei. Development of the root system was concomitan tly accompanied by an enhancement of shoot growth, except for plantlet s inoculated with S. grevillei, which died after three to six months. At the ultrastructural level, cross-sections of lateral roots made six months after inoculation, showed the presence of a Hartig net in the case oft. laccata and H. cylindrosporum, whereas no such net was obser ved with the two other fungi. With S. grevillei, some perforating: hyp hae penetrated the cortical cells and resulted in cell death. Evidence of a relationship between structure and efficiency of the association was found.