Controlled mycorrhizal initiation as a means to improve root development in somatic embryo plantlets of hybrid larch (Larix × eurolepis)

Abstract

This study represents the first report of in vitro mycorrhization of somatic embryo‐derived plantlets. Effects of fungi on the rooting of plantlets as well as on their ultra‐structure were studied. Embryo‐derived plantlets of a hybrid larch (Larix × eurolepis Henry) were grown aseptically in the presence of four ectomycorrhizal fungi. One month after inoculation with Laccaria laccata, Hebeloma cylindrosporum and Pisolithus tinctorius, the number of plantlets which had developed a well‐growing 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. tinctorius and Suillus grevillei. Development of the root system was concomitantly accompanied by an enhancement of shoot growth, except for plantlets 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 of L. laccata and H. cylindrosporum, whereas no such net was observed with the two other fungi. With S. grevillei, some perforating hyphae penetrated the cortical cells and resulted in cell death. Evidence of a relationship between structure and efficiency of the association was found.