PHYSIOLOGIA PLANTARUM 95: 575-580. 1995 Copyright Printed in Denmark ~ all righti: reserved ISS^ 0031-9317 Controlled mycorrhizal initiation as a means to improve root development in somatic embryo plantlets of hybrid larch {Larix X eurolepis) ¥. Piola, R. Rohr and P. von Aderkas Piola, F., Rohr, R. and von Aderkas, P. 1995. Controlled mycorrhizal initiation as a means to improve root development in somatic embryo plantlets of hybrid larch (Larix x eurolepis). - Physioi. Plant. 95: 575-580. This study represents the first report of in vitro myconhization of somatic embrj'O-de- rived plantlets. Effects of fungi on the rooting of plantlets as well as on their ultra- struaure were studied. Embryo-de rived plantlets of a hybrid larch (Larix x eurolepis Henry) were grown aseptically in the presence of four ectomycorrhizal ftingi. One montfi 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. Furthertnore, root branching was strongly stimulated by L. laccata and Ft. cylindrosporum, which were more efficient species than P. tinctorius and Suiilus grevillei. Development of the root system was concomitantly accompanied by an enhancement of shoot growth, except for plantlets inoculated with S. greviliei. 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 Hardg net in the case of L. laccata and H cylindrosporum, whereas no such net was observed with the two other fungi. With 5. greviliei, 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. Key words — Larch, Larix x eurolepis, myconhization, plantlets, somatic embryos. F. Piola and R. Rohr (corresponding author). Lab. Interactions Planles-Champignons et Micropropagation. Univ. de Lyon 1. F-69622 Vitleurbanne Cedex. France: P. von Aderkas. Centre For Forest Biology. Dept of Biology, Univ. of Victoria, Victoria B.C. V8W2Y2. Canada. . hanced by the presence of mycorrhizae. Ectomycor- Introduction rhizae occur naturally on many if not most of the com- Somatic embryogenesis allows the production of quanti- mercially important forest tree species. The stimulating ties of conifer propagules. Important progress has been influence of fungi on tree growth and physiology has made since the first reports on somatic embryogenesis of been studied by many authors (see Harley and Smith Norway spruce {Picea abies) by Chalupa (1985) and 1983). In gymnosperms, several studies on mycorrhizal Hakman et al, (1985) and hapioid embryogenesis of establishment in vitro have been undertaken (David et larch by Nagmani and Bonga (1985), Improvements to al, 1983, Dudddge and Read 1984, Debaud et al, 1987). embryo initiatioti, maturation and plant regeneration In the genus Larix, mycorrhizal establishment in vitro have been made (von Arnold 1987, Hakman and von Ar- has been attempted with seedlings of L. taricina (Sam- nold 1988, Lelu etal, 1994), but root development is fre- son and Fortin 1986) and L. decidua (Dudddge 1986) quently a limiting factor in plant regeneration via so- but not with somatic embryo piantlets. Only one study matic embryogenesis (Hakman and von Arnold 1988), was carded out with somatic embryo-dedved plantlets Growth of the newly-initiated root system may be en- of a conifer, namely, Picea sitchensis, but no symbiotic Received 12 June, 1995; revised 29 September, t995 Ptiysiot. Planl. 95, t995 575