Induction of secondary somatic embryogenesis in hybrid larch (Larix x leptoeuropaea) as related to ethylene

Induction of secondary somatic embryogenesis was studied with hybrid larch (Larix x leptoeuropaea) cotyledonary somatic embryos obtained after 3, 4, 5 and 6 weeks of culture on a maturation medium supplemented with abscisic acid. Almost all 3-week matured cotyledonary somatic embryos can develop embryonal masses whereas only 78, 27 and 12% of them are able to do so after 4, 5 and 6 weeks of maturation, respectively. During the first week of culture on the induction medium, somatic embryos with high embryogenic potential (i.e. 3-week matured) release little ethylene (less than 1.5 nL h(-1) g(-1) FW), whereas those which have almost completely lost their ability to induce embryonal masses (i.e. 6-week matured) produce much more ethylene. Thereafter, ethylene production by both types of embryos is very similar at around 5-6 nL h(-1) g(-1) FW. Enrichment of the atmosphere with ethylene, or addition of 2-chloroethylphosphonic acid (ethephon) or ACC in the induction medium strongly reduced the induction of secondary somatic embryogenesis. Moreover, inhibitors of ethylene action (AgNO3 and 2,5-norbornadiene) improved the development of embryonal masses from somatic embryos, particularly from the 6-week maturated ones. The results obtained clearly suggest that ethylene is involved in the regulation of somatic embryogenesis in hybrid larch. The possible relationship between somatic embryogenic potential and ethylene biosynthesis by the explants or sensitivity of the latter to ethylene is discussed.