In vitro Propagation of Fraxinus Species

The genus Fraxinus, a member of the Oleaceae family, includes over 65 ash species native to the temperate regions of the northern hemisphere (Miller, 1955). Several of the ash species are important forest trees noted for their tough, highly resistant to shock, straight grained wood as well as being excellent shade trees for parks and residential areas (Dirr, 1998). Economically, the most important species include white ash (F. americana L.) and green or red ash (F. pennsylvanica Marsh.) in the United States and Europe or common ash (F. excelsior L.), flowering ash (F. ornus L.), and narrow leaf ash (F. augustifolia Vahl.) in Europe and Asia Minor.

Propagation is usually by seed collected in the fall and sown immediately or artificially stratified for 90 to 120 days before sowing in the spring. Reliance on seed propagation for conventional breeding is problematic as it may take 10 to 25 years for trees to attain reproductive maturity and then abundant seed crops may only be produced every 3 to 5 years (Bonner, 1974). Although there are no reliable methods for rooting softwood cuttings, ash cultivars can be propagated by budding, grafting, and possibly layering (Hartmann et al., 1997). In vitro propagation through axillary shoot micropropagation, adventitious shoot organogenesis, or somatic embryogenesis is promising for several of the ash species. The objective of this chapter is to describe the procedures we have used and to compare them to some of the most promising in vitro approaches used by other researchers for the different ash species.