Shoot initiation in the leaflet axils of compound leaves from micropropagated shoots of juvenile and mature common ash (Fraxinus excelsiorL.)

Abstract

Micropropagation of a mature ash tree has been achieved for the first time. The main obstacle encountered was contamination of the initial explants with microorganisms. However, once apparently sterile shoots had been obtained, shoot proliferation was achieved most effectively by culturing nodes on Driver and Kuniyuki walnut medium containing 22.2 μM benzyladenine. After several subcultures, a species of Bacillus appeared with the mature culture line, but it did not affect shoot or root development adversely. With successive subculturing, shoots of the mature ash clone became progressively easier to root. Pinnately compound leaf explants from micropropagated shoots of two seedlings and the mature tree, placed on to Murashige and Skoog-based culture media supplemented with 4.4μM-phenyl-N-1,2,3-thiadiazol-5-ylurea, produced shoot buds. Most buds developed from the rachis at the points of attachment of the leaflets. When transferred to Driver and Kuniyuki walnut medium, buds from one of the seedling lines and from the mature tree, elongated into shoots, and were subsequently transferred to media for micropropagation and rooting. Shoot initiation is discussed in relation to possible occurrence of vestigial meristems in the axils of leaflets and the partial shoot theory of leaf structure.