Histological study of in vitro development of adventitious buds on leaf explants of oriental beech (Fagus orientalis lipsky)

Summary  Adventitious shoots were induced on the proximal portion of leaves excised from Fagus orientalis shoot cultures derived from a 2-mo.-old or a 4-yr-old seedling. Up to 90% of the explants formed between 13 and 19 buds after culture on Woody Plant Medium containing 2.9 μM indole-3-acetic acid and 4.5 μM thidiazuron. Adventitious buds developed mostly on petiole stub callus, but also on the midvein. The histological events leading to shoot organogenesis were examined. Some shoots developed directly from subepidermis or epidermis, but most originated indirectly from cell file proliferation produced by periclinally dividing cells subadjacent to the epidermis. Some cells in the outermost layers of these files became meristematic and divided extensively, resulting in the formation of meristemoids after 16 d of culture. Dedifferentiation into meristematic cells, which exhibited a large, prominent nucleus, densely-stained cytoplasm, and a high nucleus-to-cell area ratio, was generally associated with anticlinal divisions in cells previously originated by periclinal division. Subepidermal cell proliferation occurred mainly in the abaxial surface of the explant, which initially formed a diffuse cambium and later evolved to a phellogenic cambium. Some meristemoids were also differentiated by lenticel phellogen. Organized cell divisions in meristemoids gave rise to bud primordia that emerged from the explant surface and differentiated a protoderm. The progressive structural differentiation of the apical meristem, leaf primordia, and procambial strands led, after about 28 d of culture, to shoots with vascular connections with treachery elements previously differentiated in adjacent tissues.