In vitro culture of Cucumis sativus L. VI. Histological analysis of leaf explants cultured on media with 2, 4-D or 2, 4, 5-T

The developmental sequence of callus initiation and somatic embryogenesis in leaf explants of Cucumis sativus cv. Borszczagowski was analysed and compared on media containing two different auxin phenoxy-derivatives (2,4-D and 2,4,5-T) and cytokinin (BAP or 2iP). During the first 20 days of culture on media with 2,4,5-T proliferation of parenchymatic tissue occurred mainly and only small meristematic centers were observed. There was an intensive detachment of parenchymatic cells and dissociation of their cell walls near vessels and in the lower part of the explant adjacent to the medium. These cells were strongly plasmolysed. On the 2,4-D containing medium mostly meristematic tissue developed, proliferating around vascular bundles and forming meristematic centers or promeristem-like structures. After 35-50 days of culture, secondary callus was formed by separation of meristematic cells from the meristem surface in explants cultured on the 2,4-D containing medium. On medium supplemented with 2, 4, 5-T the detachment of parenchymatic and meristematic cells occurred, along with formation of a gel-like substance. The gel-like callus contained multi-cellular aggregates, proembryoids and embryoids. This type of callus tissue was initiated more intensively on medium with 2, 4, 5-T, but the frequency of somatic embryogenesis was much lower. The periferial cells of aggregates, proembryoids and embryoids showed the tendency to separate from the surface of the tissue. Many embryoids formed adventitious embryos.