Sucrose-induced stress and initial days after explant excision affect the pattern and efficiency of somatic embryogenesis in the tree fern Cyathea delgadii Sternb.

The efficiency of direct somatic embryogenesis (SE) induced on internode explants of the tree fern Cyathea delgadii treated with different concentrations of sucrose at various periods after their excision from donor sporophytes was investigated. Internode explants, about 1.5 mm in length, were dissected from etiolated plantlets grown in vitro and cultured on half-strength hormone-free MS medium supplemented with 10 g l −1 sucrose, under continuous darkness. The impact of four concentrations of sucrose (0.6, 0.8, 1.0, 1.2 M) applied every second day between days 0 and 10 was studied. The highest number of somatic embryos was obtained when explants were treated with 1.0 M sucrose on the 4th day (59.8 on average) or 0.8 M sucrose on the 8th day (67.6 on average) after excision. Light and scanning electron microscopy observations revealed that a nearly 22-fold increase in SE efficiency induced by sucrose treatment was associated with a change in the origin of somatic embryos from multicellular to unicellular. Application of high sucrose concentrations on the 4th day after explants excision stimulated the accumulation of starch, a valuable energy source for intensively dividing cells and developing embryos. The study's results may be useful for the in vitro mass production of plantlets of the valuable tree fern C. delgadii and helpful in enriching the knowledge of the mechanisms governing the initial phases of SE.