High frequency plant regeneration from Eucalyptus globulus Labill. hypocotyls:: Ontogenesis and ploidy level of the regenerants

Up to 70% regenerating hypocotyls provided with 5 to 20 buds were obtained on MS medium containing 0.01 or 1 mg l(-1) NAA and 0.2 mg l(-1) BA or 0.2 mg l(-1) BA and 0.2 mg l(-1) TDZ. The ability to regenerate buds was correlated with the presence of oil glands at a stage in germination when oil secretion was not yet occurring. The regeneration of shoot meristems took place from the cells involved in the differentiation of these glands. Such glands could also appear during callus redifferentiation, giving rise to indirect regeneration. Rooting of the regenerants was efficient using a two-step procedure of induction under darkness in the presence of 3 mg l(-1) IBA, followed by root development on medium devoid of growth regulators under a 16-h photoperiod, the medium being solidified with Gelrite. Regenerated plants showed no phenotypic alterations. Nuclear DNA contents in mother-plant material and regenerants were analysed using flow cytometry. There was no evidence of polyploidy in any of the samples, indicating the absence of polyploidisation during cell differentiation and under in vitro conditions. No regeneration was obtained from leaf or stem explants from micropropagated plantlets.