Shoot and root morphogenesis from Eucalyptus grandis x urophylla callus
Eucalyptus grandis x urophylla plantlets were regenerated via indirect organogenesis. Histological assessment of their development focused on identifying the calli, the differentiation of shoots from the calli and the shoot-root junction from the nascent shoots. Vascular tissue formation within the callus preceded that of organised nodular structures, from which adventitious shoots were ultimately formed. Vascular systems linked the nodular structures and their associated cambial meristems, observed at 15 to 22 days, were hypothesized as being the centres of differentiation for shoot formation. Roots developed from isolated adventitious shoots from either the callus created as a wounding response to isolation or from a region of the stem immediately above the callus. Roots developed from the latter had integrated root and shoot vascular systems, whereas those originating from the callus were not connected vascularly; however, this connection could be created during ex vitro acclimatization. The implications of the findings for increased plant yields through indirect morphogenesis were discussed.
Key words: Eucalyptus, indirect organogenesis, microscopy.