In vitroorganogenesis from seedling explants of the ornamentalsEucalyptus erythronema,E. stricklandiiand the interspecific hybridE. erythronemaxE. stricklandiicv. ‘Urrbrae Gem’

SummaryIn vitro organogenesis and somatic embryogenesis were investigated in the ornamentals Eucalyptus erythronema, E. stricklandii and their interspecific hybrid cv. 'Urrbrae Gem' using combinations of plant growth regulators (NAA, 2,4-D, BAP and kinetin). Callus growth was consistent across all genotypes and tissue types. Calli were soft and spongy with 2,4-D alone or in combination with BAP or kinetin, while hard and friable with NAA alone or in combination with BAP or kinetin. Callus growth was generally greater with auxin or auxin/cytokinin combinations compared to cytokinin alone. Healthy root development was observed in all genotypes and tissue types with NAA alone, while roots that developed from treatments with 2,4-D turned brown within 12 weeks. Bud development was observed on all genotypes and tissue types with BAP alone or in combination with NAA. Bud clusters were observed on E. erythronema and cv. 'Urrbrae Gem' explants, while E. stricklandii explants produced individual buds. Shoots were observed in treatments with NAA alone, or in combination with BAP or kinetin, but not in treatments with 2,4-D. No somatic embryos were observed in any treatment, or species or hybrid. Light microscopy of explants treated with 2,4-D alone showed calli consisting only of large parenchyma cells, while 2,4-D in combination with a cytokinin produced smaller parenchyma cells, with areas of vascular tissue and circular growth. BAP, kinetin and NAA alone, or in combination, produced an increase in vascular tissue growth, with vascular connections to developing shoots and roots. Shoot and root initiation appeared to occur within the callus. Organogenesis of roots, buds and shoots was achieved with NAA or NAA/BAP, NAA/kinetin combinations, while the auxin 2,4-D gave poor organogenesis.