Effects of auxins and cytokinins on direct somatic embryogenesis on leaf explants of Oncidium 'Gower Ramsey'

Four auxins (IAA, IBA, NAA and 2,4-D) and five cytokinins (2iP, zeatin, kinetin, BA and TDZ) were examined for their effects on direct somatic embryogenesis on leaf explants of a sympodial orchid Oncidium 'Gower Ramsey'. On a hormone-free basal medium, the percentages of embryo formation were 40%, 20%, 5% and 0% on leaf tips, adaxial sides, wound surfaces and abaxial sides of the leaf explants, respectively, and the average number of embryos per explant was 5.6. Embryo formation on leaf explants was retarded by all four auxins tested, but promoted by all the cytokinins. The percentages of embryo formation were reduced to 20%, 5-10%, 0% and 0%, respectively, in the same parts of leaf explants when supplemented with low dogsages of IAA (0.3-3 mg/l) on the basal medium. Furthermore, embryo formation was totally inhibited by 3 mg/l NAA, 0.3-3 mg/l IBA and 2,4-D. The sequence of embryo formation on various location of leaf explants was altered by 0.3-1 mg/l 2iP and 3 mg/l zeatin, and embryo formation on adaxial sides > leaf tips > wound surfaces > abaxial sides. The highest percentage of embryo formation on leaf tips, adaxial sides and wound surfaces of explants were 75%, 50% and 20% when supplemented with 1 mg/l TDZ, 1 mg/l 2iP and 0.3 mg/l kinetin, respectively. The highest average number of embryo per explant (10.7) was found on a basal medium containing 1 mg/l TDZ.