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P. Gercheva, R.H. Zimmerman, L.D. Owens, C. Berry, and F.A. Hammerschlag

Shoot regeneration from apple (Malus domestica Borkh.) leaf explants following particle bombardment at various acceleration pressures was studied. Basal leaf segments of micropropagated `Royal Gala' apple were bombarded with 1 μm gold particles, accelerated using helium pressures of 4.5, 6.2, 7.6, 9.3, or 13.8 MPa (650–2000 psi), and cultured on shoot regeneration medium consisting of N6 salts supplemented with 10 μM TDZ for 5, 10, or 20 days in darkness. Bombarded and control explants exhibited 63% to 100% shoot regeneration. With a 5-day dark period, average shoot production per explant ranged from 6.1 to 14; bombardments of 4.5 and 6.2 MPa significantly increased shoot production over the controls. With a 10-day dark period, average shoot production per explant ranged from 9.1 to 22 following bombardment at 9.3 and 6.2 MPa, respectively. Following bombardment at 6.2 MPa, 75% of the explants produced more than 20 regenerants per explant. With a 20-day dark period, average shoot production per explant ranged from 8.9 to 19 following bombardment at 13.8 MPa and following no bombardment, respectively. Shoot production per explant was significantly less than the controls following bombardments ranging from 6.2 to 13.8 MPa. Shoot production was highest per explant with particle bombardment at 6.2 MPa followed by incubation in darkness for 10 days. Chemical name used: thidiazuron (TDZ).

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F.A. Hammerschlag, R.H. Zimmerman, U.L. Yadava, S. Hunsucker, and P. Gercheva

A range of antibiotics was evaluated for their effect on eliminating Agrobacterium tumefaciens supervirulent strain EHA101(pEHA101) from leaf explants of `Royal Gala' apple (Malus domestica Borkh) and on regeneration. After long-term (38 days) exposure to 100-μgml–1 concentrations of either cefotaxime (cef), carbenicillin (carb), mefoxin (mef), or combinations of these antibiotics, only on carb or carb with mef was regeneration not inhibited. None of the above antibiotics or antibiotic combinations eliminated A. tumefaciens from leaf explants. Short-term (1-18 hours), vacuum infiltration with 500- to 1000-μgml–1 concentrations of either of the above antibiotics did not inhibit regeneration, but did not eliminate A. tumefaciens from leaf explants. After a 30-min vacuum infiltration with a 2000-μgml–1 concentration of either cef, carb, or mef, only cef reduced the number of leaf explants with A. tumefaciens.