PARTICLE BOMBARDMENT OF APPLE LEAF EXPLANTS INFLUENCES ADVENTITIOUS SHOOT FORMATION

Shoot regeneration from apple (Malus domestica Borkh.) leaf explants f ollowing particle bombardment at various acceleration pressures was st udied. Basal leaf segments of micropropagated 'Royal Gala' apple were bombarded with 1 mum gold particles, accelerated using helium pressure s of 4.5, 6.2, 7.6, 9.3, or 13.8 MPa (650-2000 psi), and cultured on s hoot regeneration medium consisting of N6 salts supplemented with 10 m uM TDZ for 5, 10, or 20 days in darkness. Bombarded and control explan ts exhibited 63% to 100% shoot regeneration. With a 5-day dark period, average shoot production per explant ranged from 6.1 to 14; bombardme nts of 4.5 and 6.2 MPa significantly increased shoot production over t he controls. With 10-day dark period, average shoot production per exp lant ranged from 9.1 to 22 following bombardment at 9.3 and 6.2 MPa, r espectively. Following bombardment at 6.2 MPa, 75% of the explants pro duced 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. Sh oot production per explant was significantly less than the controls fo llowing bombardments ranging from 6.2 to 13.8 MPa. Shoot production wa s highest per explant with particle bombardment at 6.2 MPa followed by incubation in darkness for 10 days. Chemical name used: thidiazuron ( TDZ).