MICROPROPAGATION OF SOME COLD-HARDY DWARFING ROOTSTOCKS FOR APPLE

The important cold-hardy dwarfing apple rootstocks B.9, 'Ottawa 3', P.2 and P.22 were micropropagated with varying degrees of success. Micropropagation was achieved readily with P.22 and 'Ottawa 3' with shoot production and rooting increasing to high levels following subculture for 6-9 months. Micropropagation was more difficult with P.2 and B.9. With P.2, shoot production and rooting was low in spite of subculture for 38 months. Nevertheless, there was some improvement in shoot production on culture medium with phloroglucinol (PG) and increased cytokinin, and 80-90% of shoots rooted on medium with PG. With B.9, shoot production was similar to that with 'Ottawa 3' but rooting remained poor in spite of subculture for 39 months. Nevertheless, with shoots of a culture line that had been subcultured for four years, 73% rooting was achieved in vitro on medium with PG and 96% by direct transfer of shoots to sand following a dip in IBA. Micropropagated plants of the rootstocks have been planted in the field for evaluation of trueness-to-type and of improved conventional propagation following micropropagation.