Rhizogenesis in stem discs of Malus pumila rootstock M9 "Jork": I. Hormonal and environmental effects on root induction and callus formation.

To establish a protocol suited for the molecular characterization of root induction the influences of explant position, etiolation state and orientation as well as temperature and light intensity on root and callus formation were investigated. Depending on these factors stem discs of Malus incubated on indole-3-butyric acid containing medium and subsequently on hormone-free medium regenerated roots and callus of "filamentous" and "smooth" texture to a varying extent. Concentration and incubation duration of indole-3butyric acid strongly affected rooting and the production of "smooth" callus. Moreover "smooth" callus was profuse at the low light levels applied during rooting. Rooting efficiency decreased and "filamentous" callus increased between 19°C and 25°C. Explants close to the shoot apex displayed reduced rooting efficiency and profuse "filamentous" callus. There was a strong effect of explant orientation on root and "filamentous" callus formation.