FACTORS AFFECTING ADVENTITIOUS ROOT-FORMATION IN MICROCUTTINGS OF MALUS

Adventitious root formation (ARF) in conventional cuttings and microcuttings is a major problem in the vegetative propagation of many crops. We studied ARF in microcuttings of Malus. The rooting ability depended on the genetic background, the ontogenetic state (juvenile vs adult), the occurrence of dormancy, vitrification, the duration of the final propagation cycle and indolebutyric-acid (IBA) concentration during propagation. The timing of IBA application during the rooting treatment had a significant effect: postponement of the application resulted in a strong decrease in rooting. When rooting occurred under continuous light, instead of the standard dark condition, the IBA dose-response curve shifted to the right (to higher IBA concentrations). The timing of the main phases of ARF was established by 24-h pulses with benzyladenine (BAP) during the standard IBA treatment (0.2 mg/l IBA; 20-degrees-C). The BAP-pulses strongly blocked rooting when given between 24-72 h. We concluded from this that the differentiation into root-(pre)meristemoids occurred during that time. In correspondence with this, 24-h pulses with auxin had a maximum effect on rooting when applied during this period. We presume that the lag from 0-24 h represents a phase of dedifferentiation. The outgrowth phase of root-(pre)meristemoids occurred after 72 h. After ca 9 d the roots emerged from the stem.