Timing of the phases in adventitous root formation in apple microcuttings

As with other regeneration processes, adventitious root formation may be divided into three phases, namely dedifferentiation, induction and differentiation. We assumed that the appropriate hormonal conditions for rooting, in particular a high level of auxin and a low level of cytokinin, are required only during the induction phase. Hence, the effect of 24 h pulses with indolebutyric acid (IBA) or benzylaminopurine (BAP) should be maximal during this phase. On this assumption, the timing of the three phases was determined in microcuttings of Malus ‘Jork 9’. The promotion of rooting was largest for 24 h IBA-pulses given 24–48 h or 48–72 h after the onset of culture. During culture of shoots on IBA-containing medium 24 h BAP-pulses were given. Inhibition of rooting was maximal for the BAP-pulses given between 24 and 96 h. An analysis of the kinetics of root emergence also indicated that added IBA acted from 24 h onwards: emergence of roots from the tissue occurred simultaneously for the IBA pulses at 0–24 h and 24–48 h, but lagged 1 d behind for the 48–72 h pulse and 2 d for the 72–96 h pulse. We concluded that dedifferentiation occurred from 0 to 24 h, induction from 24 to 72 or 96 h, and after that differentiation. Histological observations showed that after 24 h, cells with swollen nuclei and dense cytoplasm had appeared in the regions of the stem where the roots were formed. The first cell divisions were observed after 48 h. After 96 h, meristemoids of c. 30 cells had been formed. After the BAP-pulses at 24–48 h or 48–72 h, these meristematic cells formed callus.