IN VITRO ROOTING OF PORTUGUESE PEAR CULTIVARS (PYRUS COMMUNIS) IN RESPONSE TO CHANGES IN AUXIN INDUCTION AND DARK PERIOD TREATMENTS

An efficient in vitro rooting of shoots obtained from meristem tip culture was regarded as a key issue in a program aimed at virus elimination from traditional Portuguese pears. Hence, intense efforts have been made for a continuous improvement of rooting procedures. In the first set of experiments presented, shoots were submitted either to Long Induction Pre-treatments (LIPs) of ten days on media with 5 µM or 10 µM IBA or to Brief Induction Pre-treatments (BIPs) of 16 hours in 0.5 mM or 1.0 mM IBA. After LIPs or BIPs, the shoots were transferred to a similar but auxin-free media. In both cases the shoots stayed in darkness for the initial five days. Pyrus communis cv. ‘Rocha’ showed no significant differences in rooting efficiency when LIPs and BIPs were compared. In contrast, rooting was strongly enhanced by LIPs in 'Pérola' and by BIPs in 'Carapinheira'. The 16-h BIPs, however, led to excessive callusing. In subsequent experiments, the effects of shorter BIPs (2, 4, 8 or 16 h) and of restricting the dark period to the duration of each BIP were evaluated. BIP duration had no significant effect on rooting percentage but in ‘Rocha’ the 8-h BIP led to the highest number of roots per rooted shoot. A reduction in the initial period of darkness resulted in a highly significant increase in rooting efficiency of ‘Carapinheira’. Dark periods coinciding with BIPs of short duration have been thereafter applied with success in rooting of several pear cultivars and micropropagated woody virus indicators, and are a valuable tool in the ongoing sanitation program.