Influence of IBA and aphidicolin on DNA synthesis and adventitious root regeneration from Malus 'Jork 9' stem discs.

Adventitious root formation in Malus 'Jork 9' stem discs was studied through temporarily blocking DNA synthesis by application of aphidicolin (AD). Higher number of roots per disc (8.4) after 21 days of cultivation were formed after a 24-h pulse of 15 microM AD, compared to control without AD application (6.7), with significantly more roots (3.7) already appearing at day 7, compared to 1.5 roots on the control. The promotive effect of AD on rooting was lower at 5 microM, while a concentration of 30 microM was slightly inhibitory. Results show that DNA synthesis is effectively blocked by AD, and this blockage is overcome after AD withdrawal. The data indicate that AD treatment influences cell divisions, thereby, might synchronise root initiation. The effects of different treatments with and without AD were studied at the cellular level by visualising DNA replication through BrdU-labelling. BrdU labelling further revealed temporal changes in the competence of the explants to respond to applied IBA. Thus, it is shown that the proportion of replicating nuclei present during 28-32 h is significantly increased in the split IBA treatment (0-4 h and 28-32 h; treatment C3), compared with a single IBA application during 0-8 h (treatment C3.1).