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ABSTRACT
We examined the role of cytokinins in rooting of 1-mm stem slices cut from microcuttings of the apple rootstock ‘Jork 9˚s. Various types of cytokinins inhibited the rooting of apple stem slices to different extents. Highest inhibition was obtained with thidiazuron and benzylaminopurine. Remarkably, isopentenyladenine and isopentenyladenosine enhanced rooting at low concentration (at the optimal concentration of 0.1 μM by 53 and 19%, respectively). We also examined the effect of lovastatin and simvastatin. These drugs are putative cytokinin-synthesis inhibitors. Both inhibited rooting and inhibition was partially reversed by simultaneous addition of zeatin. Moreover, in the presence of lovastatin a higher concentration of zeatin had to be applied to achieve inhibition of rooting than in the absence of the drug. This data indicates that these compounds indeed inhibited cytokinin synthesis. One-day pulses with lovastatin strongly blocked rooting when given just after cutting the slices but had no effect after that. Adding zeatin simultaneously reversed inhibition completely. In conclusion, our data confirm that cytokinins may strongly inhibit rooting but they also show that at low concentration, certain cytokinins enhance rooting. Moreover, synthesis of cytokinin is essential during root formation. We hypothesise that cell division initiated by a relatively high endogenous level of cytokinins just after cutting the slices is a necessary, initial step in adventitious root formation.