Urea derivatives are synthetic compounds, some of which have proved to be positive regulators of cell division and differentiation. N-phenyl-N'-(2-chloro-4-pyridyl)urea (forchlorofenuron, CPPU) and N-phenyl-N'-(1,2,3-thiadiazol-5-yl)urea (thidiazuron, TDZ), well known urea cytokinin representatives, are extensively used in in vitro plant morphogenesis studies, as they show cytokinin-like activity often exceeding that of adenine compounds. In recent years, renewed interest in structure-activity relationship studies allowed identification of new urea cytokinins and other urea derivatives that specifically enhance adventitious root formation. In this review, we report the research history of urea derivatives, new insights into their biological activity, and recent progress on their mode of action.

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The present research investigates the possibility that 2 weak urea-type cytokinins, the N,N¢-bis-(1-naphthyl)urea and the N,N¢-bis-(2-naphthyl)urea, enhance adventitious root formation. The rooting activity was assessed using the stem slice test, the mung bean rooting test and the rooting of apple microcuttings. The two compounds influenced the adventitious rooting process differently as regards the bioassay used. In the stem slice test, in the presence of exogenous auxin, both compounds enhanced the rooted slice percentage. In mung bean shoots, the N,N¢-bis-(1-naphthyl)urea enhanced the root formation at the lowest concentration used (0.01 lM) while the N,N¢-bis-(2-naphthyl)urea enhanced rooting at higher concentrations. In the rooting test of apple microcuttings the N,N¢-bis-(1-naphthyl)urea and the N,N¢-bis-(2-naphthyl)urea slightly enhanced only the mean root number per microcutting.