Comparison between 1,3-di(benzo[d]oxazol-5-yl)urea and kinetin in enhancing adventitious rooting in difficult-to-root carob microcuttings

Carob tree (Ceratonia siliqua L.) is a typical evergreen species of the Mediterranean region, commonly cultivated since historic times. Nowadays, its importance belongs to the great ornamental value for gardens, landscape and urban architecture and to the use of locust bean gum in the food industry. As the vegetative propagation of valuable genotypes by traditional grafting and/or cuttings gave poor results, in vitro micropropagation programmes have been established to fulfil the increased market request. In this scenario, many efforts have been made to improve the adventitious rooting performance in carob difficult-to-root micropropagated cuttings. With the aim of discovering the most effective compound to induce adventitious rooting, we report preliminary data about the enhancement of adventitious root formation of in vitro micropropagated carob tree cuttings by the exogenous supplementation of either indole-3-butyric acid (IBA) alone or a mixture made by the 1,3-di(benzo[d]oxazol-5-yl)urea (5-BDPU), a synthetic urea derivative lacking of auxin- or cytokinin-like activity, and IBA. The percentage of rooted cuttings was enhanced by the latter treatment, the concentration of auxin was lower than usual, and organized root system arose from the cutting base without callus formation. Owing to these positive results, a histological time course of the adventitious rooting process was carried out to evaluate the cellular events leading to the formation of roots in the presence of IBA alone or 5-BDPU and IBA. Comparative analyses were also made by the supplementation of kinetin (Kin), a synthetic adenine derivative showing cytokinin-like activity, and IBA in the effort to evaluate differences and/or similarities in the activity of the synthetic compounds. The results show that IBA was necessary for adventitious root induction. Kin+IBA enhanced the formation of early stages of root primordia. By contrast, 5-BDPU+IBA enhanced primordia development up to protrusion from the cutting, highlighting its importance to enhance the process in comparison with Kin.