Exogenous calcium enhances the formation of vegetative buds, flowers and roots in tobacco pith explants cultured in the absence of exogenous hormones

Pith explants excised from the apical stem internodes of vegetative, flowering, and fruiting tobacco plants were cultured on hormone-free medium in the presence or absence of CaCl2 ( 3 mM). The aim was to determine the role of exogenous calcium (Ca2+), applied at the concentration normally present in the Murashige and Skoog ( 1962) medium, in organ formation obtainable in the absence of the exogenous hormonal input. Exclusive formation of vegetative buds was obtained from explants excised from vegetative plants ( pure vegetative programme); vegetative buds and flowers ( and occasionally roots) on the same sample were obtained from explants from flowering plants ( mixed flowering programme); whereas roots, very occasionally associated with vegetative buds and flowers on the same sample, were obtained from explants from fruiting plants ( mixed rooting programme). Histological analysis showed that the organs always exhibited indirect regeneration. Exogenous Ca2+ promoted the formation of meristemoids and the first phases of their growth into organs, but did not change the realization of the organogenic programme and did not affect callogenesis. Instead, the influence of exogenous Ca2+ changed with the programme, when considering the last phases of organ growth (i.e., macroscopic development and elongation), and the appearance of morphological anomalies in the organs.