Investigations of a two-step process for potato (Solanum tuberosum L.) microtuber production

Standard protocols for potato plantlet multiplication from nodal explants and for subsequent microtuberization were established. Liquid Murashige and Skoog (1962) basal media containing 3% (w/v) or 8% (w/v) sucrose without any exogenous plant growth regulators were used for plantlet multiplication or microtuberization respectively. More than 20 variations to the standard protocols, either during the plantlet multiplication step or the microtuberization step were investigated in relation to plantlet growth, microtuber number, average fresh microtuber weight and microtuber weight distribution. The responses of two potato cultivars ('Iwa' and 'Daeji') were compared. Time courses of some major changes in the media were also studied. Initially, it was found that sucrose disappearance from the standard microtuberization medium, microtuber initiation, development and cessation of further growth, invertase activity development in the medium, osmotic potential changes and pH changes in the medium appeared to be correlative events. However, the data from the different experiments in this study indicate that most of these changes are associated with the 8% sucrose medium but are not strictly related to microtuberization. Among the 21 variations to the standard protocols, whether during plantlet multiplication or during in vitro tuberization, medium replacement was most effective in inducing the formation of bigger and more microtubers. In the course of this study, it was observed that at the end of the plantlet multiplication step the root had turned green. Even more interesting is that some of these green roots remained green after 10 weeks in darkness for the microtuberization step. A small-scale ultrastructural study confirms the occurrence of chloroplasts in the green roots during plantlet growth and also at the end of the microtuberization step in the dark