Tissue culture and frost tolerance studies in Solanum
In vitro shoot cultures of two commercial cultivars of Solanum tuberosum, Desiree and Mans Piper, and of two wild species S. commersonil and S. acaule, were established from single nodal explants and seedling tissues respectively. Callus cultures were initiated from potato stem, leaf and tuber explants. Cell suspension cultures were obtained from tuber and stem-derived calluses of S. tuberosum. Protoplasts were isolated from leaves of greenhouse-grown plants and from suspension-cultured cells of S. tuberosum, and in vitro shoot cultures of S. tube rosum and S. commersonii. Plantlets were regenerated from tuber discs, internodal explants, growing calluses, suspension-cultured cells and protoplast-derived calluses. Microtubers were induced from nodal explants of these Solanum species. Attempts were made to select frost-tolerant cell lines through resistance to hydroxyproline, by direct transfer of axillary buds, callus cultures and suspensioncultured cells to media containing different concentrations of hydroxyproline. Selection was also made after exposure of suspension-cultured cells to a freezing temperature (-6° C), and to gamma-irradiation (20 Gy). Several hyp-resistant cell lines were established from callus cultures and suspension-cultured cells but not from axillary buds. Most of these selected cell lines were found to show increased tolerance to frost. Plants were regenerated from one of the hyp-tolerant, frosttolerant cell lines. The cellular damage to S. tuberosum cv. Desiree callus cells due to freezing temperatures was examined, and the cellular structure of the callus of three Solanum species and one frost-tolerant cell line was compared by electron microscopy.