Ploidy stability in embryogenic cultures and regenerated plantlets of tamarillo

Ploidy levels of short-term (1 and 2 years) and long-term (7 and 10 years) embryogenic cultures as well as of regenerated plantlets of tamarillo were analyzed by flow cytometry and chromosome counts. Embryogenic cultures were induced from expanding leaves cultured in the presence of Picloram or 2,4-dichlorophenoxyacetic acid (2,4-D) and monthly subcultured on the same media. Embryo development and plantlets were obtained following subculture of the embryogenic tissue in auxin free medium containing gibberellic acid (GA(3)). Seedlings and rooted shoots from axillary shoot proliferation were used as controls. The results showed that in long-term embryogenic cultures the ability to develop somatic embryos and plantlets was reduced. Embryogenic tissues maintained for 10 years were mostly aneuploids of the tetraploid (2n = 4x = 48) level whereas those kept in culture for 7 years or less were also mostly aneuploids but of the diploid (2n = 2x = 24) level. The results obtained by flow cytometry were, in general, consistent with those obtained by chromosome counts. The chromosome alteration observed in the embryogenic tissues was already present after 1 year of culture and increased with culture age, hence impairing the maintenance of these tissues for long periods without affecting chromosome stability of the regenerated plantlets. However, the occurrence of triploids and tetraploids as well as aneuploids can be useful for breeding purposes. A value around 23 pg/2C was found for the genome size of tamarillo largely exceeding the value previously published (15.50 pg/2C).