Genetic-evolutionary studies on cultivated cannas-IV: parallelism between natural and induced somatic mutations

High level heterozygosity, sterility and the rather long duration between sexual generations, all restrict improvement of canna through hybridization and selection. Thus natural and induced somatic mutations may offer an easier method of producing new variation. The more common flower colour changes are spots and stripes, which are generally unstable. However, one whole-colour change in 3x 'Rosamund Coles' treated with 1 kR proved to be stable. Colour mutations in canna usually involve a change from red to yellow. This course also underlies the origin of yellow coloured 'Queen of Italy' from the pure red 'King Humbert' in nature. The former cultivar is very unstable and its flower colour ranges from pure yellow, yellow with red spots, streaks and sectors, to pure red. The number, length and breadth of the red spots, stripes and sectors show the number of mutations, and the stage in ontogeny at which they occur. Such an interconversion of 'King Humbert' and 'Queen of Italy' through somatic mutations may be the result of instability of the relevant colour gene(s). This example together with similarity in leaf variegations and flower abnormalities between natural and induced mutants investigated here, show a parallelism between the two, and also the role somatic mutations can play in the origin of new cultivars. Interchanges and reduction of pairing in diploids and autotriploids in the treated plants are the result of rearrangements caused by radiation, but such detectable changes do not account for all their sterility. There is also no strict correlation between the extent and nature of induced phenotypic and chromosomal changes. Using lethality or severe growth inhibition as an index, the triploids are more radiation-resistant than diploids. This may be due to either the buffering effect of the presence of duplicate loci or a reduced interphase chromosome volume in triploids