[Investigations on the inheritance of the charactergrandiflora inPetunia×hybrida Vilm : II. The use of tetraploidgrandiflora lines for the breeding of newsuperbissima forms, especially those with yellow flowers].
1. In the diploid petunias of the class ofgrandiflora the dominant alleleG at thegrandiflora locus which determines the character of large flowers is normally linked to a recessive lethal genel which eliminates zygotes and simultaneously diminishes the chance of fertilization of the pollen transferring it. Large flowered petunia plants in the class ofgrandiflora are therefore normally heterozygous with respect to the alleles of thegrandiflora locus and have the genotypeGl/gL. Grandiflora homozygotesGl/GL orGL/GL originate from rare crossover events, and so does the genotypeGL/gl. 2. Since the very bright leaf color of thegrandiflora homozygotes reduces their vigor, the breeding of homozygous large flowered varieties has no practical importance. A special interest therefore arises for the genotypeGL/gl because, in spite of heterozygosity, the offspring do not include undesirable small-flowered plants with the exception of some originating from rare combinations of crossover gametes. 3. The tetraploid genotypeGL/GL/gl/gl was produced by colchicine treatment. Analysis of the offsprings of the next two generations showed that in tetraploid material the zygotic effect of the lethal gene works in the same way as in diploids, producing again automatic elimination of small-flowered plants. Moreover, it was found that gametes of the genotypegl/gl can be transferred not only by egg-cells but also by the pollen. 4. Consequently, it is possible to achieve an automatic elimination of the small-flowered plants also in the tetraploid class ofsuperbissima. This may be done by replacing the linkage groupgL ofsuperbissima petunias by thegl type of the artificially produced new tetraploid material. Homozygous large-flowered plants do not occur in tetraploid material because gametes of the genotypeGG are not transferable by the pollen. 5. Particularly the above mentioned qualities, that is the crossability to tetraploid petunias of the class ofsuperbissima and the possibility of transferring some desired characters, as for example the linkage groupgl or the yellow flower color to the latter, determine the value of the newly produced tetraploid material. Since some of the typical characters ofSuperbissima petunias, particularly those concerning flower shape, do not occur in the new tetraploid material the latter is of no direct ornamental interest, except for the yellow-flowered lines. 6. The alleleG which produces large flowers both insuperbissima and in the new tetraploid material brightens, in addition, the leaf color in the new tetraploid material. Since the degree of brightness increases with the number ofG alleles in the genotype, it is rather easy to identify the different tetraploid heterozygotes in the new material. 7. Yellow-flowered lines were also produced as starting material for the polyploidization of the genotypeGL/gl. Tetraploid yellow-flowered lines were produced in order to examine whether the tint of the yellow flower color, unsatisfactory in large-flowered diploid varieties, could be intensified by tetraploidy. Because the yellow flower color did occur insuperbissima, the newly produced tetraploid yellow-flowered material is important also for transmission of this character to the latter. The experiments showed that the intensity of yellow flower color in the new tetraploid yellow-flowered and very large-flowered material still does not reach that of modern diploid small-floweredmultiflora petunias. However, for the production of both practically pure, large, and yellow-flowered tetraploid varieties and of tetraploid large and yellow-flowered F1-hybrids the new material offers interesting possibilities, which are discussed in detail.
Max-Planck-Institut für Kulturpflanzenzüchtung, Hamburg-Volksdorf.
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Reimann-Philipp