Mutagenesis and Biotechnology Techniques as Tools for Selecting New Stable Diploid and Tetraploid Olive Genotypes and Their Dwarfing Agronomical Characterization

In Olea europaea L. subsp. europaea , tetraploid genotypes do not exist in nature. Herein, we report the first example of selection of tetraploid olive plants, obtained by combining physical mutagenesis and biotechnology techniques. Stable tetraploid (4 n ) and diploid (2 n ) genotypes were isolated in vitro through shoot-tip fragmentation of two mixoploid mutants derived from the gamma irradiation of self-incompatible Leccino and self-compatible Frantoio cultivars. In this study, the stable mutants FRM5-4 n , FRM5-2 n , LM3-4 n , and LM3-2 n were characterized in the field for vegetative and reproductive behavior with the aim to use them as varieties or dwarfing rootstocks. The stable 4 n genotype of Leccino acquired self-fertility whereas the 4 n Frantoio maintained it. A high and constant yield was showed by LM3-2 n during 9 years of observation, maintaining the same oil quality as the Leccino wild type ( wt ). Moreover, the LM3-2 n acquired the capacity to be intercompatible with the diploid mutant Leccino dwarf (LD) and with the Leccino wt . This acquired property would allow for a reduction of heterozygosity in the offspring, if crossed with each other for some generations and with the Leccino wt , because it is a sort of self-fertilization. When used as rootstocks, both 4 n and 2 n Leccino mutants proved to be very effective for reducing the scion size of the high-vigor Canino cultivar, which is well known for its excellent extravirgin oil. Finally, it was demonstrated that the self-grafting of vigorous cultivar caused a reduction in plant size, thus suggesting that it is possible to produce semidwarf plants from vigorous genotypes to consider them in high-density olive orchards.