SummaryThe incidence of fortuitous pollination on interspecific hybridizations of the plum rootstock Myrobalan with the apricot cultivars 'Moniquí' and 'Moniquí Borde' was assessed in this work. Progeny was originated through hand pollination of emasculated flowers of three Myrobalan clones, without bagging, in 1998 and 1999. Fruit set was low and variable among years (1.8-8.0%), but higher than the level of accidental pollination measured with emasculated and non-pollinated flowers (1.2%). Molecular characterization of the progeny was performed with three SSR markers showing that only 28% of the seedlings, obtained by in vitro germination and culture of immature embryos, were hybrids. This represents a lower percentage than expected, and is explained here by the low viability of hybrid embryos and seedlings. The use of molecular markers is discussed in terms of a convenient method for an early identification of putative hybrids in breeding programs with low setting crosses, where the proportion of non-hybrids is magnified.

Rapid obtaining of clonal plants is desirable to shorten crossing programs in fruit tree breeding. In this work, seedling germination and multiplication in vitro allowed us to establish successful in vitro cultures of hybrid seedlings. As a part of a breeding program in apricot rootstocks, we have obtained interespecific hybrid seeds after cross-pollination: myrobalan x apricot (Prunus cerasifera  armeniaca). Immature fruits were harvested and embryos, isolated in aseptic conditions, were grown in culture medium.

In vitro embryo culturing has arisen as a powerful tool for embryo germination of low-viability seeds. This tool has been used to germinate seeds of early-maturing or hybrid Prunus species. ‘Myrobalan’ (Prunus cerasifera Ehrh.) is a widely used rootstock for plum and apricot cultivars and its interspecific hybrids have a clear potential for breeding purposes. However, early seed abortion is often a problem in interspecific crosses because no protocol has been established yet for ‘Myrobalan’ seeds. In this work, we developed a procedure for in vitro germination of embryos of different sizes. Various factors affecting embryo germination such as the culture media, the presence of cotyledons, the stratification temperature, and the embryo size were tested in three different ‘Myrobalan’ clones. The developed protocol includes the use of full embryos that were stratified at 4 °C and cultured in C2d culture medium. The germination rate was strongly affected by the embryo size and reached 90% germination with intermediate- to large-sized embryos (6.5 to 10 mm). However, smaller embryos could also be germinated, and up to 30% germination was achieved with 0.5- to 2-mm long embryos. The results obtained here provide a protocol for in vitro germination of ‘Myrobalan’ embryos that will likely be helpful in breeding programs.

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