This technique has been used, particularly in fruit trees, to grow embryos that normally would abort due to incompatibility between ovule and embryo development, and also for the rescue of the zygotic embryos of apomictic species (Ramming 1983). In peach and grape breeding programmes, for example, embryo rescue has been used to rescue immature embryos which would not normally mature due to early fruit ripening and late embryo development (peach), and a high degree of abortion six weeks after fertilization (grapes) (Rosas 1992).
Embryo rescue has been applied also to some forest tree species. The culture of embryos with associated gametophyte tissue was used to raise seedlings of a hybrid between Pinus lambertiana and P. armandii, a hybrid for which attempts at normal germination had been unsuccessful (Stone & Duffield 1950). Culture of entire ovules successfully rescued the embryos of a hybrid between Populus simonii and P. pyramidalis, in which endosperm typically degenerates at the free nucleate stage (Ho 1987). In French poplar breeding programmes, embryo rescue is used routinely in the production of hybrids of Populus trichocarpa × P. deltoides, to minimize the post-fertilization attrition of ovules usually seen in controlled crosses between these species (D. Cornu pers. com.). In Brazil, the in vitro rescue of embryos 90 days after pollination was used to improve the yield of hybrids between Eucalyptus pellita and E. cloeziana (T. De Assis, pers. com.). Field growth of the resultant plants, however, has not been good, suggesting that genetic incompatibility in the zygote may be a feature of at least a proportion of hybrid genotypes resulting from the cross-fertilization of these two species. Embryo rescue does not provide a solution to compatibility problems at this level.
The techniques involved in embryo culture are not difficult, and development of protocols for a new species generally would be a minor research task - short term with a high expectation of success.
Although hybridization will probably become more important in forest tree improvement, it is likely that the application of embryo rescue will be very limited - to what probably will be a small number of hybrids which are sufficiently close to produce a normal embryo but for which embryo development in vivo is restricted. In the short term, research projects aimed at circumventing post-fertilization, or even pre-fertilization, barriers to hybrid production using in vitro methods is not likely to be a high research priority in non-industrial species, where barriers to natural hybridization are as yet poorly defined. In the longer term, work should be targetted at hybrids which species testing has identified as potentially of interest and for which other studies suggest that in vitro approaches might provide a solution.