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Crosses between genotypes of the 3 commercially important groups of lilies, i.e. Lilium longiflorum Thunb., the Asiatic hybrids and the Oriental hybrids, are hampered by interspecific crossing barriers. Several methods have been developed to bypass pre-fertilization barriers or post-fertilization barriers in lily, e.g. the cut-style method and embryo rescue techniques. Five techniques were studied comparatively for their capacity to bypass barriers between different types of lilies: the grafted-ovary method, placental pollination, warm-water treatment, the mentor pollen technique and hormone treatment. For the grafted-ovary method, a compatible pollinated pistil was grafted one day after pollination on the ovary of another species. It appeared that the pollen tubes were not able to penetrate from one ovary into another ovary. After placental pollination, no embryo germination was found, even not when pre-pollinated styles were placed near the ovules. After a warm-water treatment (6 minutes of 50 0C) of pistils, many seeds were harvested of self-pollinated flowers of L. longiflorum. However, interspecific crossing barriers could not be bypassed. The percentage of ovules with pollen tube penetration did not change by mixing pollen with radiated mentor pollen. It appeared that the mentor pollen were able to penetrate the ovules. After treating ovaries with 0.1% BAP, on average 6 embryos per flower of the cross L. longiflorum 'Gelria' x Asiatic hybrid 'Mont Blanc' could be rescued when ovule culture was started around 40 days after pollination.

The development of the embryo and endosperm has been investigated in an intraspecific Tulipa gesneriana cross and in the incongruent cross T. gesneriana × T. agenensis at intervals of 10 days, from 12 to 82 days after pollination (DAP). In both tulip crosses, the zygote gives rise to an apparently undifferentiated cell mass, the proembryonal cell mass, on which a suspensor then develops. Subsequently, a globular embryo is formed on top of the suspensor. This embryo finally elongates, giving rise to a spindle-shaped embryo. The cellular endosperm fills the whole embryo sac in mature seeds, except for a region immediately around the embryo. In both crosses, aberrant developments were found. In the intraspecific T. gesneriana cross, the pollen tubes did not open in a number of ovules. In other ovules, the pollen tubes seemed to have opened, but an embryo or endosperm was not found or only endosperm was observed. In the cross T. gesneriana × T. agenensis, fewer pollen tubes entered the ovules than in the intraspecific T. gesneriana cross. The ovules with embryo and endosperm formation of the incongruent interspecific cross showed, in general, retarded development in comparison with the intraspecific T. gesneriana cross. The first globular embryos and spindle-shaped embryos were found at the later fixation dates and the relatively lower number of spindle-shaped embryos at 82 DAP had a shorter average length. The number of ovules with deformations in embryo and/or endosperm development was also higher in the cross T. gesneriana × T. agenensis in comparison with the intraspecific T. gesneriana cross. Between 87% and 100% of the ovules with embryo and endosperm development showed normal development in the intraspe-cific T. gesneriana cross, while in the incongruent interspecific cross, from 22 DAP, between 17% and 56% of the ovules showed normal development. Of those ovules with aberrations in embryo and/or endosperm formation, about 80% had a deformed endosperm, of which more than 50% also contained a deformed embryo. Embryos of the incongruent cross might be saved by the application of embryo rescue techniques.& k w d : Key words Interspecific hybridization · Embryo development · Endosperm development · Tulipa& b d y :

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