The reproductive biology of Xanthoceras sorbifolia,an energy crop,was investigated through field observation,artifical pollination,anatomical techniques and fluorescence microscopy.Inflorescence buds of X.sorbifolia emerge at the beginning of March and flowers bloom in the following two months in Beijing,China.Flowering of individual trees lasts 25 to 30 days with the peak in the mid and late April.The inflorescence belongs to raceme with 20 to 50 flowers,which develop acropetally.The breeding system of X.sorbifolia is andromonoecious,i.e.,hermaphroditic and male flowers appear on the same individual.Most inflorescences bear entirely male flowers,and a few inflorescences develop varying proportions of male and hermaphroditic flowers.Occasionally male flowers frequency is zero.Male flowers have obviously reduced nonfunctional pistils and are incapable of setting fruit.The pistils of male flower bear ovules prior to abortion.Dehiscence of five anthers in male flowers is asynchronous and anthers of only a few hermaphroditic flowers dehisce to shed pollens at anthesis.Although the pollen grains of both male and hermaphroditic flowers are viable,germination frequency of pollen of the hermaphroditic flowers on stigmas is extremely low.The most commonly observed visiting insects in this study area include honeybees,coccinellids and flies.Stigmas of the hermaphroditic flowers are receptive to self-and cross-pollens during the first 3 days of anthesis.Mature fruits merely come from cross-fertilization while the development of self-pollinated ovaries is arrested soon after initially expanded growth.A free-nuclear endosperm occurs in self-fertilized ovules before abortion.Two-celled proembryoes were occasionally observed in the self-fertilized ovules,but their development did not proceed and they degenerated after several days.This study revealed the trait of self-sterility in X.sorbifolia for the first time.
