Pollen biology and hybridization process: open problem in walnut

In this study the lacking of suitable, cheap and reliable methods for pollen storage and viability and the correspondent difficulty to produce progeny in breeding and research programs using controlled crosses in Juglans genus is presented. In particular Juglans nigra (Eastern Black walnut) and Juglans regia (common or Persian walnut) are highly economically important species in Europe, Asia and North America. Both species are monoecious, heterodichogamous and wind -pollinated. Their mating system is predominantly outcrossing, although under particular environmental conditions self-pollination is possible. Natural hybrids between the two species, Juglans x intermedia (Carr) can occur naturally. Compared to the parental species, most J. × intermedia (J. nigra × J. regia) hybrids show increased vegetative vigor, distinct disease resistance, high wood quality, and greater winter-hardiness. For these reasons there is great demand for J. × intermedia for forestry, especially in Northern Europe. One of the main problem in black, and common walnut production is that breeders have encountered difficulties obtaining sufficient quantities of desired pollen at the time pistillate flowers are receptive because of the dichogamous nature of the species. Several long-term walnut pollen storage techniques were developed but no suitable and simple method is available in Juglans until now. In addition, pistillate flower abortion (PFA), caused by excessive pollen load, has been reported in Persian and black walnut. PFA may severely affects the final nut set. Moreover for production of hybrid progeny it also requested the synchrony of flowering and genetic compatibility between parental trees. Several studies has been focused on biological mechanisms underlying the pistillate flower abortion, the low pollen viability, fertilization and embryogenesis in walnut . In the last two decades the scientists were seeking to overcome practical problems in programs for the genetic improvement of walnut, using open-pollinated progeny tests instead of controlled cross progeny tests. In particular we reported a new method for retrospective identification of hybridogenc walnut trees based on SSR fingerprinting and parentage analysis in order to establish new for seed orchards for hybrid production.