POLLEN BIOLOGY AND HYBRIDIZATION PROCESS: OPEN PROBLEM IN WALNUT pp. 65-99
Authors: (Paola Pollegioni, Keith Woeste, Irene Olimpieri, Fulvio Ducci, Maria Emilia Malvolti, C.N.R. Institute of Agro-Environmental and Forest Biology, Porano, Terni, Italy, and others)
Abstract: This review focuses on the pollen biology of Juglans, and in particular Juglans nigra (Eastern Black walnut) and Juglans regia (Persian or English walnut), which are 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. Hybrids between the two species, Juglans × intermedia (Carr) can occur naturally, although they often have reduced fecundity. 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 here is great demand for J. × intermedia for forestry, especially in Northern Europe. We review several aspects of Juglans pollen biology that frustrate the production of J. × intermedia and limit the progress of researchers and plant breeders who work with this genus. We also discuss the ways in which scientists and breeders are working to overcome problems related to pollen storage and viability testing, pistillate flower abscission (PFA), fertilization and embryogenesis in Juglans, and the use of microsatellites to monitor gene flow, ploidy, parentage, and hybridogenesis all with an eye toward practical solutions to the current shortage of J. × intermedia for research and applied forestry.
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