Aspects of sexual reproduction in rare monoecious Populus nigra var. nigra trees

Individuals of the genus Populus are mostly dioecious, although deviations from strict dioecism occasionally occur. The aim of this study was to determine whether six, rare monoecious Populus nigra var. nigra trees, identified in trial plots, were fertile and capable of sexually reproducing. Controlled pollinations were carried out and compared with the results of open pollination. Viable seeds were obtained from all combinations of pollination possibilities, suggesting these monoecious trees could be used as either male or female parent and were self-fertile. However, no individuals produced by selfpollination were found among the offspring from openpollinated, monoecious trees. The parentage of some of the crosses was verified using microsatellite markers. The germination energy (GE) and germination capacity (GC) were in most cases higher for seeds produced from controlled pollinations than from open pollination. Furthermore, GE and GC were both higher when pollen from the monoecious trees was used. An association between GE and GC rates on the one hand, and the pollination method and year it took place on the other, was found in most crosses made. This study extends knowledge of the reproductive biology of P. nigra that could be utilized in conservation genetic of this endangered allogamous species.