MORPHOLOGICAL ASPECTS OF EARLY HOST-PARASITE INTERACTIONS IN INFECTIONS OF MELAMPSORA-PINITORQUA AND MELAMPSORA-LARICI-TREMULAE ON PINUS-SYLVESTRIS - IMPLICATIONS IN THE TAXONOMICAL RELATIONSHIP OF THE 2 RUST FUNGI

A cyto-histological study was carried out on Pinus sylvestris seedling s artificially infected with basidiospores of Melampsora pinitorqua an d Melampsora laricitremulae, in order to differentiate between the two rust fungi and delineate their taxonomical relationships. The general morphology of the early phases of the majority of infections of Melam psora larici-tremulae on P. sylvestris show similar characteristics to those of M, pinitorqua. Moreover, in M, larici-tremulae /P. sylvestri s interactions, the following were the specific characteristics: abnor mal germlings and secondary basidiospore production, ''cell wall-like appositions,, encasing the fungal structures in the host, necrosis of the infection structures, necrosis of the host tissue colonized by myc elium from epidermal to parenchymal cells. Such reaction types reveal the coexistence of pre-and posthaustorial defence mechanisms, typical, respectively, of a nonhost and host resistance. This coexistence whic h is useful in understanding the evolution of this host-rust interacti on, was examined. The described defence mechanisms of P. sylvestris vs . M. larici-tuemulae could indicate that the two rust fungi are still closely related entities, however diverging, which evolve from ''forma e speciales'' to ''species''.