Sexual reproduction in the cork oak (Quercus sober L). - II. Crossing intra- and interspecific barriers

Intraspecific barriers promote outcrossing while interspecific mechanisms m ay contribute to the isolation of species; both control the exchange of gen es between plants. In this paper we establish that post-pollination mechani sms promote outcross and act at different temporal and spatial levels to co ntrol seed set and quality in Quercus species. Controlled pollinations were performed to investigate intraspecific crossing barriers in Q. suber and p ollen-pistil interactions following interspecific pollinations with some si mpatric Quercus species. Cytological data of intraspecific crosses in Q. so ber and Q. ilex have shown different kinetics on pollen tube growth after s elf and outcross pollination, but pollen tubes were able to reach the base of the styles in both species and seed set was successful. Although some pr ezygotic interaction is occurring at the style, the most important interact ion takes place at ovary. The cross between Q. Hay and Q. super is possible only in one direction, revealing a case of unilateral incongruity. We show that the lack of seed set observed in the cross Q. sober x Q. ilex is due to the inability of pollen tubes to penetrate the transmitting tissue after germination. With Q. suber mainly as female parent, pollinations with othe r simpatric Quercus species have shown different levels of constraint on po llen tube progression at stigma and style levels. Eventual hybridisation be tween Quercus species will depend on the compatibility of pollen-pistil int eractions, on the competitive ability of pollen genotypes, and, most import ant, on the overlapping of geographical, phenological and ecological factor s. Differences in seed set and seed allocation was evident in all crosses, and was particularly outstanding in interspecific and in self intraspecific crosses, determining the ultimate level of seed production and quality in Quercus species.