Interspecific pollen–pistil incongruity in Salix

Abstract

Pollination barriers that restrict hybridization between six Salix spp. (S. amygdaloides Anderss., S. exigua Nutt., and S.lucida Muhl., of subgenus Salix, and S. discolor Muhl., S. eriocephala Michx., and S. petiolaris Smith of subgenus Vetrix) and that are sympatric over much of their ranges in central North America were investigated through artificial cross pollination. Foreign species' pollen generally adhered to and germinated on the stigma. Pollen–pistil incongruity was most often expressed as reduced pollen tube growth rate, but the degree of incongruity was highly variable within and between different species combinations. Morphological abnormalities such as swollen, coiled, and undirected pollen tube growth were observed in some crosses but were not common. The stigma of S. eriocephala Michx. exhibited a particularly strong and characteristic inhibition of all foreign species' pollen tube growth. Despite the presence of some form of pollen–pistil incongruity in most interspecific crosses, pollen tube penetration of the ovule micropyle (fertilization) was successful in several species combinations.