Histology of sterile male and female cones in Pinus monticola (western white pine)

Two years of histological samples were collected from a Pinus monticola Dougl. (western white pine) tree identified as not producing mature pollen or seed cones. Anatomical information was collected to the ultrastructural level, to assess possible mechanisms for pollen and cone abortion resulting in sterility. Development of male and female gametophytes in the sterile western white pine tree was arrested after meiosis and before further cell divisions could take place. Sterile male gametophytes (pollen grains) had poorly developed pollen walls and sacci, reduced and degenerative cytoplasm, and no evidence of stored starch grains. The pollen cone aborted prior to pollen dehiscence. Meiosis of the megaspore mother cell in the ovule produced four megaspores, but development was stopped at the functional megaspore stage. The seed cone aborted in the first year of growth before winter dormancy. Tapetal tissue in sterile microsporangia appeared similar to that of fertile microsporangia, until the vacuolate, uninucleate microspore stage. Tapetal cells and thecal fluid surrounding the sterile microspores persisted well past the time when microsporangia on fertile trees started the process of maturation and desiccation. At pollen dehiscence, sterile pollen cones did not release any pollen and the microsporangia were filled with a sticky fluid. The behaviour of the tapetum in P. monticola sterile cones is compared with reports of tapetal function and malfunction reported in studies of angiosperm and other gymnosperm species. The occurrence and timing of gametophyte abortion in both cone sexes suggests a genetic rather than environmental basis for the sterility mechanism.