The pollination mechanism of western white pine

Abstract

The phenology of post-dormancy pollen cone, pollen, seed cone, and ovule development was determined for western white pine (Pinus monticola Dougl. ex D. Don) growing at a coastal and an interior seed orchard in British Columbia. Pollen cones were preformed and overwintered at the sporogenous cell stage before resuming development in early April. Meiosis occurred in May, and pollen was mature by late May or early June. Potential seed-cone buds resumed development in mid-April, and all bracts and ovuliferous scales were initiated before seed-cone buds burst. Comparable stages of development occurred about 2 weeks earlier at the coastal orchard. Numerical phenological stages were assigned to both pollen-cone and seed-cone development. Pollen-cone and seed-cone phenology were very dependent on temperature, whereas pollen release was dependent on both temperature and drying. Degree-days are a useful way to predict seed cone receptivity and to a lesser extent pollen shed at one site in successive years but not between sites. Receptivity and pollen release were fairly synchronized at both sites in both years of the study. The pollination mechanism is described and was similar to other pines studied in a similar manner. Seed cones were erect at receptivity and ovules inverted. Lipid microdrops were secreted on the micropylar arms to which the saccate pollen adhere. After several days of pollen collection on the arms and other cone surfaces, a pollination drop was secreted from the ovule, and if it was large it filled the space between the micropylar arms. Many large pollination drops were observed in cones at the coastal orchard, and the drops scavenged pollen from the arms and nearby cone surfaces. Few and small pollination drops were observed in cones at the interior orchard where low humidity may cause rapid evaporation of the drop. Water applied as a spray at the interior site increased the size of pollination drops or created artificial drops between the arms. At both sites water applied as a fine spray after pollen shed carried pollen from the hairs on the margins of the bracts and scales into the cone and to the ovules. The pollination mechanism in western white pine is well adapted to the wet humid coastal environment but not as well adapted to the dry interior.