Sexual reproduction of white spruce (Picea glauca)

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Abstract
Reproductive buds broke dormancy at the same time as vegetative buds. Pollen mother cells entered prophase of meiosis immediately after dormancy and five-celled, winged pollen was mature about 6 weeks later. Megasporogenesis occurred 3 weeks after microsporogenesis and the female gametophyte was mature in about 6 weeks. Pollination occurred over about 1 week in late May or early June and fertilization occurred about 3 weeks after pollination. One to four archegonia developed. A comparable number of 16-celled proembryos usually developed within 1 week after fertilization and cotyledons began to develop about 1 month after fertilization. Simple polyembryony occurred in most ovules but cleavage polyembryony was not observed. Embryos were fully developed in late August and seeds were mature and shed in September.The small number of archegonia often present, the high incidence of self-pollination, which may have been the cause of the high frequency of early embryo abortion, and the failure of basal and distal ovules to become pollinated were major causes of empty seed.The phenology of reproductive development varied with the site and the elevation but varied little at one site in successive years. Differences were greatest early in the growing season, but by the time of fertilization, higher elevation trees which began development much later had nearly caught up with lower elevation trees and seeds from all sites were mature and shed at about the same time.Reproductive bud dormancy like vegetative bud dormancy was broken in response to photo-period rather than temperature; however, subsequent cone development was greatly affected by temperature.