The pollination mechanism of Engelmann spruce (Picea engelmannii)

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Abstract
The pollination mechanism of Picea engelmannii (Parry) was studied on small potted scions under natural conditions and in controlled environment chambers. Six stages of conelet development were recognized and related to pollen receptivity. Cone-lets appeared receptive for about 2 weeks but were actually receptive for only about 1 week. Secretory droplets appearing on the micropylar arms collected pollen for several days before pollination drops formed. Pollination drops formed acropetally in the conelet and only once from each ovule. Pollination caused rapid recession of the pollination drop, whereas the drop remained for several days on unpollinated ovules. There was some decrease in size of pollination drops during midday and reemergence the following night. Pollination drops were secreted by the nucellar tip in a manner similar to nectaries. Secretory cells collapsed following secretion. The drop contained 4.3% glucose and 3.8% fructose but no sucrose. High humidity increased the longevity and decreased the diurnal fluctuation in size of pollination drops. Conelets from trees with low leaf water potential developed more slowly and produced smaller and more viscous pollination drops. Cones averaged 103 ovuliferous scales, 90% of which were fertile. However, usually less than 50% of the potential seed set was achieved. One of the major causes for low seed set is inadequate pollination. A better understanding of the pollination mechanism and the receptive period may improve seed efficiency in controlled and supplemental mass pollinations.