Pollination mechanisms in conifers

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  • 針葉樹類の受粉機構
  • シンヨウジュルイ ノ ジュフン キコウ

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Abstract

Pollination mechanisms in conifers are diversified and generally correspond with taxonomic boundaries. In Tsuga, Hesperopeuce(Pinaceae), Saxegothaea(Podocarpaceae)and Araucariaceae pollen germinates outside the ovule and a long pollen tube penetrates the micropyle. In the remaining genera so far examined in conifers, pollen is captured by ovules and germinates in the micropylar canal. The pollen capture by ovules usually involves a pollination drop secreted from the ovule Non-saccate pollen in Cephalotaxaceae, Cupressaceae, Sciadopityaceae, Taxaceae and Taxodiaceae lands on a pollination drop. It simply sinks in the drop and reaches the nucellus if ovules are erect, or it moves upward carried by the meniscus of the drop if ovules are inverted. Since the nucellar apex is hydrophilic and the inner surface of the integument is hydrophobic, the meniscus of the pollintion drop moves upward when it recedes in the micropylar canal of the inverted ovules. Saccate pollen of Picea, Pinus(Pinaceae)and most genera of Podocarpaceae is also captured by a pollination drop. In these genera ovules are inverted, and pollen in contact with the drop floats upward to the micropyle and subsequently carried into the micropylar canal perhaps by the drop meniscus. In Cedrus rain water, instead of a pollination drop, is the main vehicle for pollen movement to the nucellus, and this may be also true in Picea and Pinus. In Larix and Pseudotsuga(Pinaceae)pollen adheres to the integument tip, and it is carried into the micropylar canal when the tip invaginates into the canal. Our knowledge on the pollination mechanisms in conifers thus far is based on about half of the extant genera. Clearly research efforts should be concentrated on the mechanisms in the remaining genera and poorly examined genera. Available literature suggests further intensive studies in such aspects as the chemical composition of the pollination drop, mechanisms of secretion and receding of the drop and the relation between pollen size and pollen movement in the micropylar canal. These studies would provide insight into evolutionary considerations and essential information for seed production which often limits reforestation programs in conifers.

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