Sexual reproduction of Abies amabilis

Abstract

Pollen-cone and seed-cone buds which differentiated in the summer and fall of the year before pollination resumed development in early April after about 5 months of dormancy. The sporogenous cells present in the dormant pollen-cone bud divided and formed pollen mother cells which began meiosis by mid-April. Meiosis was completed by late April and mature five-celled pollen was formed by the 3rd week of May. Wind pollination occurred in late May.Each ovule developed a curved funnel-shaped integument tip with a shallowly notched flange-like lip to which pollen readily adhered. No pollination drop was observed. Pollen remained attached to the integument tip but did not germinate for about 1 month, during which time the nucellus grew out nearly to the micropyle and the lip of the integument folded slightly inward bringing the pollen close to the nucellar tip. Pollen tubes then developed rapidly and penetrated the nucellus during the 1st week of July.Each ovule contained a single large megaspore mother cell in the dormant seed-cone bud. Megaspore mother cells underwent meiosis in early May and the single functional megaspore began free nuclear division in mid-May. Female gametophyte development was similar to other members of the Pinaceae and was completed by the end of June. Archegonia varied from two to three and each had two tiers of neck cells.Fertilization occurred in mid-July and early embryo development was rapid. Both simple and cleavage polyembryony occurred and 16-celled proembryos formed. By late July, usually only one club-shaped embryo remained in each ovule. Distinct meristems and cotyledons developed in the embryos in early August and embryos were mature by late August. Ovuliferous scales, usually with two seeds attached, began to be shed in late September. All ovuliferous scales were shed by November.Poor seed production in Abies amabilis (Dougl.) Forbes results partly from the low cone-bearing capacity of the trees, the infrequent occurrence of abundant pollen and seed cones at the high elevation sites, and the high incidence of insect damage to seeds and cones. In addition, A. amabilis and other species of Abies have an unspecialized pollination mechanism, a long period of pollen dormancy after pollination, and a very short time after germination when pollen tubes must develop and penetrate the long nucellar tip. In A. amabilis the archegonia abort very quickly if they are not fertilized. This period when fertilization can effectively occur is very short and remarkably synchronized within a cone and a tree. These, plus the low number of archegonia, may be reasons for the low percentage of viable seed in A. amabilis.