Seed-cone differentiation and sexual reproduction in western white pine (Pinus monticola)

Abstract

Pollen-cone buds differentiated in the fall but meiosis and pollen development did not occur until the next spring. Pollen cones resumed development in mid-April. Meiosis occurred in mid-May and pollination about 1 mo. later. Mature 4-celled pollen entered the micropyle by means of a pollination drop and germinated on the nucellus about 1 mo. after pollination. Pollen tubes grew part way through the nucellus and became dormant in late summer before male gametes formed. Seed-cone buds differentiated in the spring from distal axillary apices which were initiated in the long-shoot terminal buds in the fall. Apices resumed development in early April and bracts and ovuliferous scales were initiated from mid-April until early June. Meiosis of the megaspore mother cell occurred in early July and the functional megaspore underwent free nuclear division until late August when the seed cones became dormant. Ovule and pollen-tube development resumed in early April. Free nuclear division occurred in the female gametophyte until early May when cell walls formed and 3-5 archegonia differentiated. Two male gametes formed in each pollen tube before it reached the archegonium. Fertilization occurred in early June. Simple polyembryony was rare but cleavage polyembryony was a regular feature. Early embryo development was rapid and easily distinguishable embryos were present by mid-July. Embryos and seeds were mature by early September when seeds were normally shed. The reproductive phenology and morphology of P. monticola is similar to other soft pines. Stages in the reproductive cycle are noted where abortion of cones or ovules or adverse weather conditions may result in a poor cone crop or poor seed set.