Embryo development, megagametophyte storage product accumulation, and seed efficiency in Taxus brevifolia

Abstract

Taxus brevifolia Nutt. has a reduced ovulate structure that consists of a single ovule in a leaf axil instead of a compound ovulate strobilus. Taxus brevifolia on southern Vancouver Island, British Columbia, were studied over three seasons. Proembryos occurred from mid-May to mid-June. They underwent four free nuclear divisions forming 16 nuclei before cellularization. Early embryos were present from mid-May to mid-August. Simple polyembryony was observed up to the massive embryo stage, and differential growth of the embryonal cells was interpreted as incomplete cleavage polyembryony. Mid-embryos were present from mid-June to late August and had a distinct protoderm and focal zone. Late embryos were visible from mid-July onwards. Carbohydrates began accumulating at the early embryo stage, whereas proteins and lipids accumulated in the late embryo stage. The presence of a red aril corresponded to increased amounts of lipid in the megagametophyte cells. Individual seeds matured from July until November. The seed efficiency ranged from 0 to 16% and averaged 5%. Prezygotic loss was the most common fate of ovules, followed by postzygotic loss. Possible causes of this poor seed efficiency are poor pollination success, insect damage, or light limitation.