Zygotic Embryo Development in a TropicalAcaciaHybrid (Acacia mangiumWilld ×A. auriculiformisA. Cunn. ex Benth.)

Abstract

Zygotic embryo development in the Acacia hybrid (A. mangium Willd. × A. auriculiformis A. Cunn. ex Benth.) is examined using light and electron microscopy and histochemistry. About 1 wk after pollination, the fertilized embryo sac contains abundant stored products, and wall ingrowths are observed along the central cell wall. The zygote contains abundant starch but appears metabolically inactive. The zygote has a 2‐mo dormant period, and this coincides with the time of most abundant fruit abortion. The first zygotic division is not observed until both integuments completely enclose the nucellus. This is followed by degeneration of most nucellar cells, except in the micropylar region. The early embryo lacks a distinct suspensor and is attached to the micropylar nucellus. A small suspensor then becomes noticeable in the late embryo stage. Endosperm formation is of the nuclear type. During early development, the endosperm is nourished mainly by stored products from the central cell and digested nucellus. The endosperm becomes the main nutrient source for the growing embryo during late embryo development. The mature hybrid embryo consists of a small, oval embryo axis with a well‐developed plumule, hypocotyl, and radicle and two large, broad, fleshy cotyledons. Hybrid seeds are exalbuminous. Lipids and starch are the main stored products in the cotyledons, whereas proteins accumulate in the cotyledonary epidermis, subepidermal layer, and embryo axis. The mature seed coat is formed from both the inner and outer integuments and is composed of macrosclereids, hypodermal and inner osteosclereids, and multilayered parenchymatous cells.