Anatomical Changes During Ontogeny of the Peanut (Arachis hypogaea L.) Fruit: Mature Megagametophyte through Heart-Shaped Embryo

Abstract

Correlative light (LM) and scanning electron microscopy (SEM) of the same microtomed section was used to investigate components within the embryo sac of Arachis to the heart-shaped embryo phase of development. Morphometric data on the locule, ovule, embryo sac, and proembryo show that they reach maximum size during aerial development ca. 5 days after anthesis. After the onset of peg elongation, these components contract in size, and the apical locule and its components contract more than the basal. At the time of maximum aerial size, the proembryo is in the eight-cell stage and remains at that stage until the peg tip penetrates the soil surface. The first observed changes in the proembryo on reinitiation of growth is the swelling of the two basal tiers of cells from which the suspensor forms. Correlative LM and SEM of endosperm show physical contacts, in the form of cytoplasmic strands, between the connective cell layer at the base of the proembryo and the free-nuclear endosperm. These cytoplasmic strands are also layered against the integumentary tapetum and extend to the chalazal end of the embryo sac. These connections may have a role in the supply of nutrients to the developing embryo.