Functional Anatomy of the Ovule in Broad Bean (Vicia faba L.): Ultrastructural Seed Development and Nutrient Pathways

Abstract

Ovules of broad bean (Vicia faba L.) were studied to disclose ultrastructural features, which can facilitate nutrient transport to the embryo sac from 10 d after pollination (DAP) to the mature seed. Fertilization occurs during the first 24 h after pollination. The endosperm is a coenocyte, which is eventually consumed by the embryo. By 10 DAP the inner integument is degraded and the outer integument adjoins the embryo sac boundary. The heart-shaped embryo approaches the embryo sac boundary at two sites, which here are named contact zones. Small integument cells in the neighbourhood of the first formed contact zones become separated by prominent intercellular spaces. A heterogenous scattering material, probably representing secretion products accumulates in these spaces. By 14-16 DAP the integument exudate disappears, and the suspensor degenerates. As the contact zones increase in size, wall ingrowths form a bridging network in the narrow space between the embryo sac boundary and the extra-embryonic part of the endosperm wall. The epidermal cells of the embryo separate adjacent to these zones, and develop conspicuous wall ingrowths. At 20 DAP vacuoles showing various stages in formation of protein bodies appear in the cells of the embryo.