Floral Nectary Fine Structure and Development in Glycine max L. (Fabaceae)

Abstract

<p>Floral nectaries of annual cultivated Glycine max develop between the bases of the central gynoecium and lateral stamen ring. Each discoid nectary forms immediately before flower opening and degenerates within 24 h. Three stages of nectary development are identified: preactive, active, and postactive. Preactive and active nectaries are composed of a single-layered epidermis that contains many open stomata, with guard cells having thickened walls, starch-engorged plastids, and other organelles. The major portion of each nectary consists of thin-walled special parenchyma cells, each having dense cytoplasm with a nucleus, Golgi bodies and vesicles, mitochondria, plastids, endoplasmic reticulum, many ribosomes, and one or more vacuoles. Fingers of phloem consisting of sieve tubes and companion cells, both with very small wall ingrowths and thought to provide sugars, penetrate the nectary at its base. The fingers originate from 10 vascular bundles with xylem, which innervate the stamen ring peripheral to the nectary. At the beginning of the active stage, special parenchyma around the phloem fingers become highly vacuolated first by each vacuole filling with non-water-soluble material and ribosome-like particles. In many of these cells and in the nonstomata epidermal cells, cytoplasmic bridges are associated with the vacuoles, and straight tubes containing single files of ribosome-like particles occur in the cytoplasm, or traverse plasmodesmata. In addition, bundles of tubules are often seen pressed to the outside of the vacuole tonoplast and in the cytoplasm before and during the time the tonoplast fragments and the vacuole contents mix with the cytoplasmic organelles. These cells then collapse, releasing their contents through the pores of the guard cells and onto the nectary surface. This holocrine secretion is different from that reported for other legume taxa and most other nonlegume taxa and suggests apoptosis. Remaining nectary special parenchyma cells follow the same fate, along with the epidermal cells, so that the entire nectary collapses, leaving only some of the guard cells intact. There are two types of elongate nonglandular trichomes and one type of short five- to seven-celled glandular trichome on the gynoecium adjacent to the nectary. These latter trichomes seem to be developed and functional during the active and postactive stages and following nectary collapse, suggesting that the nectar may consist of a variety of compounds originating from both the nectary and the glandular trichomes.</p>