Structure, function and secretory products of the peltate glands of Centrolobium tomentosum (Fabaceae, Faboideae)

Abstract

The glandular structures of Centrolobium tomentosum Guill. ex Benth. have been little studied despite the economic importance of this species. We describe here the distribution, development, structure and ultrastructure of the secretory cells of the peltate glands found on the vegetative organs of this species. Stem apices and leaves in various stages of development were collected and prepared for examination by light, scanning and transmission electron microscopy. Chemical analyses and conventional histochemical tests to determine the chemical nature of the secretory products were also carried out. Peltate glands occur on aerial vegetative organs during their primary growth stage. These trichomes are structurally stable, persisting throughout the development of the organ. During the initial stages of the gland development, cell separation creates a central space that expands as secretions accumulate. Maximum secretion rates occur during this phase and the secreting cells characteristically have well developed smooth and rough endoplasmic reticulum, and high numbers of plastids and mitochondria. During the later stages of the secretory phase, the central cells show symptoms of cell death and are incorporated in to the secretions. At trichome maturity, the central space is delimited by a uniseriate epithelium. In addition to the resin, which is the main secretory product, an extensive three-dimensional carbohydrate matrix was observed that extended throughout the central space, apparently giving support to the resin droplets. The terpenic nature of the secretion was confirmed by thin-layer chromatography. Given the terpenic nature of the secretion and the permanence of trichomes throughout all phases of leaf development, it is postulated that the resin-secreting trichomes act to protect the plant against herbivores.