Beyond the physical protection: What can Asplenium glandular scales secrete?

Abstract

Ferns have two types of trichomes (i.e., induments of protodermal origin): hairs and scales, which are frequently found on the surface of stems and leaves. Scales are flattened, not stratified, with different shapes. The presence of glandular cells associated with the margins and apex of the scales has been reported in taxonomic studies, as for Asplenium spp. (Aspleniaceae), although the structure, secretion and function of these glandular scales is rarely addressed or unknown. To assess the morphology and histochemistry of Asplenium spp. glandular scales, induments obtained from fiddlehead apex and stems apex of the following species with different habits were investigated: Asplenium austrobrasiliense (terricolous), Asplenium coronatum (rupicolous), and Asplenium scandicinum (epiphyte), under stereoscopic microscope, light microscope, and scanning electron microscope (SEM). Scales were more abundant in the fiddleheads and stem apices, close to young portions. Swollen glandular cells were observed at the apex and margins of the scales and they can be elevated or not by a stalk with variable number of cells. Mucilage, pectins, phenolics, essential oils, and essential oils and oleoresins were detected in the glandular cells. The abundance of scales near young portions and secretion of mucilage and pectins by the glandular cells may be related to resistance to desiccation, while phenols, essential oils and oleoresins could provide protection against herbivores and pathogens. Differences were detected between the species, and between scales of stems and fiddleheads. When compared to A. austrobrasiliense and A. scandicinum, the rupicolous species A. coronatum presented distinctions in its secretory composition, with only mucilage and pectin detected in its stems scales marginal glandular cells, which seems to be related to protection against desiccation. Studies with a greater sample of Asplenium species could indicate whether the distinctions in secretion are usually related to distinct habits or whether they are phylogenetically determined.