The secretory apparatus of Xerophyta viscosa (Velloziaceae): Epidermis anatomy and chemical composition of the secretory product

In this study, secretory activity on the adaxial surface of the leaves of the desiccation tolerant plant, Xerophyta viscosa Baker was investigated, using light, scanning and transmission electron microscopy. Glandular activity was associated with sunken cavities which appear to be modifications of infolded epidermal cells. The secretory cavity consisted of a globose lumen surrounded by two layers of cells. The cells of the outer layer were flattened with thickened walls, while those of the inner layer (epithelium), exhibited ultrastructural features of intense metabolic activity. Epithelial cells were larger with a large nucleus, numerous vacuoles, and a dense cytoplasm with abundant mitochondria, endoplasmic reticulum (ER), plastids and a few dictysomes. Lipophilic droplets were abundant in the cytoplasm, mitochondria, ER, plastids and in infolding sites of the plasmalemma outside the protoplast. ER appeared to be involved in the synthesis and transport of lipophilic substances. The mechanism of secretion in X. viscosa appeared to be granulocrine. The chemical composition of hexane and methanol extracts of the leaves, analysed by gas chromatography coupled with mass spectrometry (GC–MS), revealed the presence of diterpenes, phenolic compounds and fatty acids. Compounds in the hexane fraction included velloziolone, manoyl oxide, asperdiol and an unknown substance. Compounds in the methanol fraction included 5-(hydroxymethyl)-2-furancarboxaldehyde, 2,6-dimethoxyphenol, 4-(3-hydroxy-1-propenyl)-2-methoxyphenol and stearic acid.