Secretory and ecological function of petiolar glands in Solanum fernandesii: first description of resin glands in the genus Solanum

Extrafloral nectaries are known from many plant groups but have rarely been recorded in the large genus Solanum or, in fact, in the family Solanaceae. This study set out to explore the functional role of the extrafloral nectaries recently described in Solanum fernandesii, a species endemic to north-eastern Brazil. Light and scanning electron microscopy was used to study the morphoanatomical structure of the nectaries and histochemical analyses were performed to study the chemical composition of the exudates recovered from the glands on the basis of field studies. Light and scanning electron microscopy show that although the petiolar glands in S. fernandesii appear sessile to the naked eye, the glands are short stalked. The epidermis of the glands is composed of short, tightly packed multicellular trichomes. The gland secretions contain a mixture of polysaccharides, pectins, mucilage, proteins, lipids, essential oils, resins, and phenolic compounds on the basis of histochemical tests performed. These findings confirm that the petiolar glands in S. fernandesii are in fact resin glands and not extrafloral nectaries as previously claimed. Our study is the first report of resin glands in the large genus Solanum and we confirm that the glands found in S. fernandesii are unique in the genus.