Investigation of the black dots on leaves of Stictocardia species (Convolvulaceae) using anatomical and histochemical analyses

The presence of black dots in leaves and flowers has historically been regarded as a taxonomically useful character to distinguish Stictocardia Hallier f. from other genera in Convolvulaceae. However, the origin and structure of these dots are unknown and available data about them are unclear. This research investigated the morphology, anatomy, and histochemistry of the black dots in order to clarify their structure and possible functions for the species. Black dots were examined in Stictocardia beraviensis (Vatke) Hallier f. and S. tiliifolia (Desr.) Hallier f. as representative species of the genus. Leaves and flowers were studied using crosswise micro-sectioning, Scanning Electron Microscopy, clearing techniques, and histochemical screening. The results revealed that the black dots were multicellular head-like structures better defined as peltate glandular trichomes, usually secreting chemical compounds. The peltate glands were composed of 10–20 secretory cells with 1–2 basal cells and were sunken in pits in the petiole and both surfaces of the leaf blades, especially on the abaxial surface. They were also observed on the sepals and petals of the Stictocardia spp. Histochemistry for Stictocardia spp. indicated a positive reaction for the presence of pectin substances, polysaccharides, unsaturated lipids, terpenes and flavonoids. We believe that these glandular trichomes play an important role in protecting the plants from pests and herbivores.