Deciphering histochemical and ultrastructural features of calyx nectaries to understand the secretory process in two species of Clerodendrum (Lamiaceae).

Calyx nectaries are common secretory structures in Clerodendrum (Lamiaceae) that play an important role in plant indirect defence. These structures possess intricate morphological and anatomical features that are associated with the secretion physiology. This study intended to elucidate the structurally homologous calyx nectary glands of two Clerodendrum species that undergo morphological and anatomical changes facilitating nectar secretion. The morpho-anatomical, histochemical and ultrastructural features of the calyx nectary glands in two Clerodendrum species, viz., C. chinense and C. infortunatum, were studied throughout floral maturation stages using light and electron microscopy. The flower calyx possesses distinct disc-shaped patelliform nectary glands with different morphologies in both species. Histological analyses revealed distinct tissue regions in the nectary glands in both species. The nectary glands showed well-defined palisade-like secretory epidermis bounded by cuticular covering, lipid-rich intermediate layer, nectary parenchyma with prominent vascular bundles and photosynthetically active sub-nectary parenchyma tissue. Ultrastructural analysis revealed the presence of dense cytoplasm with a large number of mitochondria and chloroplasts in the nectariferous region in both species. Starch granules were found to be present within the chloroplasts and leucoplasts, and their depletion was noted in both the species, more prominently in C. infortunatum, suggesting their possible role in nectar biosynthesis. Anatomically, the calyx nectaries in both species were shown to be homologous and conserved which showed differential changes in both cell and tissue levels throughout floral maturation. Synthesis of nectar components in the nectary parenchyma supplemented with vascular supply and their regulation by hydrophobic barrier could shed light on the nectar biosynthesis and secretion process. Also understanding these nectary structures in taxonomic groups can provide valuable insights into phylogenetic relationships within the genus.