Ontogeny resolves gland classification in two caesalpinoid legumes

Robust glandular appendages are reported in legumes of the Caesalpinieae tribe. Most studies only attempt to describe the external morphology of these structures, without providing a distinction between glandular trichomes and emergences. This study employed ontogeny to resolve the terminology of these structures present in flowers of two tropical woody legumes of Caesalpinieae, Erythrostemon gilliesii and Poincianella pluviosa, through surface, anatomical and ultrastructural analyses. Flowers of both species exhibit branched and non-branched glandular trichomes since these structures originate from a single protodermal cell. Non-branched glandular trichomes occur on the inflorescence axis, pedicel, sepals and ovary; in P. pluviosa, they also occur in the unguicle of wings and standard, filaments, anthers and style. This type of trichome shows a non-secretory multiseriate stalk and a secretory multicellular head. Branched glandular trichomes, with similar morphology but exhibiting non-secretory branches, occur in the inflorescence axis, pedicel and sepals; in P. pluviosa, they also occur in the unguicle of wings. During the secretory phase, the trichome head cells have large nuclei, cytoplasm rich in vacuoles, oleoplasts, mitochondria, rough endoplasmic reticulum and free ribosomes. The content is released in the intercellular spaces of the head in a merocrinous mechanism and reaches the surface through cuticle rupture. We emphasized the importance of ontogenetic studies to clarify the terminology of secretory structures. This type of study should be performed in other caesalpinoids so that such robust glandular appendages can be correctly interpreted and used with phylogenetic value in the group.