Novel insight into structural and functional features of the receptacle glands of Pseudobombax longiflorum (Malvaceae)

Abstract

Floral glands, which produce diverse metabolites, have become the focus of increasing interest because of their important roles in plant-animal interactions. The glands in the receptacle of Pseudobombax longiflorum (Malvaceae) are widely accepted as nectaries. Using light, scanning, and transmission electron microscopy, we studied the anatomy, histochemistry, ultrastructure, and secretory dynamics of the receptacle glands in this species. The glands were active in secretion from young flower buds to immature fruits. Trigona spinipes was the most abundant and frequent visitor. The glands were composed of a uniseriate epidermis covered with a thick cuticle and a multilayered parenchyma, exhibiting two subregions with abundant phloem in the innermost region. These glands produced sugars, resins, oils, and phenolic compounds. Glandular cells exhibited large nuclei with nucleoli, polymorphic plastids with lipophilic inclusions, numerous mitochondria, endoplasmic reticulum profiles, cytoplasmic oil bodies, and vacuoles containing phenolic compounds. Plasmodesmata indicate that secretory components are transported from cell to cell via the symplast. There is also evidence for granulocrine and eccrine secretion mechanisms. The accumulated secretions form a bulge in the central region of the gland. The bulged epidermis detached from the parenchyma cells, and the rupture of the epidermal cells released the accumulated hydrophilic components. Residual lipophilic secretions were embedded in the epidermal cell wall and cuticle. The receptacle glands of P. longiflorum are not typical nectaries but mixed glands that produce both hydrophilic and lipophilic secretions during floral development until young fruits.