Development and functioning of the floral nectary in Pseudobombax longiflorum (Bombacoideae, Malvaceae).

Abstract

Floral nectaries are generally linked to an exchange that facilitates pollen transport, and consequently, pollination. We have characterized the ontogenesis, histochemistry, and ultrastructure of the floral nectary of Pseudobombax longiflorum (Mart.) A. Robyns (Bombacoideae, Malvaceae) from a developmental perspective and related with secretion dynamics. Light and electron microscopies were used. The floral nectary is on the proximal and median portions of the adaxial face of the calyx. Protoderm, fundamental meristem, and procambial strands form the nectary. At maturity, it is composed of glandular claviform trichomes and nectariferous and subnectariferous parenchyma vascularized predominantly by phloem. Phenolic compounds, oils, and proteins were observed in the trichome and parenchyma cells. Druse crystals occurred in the parenchyma. Starch grains decreased in the parenchyma in pre-anthesis buds, and with the nectar exudation began. Mitochondria, ribosomes, endoplasmic reticulum, Golgi bodies, and plastids containing starch grains characterize the cytoplasm of secretory cells and the apparatus compatible with nectar production. Plamodesmata occurred between parenchyma cells, parenchyma cell and basal trichome cell, and trichome cells, indicating a symplastic pathway of the pre-nectar. The downward flow of nectar through the apoplast could be prevented by the impregnation of lipids into the anticlinal walls of the stalk cell. In trichome apical cell, nectar accumulation occurred in periplasmic and subcuticular spaces. Nectar appeared to be externalized through the cell wall and cuticle. Insights into trichome development have enhanced our understanding of the formation of functional floral nectary components and nectar secretion in Malvaceae, marking the first ontogenetic and ultrastructural study in Bombacoideae.