Structure and ultrastructure of nuptial and extranuptial nectaries explain secretion changes throughout flower lifetime and allow for multiple ecological interactions

Abstract

Nectaries are specialized nectar-producing structures. Nectar traits affect animal behaviour and ecological and evolutionary processes, such as pollination and biotic defence. Previously, we found that there are differences in the characteristics of nuptial nectar (NN) and extranuptial nectar (ENN) and in the types of animals that visit each nectary in <i>Amphilophium mansoanum</i> (Bignoniaceae) flowers. We now hypothesize that nectar traits reflect the anatomical, histochemical, and subcellular characteristics of each nectary type. Using routine light microscopy, scanning electron microscopy, and transmission electron microscopy methods, we studied NN and ENN, respectively in young flower buds, pre-anthesis buds, and first- and second-day flowers. NN was a prominent annular disk; whereas, ENN was a concave, patelliform trichome. Only ENN contained alkaloids, while both nectaries contained starch grains, lipid droplets, proteins, terpenes, and phenolic compounds. Both nectaries showed subcellular organization consistent with hydrophilic and lipophilic secretion, the latter being predominant in second-day flowers. In NN, the subnectary parenchyma had phloem and amyloplasts until pre-anthesis. Starch grains decreased and tracheary elements were seen in newly opened flowers. ENN are not vascularized, with vascular bundles from the calyx approaching the base of the nectary. Starch grains were scarce and very small in the ENN secretory head cells. Fibrillary proteins were found only in NN and periplastidial reticulum was observed only in ENN. In NN, nectar secretion begins shortly before anthesis, being released through the raised stomata and the reticulate cuticle. In ENN, secretion extends from the young flower bud stage to senescent flowers, accumulating in small subcuticular spaces and being continuously released through the intact cuticle. Temporal differences in NN and ENN production, as well as in the structural characteristics and nectar release mechanisms between NN and ENN, may explain the differences in nectar characteristics of <i>A. mansoanum</i>.