Anatomy and ultrastructure of the floral nectary of Tontelea micrantha (Celastraceae: Salacioideae)

Abstract

Among several native species of the Brazilian cerrado, a shrub, Tontelea micrantha, is exploited by traditional communities for the valuable oil extracted from its seeds, which has anti-inflammatory properties. There have been no studies on the anatomy of its flower, and so the aim of this study is to describe the anatomy and ultrastructure of its floral nectary. Flower buds and flowers in anthesis were collected, fixed and processed for light and electron microscopy. The discoid floral nectary is composed of epidermis and a secretory parenchyma. Secretory cells are rich in plastids with starch grains and mitochondria. The nectar, sucrose dominant, is just sufficient to form a thin film on the nectary. The secretory cells show starch and oil droplets; however, during nectar production there is no evidence of hydrolysis of starch and some lipid reserves remain unchanged. Our results suggest a reduction in the amount of oil in the secretory cells during the secretory phase but this does not appear to imply a release of oil as a nectar component. In addition to maintaining part of the reserves, the lower frequency of organelles involved in nectar synthesis reinforces the hypothesis that phloem sap is the origin of nectar sugars. The tiny nectar film, released through modified stomata, is attractive to small insects such as flies. Considering the importance and intensity of use of T. micrantha in the Brazilian cerrado, we think that these data about its floral nectary can help to better explain its reproductive biology with positive impacts on its management and conservation. Among several native species of the Brazilian cerrado, a shrub, Tontelea micrantha, is exploited for the valuable oil extracted from its seeds. In this article we describe the anatomy and ultrastructure of its floral nectary. The scarce nectar just forms a thin film on the nectary surface and attracts small insects for pollination. The ultrastructure of nectar-secreting cells is coherent with the origin of nectar in phloem.