Extrafloral nectaries of annatto ( Bixa orellana L.): anatomy, nectar composition and activity during organ development

Abstract

This study aimed to anatomically characterize the extrafloral nectaries (EFNs) of annatto (Bixa orellana) and determine the composition of its nectar in order to better understand their structure and function during organ development. Standard light and scanning electron microscopy techniques were used for anatomical analysis, and test-strips and a refractometer for determining nectar composition. Both receptacle and stem EFNs were found to possess similar anatomy and nectar composition, and to secrete from early to advanced developmental stages of the organs to which they are associated. EFNs consisting of uniseriate epidermis, nectariferous parenchyma and subnectariferous parenchyma were found located where vascular tissue is immersed. Some layers of nectariferous parenchyma exhibited sclerification and cells with phenolic compounds or calcium oxalate druses were present. Nectar exuded by stomata was acidic, diluted and found to contain sugar. The anatomical and histochemical features of annatto EFNs ensure their integrity and nectar secretion function during the development of buds, flowers and fruits.