The structural diversity of floral nectaries does not mean ontogenic diversity in Rhamnaceae species

Abstract

Rhamnaceae is a family with small, radially symmetrical flowers, which present nectaries of diverse morphology. Therefore, studies to explain the developmental pathways of this diversity are welcome in understanding the evolutionary biology of this gland. Thus, the objective of the present study was to compare the developing floral nectary of six species of Rhamnaceae belonging to different tribes to check its ontogeny and improve the descriptions of nectary types. Buds and flowers were collected and processed for surface, histological and ultrastructural analyses. The nectaries originate from the hypanthium soon after the gynoecium primordium and are generally positioned above the ovary. All nectaries are structured and vary in terms of surface, shape, and presence of indumentum. Two types of nectary were identified: annular and adpressed. The intensity of intercalary growth in the intrastaminal area has a fundamental role in the hypanthium development and consequently, in the differentiation of nectary types. This phenomenon is also responsible for the different positions of annular and adpressed nectaries in flowers of the Rhamnaceae species studied. We conclude that the origin of nectaries in Rhamnaceae seems to be homologous, despite of their great morphological diversity. Also, this study was successful in testing the value of previous nectary classification, placing the investigated species in the categories already defined for the family.