Structural diversity of extrafloral nectaries in Chamaecrista sect. Apoucouita

Section Apoucouita (Chamaecrista (L.) Moench – Caesalpinioideae) is an arboreal group that is most diverse in the Amazon and Brazilian Atlantic forests. These species typically bear petiolar and (or) rachis glands called extrafloral nectaries (EFNs). However, no detailed anatomical studies or histochemical analyses have been conducted to confirm nectar secretion. We aimed at describing the structure of such EFNs, as well as determining the chemical nature of the secretion. Eighteen species (23 taxa) were studied using standard light microscopy techniques. We describe 13 types of EFNs with variable morphology. Such EFNs may be impressed, sessile, or stalked; with concave, flat or truncate, or convex secretory surfaces. Cupuliform EFNs (stalked or not) were the most common type observed and patelliform the least common. Despite the morphological variation, differences in the anatomical structure of the EFNs and the chemical composition of the secretion were not observed. EFNs with concave secretory surfaces appear to be more effective as nectar may become accumulated in the concavity, increasing the volume of available nectar. Our results show that despite the variable morphology of the EFNs, such structures share similarities on the anatomy and composition of the secretion and development of a wound-healing periderm in older EFNs. We also indicate the importance of including the morphological variation observed in the EFNs in species of sect. Apoucouita in future taxonomic evaluations