Glands up! Novel Secretory Structures from Cuphea melvilla in Amazon Floodplain and Paradigm Shifts of their...

The diversity of secretory structures in Lythraceae remains poorly understood. We thus present a comprehensive morphoanatomical description of vegetative and reproductive glands in Cuphea melvilla to address fundamental questions about their secretions and functions.

Buds, leaves and flowers of C. melvilla were sampled from floodplain habitats in the Brazilian Amazon. Field observations of visitors were conducted, and seven distinct gland types were characterized using light and scanning electron microscopy alongside comprehensive histochemical testing of glandular tissues and exudates.

An unexpected diversity of secretory structures was identified within C. melvilla. Conical colleters occur at both vegetative and reproductive shoot apical meristems, exhibiting a novel pattern consisting of a central secretory axis enclosed by a non-secretory epidermis. The yellow spots on the leaves are epithemal hydathodes, and the epicalyx revealed the unprecedented discovery of floral hydathodes associated with non-vascularized glandular projections. Additionally, the floral spur exhibits a vascularized nectary. Four main functional roles are proposed: (i) leaf and floral hydathodes mediate water homeostasis via guttation/absorption; (ii) colleters produce a predominantly mucilaginous exudate that protects developing leaves and flowers from desiccation and pathogen invasion; (iii) an oleoresinous, sticky secretion from floral glandular projections helps in defense against herbivores and facilitates pollen adhesion; and (iv) the floral nectary produces a sugary, aromatic nectar as hummingbirds reward.

The remarkable diversity of secretory structures in C. melvilla - most notably the first report of colleters in Lythraceae and of floral hydathodes in angiosperm - underscores the need for a broader reassessment of anatomy descriptions across the family. These secretory structures revealed unprecedented morphoanatomical and functional evidence that expand significantly the understanding of ecological adaptation within the family, and likely play pivotal roles in mediating plant-animal interactions and aiding survival during seasonal drought and flooding regimes in Amazon wetlands.