Leaf anatomical traits and their ecological significance for Acrostichum aureum (Pteridaceae), a remarkable fern species growing in mangroves

Abstract

Mangroves are naturally stressful habitats, with high salinity, regular flooding, strong winds, high temperatures, and muddy, hypoxic soils, requiring adaptations in plant anatomy. In this sense, we present a description of the leaf anatomy of Acrostichum aureum, a remarkable fern species growing in mangroves, focusing on novelties and the significance of the traits for adaptation to mangrove conditions. The leaves were amphistomatic with anomocytic stomata at the same level as the other epidemic cells, and there was a uniseriate hypodermis on both surfaces of the lamina. A dense spongy parenchyma was present in the lamina with limited number of intercellular spaces. We observed sclerenchyma fibers in the costae (3–5 layers) immediately below the epidermis and surrounding the vascular bundles, which were amphicrival (1) and bicollateral (4) in the costae, and collateral and bicollateral in the mesophyll (4–6). In the costae, the endodermis was uniseriate with Casparian strip. The pericycle was interrupted, uniseriate, with varied cell sizes, indicating activity. Idioblasts containing isolated calcium oxalate crystals with a prismatic shape occurred in the epidermis, hypodermis, and sclerenchyma. Some anatomical traits of A. aureum are similar to those reported to angiosperms in mangroves, such as the presence of a thick cuticle, amphistomaty, hypodermis, CaOx crystals, and intercellular spaces, indicative of ecological convergence. These traits are potentially involved in promoting resistance to water loss, enhanced photosynthesis, avoidance of photoinhibition, mechanical resistance, resistance to herbivory, homeostasis, and detoxification.