Double defense: chemical and mechanical protection of the vegetative apices of Begonia curtii (Begoniaceae)-an endemic species of the Atlantic forest.

The stem apical meristems (SAM) are essential for the primary growth of plants and possess protective mechanisms to safeguard their fragile structures, including stipules and chemical secretions such as mucilage. This study examines Begonia curtii, focusing on the origin and composition of its protective sticky secretions. The research is set within the Atlantic Forest-a biodiversity hotspot-highlighting the plant's adaptive survival strategies. Samples of leaves and stipules at three stages of development were collected and analyzed using micromorphological, anatomical and histochemical techniques. The stem apices of B. curtii are covered by large, green stipules, which protect developing leaves and stipules with a mucilaginous secretion produced by trichomes and idioblasts present in both organs. Mucilage, originating from idioblasts present in the stipules, accumulates in the substomatal chambers and is released by the stomata, an atypical phenomena. The exudate from the hydathodes increases the volume of secretion to hydrate young organs and meristematic regions, protecting them from desiccation. The adult stipules remain attached to the stem, become dry, and without secretions, while the multiple epidermis and idioblasts, in adult leaves, store water and mucilage, respectively. The morphological and chemical differences between the leaves and stipules of B. curtii at different stages of development, reinforce both their physical and chemical protective roles in the face of biotic and abiotic factors typical of the Atlantic Forest's rocky outcrops.