Structural adaptations of aboveground vegetative organs to cangas: A case study with the Amazonian species Carajasia cangae R.M.Salas, E.L.Cabral & Dessein (Rubiaceae), which grows on ferruginous outcrops.

Abstract

The Brazilian ironstone outcrops (cangas) impose challenging conditions for establishing species. Adapted species should exhibit different responses to xeric conditions of cangas. To understand these adaptations, our plant model was Carajasia cangae R.M.Salas, E.L.Cabral & Dessein, an endemic and threatened species restricted to the cangas of the Amazon Forest. Herein, we investigate structural adaptations that are important for its conservation, including anatomical characteristics of stems and leaves. To this goal, we evaluated the plant anatomy, including developmental aspects, between plants from the natural environment (in situ) and cultivated specimens (ex situ), providing information on phenotypic plasticity. Developmental aspects and the effects of seasonality on leaf anatomy and ultrastructure were also analyzed. Under natural conditions, the plants exhibit rigid stems with secondary growth and leaves with a thicker blade and mesophyll. In these plants, mucilaginous cells are associated with the vascular bundles. Such characteristics, however, do not occur in the individuals under cultivation, which are fragile and etiolated. The leaf structure, in particular, is well adapted to the high radiation of the cangas. This condition is related to the rapid development of epidermal and mesophyll characteristics during regrowth in the rainy season. Seasonality modulates the arrangement of the mesophyll and the synthesis of phenolic compounds. In the dry season, the fibrous vascular bundles are more lignified, contributing to the support of the leaves, including senescent ones. The ultrastructure reveals that subcellular dissolution of the mesophyll occurs during leaf death. The results showed that C. cangae presents little adaptability to the cultivation conditions tested and that its life cycle is mediated by seasonality. Therefore, the integrity of the canga areas where the species occurs must be maintained for its conservation, or its relocation must be made to areas with similar soil and climate conditions.