Plasticidade estrutural das raízes da hemiepífita Vanilla bahiana Hoehne (Orchidaceae) em função de suas mudanças funcionais

Abstract

The epiphytic environment offers several advantages for plants, highlighting the greater availability of light in the canopy. However, this advantage is accompanied by limitations on water and nutrient availability. Orchidaceae are successful in this context, highlighting strategies of the vegetative organs to overcome water deficiency. For example, these plants develop a complex root system with a specialized epidermis (velamen) capable of increasing water absorption. The velamen occurs most commonly as a multiple epidermis composed of dead cells. However, the hemiepiphyte Vanilla bahiana has a simple epidermis also considered by some authors as velamen. In this sense, we seek to confirm whether or not the epidermis cells in this species fit this definition. In addition, the present work compared the adventitious roots of this species, grown in the air habit (free roots and attached to the phorophyte) and terrestrial (fixed in the soil), in order to investigate if the different phenotypes, such as the root cross-sectional area, cortex, vascular cylinder, gaps in the inner layers of the cortex, epidermis thickness, exodermis, internal periclinal thickening, and protoxylem cords, are environmental products. The material was collected in a gallery forest and fixed in FAA. Transverse sections were obtained by freehand or in infiltrated material, and the slides were used to characterize the different tissues. The epidermis could not be considered as velamen because, unlike the one proposed in the literature, it has living cells at maturity. The parameters that had significant differences were the root cross-sectional area, the parietal thickenings of the epidermis, the cross-sectional area of the cortex and the vascular cylinder, the investment in air gaps (aerenchyma) and protoxylem strands. The characteristics with greater plasticity were the internal periclinal thickening of the epidermis when comparing the adhered roots with those fixed to the soil, and the total area occupied by the gaps in the inner cortex layers between the free and adhered roots The observed plasticity reflects the habit in which the free roots or the roots in contact with the phorophyte (aerial) and terrestrial (fixed in the soil) grow.