Anatomia radicular em Orchidaceae epífitas: estratégias estruturais para maximizar a absorção de recursos

Abstract

The epiphytic environment provides limited content of nutrients to plants, marked by constantly water fluctuations. However, the Orchidaceae are successful in this ambience, showing structural and physiological strategies of the vegetative organs, mainly to supply water deficiency. The aerial roots of these plants exhibit anatomic special features, being constituted by a stele surrounded by the endodermis, followed by multiple layers of cortical parenchyma bounded externally by the exodermis, and a dermal tissue composed of multiple cell layers called velamen. This anatomical apparatus reveals different strategies for maximizing water and nutrients uptake, with greater investment in velamen area (facilitates water passage) or greater investment in internal structures, such as cortex and vascular cylinder associated with the higher number of passage cells from endodermis (promote effective management of these resources into the plant). The velamen, regardless of being composed by dead cells, retains its pectin matrix. The negative character of this matrix retains cations and contributes to attract negative ions, which makes velamen a tissue with higher nutritional intake when the environment supply is limited. The higher number of xylem strands aligned with the passage cells of the endodermis can also increase the transport efficiency, since it facilitates these ions entrance on vascular cylinder. The exodermis and endodermis help on mechanical support in cases of desiccation, and constitute an apoplastic barrier due to the high suberin and/or lignin impregnation, except for the passage cells, where its induce the symplastic transport. In cortical parenchyma, the high methyl-esterification of the pectins contributes to the porosity of the cell wall (increasing the flow of nutrients among the cells) and its flexibility (increasing its elasticity and water retention capacity). In this tissue, there were cells with phi, uniform and reticulated thickenings, besides cells with high suberin lamellae. The lignin reinforcement help to prevent the cortex collapse in low humidity conditions, while the suberin impregnation generates a hydrophobic character of the cortex, which contributes to the water flow toward the stele.