Estratégias anatômicas de epíftas vasculares do Parque Estadual do Ibitipoca, MG, Brasil

Abstract

Anatomical alterations are decisive in the capability of adaptation of the species to distinct environmental conditions, that result in exploitation of new niches and enhancement of the environmental tolerance. The anatomical characters can vary due to abiotic features as temperature, luminosity, air humidity, wind speed and water availability. The leaves reveal that stress, mainly hydric, is an important selective force to the development of strategies to acquisition and storage of water by plants. The epiphytes are submitted to adverse conditions of low luminosity, as well as hydric and nutritional deficit, that lead to the development of anatomical adaptations of the leaves, and ensure the survival of the species in shaded environments and with low levels of water and nutrients. The anatomy is an important tool for understanding the adaptive strategies of the plants, once quantitative and qualitative evaluations of the tissues can elucidate the relationship between gas exchange, water absorption and the leaf structure. Due to the scarcity of informations about the anatomy of epiphyte, we conducted the leaf characterization of 40 epiphytic species occurring in the cloud dwarf-forests of the Parque Estadual do Ibitipoca, relating the anatomical characters to the taxonomy and epiphytism conditions. We performed freehand cross sections and epidermis dissociations of the median region of the leaves of five individuals per species. The material was processed, stained and mounted according to usual techniques of Plant Anatomy. We performed micromorphometrical measurements of the samples. Cluster and ordination analyses of the qualitative and quantitative characters were applied, through UPGMA and PCA methods, respectively. Among the 40 evaluated species, respectively, 19 and 17 present present thin and thick cuticles. Three species of Orchidaceae have thick adaxial cuticle and thin abaxial cuticle, and the inverse occurs in Nematanthus strigillosus. The majority of the species is hypoestomatic, presenting the stomata in the same level of the other epidermal cells. The leaves can be glabrous or present several types of trichomes: glandular trichomes in several species of Orchidaceae, Peperomia and Nematanthus strigillosus; tector trichomes in N. strigillosus, P. mandioccana, Pecluma pectinatiformis and Hymenophyllum polyanthos; claviform trichomes in four species of ferns, and scales were observed in all Bromeliaceae and Pleopeltis. We found aquifer hypodermis or water storage parenchyma in all species of Orchidaceae and Bromeliaceae, Hillia parasitica, N. strigillosus and Microgramma squamulosa. Homogenous chlorenchyma was found in Orchidaceae, Tillandsia, Peperomia, Pecluma pectinatiformis, Asplenium auritum and Cochlidium punctatum. The remaining species have dorsiventral chlorenchyma. Mechanical tissue, fibers or mechanic hypodermis were found in the majority of the species, as well as low density of stomata and trichomes. Twenty species present thick leaves and 19 present high ratio between chlorophyllous and non-chlorophyllous parenchyma. According to the statistical analyses, we found that the main character sustaining the separation of the epiphytes in this study is the stomatal density. The epiphytes present several adaptations related to the economy and absorption of water, also presenting anatomical particularities that classify them as a functional group. However, there is not a set of anatomical attributes that characterize the epiphytes, suggesting that other features would be involved in the adaptation of these species to the epiphytic environment.