83 Vol.51, n. 1 : pp.83-93, January-February 2008 BRAZILIAN ARCHIVES OF ISSN 1516-8913 Printed in Brazil BIOLOGY AND TECHNOLOGY AN INTERNATIONAL JOURNAL Comparative Anatomy of the Absorption Roots of Terrestrial and Epiphytic Orchids Ana Sílvia Franco Pinheiro Moreira* and Rosy Mary dos Santos Isaias Universidade Federal de Minas Gerais; Instituto de Ciências Biológicas; Departamento de Botânica; [email protected]; 31270-901; Belo Horizonte - MG - Brasil ABSTRACT The present study compared roots of terrestrial and epiphytic Orchidaceae, analyzing the anatomical characteristics from an ecological point of view. The material was collected at three different sites in Minas Gerais / Brazil and was fixed in FAA. Transverse sections were obtained by freehand sections or from material previously embedded in Paraplast or Historesin. The prominent characteristics of the epiphytic group were: significant smaller perimeter, epidermis with 3 or more cell layers, U-thickened exodermal cell walls, O-thickened endodermal cell walls, and a low ratio between the caliber and the number of protoxylem arches. The terrestrial group presented simple or multiseriate epidermis, and exodermis and endodermis with typical Casparian strips. The anatomical characteristics should have evolved with several adaptations to distinct environments during evolutionary process. Key words: Orchids, terrestrial, epiphytic, anatomy, adaptations INTRODUCTION They have aerial roots with the function of fixation on substrate, and water and minerals absorption. The Orchidaceae constitute one of the largest The terrestrial species absorb their nutrients direct families of flowering plants, having around 20,000 from the soil, and may present three kinds of roots: species. They are unique in forms, colors and (1) one adapted to absorption, and fixation; (2) a flower structure. However, the vegetative tuber adapted to nutritive substances storage, and organization is variable among the species (3) one specialized in storage, absorption, and (Dressler, 1993). In general, they are composed of fixation (Black, 1973). According to Dressler adventitious roots, rhizome, stem, leaves, floral (1993), the tubers are able to survive during dry ramets, and flowers (Kramer, 1975). season, and nourish the plant when it is necessary. During evolutionary process, the orchids have The Orchidaceae are subdivided into five become adapted to distinct environments, so they subfamilies: Apostasioideae, Cypripedioideae, can be epiphytes, terrestrials, saprophytes or Epidendroideae, Spiranthoideae, and Orchidoideae lithophytes (Black, 1973). These environmental (Dressler, 1993). The ten species presently studied variations contribute for structural alterations in belongs to the last three groups. Spiranthoideae vegetative organs (Pabst and Dungs, 1975). and Orchidoideae are constituted mostly of The epiphytic orchids constitute at about 75% of terrestrial species, and so have adaptive all the other Orchidaceae species (Atwood, 1986). characteristics to soil environment, while * Author for correspondence Braz. arch. biol. technol. v.51 n.1: pp.83-93, Jan./Feb. 2008 84 Moreira, A. S. F. P. and Isaias, R. M. S. Epidendroideae species are mostly epiphytes mounted in Haupt glycerinate gelatin (Kraus and (Dressler, 1993). Arduin, 1997). Even though the orchids are considered a derived Root fragments were dehydrated in buthyl or ethyl group, some primitive characteristics may be series (Johansen, 1940) and embedded in observed. The reduced size of their seeds is Paraplast (Kraus and Arduin, 1997) or Leica efficient for plant dispersion in environments of Historesin. Transverse sections were cut on a difficult colonization, and the flowers Leica Jung Biocut microtome, with 5-6µm specialization to ornitophyly and entomophyly are thickness for Historesin and 10-14µm for characteristics to be considered (Robinson and Paraplast embedded material. The sections were Burns-Balogh, 1982). These authors suggested that stained with 0.5% astra blue and safranin (2:8) or the subfamilies Spiranthoideae, Apostasioideae, 0.5% toluidine blue (Kraus and Arduin, 1997), and and Orchidoideae can be viewed as a succession of mounted in Entellan. forms that have reverted to the terrestrial habits Root sections were analyzed in light microscope. from epiphytic ancestry. Photomicrographs were obtained in an Olympus This research compares terrestrial and epiphytic BH2-BHS photomicroscope. Orchidaceae absorption roots, analyzing their anatomical characteristics in relation to the environment. RESULTS Terrestrial orchids MATERIALS AND METHODS Most of the terrestrial orchids have tubers, while Habenaria petalodes has dimorphic roots; one for The terrestrial orchids: Habenaria petalodes absorption and soil fixation, and other for nutrient Lindl., Prescottia montana Barb.Rodrig., storage. In this research just the absorption roots Prescottia oligantha Lindl. and Sauroglossum were analyzed and are described in sequence. nitidum (Vell.) Schltr., and the epiphytic orchids: Habenaria petalodes (fig. 1). Hairs unicellular. Epidendrum saxatile Lindl., Encyclia patens epidermis uniseriate (fig. 11). Exodermis Hook., Lanium avicula Lindl., Polystachya uniseriate. Cortex 7-9 cells wide, parenchymatic, estrellensis Rchb. f., Sophronitis pumila (Hook) with mycorrhizae sparsely disposed inside cells; Van den Berg and M.W.Chase and Scaphyglottis cells of 3-4 layers subtending epidermis with modesta Schltr. were collected at Serra da reduced intercellular spaces, small size, thin Calçada, Brumadinho/MG, and Serra do Caraça, walled, with raphids. Cells of inner 4-5 layers Catas Altas/MG. Twenty samples of the roots of circular, with triangular intercellular spaces, higher each species were fixed in FAA 50 for 48 hours sizes. Endodermis uniseriate, with inconspicuous and then transferred to ethanol 50% (Johansen, Casparian strips. Vascular cylinder 10-arch, 1940). inconspicuous xylem and phloem strands. Pith The perimeter (p) of twenty roots of each species cells mostly circular, thin walled, reduced π were calculated by the formula p = 2 r. The intercellular spaces. The perimeter:protoxylem diameter of the roots at 2cm from the apex was arches ratio was 0.63. ® obtained with a caliper Digimess and used to Prescottia oligantha (fig. 2). Velamen 3-4 cell calculate de radius by the formula d = 2r. The layers, with thick walls and supporting strips. perimeter:protoxylem arches ratio (p/px) was Exodermis uniseriate. Cortex 15-17 cells wide. calculated to estimate the relative water and Cells thin walled, variable in size. Cells 2-3 layers nutrients conduction from the root cortex to the subtending exodermis and 3-4 layers off other parts of the plant. endodermis with raphids. Little storage starch Histological slides were prepared by freehand grains. Endodermis uniseriate with conspicuous sections or from material previously embedded in Casparian strips (fig. 15). Vascular cylinder 4- Paraplast and/or Leica historesin. The freehand arch, xylem and phloem easily distinguishable. sections were obtained using a razor blade, and Pith cells mostly circular with no intercellular stained with 0.5% astra blue and safranin (2:8) spaces. The perimeter:protoxylem arches ratio was (Kraus and Arduin, 1997). The slides were 3.94. Braz. arch. biol. technol. v.51 n.1: pp.83-93, Jan./Feb. 2008 Comparative Anatomy of the Absorption Roots of Terrestrial and Epiphytic Orchids 85 Prescottia montana (fig. 3). Velamen 2-3 cell walled, with raphids. Endodermis O-thickened, layers, with thin walls and supporting strips. passage cells opposite to the protoxylem arches Exodermis uniseriate. Cortex 14-16 cells wide, (fig. 17). Vascular cylinder 14-arch, with parenchymatic; cells of regular shapes and sizes, intercalated strands of xylem and phloem. PITH with triangular intercellular spaces. All cells thin- cells mostly circular with triangular intercellular walled. Raphids in 1-2 cell layers subtending spaces. The perimeter:protoxylem arches ratio was exodermis and nearby endodermis. Pelotons in 0.52. parenchymatic cells next to the site of fixation to Sophronitis pumila (fig. 7). Velamen 4-5 cell the substrate and storage starch grains. 2-6 layers layers, with thin walls and supporting strips. nearby endodermis with raphids (fig. 14). Exodermis uniseriate, U-thickened. Cortex 8-12 Endodermis uniseriate, cells isodiametric, with cell wide, parenchymatic; cells variable in size, conspicuous Casparian strips. Vascular cylinder with thin walls, raphids, little starch grains and 11-arch, xylem scarcely distinguished within mycorrhizae in cells closer to the substrate (fig. phloem strands. PITH cells mostly circular with 18). Endodermis uniseriate, O-thickened. Clusters thick walls and reduced intercellular spaces. The of passage cells opposite to xylem arches. perimeter:protoxylem arches ratio was 2.40. Vascular cylinder 16-arch, intercalated strands of Sauroglossum nitidum (fig. 4). Hairs unicellular. xylem and phloem surrounded by lignified cells. velamen 9-10 cell layers (fig. 12), with thick walls Pith cells circular, with thin walls and triangular and supporting strips. Exodermis uniseriate, cells intercellular spaces. The perimeter:protoxylem polygonal, with no apparent thickening. Cortex arches ratio was 0.47. 17-24 cells wide, parenchymatic; small cells of 1-3 Polystachia estrellensis (fig. 8) roots were star cell layers subtending exodermis with some shaped in transverse section. Velamen 3 cell layers raphids; the next 10-12 cell layers circular and of with