Annals of West University of Timişoara, ser. Biology, 2015, vol XVIII (2), pp. 67-72

HISTOANATOMICAL FEATURES OF THE AQUATIC TENELLUM (MART.) BRITT. ()

Rodica BERCU Faculty of Natural and Agricultural Sciences,”Ovidius” University, Constantza University Alley, No. 1, B, 900470, Constantza Corresponding author e-mail:[email protected] Received 14 October 2015; accepted 11 November 2015

ABSTRACT The paper concerns a histological study of the main features of the stem (rhizome) and ( and blade) of a perennial herb native to North, Central and South America, Helanthium tenellum (Mart.) Britt., growing both submerged and emerged in its native region. In our country the plant is known as an aquarium plant. However, the stem cortex is well-developed with epidermis, hypodermis and an aerenchyma tissue with a number of air chambers. The vascular system consists of amphivasal vascular bundles to the center and peripheral few collateral bundles. The leaf mesophyll is heterogenous, represented by a palisade and a spongy tissue. In the mesophyll are embedded few poor developed vascular bundles. Excepting some taxonomic studies, a study concerning the anatomy of this species is lacking. KEY WORDS: histo-anatomy, aerenchyma, stem, leaf, Helanthium tenellum

INTRODUCTION Helanthium tenellum (Mart.) Britt. [syn. tenellus (Mart. ex Schult. f. Buchenau], common known as dwarf sword, is a submerged aquatic plant, native from North, Central and South America, which grows up to 5-8 cm. The phylogeny of this genus was studied on the basis of 96 morphological characters. The analysis resulted in a polyphyletic origin of the genus (Lehtonen, 2006). It belongs to the Alismataceae family (Charlton, 1968). Helanthium tenellum is the smallest of all sword . The plant can grow both submerged and emerged. At favorable conditions in the aquarium, H. tenellum forms a dense carpet and it is thus very suitable as a foreground plant. In large aquaria care should be taken to provide enough light because H. tenellum is rather light demanding. It has dark green lanceolate with a pointed tip, arranged in rosette. At young plants the blade is barely distinguishes by the petiole. Later, at maturity, the petiole becomes 10 to 15 times longer than the blade. At this point, in the life cycle of the plant, the blade is 1-4 cm long and 0.2-1 cm wide with parallel veins and decurrently base. In bright light it becomes reddish (Jérémie et al, 2001; Rataj, 2004; Costa & Forni-Martins, 2004). In literature is generally little information on this aquatic plant anatomy, many papers having only morphological and taxonomic approaches (Fassett, 1955; Haynes & Holm-Nielsen, 1986; Schrott & Sinitean, 1999; Somogyi, 2006). In our country, studies of aquatic spontaneous plants were performed by Toma et al. (1996), Bercu (2003), Sârbu et al. (2005), Stănescu & Toma (2007), Bercu (2009), 67

BERCU: Histoanatomical features of the aquatic plant Helanthium tenellum (Mart.) Britt. (Alismataceae)

Petruș-Vancea et al . (2013), Săndulescu et al. (2014, 2015) and Bercu (2010), the latter having few studies on the anatomy of some aquarium plants. In the freshwater herbaceous plants, the biologist sees clear examples of structural modifications that adapt the plants to the varying degrees of water availability. Aquatic plants present anatomical specificities such as the few supporting tissues, which are concentrated in the stem (in an endodermis) (Rascio, 2002; Raven, 1996). Around the central cylinder, a cortex of lacunar parenchyma develops and consists of a system of interconnected airspaces, allowing plant flotation and gas accumulation. This tissue facilitates gas storage and provides a lower resistance pathway for oxygen, allowing a better circulation through the plant diffusion (Toma & Gostin, 2000; Faur & Ianovici, 2005; Ianovici et al., 2010; Lamberti-Raverot & Puijalon, 2012; Grudnicki & Ianovici, 2014). A well- developed lacunar parenchyma (also called aerenchyma) is a major characteristic of aquatic plants (Jung et al ., 2008).

MATERIALS AND METHODS Small pieces of rhizome and leaf (petiole and lamina) were fixed in FAA (formalin: glacial acetic acid: alcohol 5:5:90). Cross sections of the leaf were performed by free hand made technique (Bercu & Jianu, 2003). The samples were stained with alum-carmine and iodine green. Anatomical observations and micrographs were performed with a BIOROM–T bright field microscope, equipped with a TOPICA 6001A video camera.

RESULTS AND DISCUSSIONS The rhizome, in cross section, discloses an almost circular shape. Externally is a single-layered epidermis, with rectangular cells, closely linked together, with slightly thickened cells walls. It is followed by a hypodermis, composed of a single layer of slightly radial elongated cells, regularly arranged, larger than those of the epidermis. The cortex is multilayered, with round-oval parenchymatous cells. Remarkable are the cortical and medullary different size air chambers. Centrally located is the pith with three air chambers (Fig. 1). The centrally located vascular system is composed by five concentric amphivasal (leptocentric) vascular bundles, one larger than the other and, peripheral, several small collateral bundles are present. The amphivasal vascular bundles have inside a well-developed phloem (phloem vessels, companion cells and phloem parenchyma), surrounded by 10-14 xylem vessels. In the rhizome basic tissue starch grains and calcium oxalate crystals (druses) are present (Fig. 2). The petiole, in cross-section, has an almost quadratic shape. The outer one- layered epidermis has regularly arranged large cell, without intercellular spaces, with slightly radial elongated cells, containing chloroplasts. The cortex lies beneath the epidermis and is differentiated in two distinct zones: a hypodermis composed of thin walled compactly arranged parenchymatous cells with chloroplasts and the inner region which is made by symmetrically arranged air chambers separated by thin partitions made of one or two layer of thin-walled cells. Rare small

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Annals of West University of Timişoara, ser. Biology, 2015, vol XVIII (2), pp. 67-72 insignificant vascular bundles occur (Fig. 3, A, B). Such as other aquatic plant species (Bercu, 2007), the air chambers are kept open by a diaphragmatic tissue.

FIG. 1. Cross section of the rhizome - ensamble: FIG. 2. Cross section of the rhizome with vascular ac- air chambers, avb- amphivasal vascular bundles, bundles – detail: avb- amphivasal vascular bundle, c- cortex, e- epidermis, h- hypodermis. ac- air chamber, bt- basic tissue, cb- collateral bundle.

A B

FIG. 3. Cross sections of the petiole. Ensemble (A). Section with epidermis and cortex (B ): ac- air chamber, co- collenchyma, e- epidermis, h- hypodermis, vb- vascular bundle.

The vascular system is fascicular with a number of poor developed closed collateral bundles. They possess one or two xylem vessels inside and four or five phloem vessels outside (Fig. 3, A, B). Cross section of the blade exhibits the usually succession of tissues: upper epidermis, mesophyll and lower epidermis. The upper epidermis, such as the lower one, is formed by a single layer of thin-walled cells. The upper epidermis forms a slightly prominence and is interrupted by stomata with large sub-stomatal cavities (Fig. 4, A, B). Unlike the mesophyll of other Echinodorus species (Bercu, 2009), the Helanthium tenellum mesophyll tissues are clearly differentiated into two layers of palisade tissue and 5-6 layers of spongy tissue, the later possessing a large number of small air chambers. However, the palisade cells placed just below the upper epidermis, are having abundant chloroplasts whereas the rest of the layers of cells are having only few chloroplasts (Fig. 4, A, B; 5, A).

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BERCU: Histoanatomical features of the aquatic plant Helanthium tenellum (Mart.) Britt. (Alismataceae)

The vascular system of the blade is composed by a large vascular bundle of the mid rib and small secondary veins in the rest. The large bundle of the mid rib is poor developed, composed of xylem placed to the upper epidermis and phloem to the lower one. Xylem is represented by one or two xylem vessels next to a larger number of phloem vessels. Each vascular bundle is protected by a parenchymatous sheath. The lower epidermis forms, right to the mid rib, three small crests due to the presence of some groups with few collenchymatous cells (Fig. 4, A; 5, B).

A

FIG. 4. Cross sections of the blade.Ensemble (A). Section of mesophyll (B): ac- air chamber, le- lower epidermis, m- mesophyll, pt- palisade tissue, s- stoma, st- spongy tissue, ue- upper epidermis, vb- vascular bundle.

B

A B FIG. 5. Cross sections of the blade. Sections of upper epidermis and mesophyll (A). Section of lower epidermis and vascular bundles (B): ac- air chamber, co- collenchyma, le- lower epidermis, ph- phloem, pt- palisade tissue, s- stoma, st- spongy tissue, ue- upper epidermis, x- xylem.

CONCLUSIONS The rhizome possesses a characteristic dicot’s structure with an aerenchyma cortex and amphivasal and collateral bundles in the vascular system. 70

Annals of West University of Timişoara, ser. Biology, 2015, vol XVIII (2), pp. 67-72

The leaf petiole epidermis has thin-walled cells, covered by a thin cuticle. The cortex possesses air chambers and a number of poor closed collateral vascular bundles embedded in the aquatic parenchyma tissue. Phloem tissue is more developed than the xylem one. The leaf blade mesophyll is heterogenous and amphystomatic, possessing air chambers, mostly near by the mid rib vein, and poor developed vascular bundles. Stomata have large sub-stomatal cavities. Mechanical tissues almost lack, only the blade possesses some groups of collenchymatous cells to the lower epidermis. The histoanatomical features of the plant organs are in accordance with their hydrophytic nature. The presence of stomata provides the plant resistance to decreases in water levels periods.

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