A comparative study on vascular and supporting systems in several leaf types of arecaceae species

107 HNUE JOURNAL OF SCIENCE DOI: 10.18173/2354-1059.2017-61 Chemical and Biological Science 2017, Vol. 62, Issue 10, pp. 107-116 This paper is available online at A COMPARATIVE STUDY ON VASCULAR AND SUPPORTING SYSTEMS IN SEVERAL LEAF TYPES OF ARECACEAE SPECIES Nguyen Van Quyen 1,2 , Bui Thu Ha 1 , Vu Thi Dung 1 , Nguyen Thi Yen Ngoc 1 and Tran Van Ba 1 1 Faculty of Biology, Hanoi National University of Education 2 Center for Environmental Research and Education, Hanoi National University of Education Abstract. Vascular and supporting systems play important roles and are closely related to leaf morphology. To understand and compare vascular and supporting systems in several leaf types in Arecaceae, we studied leaf structure of species in different genera, including Cocos nucifera, Phoenix roebelenii and Calamus tetradactylus. The vascular bundles, from different leaf types, vary in size and number of tissue components. There is an additional supporting system, made up of fiber strands which are parallel to the leaf surfaces, in the leaf blades of all studied species. In addition, there are bulliform cells in the leaf. These indicate the functional and mechanical adaptation of the leaves of the studied species, especially the Cocos with large leaves. Keywords: Vascular system, vascular bundle, supporting system, leaf anatomy, Arecaceae. 1. Introduction The leaf is vegetative organ which is very important for plants, in particular, and biosphere, in general, since it is the main source of food production in terrestrial ecosystems. Beside the photosynthesis, the mechanical support and nutrient transportation are essential for the leaf. Vascular system is usually responsible for the mechanical support and nutrient transportation in the leaf. In dicots, vascular bundles (VBs) occur in a netted pattern called reticulate venation. In monocots with long, strap-shaped leaves, the larger veins (with vascular bundles inside) run side by side with few obvious interconnections, called parallel venation [1-3]. The vascular bundles, except in the midrib, in dicots such as Ilex, Larrea and Dianthus, have little or no sclerenchyma. In contrast, in monocots, such as Linum and Saccharum, vascular bundles usually consist of one or two fiber strands, which provide mechanical support for the leaf blade [1]. Minor vascular bundles are smaller and less complicated than larger ones, and their structure is suitable for the water, minerals and sugar exchange with leaf parenchyma [4]. Received June 1, 2017. Revised November 22, 2017. Accepted November 30, 2017. Contact: Nguyen Van Quyen, e-mail address: quyennv@hnue.edu.vn Nguyen Van Quyen, Bui Thu Ha, Vu Thi Dung, Nguyen Thi Yen Ngoc and Tran Van Ba 108 The vascular system in the stem of some plant species in Arecaceae was previously studied, showing differences in VBs structure among species [5-7]. However, little is known about fiber structure in palms [5]. Xylem structure in several climbing plants, such as Calamus insignis, Daemonorops hystrix, and Korthalsia rostrata, was also investigated [8]. Xylem structure in the stem and its conductivity in some monocots, including species in Arecaceae, were discussed by Zimmermann [9, 10]. However, those in the leaves, especially leaf blades, were not mentioned. In the genus Syagrus, Glassman compared vascular system in the midribs and large veins and showed that there were differences in shape and size between species [11]. The sclereids in the leaves of species in many genera of Arecaceae, such as Bactris, Etigeissona and Liciiala, were also studied by Tomlimson. These structures are unicellular and scattered in the leaf which mechanically support the leaf blade [12]. However, vascular and fiber structures in the leaf were poorly studied. Arecaceae is a large family of Monocotyledonae with 150 genera and more than 1500 species [13]. In Vietnam, there are 38 genera with more than 100 species [14]. Species in Arecaceae have notable evergreen compound leaves, either pinnately compound (feather palms) or palmately compound [1]. Another notable characteristic is that the leaflet is folded at the midrib either in ٨ or ٧ shapes in transverse section. Leaves vary in size, usually 1 – 2 m; some may reach 5 – 7 m [15]. Leaves of species in a genus may have common characteristics, but may also differ, showing species-specific characteristics [11]. However, it is not reported whether the anatomical structures, especially vascular and supporting tissues, of different or similar types and sizes of leaves differ. In this study, we present and compare the vascular and supporting systems in several leaf types of species in Arecaceae, including Cocos nucifera, Phoenix roebelenii and Calamus tetradactylus. We focus on the investigation of vascular system, especially vascular bundle’s shape, structure, number and size of its components, and supporting system in petioles and leaf blades of studied species. 2. Content 2.1. Materials and methods 2.1.1. Plant materials We used leaves of three Arecaceae species, which are popular in Vietnam, including coconut (Cocos nucifera L., hereafter Cocos), Phoenix roebelenii O’ Brien (hereafter Phoenix) and the rattan Calamus tetradactylus Hance (hereafter Calamus). These species’ leaves are pinnately compound with very short petiolules [16], which some authors consider as deeply divided single leaf [13, 15]. Cocos leaf is largest (up to 5 m), much larger than the others (0.6 - 1 m), and its leaflets are long strap-shaped, and each has two opposite halves (˄-shaped in cross section) and reaches 0.5 m. Phoenix leaflet is similar to Cocus leaflet but is ˅-shaped in cross section, and is 10 - 15 cm in length. Calamus leaflet is streamlined, although petiolule is ˅-shaped, and is 15 - 20 cm in length. 2.1.2. Microsectioning We used both fresh and ethanol-stored leaves. Leaves were excised to microsections at the middle of leaf sheaths, petioles and leaf blades. We employed cross-sectioning to analyzed leaf vascular and supporting systems, especially fibers. A comparative study on vascular and supporting systems in several leaf types of species in Arecaceae 109 2.1.3. Histological staining We used double staining method which was widely applied [17]. Briefly, microsections were stained with methylene blue (to detect lignified structures) and carmine (to detect cellulose thickening structures). The stained microsections were then observed using light microscopes. 2.1.4. Size measurements of observed structures Plant sections were photographed by using a mounted camera on the microscope. The images were then analyzed using ImageJ software (imagej.nih.gov/ij) [18]. For vascular bundles, which are ellipse-like in cross section, we also measured two sizes: (1) The length, which is in xylem-phloem direction and is usually radial direction, and (2) the width, which is perpendicular to the length and is usually in tangential direction. 2.1.5. Data analysis Each experiment was conducted with 10 - 13 replicates. We used one-way analysis of variance (ANOVA), applied Duncan multiple range test (DMRT) to test the difference between means at p < 0.05, using SPSS software (v.16). 2.2. Results and discussion 2.2.1. Vascular and supporting systems in leaf sheath and petiole Leaf sheath and petiole play two principal roles, including mechanical support for the leaf blade and transportation [1]. In Cocos and Phoenix, leaf sheath is not obviously distinguished from petiole, but an expansion of the petiole. In Calamus, the leaf sheath is tubular and c.a. 20 - 30 cm in length. Anatomically, there are large VBs with fiber strands outward (Figure 1). These VBs are in radial arrangement and could reach 2/3 leaf sheath thickness. There are also smaller VBs which are elliptical or nearly circular. These VBs have little fibers, and some VBs may have a common fiber strand. Additionally, there are small VBs and fiber strands. The VBs in the leaf sheath, which have a mass of fibers outward, are very close to each other. These VBs are the main support for the leaf and the protective layer for inner stem and leaves. The petioles of all studied species, in cross section, are semi-circular (or “D” shaped), more convex on the lower part. There are numerous VBs in the petiole scattered in the parenchyma. This is similar to that in Attalea previously mentioned [1]. VBs are denser near the surface compared to the center. There are numerous small fiber strands near the surface, they are probably undeveloped VBs and play a role in mechanical support. The VBs near the surface have higher amount of fibers compared to those in the center, while those in the center have very developed conducting tissues. This petiole structure is different from that in dicots where the VBs are arranged in a ring or in a V or U shapes in the center of the petiole [1, 19]. In addition, there are numerous fiber strands scattered in the parenchyma, mechanically stabling the parenchyma structure. There is no collenchyma in the petioles of all studied species. This is in agreement with many monocots, especially species in Poaceae, except some in Araceae species [20], and is different from dicots [1, 21]. Nguyen Van Quyen, Bui Thu Ha, Vu Thi Dung, Nguyen Thi Yen Ngoc and Tran Van Ba 110 Figure 1. Transverse sections through leaf sheath and petiole A-C: Cocos petiole, D-F: Phoenix petiole, G-I: Calamus petiole, J: Calamus leaf sheath. A/D/G, B/E/H and C/F/I are the sections near the upper, in the center and near the lower surface, respectively. 1 - Vascular bundle, 2 - Small fiber strand. Bars 200 µm. 2.2.2. Structure of vascular bundles in the petiole In monocots, VBs typically consist of 1 xylem and 1 phloem strand, surrounded by a fiber ring [1]. However, in the studied species, VBs vary between species (Figure 2, Table 1). In the center, each VB consists of 1 xylem strand and variable number of phloem strands, which is 1 (Phoenix) or 2 (Cocos and Calamus). Sclerenchyma (fibers) is more numerous in outer VBs, and fiber cell wall is thicker. In Cocos, outer VBs have one phloem strand, but VBs in and near the center may have two more phloem strands with much smaller size. The number of metaxylem vessels varies between species. In cross section, VBs have 1 (Calamus), 2 (Phoenix) or 1 - 2 (Cocos) metaxylem vessels (Table 1). The number of protoxylem cells is usually 3 - 5. Number of sieve tubes in Cocos and Phoenix (more than 10) is higher than those in Calamus (3 - 4). A comparative study on vascular and supporting systems in several leaf types of species in Arecaceae 111 Figure 2. Cross sections of vascular bundles in the middle of the petiole A: Cocos, B: Phoenix, C: Calamus. VBs and fiber strands are scattered in parenchyma. 1 - Fiber; 2 - Phloem; 3 - Xylem; 4 - Fiber strand. Bar 100 µm. Table 1. Characteristic of typical vascular bundle in the middle of the petiole Species Phloem strands Xylem strands Metaxylem vessels Cocos nucifera 2 1 1 – 2 Phoenix roebelenii 1 1 2 Calamus tetradactylus 2 1 1 VBs, especially in the center, are ellipse-like with longer axis in radial direction (Figure 2, Table 2). This shape and the arrangement enhance VBs mechanical support [22]. VBs in Cocos are much larger (4 - 6 times in laminar area) than those in Phoenix and Calamus (p < 0.05, Table 2), showing the correlation between leaf size and VB size. The VBs in Calamus are larger than in Phoenix (p < 0.05). Metaxylem vessels in Cocos are also larger (2 – 5 times) than those in the other species, leading to much higher conductivity, because when the diameter increases 2 times, the water conduction capacity could increase 16 times [10]. The larger size of xylem vessels in Cocos makes it possible to supply water to the large leaf. Table 2. Size of VBs in the petiole Structure/Character Cocos Phoenix Calamus Central VB Radial axis (RA, µm) 701.0 ± 51.1 284.0 ± 26.3 320.6 ± 27.6 Tangential axis (TA, µm) 528.0 ± 49.4 210.0 ± 10.5 262.4 ± 10.5 RA/TA 1.3 1.4 1.2 VB area (10 3 µm 2 ) 302.6 ± 32.7 a 51.2 ± 2.1 c 75.9 ± 5.6 b Metaxylem area (10 3 µm 2 ) 17.4 ± 1.9 a 3.5 ± 0.4 c 7.8 ± 1.5 b Data show mean and SD. Letters following the means in a same row indicate statistically significant difference (DMRT, p < 0.05). 2.2.3. Vascular and supporting systems in the midrib vary between species Leaf midribs play important roles in mechanical support and nutrients transportation. Anatomically, midribs vary between studied species. In Cocus and Phoenix, each leaflet has two opposite halves. In Cocos, there is a midrib which is convex in upper surface. There are many VBs (c.a. 10) in the midrib. Typically, VBs have 1 xylem strand with 1 metaxylem vessel and 2 phloem strands, and lack of sclerenchyma. Instead, there is a complete ring of fiber surrounding all VBs Nguyen Van Quyen, Bui Thu Ha, Vu Thi Dung, Nguyen Thi Yen Ngoc and Tran Van Ba 112 (Figure 3A). This fiber ring functions in mechanical support for the leaflet. Since the midrib with fibers develops in upper site of leaflet, it acts against the tensile stresses. This structure in Cocos is different from most of plant species in which the midrib develops in lower site of the leaf [1, 23]. In contrast to Cocos, there is no midvein with VBs in Phoenix; instead there are many fiber strands near the lower surface in the center of the leaflet (Figure 3B). These fiber strands are parallel along the leaflet. They are the main mechanical supporter of the leaflet. In Calamus, leaflet usually has 5 main veins convex in upper surface, whereas the largest is in the center. Main vein has 3 – 5 VBs with one common fiber strand outward (Figure 3C). The mid-VB has 1 xylem strand (with 1 large metaxylem vessel) and two phloem strands. Other VBs may structurally differ from this mid-VB. Figure 3. Cross section of midribs in Cocos (A), Phoenix (B) and Calamus (C) Cross sections are shown as natural leaf upper-lower sides. There is no VB in the middle of the Phoenix leaflet; this is different from Cocos and Calamus. 1 - A thick sclerenchyma (fibers) ring in Cocos; 2 - VBs in the midrib. 3 - Bulliform cells; 4 - Fiber strands; 5 - VBs in leaf blade. Bars 200 µm. 2.2.4. Vascular bundles vary in the leaf blade and differ between species Generally, vascular system in the leaf blade consists of mostly longitudinal and parallel vascular bundles (LVBs). These VBs vary in size and structure. There are small and short transverse vascular bundles (TVBs) connect two larger LVBs. This structure is typically found in monocots [1, 23-25]. The leaf structure in all three species shows typical leaf structure of C3 plants [24]. However, detail structure of vascular system varies between studied species. In Cocos (Figure 4A), there are 4 - 6 small TVBs between two large LVBs. All VBs are near the lower leaf surface. Large LVBs have 2 – 5 phloem strands and 1 xylem strand which has 1 - 2 metaxylem cells and several much smaller protoxylem cells. Small VBs A comparative study on vascular and supporting systems in several leaf types of species in Arecaceae 113 have 1 phloem strand and 1 xylem strand with few small protoxylem cells. Surrounding large VB is a continuous ring of fibers, forming a closed VB. There is a ring of large bundle sheath cells surrounding small VBs, facilitating the nutrients exchange between VBs and leaf parenchyma. This is similar to VBs in herbs [4]. In Phoenix (Figure 4B), there are 3 - 4 small VBs between two large VBs. Large VBs are ellipse-like, each consists of 1 phloem strand (in crescent shape) and 1 xylem strand with 1 metaxylem cell. These are much different compared to those in Cocos. There is a continuous ring of fibers surrounding xylem and phloem strands in the large VBs. Small VBs are quite similar to those in Cocos. In Calamus (Figure 4C), there are 5 main veins in the leaflet. Between those main veins are smaller veins, each has one VB. The small VBs are slightly different in size, and there are 2 - 3 smaller VBs between two larger ones. There is no metaxylem vessel in these small VBs. These VBs are nearly circular in cross section, and are quite similar to those in Cocos and Phoenix, except the less developed fiber strands. In addition, there are fiber strands alternative to VBs. This structure is different from the other studied species. Figure 4. Transverse sections of leaf blades in Cocos (A), Phoenix (B) and Calamus (C) Cross sections are shown as natural leaf upper-lower sides. The large VBs in Cocos have 4 - 5 phloem strands, while those in the other species have only 1 strand. 1 - Xylem; 2 - Phloem; 3 - VB fibers; 4 - Small VBs; 5 - Individual fiber strands. Bars 100 µm. Table 3. Size of several leaf blades components in cross section Species (a) Leaf thickness (µm) (b) Large VB longitudinal axis (µm) Large VB area (10 3 µm 2 ) b/a (%) Cocos nucifera 309.2 ± 10.4 a 217.8 ± 9.5 a 26.2 ± 3.3 a 70.4 Phoenix roebelenii 145.6 ± 9.4 c 101.1 ± 10.9 b 5.2 ± 0.6 b 69.4 Calamus tetradactylus 153.2 ± 3.0 b 63.5 ± 6.1 c 2.4 ± 0.6 b 41.5 Data show mean and SD. Letters following the means in a same column indicate statistically significant difference (DMRT, p < 0.05). In term of size, large VBs in Cocos, in cross section, are larger (2 – 3 times in longitudinal axis) compared to those in the other species (p < 0.05, Table 3). The longitudinal axis of large VBs in Cocos and Phoenix is approximately 70% leaf thickness, but is only 40% in Calamus. Large VB area in cross section is much bigger (5 – 10 times) Nguyen Van Quyen, Bui Thu Ha, Vu Thi Dung, Nguyen Thi Yen Ngoc and Tran Van Ba 114 compared to those in Phoenix and Calamus (p < 0.05), whereas the smallest is in Calamus. Larger size of VBs makes them possible to transport larger amount of nutrient and provide more mechanical support. 2.2.5. A specific supporting system in the leaf blade VBs also play an important role in mechanical support for leaf blade. In addition to vascular system, there is a specific supporting system in the leaf of studied species. The supporting system consists of individual fiber strands parallel to VBs. These fiber strands form two “nets”, one near the upper leaf surface and one near the lower surface (Figure 4, component number 5). The number of these fiber strands in each leaf side is 2 – 4 times higher than number of VBs. The fiber strands are in between epidermis (outer) and mesophylls (inner) in both sides of the leaf lamina. The number of fiber cell in each strand varies, mostly 3 – 10 cells, whereas smallest number is in Calamus. The fiber cells have very thick cell walls possessing nearly whole cell lumen. Fiber cell diameter is 11.9, 6.3 and 7.0 µm in Cocos, Phoenix and Calamus, respectively. In Calamus, there are fiber strands in the middle lamella of leaf. These fiber strands’ position is similar to VBs, and each fiber strand alternates one VB. Each fiber strand consists of 6 – 15 cells, higher than number of cells in fiber strands near the leaf surfaces as mentioned above. This is a species-specific feature which only found in Calamus. This sclerenchymatous supporting system (the two fiber “nets”) in the studied species is different from supporting system in dicots and other monocots, in which the mechanical support mainly relies on VBs [1, 26]. This system strengthens the mechanical support for the leaf blades of studied species. Additionally, it may also play role in anti-feeding against herbivores [27, 28]. Furthermore, this may be a specific characteristic of Arecaceae species, since all species are from different genera, although number of studied species is small. However, further data are needed to prove this conclusion. 2.2.6. Bulliform cells in the leaf Although bulliform cells (also called motor cells, the enlarged epidermal cells in grasses [21]) are not typical supporting tissue, they play a role in folding/unfolding of the leaf blade, which demands a mechanical force. Hence, we mention bulliform cells in this study. We found bulliform cells in all three species. However, the position of bulliform cells varies between species (Figure 3). The bulliform cells are usually situated near the midribs, which might have 1 - 3 cell layers below the epidermis. In Cocos, they are situated in lower leaf surface, which form two strands along the midrib, and have 2 – 3 cell layers in cross section below the outermost layer of epidermis. In Phoenix, they are in upper leaf surface and usually have two cell layers under outermost epidermal layer. In Calamus, they form two strands in lower side of the leaf near main veins, and some might be found in upper side. The cell size in largest cell layer is 94.5, 56.9 and 42.1 µm in Cocos, Phoenix and Calamus, respectively. The structure of bulliform cells in the leaf of studied species is different from those in other monocots. In grasses, there are many strands of bulliform cells in the leaf blade (i), and these cells arrange in only one layer (ii) and in the outermost cell layer of the leaf (iii) [21, 23, 29]. However, in studied species, especially Cocos and Phoenix, there are two strands (i), with 1 - 3 cell layers (ii) below an outermost epidermal cell layer that is A comparative study on vascular and supporting systems in several leaf types of species in Arecaceae 115 similar with normal epidermis (iii). These bulliform cells, especially two strands near the midrib, when flaccid or turgid, might slightly reduce or increase leaf area exposed to certain environmental factors (i.e. intense sunlight). 3. Conclusion In conclusion, the vascular systems in the leaves of Cocos nucifera, Phoenix roebelenii and Calamus tetradactylus, were showed and compared. General structure of vascular systems in the petioles of these leaves shares some similarity; petiole has numerous VBs, with developed fiber sheath, scattered in parenchyma and VBs structure changes radially. However, there are differences in VB components, xylem and phloem structure, and difference in size and slight difference in shape, relatively related to leaf sizes. In leaflets, the midrib structure varies between species; the fibers are extensively developed in Cocos, giving a strong mechanical support for the large leaf blade. The vascular systems in the leaf blades share many common characteristics, but differ in VBs structure, especially larger ones, between studied species. Interestingly, there is a special fiber strand system in the leaf blades of all studied species. These fiber strands are in between epidermis and mesophylls in both sides of the leaf, and are longitudinally parallel to leaf surface. This may be a common characteristic of species in Arecaceae. However, the origin and development of these fiber strands remain unknown. Furthermore, the chemical compositions of xylem, phloem and fiber in these species’ leaves need to be investigated. Acknowledgements. 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