A comparative study on the root anatomical adaptation of different species in arecaceae

147 HNUE JOURNAL OF SCIENCE DOI: 10.18173/2354-1059.2018-0083 Natural Sciences 2018, Volume 63, Issue 11, pp. 147-153 This paper is available online at A COMPARATIVE STUDY ON THE ROOT ANATOMICAL ADAPTATION OF DIFFERENT SPECIES IN ARECACEAE Nguyen Van Quyen 1,2 , Ha Kieu Oanh 1 and Khuat Thi Hang 1 1 Faculty of Biology, Hanoi National University of Education 2 Center for Environmental Research and Education, Hanoi National University of Education Abstract. In this study, we compare anatomical characteristics of species with different life forms in Arecaceae family, including Cocos nucifera, Phoenix roebelenii and Calamus tetradactylus. The roots of these species all had primary structure, and shared many similar characteristics, including the distinctive stele and cortex, developed Casparian strip, scleroid stele, aerenchyma in the cortex and a thick layer of scleroid exodermis. The root cortex of all species has numerous air spaces, indicating that these species have adapted to the wet soil and waterlogged environment. Although these species share similar characteristics, there are many differences between them, including the relative ratio of root components’ size to the root diameter, cortex structure, and the number of vascular tissue and cell components. Interestingly, in Phoenix roebelenii, there are numerous fiber strands in the aerenchyma of the cortex, and this feature is uncommon in plants. These indicate the relationship between studied species and their anatomical adaptation to the environment. Keywords: Root, anatomy, adaptation, Arecaceae. 1. Introduction The root is an important organ that absorbs and transports water and minerals, anchors and supports for shoot system in the plant. The interaction between the roots and the soil not only affects plants but also the soil characteristics, especially those related to carbon and nitrogen cycles [1, 2]. The root is sensitive to environmental changes, thus it is important to address its adaptation to the soil environment, especially in term of water and nutrient relation [3]. Arecaceae is a monocot plant family with quite distinct forms, including large monoaxial “woody” palm such as coconut (Cocos nucifera L.), small “woody” palm such as Phoenix roebelenii O’ Brien, and “woody” liana such as Calamus tetradactylus Hance [4]. Monocots, unlike dicots, have fibrous root system made up of numerous small lateral roots. However, it is not understood whether the root structure varies between plants with different sizes and if there is difference, such as tissue structure and organization, between plant genera, including Cocos, Phoenix and Calamus. The root characteristics, especially vascular system and the structure of cortex, of different genera are useful evidences for classification of close relative species and to understand their adaptation [5-7]. Received October 9, 2018. Revised November 14, 2018. Accepted November 21, 2018. Contact Nguyen Van Quyen, e-mail address: quyennv@hnue.edu.vn Nguyen Van Quyen, Ha Kieu Oanh and Khuat Thi Hang 148 In this study, we analyze and compare the anatomical characteristics of the roots of Cocos nucifera L., Phoenix roebelenii O’ Brien and Calamus tetradactylus Hance, and their adaptive features. 2. Content 2.1. Materials and methods * Materials We used three species in different genera in Arecaceae. These included Cocos nucifera L., which had large “woody” upright trunk, 15 - 20 m in height and 20 - 30 cm in diameter, Phoenix roebelenii O’ Brien which had small upright trunk, 0.5 - 2 m in height and 10 - 15 cm in diameter, and Calamus tetradactylus Hance which was an liana, 1 - 6 m in length and 0.5 - 1 cm in diameter, and grew in clumps. Root samples were collected from Thanhhoa, Bacgiang and Hanoi, and preserved in 50% ethanol. * Plant histology and anatomical analysis We employed first-level lateral roots (ca. 20 - 50 cm in length) of studied species. The roots were preserved in alcohol prior to sectioning. Roots were cross-sectioned at approximately 10 - 15 cm from the root tips, and were cleaned and stained using previous methods [8, 9]. We applied carmine and methylene blue dyes for root sections to stain cell primary and secondary walls of different tissues. The stained root cross-sections were then observed on a light microscope and the size of anatomical components was measured with a ruler attached to the eyepiece [8]. * Data analysis Experiments were conducted in 10 replicates. Data were analyzed using the software Microsoft Excel and SPSS (IBM). We applied ANOVA, followed by Tukey test to compare means. In general, mean difference was considered as significant at P < 0.05. 2.2. Results and discussion 2.2.1. Tissues organization of the roots of the studied species The roots of studied species were quite similar and all showed primary structure with a distinct stele and a cortex (Figure 1). The root structure of all studied species had a scleroid epidermis-exodermis, large cortex mostly made up of aerenchyma, a circle of developed endodermis, and scleroid stele with numerous vascular arches (xylem and phloem strands) and large metaxylem vessels. This tissue organization in studied species was also similar to that of other rattans such as Calamus platyacanthus [10]. This indicates a close relationship between these species. However, the cortex of Phoenix roebelenii had a specific feature that was the presence of sclerenchymatous (fiber) strands in the aerenchyma. A comparative study on the root anatomical adaptation of different species in Arecaceae 149 Figure 1. Root cross-sections of studied species A, B - Cocos nucifera L.; C, D - Phoenix roebelenii O’ Brien; E, F - Calamus tetradactylus Hance. B, D, F - Close-up view of endodermis and vascular tissue. 1- sclerenchyma-like exodermis and epidermis; 2- Aerenchyma in the cortex; 3 - Casparian strip; 4- Phloem; 5- Xylem; 6- Sclerenchyma in the stele; 7- Fiber strand in the cortex. Bars - 200 µm (A, C and E) and 100 µm (B, D and F) 2.2.2. Scleroid exodermis and epidermis Epidermis and exodermis play roles in the exchange of water, gases, and minerals between the root and the environment, and mechanical support [11, 12]. In the studied species, epidermis and exodermis were tightly attached and the exodermis had numerous cell layers (10 - 20 or more cell layers), which were small, with thick-walls, and were closely compacted. The thickness of the epidermis and exodermis layer was 188.4 - 435.5 µm. This layer in coconut was much thicker than those in the other species. This thickness was associated with large diameter of Cocos nucifera (Figures 2A and 2B, P < 0.05). However, the percentage of epidermis-exodermis layer thickness to root diameter in Calamus tetradactylus (12.2 ± 0.7%) was the highest among studied species (Figure 3A, P < 0.05), while those in Cocos nucifera and Phoenix roebelenii were not significantly different. These structures provide a significant mechanical support for the weak cortex with numerous air spaces. Nguyen Van Quyen, Ha Kieu Oanh and Khuat Thi Hang 150 Figure 2. Comparing the size of major root anatomical components. A- Root diameter, B- Epidermis and exodermis thickness, C- Aerenchyma thickness, D- Stele diameter. Cn - Cocos nucifera, Pr - Phoenix roebelenii, Ct - Calamus tetradactylus. Data show means and standard deviations (SD). In the same category, letters following the means indicate significant difference at P < 0.05 Figure 3. Percentage of the thickness of major root anatomical components to its diameter Cn - Cocos nucifera, Pr - Phoenix roebelenii, Ct - Calamus tetradactylus. Data show means and SDs. In the same category, letters following the means indicate significant difference at P < 0.05 2.2.3. Aerenchyma outlier in the root cortex In all studied species, there were numerous air spaces in the parenchyma of the cortex (Figure 1). This is a typical characteristic of plants adapted to waterlogging or wet soil [1, 13, 14]. In the differentiated region, parenchymatous cells were collapsed, forming air spaces. In Calamus tetradactylus, the air spaces, in the cross-section, elongated radially, while in Cocos nucifera and Phoenix roebelenii, there were many strands of parenchyma that separated small air spaces. The degree of parenchyma collapse was higher in Calamus tetradactylus and Cocos nucifera, than that in Phoenix roebelenii, probably due to the presence of fiber strands in Phoenix roebelenii. A comparative study on the root anatomical adaptation of different species in Arecaceae 151 The thickness of aerenchyma is an indicator which evaluates the response of the root to the environment [2]. In the studied species, the aerenchyma thickness was 601.3 – 1697.5 µm (36.3 – 41.3% of root diameter). The aerenchyma thickness in Cocos nucifera was the greatest (1697.5 ± 113.9 µm) and the smallest was in Calamus tetradactylus (601.3 ± 25.3 µm, Figure 2, P < 0.001). However, relative ratio of aerenchyma to root diameter in Phoenix roebelenii was the highest (41.3 ± 4.7%, Figure 3B, P < 0.05). 2.2.4. Fiber strands in the cortex of Phoenix roebelenii O’ Brien An obvious difference in the anatomical structure of the roots of studied species was the presence of fiber strands in the root cortex of Phoenix roebelenii (Figure 1C). The fiber strands scattered in the aerenchyma, and likely arranged into 4-6 rings, which reduced the collapse of parenchyma cells compared to the other species. The fiber cells were much smaller compared to parenchymatous cells. Average diameter of fiber strands was 73.6 ± 9.7 µm. The presence of fiber strands is rarely observed in plants, instead the presence of sclereids is more common [1, 7]. This characteristic is also a useful feature for classification of close relative species. 2.2.5. Scleroid stele The root stele plays important role in transporting and mechanical supporting for the root and shoot systems [15]. The anatomical characteristics in the roots of the studied species showed adaptation to these functions. In the cross section, there were numerous xylem and phloem strands, large metaxylem vessels, and massive sclerenchyma (Figure 1B, 1D and 1F). The pith was made up of mainly parenchyma, including lignified parts. The stele diameter and its ratio to root diameter were negatively correlated. The stele diameter was 1390 - 3030 µm, which was 32.4 - 41.9% of root diameter in the studied species. The stele diameter of Cocos nucifera root was the largest (3030 ± 109.8 µm) and comparatively, more than two times as large as the size of the others. However, the ratio of stele diameter to root diameter was the highest in Calamus tetradactylus (41.9 ± 1.1%, Figures 2D and 3C). The stele of studied species had large number of vascular strands, with 30.4 - 50.1 xylem (or phloem) arches. The largest number of arches was in Cocos nucifera, then Calamus tetradactylus, and the least was in Phoenix roebelenii (Figure 4A, P < 0.001). In general, the number of vascular strands in these species was higher than other monocots. Figure 4. Root vascular system. A-Number of xylem and phloem strands (arches) and number of metaxylem vessels. B-The size (radial and tangential length) of inner metaxylem vessels in the cross-section. Cn- Cocos nucifera, Pr- Phoenix roebelenii, Ct - Calamus tetradactylus. Data show means and SDs. In the same category, letters following the means indicate significant difference at P < 0.05 Nguyen Van Quyen, Ha Kieu Oanh and Khuat Thi Hang 152 There were a large number of metaxylem vessels in the stele (30.2 - 38.1 vessels per stele). The number of metaxylem vessels in Cocos nucifera was smaller than the other species (Figure 4B, P < 0.001). The density of metaxylem vessels in Cocos nucifera (4.2 ± 0.4 vessels/mm 2 stele) was also smaller than Phoenix roebelenii (24.2 ± 3.9 vessels/mm 2 stele) and Calamus tetradactylus (25.2 ± 2.2 vessels/mm 2 stele). The size of the innermost metaxylem vessels in the cross-section was 107 - 181 µm in radial length and 85 - 126 µm in tangential length. Metaxylem vessel in Cocos nucifera was the largest, followed by Calamus tetradactylus, and then Phoenix roebelenii (Figure 4B, P < 0.007). This indicates that xylem vessel in Cocos nucifera has the most efficient conductive capacity as, in general, when the length increases 2 times, the conductive capacity could increase 16 times [16]. This allows the roots of Cocos nucifera to transport sufficient water for the large stem and foliage above the ground. The metaxylem vessels in Calamus tetradactylus were also larger than those in Phoenix roebelenii. This might be due to the distance of the transport, from the root through a long stem to the leaves in Calamus tetradactylus. 3. Conclusions In conclusion, the root structure in terms of tissue arrangement in the studied species showed similarities in major tissue arrangement, including the major components hardened epidermis- exodermis, large cortex with massive aerenchyma, “ring” of developed endodermis, and scleroid stele with large metaxylem vessels. However, there are a few crucial distinctions. In Phoenix roebelenii, there are numerous fiber strands scattered in the aerenchyma of the cortex. In addition, the numbers of vascular tissue and cell components were slightly different between studied species. The development of aerenchyma in the cortex of all studied species indicates the adaptation of these species to wet soil or waterlogging, and the hardened exodermis and stele were adapted for the mechanical support. The ratio of epidermis-exodermis thickness and stele diameter to root diameter in Calamus tetradactylus were higher than those in the other species, while the ratio of aerenchyma thickness to root diameter in Phoenix roebelenii was higher than that in the remaining species. These indicate the relationship between studied species and the anatomical adaptation of their roots to the environment. Acknowledgements. This study was supported by Hanoi National University of Education (grant number SPHN17-11). We would like to thank Alyssa Meyer (Resource Exchange International) for English editing of the manuscript. REFERENCES [1] White, P. J., George, T. S., Gregory, P. J., Bengough, A. G., Hallett, P. D., & McKenzie, B. M., 2013. Matching roots to their environment. Annals of Botany 112 (2):207-222. [2] Vasellati, V., Oesterheld, M., Medan, D., & Loreti, J., 2001. Effects of flooding and drought on the anatomy of Paspalum dilatatum. Annals of Botany 88 (3):355-360. [3] McDonald, M., N. Galwey, and T. Colmer, 2002. Similarity and diversity in adventitious root anatomy as related to root aeration among a range of wetland and dryland grass species. Plant, Cell & Environment 25 (3):441-451. [4] Ho, P. H., 2000. An Illustrated flora of Vietnam, Vol. 2. Ho Chi Minh city, Youth Publishing House (in Vietnamese). [5] Basconsuelo, S., Grosso, M., Molina, M. G., Malpassi, R., Kraus, T., & Bianco, C., 2011. Comparative root anatomy of papilionoid legumes. Flora-Morphology, Distribution, Functional Ecology of Plants 206 (9):799-807. A comparative study on the root anatomical adaptation of different species in Arecaceae 153 [6] Proença, S.L. and M. das Graças Sajo, 2008. Rhizome and root anatomy of 14 species of Bromeliaceae. Rodriguésia:113-128 [7] Seago, J.L. and D.D. Fernando, 2013. Anatomical aspects of angiosperm root evolution. Annals of Botany:mcs266. [8] San, H. T. and T. V. Ba, 2001. Plant Morphology - Anatomy. Education Publishing House, Hanoi (in Vietnamese). [9] Quyen, N. V., Ha, B. T., Dung, V. T., Ngoc, N. T. Y., & Ba, T. V., 2017. A comparative study on vascular and supporting systems in several leaf types of Arecaceae species. Chemical and Biological Sciences 62 (10):107-116. doi:10.18173/2354-1059.2017-0048, HNUE Journal of Science. [10] Ha, N.C., N.T.H. Lien, and T.T. Trang, 2017. Study the adaptive morphological characteristics of the root of natural Calamus platyacanthus Warb. ex Becc. in Tam Dao National Park in different soil environments. Chemical and Biological Sciences 62 (10):117- 126, HNUE Journal of Science. [11] Vaculík, M., Konlechner, C., Langer, I., Adlassnig, W., Puschenreiter, M., Lux, A., & Hauser, M.-T., 2012. Root anatomy and element distribution vary between two Salix caprea isolates with different Cd accumulation capacities. Environmental Pollution 163:117-126. [12] Pi, N., Tam, N., Wu, Y., & Wong, M., 2009. Root anatomy and spatial pattern of radial oxygen loss of eight true mangrove species. Aquatic Botany 90 (3):222-230. [13] Jung, J., S.C. Lee, and H. K. Choi, 2008. Anatomical patterns of aerenchyma in aquatic and wetland plants. Journal of Plant Biology 51 (6):428-439. [14] Parreno-de Guzman, L.E. and O.B. Zamora, 2008. Formation of root air spaces (aerenchyma) and root growth of lowland rice (Oryza sativa L.) varieties under different water regimes. Asia Life Sci. 17:309-323. [15] Mauseth, J.D., 1998. Botany: An introduction to plant biology. Jones & Bartlett Publishers, USA. [16] Tyree, M.T. and M.H. Zimmermann, 2002. Xylem structure and the ascent of sap. Springer Science & Business Media, Berlin Heidelberg.