358 S.Afr.J.Bot., 1992, 58(5): 358 - 362 Ultrastructure of the pneumatophores of the mangrove A vicennia marina

Naomi Ish-Shalom-Gordon* and Z. Dubinsky Department of Life Sciences, Bar-lian University, Ramat-Gan 52900, Israel ·Present address: Golan Research Institute, P.O. Box 97, Qazrin 12900, Israel

Received 24 February 1992; revised 3 June 1992

Pneumatophores of Avicennia marina (Forssk.) Vierh. were studied by scanning electron microscopy (SEM) in order to relate their ultrastructure to their function as air conduits. The path of air from the atmosphere through the lenticel into the aerenchyma and then to the horizontal root is described. Different states of the lenticels were observed (closed, partially opened, fully opened), and are suggested as developmental stages of the lenticel. The nature of the complementary cells and their role in the aeration function of the lenticel are characterized.

Die pneumatofore van Avicennia marina (Forsh.) Vierh . is deur skandeerelektronmikroskopie bestudeer om die verband tussen die ultrastruktuur en hu"e funksie as lug wee vas te stel. Die pad van lug vanaf die atmosfeer deur die lentisel na die aerenchiemweefsel en daarna na die horisontale wortel word beskryf. Verski"ende toestande van die lentise"e (geslote, gedeeltelik oop en volledig oop), wat waarskynlik ontwikkelingstadia van die lentisel is, is waargeneem . Die aard van die komplementere selle en hulle rol in die belugtingsfunksie van die lentisel word gekarakteriseer.

Keywords: Avicennia marina, lenticel, mangrove, pneumatophore, ultrastructure.

Introduction short and long distances from the main trunk of the plant Avicennia marina (Forssk.) Vierh. (Verbenaceae) is a (20 cm and 12 m, respectively) were used. Preparation for common mangrove species on tropical and sUbtropical sea SEM was made according to Glauert's procedure (1980). shores, swamps and stream banks (Chapman 1976; Lipkin Slices about 3 mm thick were cut at distances of 0, 4, 10 1987; Bums & Ogdan 1985; Ish-Shalom-Gordon & Dubin­ and 15 cm from the pneumatophore apex. Similar slices sky 1990). Its root system is composed of five types of were taken from horizontal roots (cable roots). Immediately roots: primary adventitious roots, horizontal (cable) roots, after cutting, specimens were fixed in 6% glutaraldehyde in nutrition roots, anchoring roots, and the negatively geotropic phosphate buffer, pH 7.2. Dehydration was done in a series roots, the pneumatophores. While the first four are located of alcohol solutions of increasing concentration. The underground, in the anaerobic sediment, the pneumato­ adcohol was replaced by Freon-I 13, through a gradient of ph ores are inundated by sea water during high tide, and fully Freon solutions. The specimens were dried using critical­ or partially exposed during low tide. The pneumatophores point drying (CPD), gold-coated and examined with a Jeol- are covered by an impermeable periderm, and gaseous inter­ 840 SEM. Lenticels were cut and removed from the speci­ change between the atmosphere and the internal tissues is mens and gold-coated again, to get better coating. Thin accomplished through openings, called lenticels. By trans­ sections of lenticels were histochemically tested. Sudan IV porting air to the subsurface roots, the pneumatophores staining of lenticels was done according to Sherwood and function as a highly specialized ventilation mechanism, Vance (1976). Staining of the lenticels for lignin and waxes enabling the plant to survive in anaerobic soil. was made using Johansen's method (1940). The ecology and physiology of the root aeration of Avi­ cennia spp. and other mangroves have been studied pre­ Results viously (Field et al. 1981; Burchett et al. 1984; Curran 1985; Curran et al. 1986; McKee & Mendelssohn 1987; A. marina plants are surrounded by many pencil-shaped Mall et al. 1987). However, anatomical studies of the pneumatophores, protruding from the ground (Figure 1). pneumatophores of Avicennia spp. and other mangroves are These pneumatophores may reach a height of 60 cm above very few (Groom & Wilson 1925; Chapman 1939, 1976; ground and a diameter of 1.5 cm at the base, and typically Baylis 1950; Fahn 1982), and none of them focused on have many lenticels (Figure 2). Figure 3 demonstrates the ultrastructure. The aim of the present study was to describe arrangement of the different tissues of the pneumatophore, the air path from the atmosphere, through the lenticel and from the periderm to the pith (phellem, phellogen, phello­ via the pneumatophore tissues, to the horizontal root, by derm, parenchyma, aerenchyma, phloem, xylem and pith). providing ultrastructural description of the lenticel and the This arrangement was similar in the horizontal roots, all tissues of the pneumatophores through which the air passes. parts of the pneumatophore, and in pneumatophores ob­ tained at different distances from the plant trunk. The major Materials and Methods difference was that the horizontal roots had no lenticels. Pneumatophore samples were cut from A. marina plants, Some quantitative differences were also observed, but they naturally growing in Shurat el Gharqana mangal, along the did not seem to have a sugnificant effect on the air flow in Sinai shores of the Red Sea. Pneumatophores growing at the plant. S.AfrJ.Bot., 1992, 58(5) 359

Figure 1 Pneumatophores of A. marina in Shurat el Gharqana.

Figure 2 Upper part of pneumatophore of A. marina. (A) Upper part, X40; (B) lenticels in the upper part of the pneumatophore, X 18.

Figure 3 Tissues on the path of air through the pneumatophore of A. marina. Top left photo: Transverse section of pneumatophore from the periderm to the pith, x25; with cork (A), parenchyma (B), aerenchyma (C, D), phloem (E), xylem (F), and pith (G). Photo A: Cork layer with phellem in the middle, periderm to upper side (outside) and phelloderm to the inside, x350. Photo B: Parenchymal cells, x350. Photo C: Aerenchymal cells, transverse section, X350. Photo D: Aerenchymal cells, vertical section, x200. Photo E: Xylem cells, X350. Photo F: Pith cells, X400. Photo G: Phloem cells, X350. 360 S.-Afr.Tydskr.Plantk., 1992,58(5)

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~ - ," ~~ i-·... ., ... ),fr ,"""

Figure 5 Transverse section of lenticels at different states. (A) Closed state with a beginning of formation of complementary cells by the phellogen of the lenticel, X 110; (B) The mass of comple­ mentary cells pushes the cork, bu t the lenticel is still closed, X 160; (C) The cork is broken and only its residuals are seen. Lenticel is partially-open, the complementary cells are dispersed and new ones are formed by the phellogen, X40; (D) Fully­ opened lenticel, X 100.

Figure 4 States of a lenticel. (A) Closed lenticel, X 80; (B) staining for suberin/cutin, while the staining for lignin and partially-open lenticel, X65; (C) Fully-open lenticel, x80. waxes was negative.

Discussion Different states of the lenticels are shown in Figure 4: closed lenticels (Figure 4A), partially opened (Figure 4B), Air path and fully opened (Figure 4C). Transverse sections in Figure Based on our ultrastructural studies, we suggest a possible 5 show the morphology of the lenticels in these states. pathway for air flow in A. marina plant (Figure 7). Air may The complementary cells obtained from fully-opened enter through an open lenticel in the pneumatophore lenticels are shown in Figure 6. They are concave discs, (Figures 4B, 4C) and diffuse between the complementary stacked with no intercellular matrix. Fine hairs were cells (Figure 6) in the air spaces between the parenchymal observed between these complementary cells (Figures 6C, and the aerenchymal cells (Figure 3). Then, the large D). The complementary cells demonstrated a positive cortical air spaces offer a ready channel for longitudinal air

Figure 6 Complementary cells. (A) Mass of complementary cells, x600; (B) Complementary cells, X2500; (C, D) Hairs between the complementary cells, X 1000. S.Afr.J.Bot., 1992,58(5) 361

A B partially-opened one (Figure 4B), a partially-mature lenticel; ~ Air ~ and the fully-opened one (Figure 4C), a mature lentice!. The /~ Enters "",, o /Through Lenticel" 0 ultrastructural studies suggest a developmental process I'-J f'-.J taking place in the !entice!. The young immature lenticel is , , closed, with a relatively low number of complementary Pneumatophore cells. Later, more and more complementary cells are Cable formed, creating an increasing pressure inside the lentice!. Root , , • When the pressure is high enough, the cork breaks open, forming the partially-opened state of the partially-mature , '------+------' ,'------,. lenticel. As the complementary cells continue to be formed, r ...... the additional pressure enlarges the opening, creating the fully-opened state of the mature lenticel. Although gas exchange can occur in the partially-mature lenticel, the fully-opened mature lenticel is probably more effective in the aeration process, as its larger opening, and the larger Figure 7 Possible pathway of air in A. marina plant, from the atmosphere, through the lenticel, downward through the pneuma­ amount of complementary cells inside the lenticel, allow tophore, to the cable root. Pneumatophore A provides air for more air to diffuse into the lenticel and to move rapidly. fraction Al of the cable root. The same applies to B. Chapman (1939), using light microscopy, described lenticels of Avicennia nitida with and without an opeQing, but associated them with different kinds of pneumatophores. diffusion, creating aerenchymal 'tubes' down the pneumato­ Thus, the suggested development stages for lenticels of A. phore, toward the meeting point with the cable root. The marina may be similar in some or all the species in the path from the aerenchyma to the pith by diffusion through genus. the cell walls of the phloem and xylem is effectively block­ ed by the thickness of these walls (Figures 3E, 3G). Each Complementary cells A. marina plant has an abundance of pneumatophores We suggest that the presence of the mass of complementary (Figure 1). We suggest that the reason for this is that each cells with no intercellular matrix (Figures 5 and 6) encour­ pneumatophore is capable of providing oxygen for only a ages free diffusion of air between the cells, whose concave small portion of the subsurface root system associated with shape also helps to fulfil this function. Since the reaction to it, which includes horizontal roots, anchoring roots, adventi­ the Sudan IV staining was positive, it is possible that, tious roots and nutrition roots. because of their suberin and/or cutin cover, the accumula­ The pneumatophore of A. marina is adapted to function as tions of complementary cells may act as a hydrophobic layer a gaseous pathway, and this adaptation is demonstrated in its through which air may diffuse while water penetration is ultrastructural characteristics. First, the periderm prevents air loss during its downward movement. Second, the struc­ prevented. ture of the aerenchyma is ideal for fulfilling dual functions: it is a transport system and a storage tissue for oxygen for Acknowledgement high-tide respiration. Third, the thick cell walls of the xylem The authors thank Mr Y. Langzam for SEM assistance, and and the phloem avoid penetration of air into these cells and Dr Z. Malik and Dr Y. Ruthmann for their advice and dis­ possible interference with the transport of water and solutes. cussions. Fourth, the small air spaces in the aerenchyma of the pneumatophore's pith do not allow a significant diffusion of References air down this tissue. Chapman (1939) showed experi­ BA YUS, G.T.S. 1950. Root system of the New Zealand mentally that air flow in the pneumatophore is in a mangrove. Trans. R. Soc. N.Z. 78: 509 - 514. longitudinal direction: he pumped air into the surface of a BURCHETT, M.D., FIELD, CD. & PULKOWNIK, A. 1984. cut pneumatophore, and found that bubbles emerged only Salinity, growth and root respiration in the grey mangrove from the nearest lenticels. To make the aeration even more Avicennia marina. Physiol. Pl. 60: 113 - 118. effective, the cable root has small air spaces in the BURNS, B.R. & OGDAN, J. 1985. The demography of the aerenchyma of its pith. These probably prevent a significant temperate mangrove [Avicennia marina (Forsk.) Vierh.) at its flow down this tissue, allowing the air to flow horizontally southern limit in New Zealand. Aust. 1. Ecol. 10: 123 - 133 . along the cable root. Experimental data are needed to CHAPMAN, V.I. 1939. Cambridge University Expedition to Jamaica: Part 3. The morphology of Avicennia nitida Jacq. and confirm this suggested pathway for the air flow in A. the function of its pneumatophores. Linn. Soc. Bot. 52: 487 - marina, or to suggest another. Such experiments may be 533. done by using a tracer gas or by other methods. CHAPMAN, V.I. 1976. Mangrove vegetation. J. Cramer, Germany. Developmental stages of the lenticels CURRAN, M. 1985 . Gas movement in the roots of Avicennia Our SEM observations revealed different states of lenticels marina (Forsk.) Vierh. Aust. 1. Pl. Physiol. 12: 97 - 108. (Figures 4 and 5). We suggest that these are different stages CURRAN, M., COLE, M. & ALLAWAY, W.G. 1986. Root aera­ in the development of the lentice!. We believe that the tion and respiration in young mangrove plants [Avicennia closed state (Figure 4A) represents an immature lenticel; the marina (Forsk.) Vierh.) 1. Exp. Bot. 37: 1225 - 1233. 362 S.-Afr.Tydskr.Plantk., 1992, 58(5)

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