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 Avicennia marina (Forssk.) Vierh. (Verbenaceae) is a common mangrove species on tropical and sUbtropical sea shores, swamps and stream banks (Chapman 1976; Lipkin 1987; Bums & Ogdan 1985; Ish-Shalom-Gordon & Dubin­sky 1990). Its root system is composed of five types of roots: primary adventitious roots, horizontal (cable) roots, nutrition roots, anchoring roots, and the negatively geotropic roots, the pneumatophores. While the first four are located underground, in the anaerobic sediment, the pneumato­ph ores are inundated by sea water during high tide, and fully or partially exposed during low tide. The pneumatophores are covered by an impermeable periderm, and gaseous inter­change between the atmosphere and the internal tissues is accomplished through openings, called lenticels. By trans­porting air to the subsurface roots, the pneumatophores function as a highly specialized ventilation mechanism, enabling the plant to survive in anaerobic soil.

The ecology and physiology of the root aeration of Avi­cennia spp. and other mangroves have been studied pre­viously (Field et al. 1981; Burchett et al. 1984; Curran 1985; Curran et al. 1986; McKee & Mendelssohn 1987; Mall et al. 1987). However, anatomical studies of the pneumatophores of Avicennia spp. and other mangroves are very few (Groom & Wilson 1925; Chapman 1939, 1976; Baylis 1950; Fahn 1982), and none of them focused on ultrastructure. The aim of the present study was to describe the air path from the atmosphere, through the lenticel and via the pneumatophore tissues, to the horizontal root, by providing ultrastructural description of the lenticel and the tissues of the pneumatophores through which the air passes.

Materials and Methods Pneumatophore samples were cut from A. marina plants, naturally growing in Shurat el Gharqana mangal, along the Sinai shores of the Red Sea. Pneumatophores growing at

short and long distances from the main trunk of the plant (20 cm and 12 m, respectively) were used. Preparation for SEM was made according to Glauert's procedure (1980). Slices about 3 mm thick were cut at distances of 0, 4, 10 and 15 cm from the pneumatophore apex. Similar slices were taken from horizontal roots (cable roots). Immediately after cutting, specimens were fixed in 6% glutaraldehyde in phosphate buffer, pH 7.2. Dehydration was done in a series of alcohol solutions of increasing concentration. The adcohol was replaced by Freon-I 13, through a gradient of Freon solutions. The specimens were dried using critical­point drying (CPD), gold-coated and examined with a Jeol-840 SEM. Lenticels were cut and removed from the speci­mens and gold-coated again, to get better coating. Thin sections of lenticels were histochemically tested. Sudan IV staining of lenticels was done according to Sherwood and Vance (1976). Staining of the lenticels for lignin and waxes was made using Johansen's method (1940).

Results A. marina plants are surrounded by many pencil-shaped pneumatophores, protruding from the ground (Figure 1). These pneumatophores may reach a height of 60 cm above ground and a diameter of 1.5 cm at the base, and typically have many lenticels (Figure 2). Figure 3 demonstrates the arrangement of the different tissues of the pneumatophore, from the periderm to the pith (phellem, phellogen, phello­derm, parenchyma, aerenchyma, phloem, xylem and pith). This arrangement was similar in the horizontal roots, all parts of the pneumatophore, and in pneumatophores ob­

tained at different distances from the plant trunk. The major difference was that the horizontal roots had no lenticels. Some quantitative differences were also observed, but they did not seem to have a sugnificant effect on the air flow in 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

Figure 4 States of a lenticel. (A) Closed lenticel, X 80; (B)

partially-open lenticel, X65; (C) Fully-open lenticel, x80.

Different states of the lenticels are shown in Figure 4: closed lenticels (Figure 4A), partially opened (Figure 4B), and fully opened (Figure 4C). Transverse sections in Figure 5 show the morphology of the lenticels in these states.

The complementary cells obtained from fully-opened lenticels are shown in Figure 6. They are concave discs, stacked with no intercellular matrix. Fine hairs were observed between these complementary cells (Figures 6C, D). The complementary cells demonstrated a positive

S.-Afr.Tydskr.Plantk., 1992,58(5)

D '--'-

~ -,"

~~ 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 comple­mentary 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) Fully­opened lenticel, X 100.

staining for suberin/cutin, while the staining for lignin and waxes was negative.

Discussion Air path Based on our ultrastructural studies, we suggest a possible pathway for air flow in A. marina plant (Figure 7). Air may enter through an open lenticel in the pneumatophore (Figures 4B, 4C) and diffuse between the complementary cells (Figure 6) in the air spaces between the parenchymal and the aerenchymal cells (Figure 3). Then, the large 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)

Cable Root

A B ~ Air ~

/~ Enters "",, o /Through Lenticel" 0 I'-J f'-.J , , , , Pneumatophore

•

, '------+------' r .......... ,'-------,. .......... Figure 7 Possible pathway of air in A. marina plant, from the atmosphere, through the lenticel, downward through the pneuma­tophore, to the cable root. Pneumatophore A provides air for fraction Al of the cable root. The same applies to B.

diffusion, creating aerenchymal 'tubes' down the pneumato­phore, toward the meeting point with the cable root. The path from the aerenchyma to the pith by diffusion through the cell walls of the phloem and xylem is effectively block­ed by the thickness of these walls (Figures 3E, 3G). Each A. marina plant has an abundance of pneumatophores (Figure 1). We suggest that the reason for this is that each pneumatophore is capable of providing oxygen for only a small portion of the subsurface root system associated with it, which includes horizontal roots, anchoring roots, adventi­tious roots and nutrition roots.

The pneumatophore of A. marina is adapted to function as a gaseous pathway, and this adaptation is demonstrated in its ultrastructural characteristics. First, the periderm prevents air loss during its downward movement. Second, the struc­ture of the aerenchyma is ideal for fulfilling dual functions: it is a transport system and a storage tissue for oxygen for high-tide respiration. Third, the thick cell walls of the xylem and the phloem avoid penetration of air into these cells and possible interference with the transport of water and solutes . Fourth, the small air spaces in the aerenchyma of the pneumatophore's pith do not allow a significant diffusion of air down this tissue. Chapman (1939) showed experi­mentally that air flow in the pneumatophore is in a longitudinal direction: he pumped air into the surface of a cut pneumatophore, and found that bubbles emerged only from the nearest lenticels. To make the aeration even more effective, the cable root has small air spaces in the aerenchyma of its pith. These probably prevent a significant flow down this tissue, allowing the air to flow horizontally along the cable root. Experimental data are needed to confirm this suggested pathway for the air flow in A. marina, or to suggest another. Such experiments may be done by using a tracer gas or by other methods.

Developmental stages of the lenticels

Our SEM observations revealed different states of lenticels (Figures 4 and 5). We suggest that these are different stages in the development of the lentice!. We believe that the closed state (Figure 4A) represents an immature lenticel; the

361

partially-opened one (Figure 4B), a partially-mature lenticel; and the fully-opened one (Figure 4C), a mature lentice!. The ultrastructural studies suggest a developmental process taking place in the !entice!. The young immature lenticel is closed, with a relatively low number of complementary cells. Later, more and more complementary cells are formed, creating an increasing pressure inside the lentice!. 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, 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 amount of complementary cells inside the lenticel, allow more air to diffuse into the lenticel and to move rapidly. Chapman (1939), using light microscopy, described lenticels of Avicennia nitida with and without an opeQing, but associated them with different kinds of pneumatophores. Thus, the suggested development stages for lenticels of A. marina may be similar in some or all the species in the genus .

Complementary cells

We suggest that the presence of the mass of complementary cells with no intercellular matrix (Figures 5 and 6) encour­ages free diffusion of air between the cells, whose concave shape also helps to fulfil this function. Since the reaction to

the Sudan IV staining was positive, it is possible that, because of their suberin and/or cutin cover, the accumula­tions of complementary cells may act as a hydrophobic layer through which air may diffuse while water penetration is prevented.

Acknowledgement The authors thank Mr Y. Langzam for SEM assistance, and

Dr Z. Malik and Dr Y. Ruthmann for their advice and dis­cussions.

References BA YUS, G.T.S. 1950. Root system of the New Zealand

mangrove. Trans. R. Soc. N.Z. 78: 509 - 514. BURCHETT, M.D., FIELD, CD. & PULKOWNIK, A. 1984.

Salinity, growth and root respiration in the grey mangrove

Avicennia marina. Physiol. Pl. 60: 113 - 118. BURNS, B.R. & OGDAN, J. 1985. The demography of the

temperate mangrove [Avicennia marina (Forsk.) Vierh.) at its southern limit in New Zealand. Aust. 1. Ecol. 10: 123 - 133 .

CHAPMAN, V.I. 1939. Cambridge University Expedition to Jamaica: Part 3. The morphology of Avicennia nitida Jacq. and the function of its pneumatophores. Linn. Soc. Bot. 52: 487 -533.

CHAPMAN, V.I. 1976. Mangrove vegetation. J. Cramer, Germany.

CURRAN, M. 1985. Gas movement in the roots of Avicennia marina (Forsk.) Vierh. Aust. 1. Pl. Physiol. 12: 97 - 108.

CURRAN, M., COLE, M. & ALLAWAY, W.G. 1986. Root aera­tion and respiration in young mangrove plants [Avicennia marina (Forsk.) Vierh.) 1. Exp. Bot. 37: 1225 - 1233.

362

FAHN, A. 1982. Plant anatomy. Pergamon Press, Oxford, New York, Toronto, Sydney, Paris, Frankfurt.

FIELD, C.D., BURCHETT, M.D. & PULKOWNIK, A. 1981. Respiration rates in mangrove roots. Proc. Int . Congo Bot. 13: 173.

GLAUERT, A.M. 1980. Fixation, dehydration and embedding of biological specimen - practical methods in electron micros­copy. North-Holland Publishing Company, Amsterdam, New York, Oxford.

GROOM, P. & WILSON, S.E. 1925. On the pneumatophores of Paludal species of Amoora. Carapa.and Heritiera. Ann. Bot. 39: 9 - 24.

ISH-SHALOM-GORDON, N. & DUBINSKY, Z. 1990. Possible modes of salt secretion in Avicennia marina in the Sinai. Pl. Cell Physiol. 31: 27 - 32.

S.-Afr.Tydskr.Plantk., 1992, 58(5)

JOHANSEN, D.A. 1940. Plant microtechnique. McGraw Pub­

lishing Company, Bombay - New Delhi, India.

LIPKIN, Y. 1987. Marine vegetation of the Museri and Entedebir

Islands (Dahlak Archipelago, Red Sea) . Isr. 1. Basic Appl. Pl. Sci. 36: 87 - 99.

MALL, L.P., SINGH, V.P., GARGE, A. & PATHAK, S.M. 1987. Certain ecological observations on mangroves of the Andaman Islands. Trop . Ecol. 23 : 225 - 233.

MCKEE, K.L. & MENDELSSOHN, LA. 1987. Root metabolism

in the black mangrove [Avicennia marina (L.) L.J : response to hypoxia. Envir. Exper. Bot. 27: 147 - 156.

SHERWOOD, R.T. & VANCE, C.P. 1976. Histochemistry of

papillae formed in reed canarygrass leaves in response to non­infecting pathogenic fungi . Phytopathol. 66: 503 - 510.