IAWA Bulletin n.s., Vol. 11 (1), 1990: 85-96

HYPERHYDRIC TISSUE FORMATION IN FLOODED POPULUS 1REMULOIDES SEEDLINGS

by

Guillermo Angeles Universidad Nacional Aut6noma de Mexico, Instituto de Biologia, Estacion de Biologia Tropical 'Los Tuxtlas', Apdo. Postal 94, San Andres Tuxtla, Veracruz, Mexico

Summary Introduction Formation and development of hyperhy­ The most immediate morphological re­ dric tissue (HHT) were investigated morpho­ sponse of woody plants to flooding is the logically and anatomically in Populus tremu­ swelling of lenticels to produce a white, loides seedlings flooded for 5, 9, 15,22,30, spongy, fast growing tissue. This tissue ex­ 45, and 80 days. HHT was initiated after 5 tends in some cases up to 1.0 cm from the days of flooding (DF) by swelling of the fill­ stem surface. This phenomenon has been ob­ ing tissue of lenticels, probably by water in­ served in many woody species, and it was take. At the same time, cell division was ini­ reported as early as 1832 by Von Mohl. De tiated in the phellogen and phelloderm of Bary (1884) described the formation of a lenticels. Repeated divisions'of the phellogen white, spongy tissue from the lenticels of of flooded lenticels produced long files of woody stems immersed in water, and attri­ cells that pushed the filling tissue outwards. buted its formation to the hygroscopic prop­ After 9 DF the activity of the phellogen erties of the periderm. He cited previous work extended beyond the lenticels. When large, by De Candolle and other authors. KUster extensive areas of phellogen were involved, (1925) used the term 'hyperhydric tissue' HHT formed patches of short tissue covering (HHT) to designate the tissue formed by most of the stem surface. When the activity plants exposed to a water-saturated environ­ of the phellogen was restricted to a small ment. He claimed that HHT can be formed by area, long columns of HHT were produced stems, roots, leaves, and flowers, and that instead. In one case, in a stem flooded for 80 immersion in liquid water is not necessary for days, the formation of a new phellogen im­ its formation, since some species are able to mediately below the old one was observed. produce it when exposed to mist. It was not Cells produced centrifugally by the active until 1967 that the term HHT was used again phellogen of flooded seedlings were thin­ by Ginzburg, in his study on adventitious walled, not suberised, without nuclei, radial­ root formation in twigs of Tamarix aphylla L. ly elongated or with irregular shape, firmly immersed in water. He showed that adven­ connected by their tangential walls, but with titious roots originated from HHT as a re­ only a few points of contact with neighbour­ sponse to flooding ing cells by their radial walls, mainly by knob­ Several authors studying the responses of like projections. After 22 DF the cortical pa­ woody plants to flooding have reported the renchyma and rays of the secondary phloem formation of 'hypertrophied lenticels' in stems started to take part in HHT formation, pro­ immersed in water for several days (Hook ducing new, larger cells, rich in starch grains, et al. 1970; Teskey & Hinckley 1978; New­ with large aerenchyma spaces thilt greatly some et al. 1982; Hook 1984; Angeles et al. increased bark thickness and por~~ity. 1986; Topa & McLeod 1986). Claims have Key words: Hyperhydric tissue, flooding, been made of lenticel proliferation in flooded lentice1s, hypertrophy, hyperplasia, peri­ woody stems (Newsome et al. 1982; Topa derm, cortex, secondary phloem. & McLeod 1986; Tsukahara & Kozlowski

* Paper extracted from the author's doctoral dissertation.

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1985). It is suspected that HHT fonnation of For observations with the dissecting micro­ a spongy tissue by flooded plants improves scope, the bark surfaces of flooded stem por­ aeration of the immersed organs (Annstrong tions were illuminated with a fibre optics 1972, 1979; Angeles et al. 1986; Glinski & lamp. For observations with the scanning Stepniewski 1985; Hook 1984; Hook & electron microscope, tangential segments of Brown 1972; Hooketal.1972; Kawase 1981; control stems and treated stems were cut off, Koslowski 1984; Topa & McLeod 1986). freeze-dried, mounted on aluminum stubs, sputter-coated with gold-palladium, and ob­ Materials and Methods served with an ETEC SEM at 10 kv. Populus tremuloides Michx. seedlings (par­ Stem segments 1 cm long, from treated ent material: female from Clinton Co., New and control specimens, were washed in run­ York; male from Oneida Co., New York) ning tap water to remove the fixative and were obtained from the Forest Experimental softened for 24 hours in a 4% aqueous solu­ Station of the College of Environmental Sci­ tion of ethylene diamine (EDA) for 24-48 ence and Forestry at Syracuse, New York, hours under mild vacuum (Carlquist 1982). and grown in 15 x 15 cm plastic pots in a When soft enough to be cut with a razor 1 : 2 : 1 : 1 (by volume) mixture of top soil, blade, the stem segments were washed in peat moss, vermiculite, and perlite. The seed­ running tap water until the yellow colour of lings were acclimatised for one month in the the EDA disappeared. The softened stem greenhouse before starting the experiments. segments were dehydrated with a graded During this time, the seedlings were watered series of tertiary butyl alcohol (Berlyn & every day. When they were ten weeks old, a Miksche 1976), infiltrated and embedded group of 37 healthy seedlings were chosen with 65°C paraffin. Cross and longitudinal for unifonnity of height and number of sections, 8-10 Ilm thick, were made with a leaves, and randomly divided into two groups: rotary microtome. Sections were stained with one (with 21 seedlings) to be flooded, and a combination of tannic acid, ferric chloride, the other one (with 16 seedlings) to be used saffranin, and fast green (Gurr 1965). Un­ as controls. Pots containing the seedlings to stained cross sections were used for histo­ be flooded were placed in a large fish tank chemical tests. To detect suberin in the bark, filled with enough water to cover the root sections were mounted directly in a solution system and the adjacent 6 cm of stem. Water of Sudan III dissolved in equal parts of 95% in the tank was replenished every day to alcohol and glycerine (Clark 1981) and ex­ compensate for evapotranspiration. Control amined under the light microscope. To test seedlings were watered every other day. for starch, sections were mounted directly in Experimental seedlings were flooded for Lugol's solution (Gurr 1965) and observed 5, 9, 15, 22, 30, 45, and 80 days (DF). At under the light microscope. the end of each flooding period three seed­ lings were harvested. The 6 cm long flooded Results stem segments were subdivided into six 1 cm long pieces, and fixed in FAA under a mild Morphological changes vacuum for 24 hours to ensure a good fixa­ HHT development on flooded stem sur­ tion (Berlyn & Miksche 1976). Two control faces is illustrated in Figures 1-3. Its fonna­ seedlings were harvested at the beginning of tion was first observed macroscopically on the experiment, and two more at the end of lenticels of seedlings flooded for 9 days. At each experimental period. A portion 6 cm 15 DF swelling of lenticels by HHT broke long, measured from above the soil line, was the surrounding peridenn (Fig. Id). Neigh­ subdivided and fixed in the same way as the bour lenticels coalesced, fonning long fis­ treated ones. sures in the peridenn (Fig. Ie & h). Figure If The morphological changes occurring in shows an area of the peridenn through which the submerged portions of flooded stems a white, spongy tissue (HHT) is protruding. were observed with a dissecting microscope Figure 3b is a surface view of such tissue and a scanning electron microscope (SEM). with the SEM, where remnants of thick- and

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Fig. 1. Changes in the bark surface of flooded stems. - a: Control. - b-h: Stems flooded for 5, 9, 15,22,30,45, and 80 days, respectively. The arrow at lfpoints to a swollen lenticel. A SEM view of this same lenticel is shown in Fig. 3b. Each small division in the scale represents 1 mm.

thin-walled cells can be seen. HHT had cov­ the periderm remained intact (Fig. 3c), or it ered most of the stem surface in Figure lh, was formed on an extended area of the peri­ which was flooded for 80 days. derm, forming patches of a short, porous Figures 2 and 3 are SE micrographs of tissue (Fig. 3d). stem surfaces. A controllenticel is shown in Figure 2a, where it is seen as an almost flat, Anatomical changes smooth structure, with some small fissures Bark anatomy of control stems - The (arrow). Figure 2b shows a lenticel that was phellogen of control stems is composed of a immersed in water for 5 days. The cells form­ single layer of cells, producing 4-7 rows of ing the filling issue (f) had swollen. As flood­ phellem (cork) cells outwards, and a single ing proceeded, the filling tissue swelled even layer of phelloderm inwards (Fig. 4a). Histo­ more, projecting farther from the stem sur­ chemical tests with Sudan III showed that the face (Figs. 2c & d). Stems flooded for 22 days phellem cells of control stems are suberised. showed fissures in their periderms in areas The lenticels are formed by several layers of not related to lenticels, from where HHT highly suberised, tannin-rich cells, forming a emerged (Fig. 3a). After this stage, HHT for­ filling tissue; a single layer of phellogen, and mation followed two different patterns, as several layers of phelloderm, which is com­ illustrated in Figures. 3c & d. In some cases, posed of small, isodiametric cells (Fig. 4a, long, slender columns of HHT were formed arrow). The primary cortex consists of sev­ on small areas of periderm, while the rest of erallayers of tangentially elongated, elliptical

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Fig. 2. Changes in the bark surface of flooded stems (SEM). - a: Control stem with an intact lenticel. Arrow points to small fissures in the !enticel. Bar = 100 /.lm. - b: Lenticel on stem flooded for five days. The filling tissue (0 has swollen slightly. Arrow points to thin filaments (probably algae) growing on the stem surface. Bar = 100 /.lm. - c: Stem flooded for 9 days. The filling tissue (0 of the lenticel has been broken. Remnants of the epidermis (e) can be seen on the periderm (P). Bar = 200 /.lm. - d: Lenticel on stem flooded for 15 days. The filling tissue has swollen even more. Bar = 100 /.lm.

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Fig. 3. Changes in the bark surface of flooded stems (SEM). - a: Stem flooded for 22 days, with well-developed HHT (h) forcing its way through the periderm. Bar = 200 Jl1ll. - b: Surface of the swollen lenticel shown in Fig. If. At the centre of the electron micrograph several broken hyperhydric cells can be seen. The arrow points to remnants of some thick-walled cells. Bar = 20 Jl1ll. - c: Stem flooded for 45 days, forming long columns of HHT (h) perpendicularly to the stem surface. Most of the periderm remains intact. Bar = 500 Jl1ll. - d: HHT formed extensive patches on this stem flooded for 80 days. Compare with c. Bar = 200 ).lm.

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Downloaded from Brill.com10/04/2021 05:03:23AM via free access Angeles - Hyperhydric tissue in flooded seedlings 91 cells, with clusters of sclereids interspersed 6a), producing abundant HHT. The cortex be­ at the bottom of this tissue, with only small came aerenchymatous. Figure 6b is an en­ intercellular spaces. The cortex is separated largement of the region indicated by an arrow from the secondary phloem by a band of in Figure 6a. The cells produced by this ac­ primary phloem fibres. Rays in the second­ tive phellogen are radially elongated or have ary phloem are exclusively uniseriate, non irregular shapes, as the one pointed by an ar­ expanded. row. They are firmly attached to their neigh­ Bark anatomy of flooded seedlings - At bours only by their tangential walls. The 5 DF the cells of the filling tissue swelled radial contact with neighbouring cells is re­ (Fig. 4b), losing cell to cell contact. At the duced to a few points, mainly through knob­ same time, the phelloderm cells of the len­ like projections. The cells produced by this ticels divided anticlinally (Fig. 4b, arrow). At active phellogen had thin walls and were vir­ 9 DF the lenticular phellogen started to di­ tually empty. They lost their nuclei as soon vide periclinally, producing files several cells as they became separated from their mother long (Fig. 4c). Longer files of cells were· cells, and the cytoplasm, when present, was produced by the lenticular phellogen at 15 distributed parietally, forming a thin ring. DF. At this stage, the phellogen activity Also, they failed to produce suberin, as tests propagated around the lenticels, forming fila­ with Sudan III revealed. ments that lifted the layer of cork cells op­ Figure 7 shows a different pattern of HHT posing them (Fig. 4b). Between 22-30 DF formation that was observed in some flooded the cortical parenchyma cells produced abun­ seedlings. Here, a new phellogen was form­ dantly larger, rounded cells, loosely connect­ ed in a small area of the periderm, producing ed to neighbouring cells (except when the an extraordinary number of cells outwards, neighbour was a sclereid, to which they re­ forming a large column of HHT. The column mained firmly attached) (Fig. 5a-c). At the shown in Figure 7 was lined with bacteria same time, rays in the secondary phloem (darker cells at the periphery of the column). formed larger cells, and in some cases, ex­ panded rays up to five cells wide were Discussion and Conclusions formed (Fig. 5c). The cortical and phloem Even though HHT formation was initiated parenchyma cells formed at 30 DF were rich in the lenticels of flooded seedlings, its for­ in starch grains (Fig. 5c), as revealed by mation was not restricted to them, since the Lugol's test. These anatomical changes great­ flood-induced phellogen activity rapidly prop­ ly increased bark thickness and porosity of agated beyond the lenticels. As flooding pro­ flooded stems (compare Fig. 4a with 5a). ceeded, inner bark tissues (cortex, secondary Figure 6 illustrates the anatomical changes phloem parenchyma) took part in HHT for­ in a stem flooded for 80 days. The phellogen mation. Both hypertrophy (increase in vol­ activity extended to cover a large stem ume through cell expansion) and hyperplasia portion (dark line pointed by an arrow in Fig. (increase in volume through cell prolifera-

Fig. 4. Anatomical changes in the bark of flooded stems. - a: Cross section of control stem showing an intact lenticel. The arrow points to a group of small isodiametric cells, with dense cytoplasm just below the lenticel. The heavily staine4 cells of the lenticel form the filling tissue (t). Bar = 100 j.Lm. - b: Cross section of stem flooded for 5 days. The filling tissue (t) has swollen, causing the filling tissue and phellogen to break. A group of cells underneath the len­ ticel (arrow) had divided anticlinally. Bar = 100 j.Lm. - c. Cross section of stem flooded for 9 days. The phellogen underneath the lenticel produced files several cells long, pushing the filling tissue (t) in front of them. Bar = 200 j.Lffi. - d: Cross section of stem flooded for 15 days. Long files of HHT (h) were produced by repeated periclinal divisions of the phellogen. The phellogen activity has extended to other regions beyond the lenticel. Arrow points to remnants of filling tissue of the originallenticel. Bar = 200 j.Lm.

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Fig. 5. Anatomical changes in the bark of flooded stems. - a: Cross section of a stem flooded for 30 days. The cortex and secondary phloem rays had produced HHT. Long files of cortical cells loosely attached to other files, but remained firmly attached to sclereids (vertical arrows). Tilted arrows show two expanded rays in the secondary phloem. Bar = 200 11m. - b: Expansion of secondary phloem (Ph) rays was restricted by the primary phloem fibres (f). The upper tilted arrow shows an uniseriate ray between two clusters of fibres. The lower tilted arrow points to an expanded ray in an area devoid of fibres. Bar = 100 11m. - c: Expanded ray in the secondary phloem (Ph) of a stem flooded for 30 days. The arrow points to the place where the ray started to expand The bright dots in the parenchyma cells are starch grains. Bar = 50 11m.

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Fig. 6. Anatomical changes in the bark of flooded stems. - a: Cross section of stem flooded for 80 days. The activity of the phellogen (arrow) has extended considerably, The cortical paren­ chyma also produced HHT, increasing bark thickness and porosity. Bar = 200 ~m. - b: En­ largement of the portion of meristem indicated by an arrow in a. The cells produced by this meristem are elongated or irregularly shaped (arrow), thin-walled, lack nuclei, and have scarce cytoplasm distributed parietally. Bar = 25 ~.

tion) took place during HHT formation. For plants, either exclusively by gas entrapment these reasons, it is recommended here to dis­ or by a combination of entrapment and in­ continue the use of the term 'hypertrophied creased production. Cell wall expansion and lenticels' in favour of the more accurate term proliferation taking part in HHT formation can 'hyperhydric tissue' to designate that tissue be attributed to the action of ethylene. For cell formed in immersed stems of woody plants. expansion to take place, increased cell wall A direct mechanism by which flooding extensibility and turgor pression must occur. could induce cell proliferation and elongation Kawase (1981) showed that ethylene causes is not known. However, an inq.irect mecha­ an enhancement of cellulase activity, soften­ nism by which water could proouce such ef­ ing the cell wall. Ridge (1987) suggested that fects is by entrapping ethylene within the tis­ ethylene, in combination with auxin, decreas­ sues. Since diffusion of this gas in water is es cell wall pH, increasing cell wall extensi­ 10,000 times slower than in air (Glinski bility, while gibberellic acid increases the os­ & Stepniewski 1985), ethylene can build up motic potential of the protoplast, providing quickly within the tissues of flooded plants. the 'driving force' for cell expansion. Ridge Ridge (1987) concluded that this gas accu­ (1987) hypothesised that when cells extend to mulates considerably in water immersed some extent, the rate of cell division increases.

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Fig. 7. Columnar HHT. A new phellogen (n) has fonned in this stem flooded for 80 days. Resi­ dues of an old phellogen (0) can be seen. In this case, the phellogen activity is restricted to a small area, producing HHT mostly outwards, forming a large column 0.5 cm long. Bar = 200 Ilm.

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There is some evidence of anatomical Bary, Anton de. 1884. Comparative anatomy changes induced in stems of woody plants of the Phanerogams and Ferns. English exposed to exogenous ethylene closely re­ translation by F.O. Bower & D.H. Scott, sembling those observed in the present study. Clarendon Press, Oxford. Twigs of Pyrus malus var. translucent ex­ Berlyn, G.P. & J.P. Miksche. 1976. Bota­ posed to ethylene under a bell jar for several nical microtechnique and cytochemistry. days (number of days not specified) formed a Iowa Univ. Press, Ames, Iowa. white, soft tissue resembling HHT that caus­ Clark, G. (ed.) 1981. Staining procedures. ed the periderm to split longitudinally (Wal­ 4th. ed. Williams & Wilkins, Baltimore, lace 1926). Yamamoto et al. (1987) applied London. ethrel (an ethylene releasing agent) around Ginzburg, C. 1967. Organization of the ad­ the stem of Ulmus americana seedlings, ventitious root apex in Tamarix aphylla. causing proliferation of cortical cells and Am. J. Bot. 54: 4-8. large aerenchyrna spaces in the bark. Glinski, J. & W. Stepniewski. 1985. Soil aer­ KUster (1925) reported HHT formation in ation and its role for plants. CRC Press, other organs besides the stem. He observed Inc. Boca Raton, Fla. this phenomenon in water immersed leaves Gurr, E. 1965. The rational use of dyes in of Ficus elastica, and claimed that roots, biology. Leonard Hill, London. leaves, and flowers of other species also are Hook, D.D. 1984. Adaptations to flooding able to form HHT when immersed in water or with fresh water. In: Flooding and plant exposed to a water mist. Since several physi­ growth (T. T. Kozlowski, ed.): 265-294. ological changes are involved in the process Acad. Press, New York. of HHT formation, this phenomenon deserves Hook, D.D. & C.L. Brown. 1972. Permea­ more attention than it has received so far. bility of cambium to air in trees adapted to wet habitats. Bot. Gaz. 133:.304-310. Aclmowledgements Hook, D.D, C.L. Brown & P.P. Korma­ The author wishes to thank Drs. Hector nik. 1970. Lenticels and water root devel­ Hernandez (Botany Department, Instituto de opment of swamp tupelo under various Biologia, Universidad Nacional Aut6noma flooding conditions. Bot. Gaz. 131: 217- de Mexico) and Mark Engleman (Centro de 224. Botanica, Colegio de Postgraduados, Cha­ Kawase, M. 1981. Anatomical and morpho­ pingo, Mexico) for their valuable suggestions logical adaptations of plants to waterlog­ on the original manuscript. Research facilities ging. HortScience 16: 30-34. were provided by the State University of the Kozlowski, T. T.1984. Responses of Woody New York College of Environmental Science Plants to Flooding. In: Flooding and plant and Forestry at Syracuse, N. Y. This research growth (T.T. Kozlowski, ed.): 129-163. was supported entirely by the Universidad Acad. Press, New York. Nacional Aut6noma de Mexico and the Na­ KUster, E. 1925. Pathologische Pflanzen­ tional Council of Science and Technology anatomie. 3rd. ed. Gustav Fischer, Jena. (CONACYT) of Mexico. Mohl, H. von. 1832. Sind die Lenticellen als Wiirzelknospen zu betrachten? Flora 15: References 65-74. Angeles, G., R.F. Evert & T.T. Kozlow­ Newsome, R. E., T. T. Kozlowski & Z. C. ski. 1986. Development of lenticels and T~ng. 1982. Responses of Ulmus ameri­ adventitious roots in flooded Ulmus ame­ cana seedlings to flooding of soil. Can. J. ricana seedlings. Can. J. For. Res. 16: Bot. 60: 1685-1695. 585-590. Ridge, I. 1987. Ethylene and growth control Armstrong, W. 1972. A re-examination of in amphibious plants. In: Plant life in the functional significance of aerenchyma. aquatic and amphibious habitats (ed. Physiol. Plant. 27: 173-177. R.M.M. Crawford). Special Publ no. 5 Armstrong, W. 1979. Aeration in higher plants. of the British Ecological Society: 53-76 Adv. Bot. Res. 7: 225-332. Blackwell Sci. Pub!., London.

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Teskey, R.O., & T.H. Hinckley. 1978. Im­ Tsukahara, H. & T. T. Kozlowski. 1985. Im­ pact of water level changes on woody portance of adventitious roots to growth riparian and wetland communities. Vol. I. of flooded Platanus occidentalis seedlings. Plant and soil responses. Biological ser­ Plant & Soil 88: 123-132. vice program. Fish and Wildlife Service. Wallace, R.H. 1926. The production of in­ U.S. Dept. of the Interior. tumescences upon apple twigs by ethylene Topa, M.A. & W. McLeod. 1986. Aerenchy­ gas. Bull. Torrey Bot. Club 53: 385-402. rna and lenticel formation in pine seed­ Yamamoto, E, G. Angeles & T. T. Koz­ lings: a possible avoidance mechanism to lowski. 1987. Effect of ethrel on stem anaerobic growth conditions. Physiol. anatomy of Ulmus americana seedlings. Plant. 68: 540-550. IAWABull. n.s. 8: 3-9.

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