Part II Wetland Plants: Adaptations and Reproduction L1372 - Chapter 4 04/25/2001 9:34 AM Page 85 © 2001 by CRC Press LLC 4 Adaptations to Growth Conditions in Wetlands I. Introduction The greatest difference between wetland and upland plants is the ability of rooted wetland plants to survive in saturated soil. In addition, submerged plants grow with little or no exposure to the atmosphere, and exhibit adaptations to low light and low carbon dioxide levels in the water column. Free floating plants, able to absorb dissolved nutrients directly from the water, thrive without anchoring roots. While many wetland plant adaptations are unique to the wetland habitat, some are also found in upland plants, such as the enhance- ment of nutrient uptake through nitrogen fixation, or various defenses against herbivores. In this chapter, we describe the fate of an upland plant when subjected to anoxic sedi- ments, as well as the many adaptations that have evolved in wetland plants as a result of anoxia. We also discuss plant adaptations to high salt and sulfide concentrations in salt marshes and mangrove forests. We give examples of adaptations that allow for improved nutrient uptake or nutrient conservation. We describe adaptations of submerged plants to life underwater, the defenses some wetland plants have developed against herbivory, and finally, wetland plants’ adaptations to water shortages. A. Aerobic Respiration and Anaerobic Metabolism Every plant cell requires oxygen for aerobic respiration. A green plant produces more oxy- gen than it needs during daylight hours; however, the oxygen produced during photo- synthesis diffuses away from the plant and very little of it is transported to the root tips. As a consequence, the foliage of plants must take in oxygen from the atmosphere, and the roots of plants in drained soils must take in oxygen from the soil pore spaces. During aerobic respiration, the 6-carbon glucose molecule produced during photosyn- thesis is broken down to a pair of 3-carbon molecules of pyruvate in glycolysis. When oxy- gen is available, pyruvate is completely oxidized to carbon dioxide. In this process, ATP is formed from ADP and phosphate. In aerobic respiration, the oxidation of one molecule of glucose results in the optimal net yield of 36 ATP molecules. An active cell requires more than 2 million molecules of ATP per second to drive its biochemical machinery. If the pro- duction of ATP completely shuts down, the cell, and eventually the plant, will die. In the absence of oxygen, plant cells undergo anaerobic metabolism, or alcohol fermenta- tion. Glycolysis occurs as in aerobic respiration, but the resulting pyruvate molecules are broken down first into acetaldehyde and then into ethanol and CO 2 . Thus, the chain of major products of anaerobic metabolism is glucose → pyruvate → acetaldehyde → ethanol. L1372 - Chapter 4 04/25/2001 9:34 AM Page 87 © 2001 by CRC Press LLC In anaerobic metabolism, only two molecules of ATP are produced per molecule of glu- cose, and cell activities such as cell extension, cell division, and nutrient absorption decrease or stop altogether (Raven et al. 1999). Plants that cannot tolerate long periods of flooding-induced anaerobiosis usually die due to insufficient energy (ATP) generation to sustain cell integrity (Vartapetian and Jackson 1997). B. Upland Plant Responses to Flooding Much of the research on plants under the stresses of anaerobiosis has been done using crop plants, especially tomatoes, maize, and rice. In tomatoes, maize, and other upland crop plants, some of the signs of stress due to waterlogged sediments begin to appear within minutes to hours. When the roots lack oxygen, the plant’s ability to transport water decreases, leading to a decrease in water uptake and a wilted appearance. The stomata close to decrease water loss and, subsequently, photosynthetic activity decreases. In some species, the plant hormone ethylene stimulates hypertrophy, or swelling at the stem base. Hypertrophy expands the gas spaces in the stem base and may aid in the diffusion of gases to the roots. Another sign of stress is epinasty, or non-uniform elongation of cells, in which the cells on the upper side of a leaf petiole elongate at a faster rate than the cells on the lower side. Epinasty may provide an advantage in water conservation, as it tends to decrease direct insolation of leaf surfaces. Plant cells deprived of oxygen convert to anaerobic metabolism. Ethanol is the main product of anaerobic metabolism, with lactic acid and alanine produced to a lesser extent. During anaerobic metabolism, ATP production decreases, leaving less energy available for the maintenance of cellular pH and the transport of ions. The optimum pH for the activity of many plant enzymes is 7, so as the pH declines (due to processes discussed in Section II.B.2.b, Davies’ Hypothesis), cell metabolism is disturbed. This condition, called cytoplas- mic acidosis, is a secondary effect of the absence of oxygen in root cells (Roberts et al. 1984). The ultimate cause of plant death in flooded soils is drastically reduced ATP production which shuts down the cell’s metabolism (Crawford 1993; Jackson 1994; Lambers et al. 1998). II. Adaptations to Hypoxia and Anoxia A number of adaptations allow wetland plants to sequester oxygen or tolerate the conse- quences of low oxygen levels. We start our discussion with the structural adaptations that affect wetland plants’ oxygen supply. The most common adaptation is the formation of aerenchyma (porous tissue) in the shoots and roots. We also discuss other root and shoot adaptations, as well as the mechanisms by which oxygen moves through the plants and into the root zone. Following our discussion of structural adaptations, we cover plant metabolic responses to anoxia and some of the research in this field. A. Structural Adaptations 1. Aerenchyma Virtually all rooted wetland plants form internal gas-transport systems made up of large gas-filled spaces called lacunae (Crawford 1993). The lacunae are held together in a porous tissue referred to as aerenchyma (the most commonly used term), aerenchymous tissue, or L1372 - Chapter 4 04/25/2001 9:34 AM Page 88 © 2001 by CRC Press LLC aerenchymatous tissue. Gases are transported throughout the plant along the channels formed by aerenchyma and there is little or no resistance to gas movement (Figure 4.1; Laing 1940; Armstrong 1978). In emergent wetland plants, oxygen enters the aerial parts of the plant via stomata in leaves, and via lenticels in stem or woody tissue. It travels toward the roots through aerenchyma, usually via diffusion. Carbon dioxide follows the opposite route, moving upward from the roots where it is produced as a by-product of res- piration, through the aerial portion of the plant, where it is released into the atmosphere through the stomata (Armstrong 1978; Topa and McLeod 1986). Aerenchyma forms in both new and old tissue in the roots, rhizomes, stems, petioles, and leaves of both woody and herbaceous wetland plants (Jackson 1989; Arteca 1997). In some species, such as Cladium mariscus (twig rush) and Spartina alterniflora (cordgrass), a continuous air space extends from the leaves to the roots (Teal and Kanwisher 1966; Smirnoff and Crawford 1983). a. Aerenchyma Formation Aerenchyma forms in flood-tolerant species and, to a lesser extent, in many flood-intol- erant species (Armstrong 1978, 1979; Crawford 1982; Justin and Armstrong 1987). In FIGURE 4.1 Cross-sectional electron scanning micrographs of the roots of six wetland macrophytes showing large air spaces, or aerenchyma. (A) Isoetes lacustris, (B) Littorella uniflora, (C) Luronium natans, (D) Nymphoides peltata, (E) Nymphaea alba, (F) Nuphar lutea. Bars represent 100 µm. (From Smits et al. 1990. Aquatic Botany 38: 3–17. Reprinted with permission; photos courtesy of G. van der Velde.) L1372 - Chapter 4 04/25/2001 9:34 AM Page 89 © 2001 by CRC Press LLC flood-intolerant plants, the spaces may occupy 10 to 12% of the total root cross-sectional area, but in flood-tolerant plants, the total area of gas spaces may be over 50 to 60% of the root area (Smirnoff and Crawford 1983; Smits et al. 1990). The volume of aerenchyma varies considerably among species, but porosity is generally greater in emergent than in submerged plants (Sculthorpe 1967). Aerenchyma forms in two ways: 1. By cell wall separation and the collapse of cells, known as lysigeny 2. By the enlargement and separation of cells (without collapse), known as schizogeny In lysigeny, the cells disintegrate and the total number of cells in the air spaces is reduced. Lysigeny is more common than schizogeny (Arteca 1997). Smirnoff and Crawford (1983) noted lysigeny in Mentha aquatica, Ranunculus flammula, Potentilla palus- tris, Juncus effusus, Narthecium ossifragum, Glyceria maxima, and G. stricta and in some members of the Cyperaceae (sedge family), including Eriophorum vaginatum, E. angusti- folium, Carex curta, and Trichophorum cespitosum. In schizogenous plants, the number of cells is not reduced, but a honeycomb structure is produced by the enlargement of intercellular spaces. The cells move farther from one another, thus creating space, but do not disintegrate (Arteca 1997). Schizogeny has been observed in Caltha palustris, Filipendula ulmaria (Armstrong 1978; Smirnoff and Crawford 1983) and Rumex maritimus (Laan et al. 1989). The precise mechanism of aerenchyma formation is not entirely defined, but the gaseous plant hormone, ethylene, is clearly involved. When a chemical inhibitor is used to stop ethylene production, aerenchyma formation also stops (Arteca 1997). Low oxygen levels stimulate the production of the enzyme, 1-aminocyclopropane-1-carboxylate (ACC) synthase, which in turn brings about increased levels of another enzyme, ACC oxidase. ACC oxidase is directly responsible for ethylene production, which requires oxygen. ACC oxidase diffuses throughout the plant and ethylene is produced in the aerated plant parts (Jackson 1994; Arteca 1997). Ethylene normally diffuses away from plants, but diffusion is inhibited when the plant is surrounded by water. As ethylene accumulates, it stimulates cell rupture, cell wall degeneration, and an increase in the activity of compounds that degrade cell walls (Vartapetian and Jackson 1997). The amount of porosity in plant tissues increases with increasingly reduced conditions. Smirnoff and Crawford (1983) noted that several flood-tolerant species formed aerenchyma at the onset of waterlogging, and that porosity increased as the soil redox potential decreased. Plants taken from fens, bogs, and a reed swamp had from 1.2 to 33.6% porosity after 11 weeks of waterlogging, and as waterlogging time increased to 32 weeks, the percent porosity increased up to 50% in some species (Eriophorum vaginatum and E. angustifolium). As the soil water content increased from 70 to 90%, the root porosity of Senecio aquaticus increased from 10 to 35% (Figure 4.2). Lacunal space increases with increasing sediment anaerobiosis in other herbaceous plants as well, notably in the seagrass, Zostera marina (Penhale and Wetzel 1983), and in Salicornia virginica (Seliskar 1987), Oryza sativa (deep water rice; Kludze et al. 1993), Spartina patens (salt marsh hay; Burdick and Mendelssohn 1990; Kludze and DeLaune 1994), Cladium jamaicense (sawgrass) and Typha domingensis (cattail; Kludze and DeLaune 1996). Taxodium distichum (bald cypress) also forms more aerenchyma under increasingly reduced conditions (Kludze et al. 1994). Some plants, such as Oryza sativa, Schoenus nigricans, and some Juncus (rush) species, form aerenchyma even in well-aerated soils as a part of ordinary root development. This suggests that the formation of aerenchyma in these plants has a genetic component and L1372 - Chapter 4 04/25/2001 9:34 AM Page 90 © 2001 by CRC Press LLC does not require ethylene accumulation (John et al. 1974; Jackson et al. 1985; Justin and Armstrong 1987; Jackson 1990). b. Aerenchyma Function Aerenchyma decreases the resistance to flow encountered by oxygen and other gases in plant tissue, allowing oxygen to reach the buried portions of the plant relatively unimpeded (Vartapetian and Jackson 1997). Aerenchyma also allows plant-produced gases such as car- bon dioxide and ethylene to escape into the atmosphere (Visser et al. 1997). Aerenchyma is effective in aerating the roots and rhizomes of wetland plants; however the aeration is often incomplete. Even with extensive aerenchyma, roots may suffer some degree of anoxic stress and shift to anaerobic metabolism (Saglio et al. 1983). At the beginning of the growing sea- son, the roots and rhizomes of some emergent species experience oxygen deficiency until their new shoots arise and connect them to the atmosphere (Burdick and Mendelssohn 1990; Koncalova 1990; Naidoo et al. 1992; Weber and Brandle 1996). Aerenchyma also provides storage for gases. The gas storage capacity of herbaceous plants is limited, however, and can be depleted in minutes to hours. New inputs from the atmosphere are required to sustain the plant’s oxygen needs. In general, the more air space within the plant, the greater its storage capacity, and monocots tend to have greater poros- ity and storage capacity than eudicots (Crawford 1993). In Typha latifolia (broad-leaved cattail), approximately half of the total leaf volume is occupied by gas spaces and the inter- nal leaf concentration of CO 2 is up to 18 times greater than atmospheric levels (Constable et al. 1992). The internal CO 2 is assimilated by the leaves and provides the plant with a sig- nificant carbon supplement (Constable and Longstreth 1994). 2. Root Adaptations Besides the formation of aerenchyma, wetland plants may undergo other root changes in response to flooded conditions. Among these are the development of adventitious roots (roots that arise from other than root tissues) and shallow rooting (Laan et al. 1989; Koncalova 1990). In woody plants, other root adaptations include pneumatophores, prop roots, and drop roots. a. Adventitious Roots Within a few days of flooding, some plants form adventitious roots that grow laterally from the base of the main stem. They spread into the surface layers of the soil or grow above the soil surface. In standing water, adventitious roots are in contact with oxygen-containing FIGURE 4.2 The relationship between soil water content and the porosity of the root systems of Senecio aquaticus plants growing in a peatland of the Orkney Valley, United Kingdom. (From Smirnoff, N. and Crawford, R.M.M. 1983. Annals of Botany 51: 237–249. Redrawn with permission.) L1372 - Chapter 4 04/25/2001 9:34 AM Page 91 © 2001 by CRC Press LLC water, while in areas of saturated soil with no standing water, adventitious roots are in con- tact with the air. Adventitious roots replace the roots of deeper soil layers that have died due to anoxia. With fewer roots belowground, less root biomass needs to be aerated (Ernst 1990; Arteca 1997; Vartapetian and Jackson 1997). Adventitious roots form in many herbaceous wetland plants such as Rorippa nasturtium- aquaticum (=Nasturtium officinale; water cress; Sculthorpe 1967), Cladium jamaicense, Typha domingensis (Kludze and DeLaune 1996), and various species of Rumex (Laurentius et al. 1996). Adventitious roots have aerenchyma, and the entire stem/root system forms a highly porous continuum (Vartapetian and Jackson 1997). Adventitious roots form in many flood- tolerant tree and shrub species, including Salix species, Alnus glutinosa, Cephalanthus occiden- talis, Pinus contorta,Thuja picata,Tsuga heterophylla, and Ulmus americana (Crawford 1993). The plant hormone, auxin, is involved in the formation of adventitious roots. In flood- tolerant Rumex species, the diffusion of auxin into oxygen-deficient roots is slowed and auxin accumulates at the root-shoot junction where adventitious roots form (Laurentius et al. 1996). Some studies have implicated ethylene in the formation of adventitious roots as well (Kawase 1971; Jackson et al. 1981), although results are contradictory (Jackson 1990; Arteca 1997; Vartapetian and Jackson 1997). Unlike aerenchyma, adventitious roots do not increase if the substrate becomes increasingly anoxic. They are simply triggered by an ini- tial flooding (Kludze and DeLaune 1996). Adventitious roots aid in water and nutrient uptake in flood-tolerant plants. They enhance nitrate availability to plants under anoxic stress because they are in contact with oxygenated soil, air, or water. When adventitious roots are cut daily as they emerge, leaf senescence and dehydration are accelerated and survival rates are decreased (Jackson 1990). In a number of monocots, the large surface area of adventitious roots enhances the FIGURE 4.3 The shallow roots of a tree growing in saturated soil. (Photo by H. Crowell.) L1372 - Chapter 4 04/25/2001 9:35 AM Page 92 © 2001 by CRC Press LLC rapid absorption of nutrients (Koncalova 1990). Adventitious roots also allow the end product of alcoholic fermentation, ethanol, to diffuse from the plant more easily, rather than accumulating in and near the plant (Crawford 1993). b. Shallow Rooting Both herbaceous and woody species tend to have shallower root systems when in flooded conditions (Figure 4.3). Surface or sub-surface roots are above the soil or in the oxygenated portion of the soil profile, thereby alleviating the problem of oxygen shortages in the roots. In a German salt marsh dominated by Aster tripolii and Agropyretum repentis, the highest root density was found in the soil sub-surface (0 to 8 cm; Steinke et al. 1996). Phragmites australis (common reed) also concentrates root growth at or near the soil surface when in flooded sed- iments (Weisner and Strand 1996). In a study in which Taxodium distichum saplings were con- tinuously flooded, only 6% of their total root mass was found below 30 cm in the soil profile. Periodically flooded saplings had 30% of their root biomass below 30 cm. The relatively shal- low rooting zone of the continuously flooded plants allows the roots access to nitrate and oxy- gen (Megonigal and Day 1992). Trees with shallow roots are sometimes felled by high winds and such uprooted trees (“tip-ups”) are an indicator of continuous soil saturation (Figure 4.4). c. Pneumatophores Pneumatophores are modified erect roots that grow upward from the roots of Taxodium distichum and some mangrove species. In T. distichum, pneumatophores are commonly called “knees.” Cypress knees rise out of the soil wherever water covers the soil surface for extended periods (Figure 4.5). Their height often corresponds to the mean high water level and the highest part of the knee is exposed to air much of the time. Most of the oxygen brought into the plant from the knees is consumed within the knee itself. There is little oxy- gen exchange between the knee and the roots so they do not aerate the subsurface roots. Cypress knees do have a role in gas exchange, however, since they release 3 to 22 times more carbon dioxide per unit area than an equivalent area of trunk surface and account for 6 to 21% of stem respiration (Brown 1981). FIGURE 4.4 An uprooted tree, or “tip-up,” indicating shallow rooting and saturated soil conditions. (Photo by H. Crowell.) L1372 - Chapter 4 04/25/2001 9:35 AM Page 93 © 2001 by CRC Press LLC In mangroves, there are several different types of pneumatophores, variously called pneumatophores, root knees, and plank roots (Figure 4.6; Tomlinson 1986): • The pneumatophores of Avicennia and Sonneratia species are erect lateral branches of the horizontal roots. They appear at regular intervals along the root (in Avicennia, every 15 to 30 cm). A single Avicennia tree may have up to 10,000 pneumatophores. In Avicennia, pneumatophores are usually less than 30 cm high while in Sonneratia they can be up to 3 m. The pneumatophores of Laguncularia spp. are erect and blunt-tipped and rarely exceed 20 cm in height. They do not grow in all Laguncularia populations and appear to be facultative. • The root knees of Bruguiera and Ceriops are actually horizontal roots which peri- odically re-orient and grow upward through the substrate. The tip of the upward growth forms a loop and then growth continues horizontally so that the root appears to curl its way in and out of the substrate. Branching occurs at the knees and new horizontal anchoring roots are formed. Some Xylocarpus species also have root knees that are localized erect growths on the upper surface of horizon- tal roots that can grow up to 50 cm. • The plank roots of Xylocarpus granatum are horizontal roots that become extended vertically and appear to be shallow roots half in and half out of the substrate. The roots curve laterally back and forth on the soil surface in a series of S-shaped loops. In all of these root systems, the aboveground component of the root ventilates the buried portion. The entire root system is permeable to the mass flow of gases, with atmos- pheric exchange occurring through lenticels in the aboveground portions of the roots. About 40% of the root system is gas space, so gases brought in through the lenticels move FIGURE 4.5 A “knee” of a Taxodium distichum (bald cypress) in the Florida Everglades (approximately 60 cm high). The height of cypress knees usually corresponds to the mean high water level. (Photo by H. Crowell.) L1372 - Chapter 4 04/25/2001 9:35 AM Page 94 © 2001 by CRC Press LLC freely. If the lenticels are blocked, the level of oxygen in the submerged roots falls and the roots become asphyxiated (Scholander et al. 1955; Tomlinson 1986; Crawford 1993). Upward growth from underground roots is reported in other woody wetland species, notably the shrubs Myrica gale, Viminaria juncea, and Melaleuca quinquenervia. When flooded, the roots reverse direction and grow upward, toward the soil surface, rather than away from it. Although these roots do not emerge from the soil surface like pneu- matophores, they have aerenchyma and allow the deeper roots to be aerated (Jackson 1990). d. Prop Roots and Drop Roots Rhizophora species (mangroves) form prop roots that develop from the lower part of stems and branch toward the substrate and drop roots that drop from branches and upper parts of the stem into the soil (Figure 2.8). Prop roots and drop roots are covered with lenticels that allow oxygen to diffuse into the plant and carbon dioxide and other gases to diffuse out. Both drop and prop roots branch and form feeding and anchoring roots. Feeding roots are shallow and fine with many root hairs that expand the surface area of the roots. Anchoring roots are thicker with a protective cork layer and extend as deep as 1 m into the substrate (Odum and McIvor 1990). Prop and drop roots give the plant stability, particu- larly in the face of tides and shifting substrates, and they increase the root surface area, thus improving aeration (Crawford 1993). 3. Stem Adaptations In addition to the ability of wetland plant stems to form aerenchyma, they exhibit other adaptations to avoid oxygen deprivation. Total submergence stimulates the stems of some wetland plants to grow rapidly toward the water surface in order to reach atmospheric oxygen. The stems of both herbaceous and woody plants sometimes swell at the base due to increased porosity (hypertrophy). The aerenchyma within the stems of many sub- merged and floating-leaved plants allows them to float near or at the water’s surface where they have access to oxygen, light, and carbon dioxide. a. Rapid Underwater Shoot Extension Rapid underwater shoot extension, or stem elongation, has been observed in many wet- land plants including Sagittaria pygmaea, S. latifolia, Nymphaea alba, Nymphoides peltata, FIGURE 4.6 The aerial roots of mangroves: (a) Avicennia, Sonneratia, and Laguncularia have horizontal roots buried in the substrate and from them arise erect lateral branches called pneumatophores. (b) Bruguiera and Ceriops have root knees that are upward growths from the horizontal roots. Branching occurs at the root knees and new horizontal anchoring roots form. (c) Some Xylocarpus species also have root knees but without the growth of new anchoring roots. (d) Xylocarpus granatum forms horizontal roots, called plank roots, that lie half in and half out of the sediments. The plank roots curve back and forth form- ing S-shaped curves; this is shown as if from above. (From Tomlinson, P.B. 1986. The Botany of Mangroves. London. Cambridge University Press. Reprinted with permission.) L1372 - Chapter 4 04/25/2001 9:35 AM Page 95 © 2001 by CRC Press LLC [...]... the Juncaceae (rushes) and Cyperaceae (sedges), two families that had previously been thought © 2001 by CRC Press LLC L1372 - Chapter 4 04/ 25/2001 9:35 AM Page 115 to be non-mycorrhizal (Sondergaard and Laegaard 1977; Clayton and Bagyaraj 19 84; Ragupathy et al 1990; Wigand and Stevenson 19 94; Rickerl et al 19 94; Wetzel and van der Valk 1996; Christensen and Wigand 1998; Cooke and Lefor 1998; Turner... 27 µmol O2 day-1 and at +200 mv, the roots released only about 20 µmol O2 day-1 (Figure 4. 15; Kludze et al 1993) Similarly, in Taxodium distichum seedlings, radial oxygen loss is greater under flooded conditions than drained Kludze and others (19 94) measured the loss of oxygen from T distichum roots as 4. 6 mmol O2 g dry weight-1 day-1 in flooded plants and 1 .4 mmol O2 g dry weight-1 day-1 in drained... oxygen, light, and carbon dioxide levels within the water column (Sculthorpe 1967) © 2001 by CRC Press LLC L1372 - Chapter 4 04/ 25/2001 9:35 AM Page 97 FIGURE 4. 7 The thickly buttressed base of Taxodium distichum (bald cypress) (Photo by H Crowell.) 4 Gas Transport Mechanisms in Wetland Plants Aerenchyma enables gases to move relatively easily between the aerial and subterranean portions of wetland plants... iron appears as rust-colored spots in the substrate and such plaques are often found in the vicinity of plant roots (Crowder and Macfie 1986; Howes and Teal 19 94; Wigand and Stevenson 19 94) Radial oxygen loss may not be sufficient in most herbaceous wetland plants to oxidize sulfide, which is found at very low redox levels (–75 to –150 mv) Sulfide diffuses into the root tissue and exposed plants must... the rhizomes, and then up the stems of the older leaves and back out © 2001 by CRC Press LLC L1372 - Chapter 4 04/ 25/2001 9:35 AM Page 98 FIGURE 4. 8 Passive diffusion of gases in wetland plants Oxygen diffuses along a concentration gradient from the atmosphere into the aerial plant parts and down the internal gas spaces to the rhizomes and roots Carbon dioxide produced by root respiration and methane... 1986) Salt glands have been observed in a number of salt marsh species including Distichlis spicata, Spartina alterniflora, S patens, S foliosa, S townsendii, and Limonium species (Anderson 19 74) In S alterniflora, salt glands selectively secrete Na+ relative to K+ (Bradley and Morris 1991a) Salt glands are abundant on the leaves of some mangrove © 2001 by CRC Press LLC L1372 - Chapter 4 04/ 25/2001 9:35... precipitation and dry atmospheric deposition as well as the weathering of rocks and soil minerals and the decomposition of organic matter In wetlands, decomposition is slow and nutrients tend to be bound in organic matter rather than mineralized If little surface drainage enters a wetland from surrounding uplands, the plants can be completely dependent on atmospheric sources of nutrients Wetlands with... 6 m s-1, the rhizome oxygen concentration increases to 90% of its potential maximum The proportion of oxygen that enters the rhizome via Venturi-induced convection may be quite significant in high winds or when the number of dead and broken shoots per unit length of rhizome is high (Armstrong et al 1992) © 2001 by CRC Press LLC L1372 - Chapter 4 04/ 25/2001 9:35 AM Page 102 FIGURE 4. 14 Venturi-induced... L1372 - Chapter 4 04/ 25/2001 9:35 AM Page 119 TABLE 4. 3 Types of Traps and Their Distribution among the Genera of Carnivorous Plants Type of Trap Pitfall Lobster pot Passive adhesive Active adhesive Bladder Snap-trap From Lloyd 1 942 and Lowrie 1998 Genus Heliamphora Sarracenia Darlingtonia Nepenthes Cephalotus Genlisea Byblis Drosophyllum Pinguicula Drosera Utricularia Polypompholyx Dionaea Aldrovanda... phosphorus, and other plant nutrients, such as nitrogen and potassium The mycorrhizae benefit from the association because the plant roots provide carbohydrates Mycorrhizae are common in upland plants and have been found to be associated with many wetland plants as well (Sondergaard and Laegaard 1977) There are two major types of mycorrhizae: endomycorrhizae and ectomycorrhizae (Fitter and Hay 1987; . Part II Wetland Plants: Adaptations and Reproduction L1372 - Chapter 4 04/ 25/2001 9: 34 AM Page 85 © 2001 by CRC Press LLC 4 Adaptations to Growth Conditions in Wetlands I. Introduction The. appears as rust-col- ored spots in the substrate and such plaques are often found in the vicinity of plant roots (Crowder and Macfie 1986; Howes and Teal 19 94; Wigand and Stevenson 19 94) . Radial. than drained. Kludze and others (19 94) measured the loss of oxygen from T. distichum roots as 4. 6 mmol O 2 g dry weight -1 day -1 in flooded plants and 1 .4 mmol O 2 g dry weight -1 day -1 in drained