Wetland Plant Morphology
Anaerobic conditions cause death in plants grown in saturated soils. Where flooding is predictable, species may time germination to coincide with low water levels. In other areas, specialized adaptations, such as rapid shoot elongation, adventitious root production, or shoot buttressing and fluting, are strategies to counteract oxygen depletion. Aerenchyma in shoots and roots allow oxygen to move from emergent to submerged organs and may diffuse out of roots oxidizing surrounding soils. Temperature gradients drive pressurized air movement within some plants. Shallow, extensive, intertwining roots can improve oxygen penetration and provide a belowground network, limiting hurricane damage. Prop roots and pneumatophores enhance oxygen movement in some mangrove plants. Floating mats are produced in areas experiencing subsidence. Solute imbalances can create conditions where water leaves the cytoplasm. Water loss can be prevented by production of salt barriers and nontoxic solutes in saline conditions, but also by confining CO2 production to night time.
KeywordsAnaerobic avoidance strategies Aerenchyma Root adaptations Adaptation to salt Photosynthesis
Roots out of soil in wetland plants:
The vast majority of plant species on Earth cannot survive long in water-saturated soils, not because of the water itself but because the soils are anaerobic. Oxygen diffuses into water 10,000 times slower than air. Therefore, saturated, or hydric, soils become anaerobic quickly, and without oxygen as the terminal electron acceptor in the electron transport chain, aerobic metabolism ceases in plant roots. Under anoxic conditions, terrestrial plants, and some wetland plants, will begin metabolizing through fermentation, whose end product (ethanol) is toxic and generally cause death within 24 h. Many wetland plants actually accelerate rates of fermentation to maintain adequate levels of ATP but release the ethanol to the environment and thus do not accumulate this toxic byproduct. An additional stress triggered by anaerobic hydric soils involves production of other toxic byproducts, as specialized microbes are able to substitute other electron acceptors in the electron transport chain, which become toxic when reduced, such as sulfate reduction to lethal hydrogen sulfide. Many species of wetland plants have evolved shoot and root adaptations that enable delivery of oxygen to the roots, avoiding the problems caused by anaerobic soils. In areas where the timing of flood events is predictable, many species have evolved timing mechanisms enabling seeds to germinate during draw down conditions.
Coastal wetlands experience the additional stress of salt water and have evolved adaptations to either exclude salt from entering the roots, sequester, or secrete it. These mechanisms involve preventing entry of salt at the root soil interface, producing nontoxic organic solutes to compensate for osmotic imbalance, packing the salt into vacuoles, xylem tissue, or old leaf tissue, or some species may excrete salts via salt glands.
Anaerobic Avoidance Strategies
General strategies used to avoid anaerobic metabolism include (1) shoot elongation to render a portion of the plant out of the water, (2) production of adventitious roots along the aerobic portions of the stem, (3) slowing or shutting down metabolism until floodwaters retreat, (4) buttressing or fluting of the basal shoot to increase surface area exchange, and (5) specialized root adaptations (see below).
The most ubiquitous adaptation enabling shoots to deliver oxygen to the roots is the creation of air spaces called aerenchyma. When the surrounding environment becomes hypoxic, the hormone ethylene is produced, which triggers production of the enzyme cellulase. This enzyme breaks down cellulose within both shoots and roots, creating honeycomb or snorkel-like air spaces. These airspaces allow oxygen to diffuse into the roots and from the roots to the pore space lining of surrounding soils to create an oxidized rhizosphere. In the oxidized rhizosphere, reduced toxic substances, such as ammonia and hydrogen sulfide, become oxidized into nontoxic nitrate and sulfate, respectively. Aerenchymous tissue may occupy up to 60% of a wetland plant’s root tissue volume (Mitsch and Gosselink 2007).
Root adaptations that enable aerobic metabolism include shallow but extensive rooting, prop root formation, and pneumatophore or air root production. In general, forested wetland tree species kill off their taproots when young and send shallow lateral roots far beyond their drip line. In addition to enhancing oxygen penetration, these extensive lateral roots form a tapestry with one another, rendering such species as baldcypress (Taxodium distichum) and water tupelo (Nyssa aquatica) highly resistant to hurricane damage; when basal areas reach greater than 30 m2 ha−1, these two canopy species also keep readily wind-thrown midstory and herbaceous vegetation intact (Shaffer et al. 2009).
To enhance oxygen penetration to the roots, red mangroves (Rhizophora spp.) develop aboveground roots called prop roots. These roots are densely topped with spongy air-filled lenticels that allow oxygen diffusion into the roots.
Black mangrove (Avicennia spp.) similarly contain thousands of pneumatophores, which are spongy extensions of the main roots (about 30 cm long), which climb vertically through the mud and are exposed during low tides. Both prop roots and pneumatophores greatly enhance root oxygenation, as demonstrated by experiments that blocked airflow using substances such as Vaseline®. This has not been demonstrated in the vertical root extensions, or knees, of baldcypress; it does appear, however, that the knees offer structural support during tropical storm events (Conner et al. 2012).
Perhaps the most intriguing root adaptation to chronic flooding is the development of floating marsh or floatant. The world’s large delta complexes undergo cycles of progradation and degradation as their rivers periodically switch paths. As a newly forming delta progrades, the abandoned delta subsides as its sediments compact and dewater. This subsidence leads to increased flood durations and at some point the vegetative community must “sink or swim.” Vegetative species that form floatant create gaseous roots and rhizomes such that the entire root mat detaches from the subsoil and essentially becomes a hydroponic marsh (Sasser et al. 1991). While these floating marshes can be very healthy ecosystems, they do not stand up to the strong winds of tropical storms. In high winds and strong storm surge, the floatant marshes roll up in carpet-like fashion and get thrown landward where they strand and die.
Thermo-Pressurized Gas Flow
Thermo-pressurized gas flow is a mechanism used by plants to actively move oxygen from shoots to roots. To achieve this, plants use the temperature and water pressure differentials between the exterior ambient air and the internal cortical tissue to create pressure gradients, which push oxygen through aerenchymous shoots into aerenchymous roots. Pressures generated during daytime peak temperatures are up to 30 times higher than those generated at night (Brix et al. 1992). Originally, humidity was thought to be the driving force in pressurized gas flow (Brix et al. 1992). However, this process can occur in trees even when the plant is dormant, so humidity is not likely to be the driving force. First discovered by Dacey (1980, 1981) in water lilies (Nuphar luteum), thermo-pressurized flow was once thought to be unique; however, it has since been demonstrated in several other hydrophyte (Brix et al. 1992) and wetland tree species (Grosse and Schroder 1984; Grosse et al. 1992). The route for gaseous transport may differ among species. Alnus glutinosa, for example, uses meristematic tissue in lenticels for pressurized gas flow, rather than leaf tissue. This pathway, however, is 8× less efficient than gas flow through leaf aerenchyma. Nevertheless, pressurized gas flow in mature trees occurs only when the tree is dormant, but in seedlings it occurs in leaves, before the seedling establishes aerenchyma (Mitsch and Gosselink 2007).
Adaptations to Salt
In general, wetland vascular plants are faced with the same challenges as single celled organisms when the outside aqueous environment has more solutes than inside the cell. The water potential of the outside solution is lower and water escapes the cytoplasm. A suite of wetland species have solved the problem by (1) eliminating salt (NaCl) at the root surface to maintain nearly fresh internal solutions, (2) increasing nontoxic organic compounds inside the cytoplasm to reverse osmotic imbalance, (3) isolating or eliminating salt that enters through the roots, and (4) using a photosynthetic pathway that minimizes fresh water requirements.
Mangroves of the genera Sonneratia, Rhizophora, and Laguncularia have tight membranes on the root surface that reduce the salinity of internal water to 1–1.5 ppt compared to 35 ppt of the outside sea water (Mitsch and Gosselink 2007). The mangrove Avicennia spp. is also able to greatly reduce the salinity internally but only to about 10% of sea water.
To prevent salt from becoming toxic internally, some wetland plant species produce nontoxic organic compounds to more than match the osmotic potential of the outside aqueous solution. These compounds are called osmotica and include prolene and beta glycine.
At the cellular level, vacuoles can serve as trash cans for sodium chloride. Salt also can be packed into old leaf tissue and the leaves excised. More elaborate mechanisms include the salt glands on the leaves of Spartina spp., which actively secrete sodium ions relative to potassium. Remarkably, baldcypress packs salt into last year’s dead xylem tissue. Baldcypress, which live over 1,000 years, can therefore be used to reconstruct historic tropical storm events (Hupp and Morris 1990).
C4 Photosynthetic Pathway
Wetland plants inhabiting saline environs face much the same “physiological drought” (Schimper 1903) problems as plants in arid environments; in both cases plants are rooted in a substrate with very low water potential, because of either dryness or high salt content. The dilemna from a water loss perspective is that vascular plants lose water by opening their stomata to capture carbon dioxide for carbon fixation in the photosynthetic process. Water is a much smaller molecule than carbon dioxide, so it readily diffuses out of open stomata. Water loss through evapotranspiration must be compensated by water uptake through the roots. An additional problem for species that rely exclusively on the C3 pathway involves the wasteful fixing of oxygen rather than carbon dioxide, a process known as photorespiration. C3 plants obtain the CO2 needed for the initial steps in photosynthesis during daylight hours. Photorespiration occurs because the initial enzyme in the C3 pathway, known as Rubisco (ribulose biphosphate carboxylase), has a relatively low affinity for carbon dioxide and also fixes oxygen to the 3-carbon sugar glycophosphate. Vascular plants with the additional C4 pathway solve the problem by initially fixing carbon dioxide at night. Fixation occurs via the enzyme PEP (phosphoenol pyruvate) carboxylase, which has a very high affinity for CO2. Wetland plant species that use the C4 pathway are far more efficient with water use per amount of carbon fixed.
- Conner WH, Krauss KW, Shaffer GP. Restoring coastal freshwater forested wetlands following severe hurricanes. In: Stanturf J, Lamb D, Madsen P, editors. A goal-oriented approach to forest landscape restoration. World forests 16. Dordrecht: Springer Science+Business Media; 2012. p. 423–42. doi:10.1007/978-94-007-5338-9_16.CrossRefGoogle Scholar
- Mitsch W, Gosselink JG. Wetlands. 4th ed. Hoboken: Wiley; 2007.Google Scholar
- Schimper AFW. Plant-geography upon a physiological basis. Oxford: Oxford at the Clarendon Press; 1903.Google Scholar