A wetland is a harsh environment physiologically. Aquatic plants can't deal with periodic drying and temperatures tend to be more extreme because the water's shallow terrestrial plants can't deal with long floods. Stresses include anoxia and wide salinity and water fluctuations. Adaptations to these conditions have an energy cost, either because the organisms cells are working less efficiently (conformers) or because it expends energy to protect cells from external stress (regulators).
Protists and Bacteria
Protists aren't plants, of course, but this is a convenient spot to put them. They are eukaryotic (membrane bound organelles), unicellular, colonial, or simple multicellular organisms, including the protozoa, algae, and slime molds. Bacteria are generally unicellular prokaryotes and not plants either, but they are vital to the detrital energy web of a wetland, so we'll include them here too.
Problems associated with flooding:
When an organic soil is flooded, the oxygen available is quickly depleted through metabolism by organisms that use oxygen. Because oxygen diffuses much more slowly through water than through air, atmospheric oxygen doesn't help.
The bacteria that survive in this situation need to be facultative anaerobes meaning they can switch from using aerobic respiration (using oxygen as an electron receptor in the process of producing energy) to anaerobic respiration (using something other than oxygen). Some bacteria will only grow under anaerobic conditions, using sulfate rather than oxygen. These are the ones that produce hydrogen sulfide which accounts for the rotten-egg smell one associates with wetlands.
In a freshwater aquatic or soil environment, the osmotic concentration of the cytoplasm in bacterial cells is higher than that of the surrounding medium. This allows the cells to absorb water until they develop turgor which means that the pressure of the cytoplasm is balanced by the resistence of the cell walls---the cells are firm, neither collapsing nor exploding.
Salt has two dangers, osmotic and toxic. The cell will dehydrate of explode, depending on outside salinity due to osmosis, unless it is adapted. Wetland bacteria generally maintain their salt balance by actively transporting some other ion (often potassium) across their membranes to maintain osmotic balance. Taking in sodium itself would not work because it is toxic at high concentrations.
Emergent plants are more complex than protists and bacteria, so they have a wider range of adaptations, but evolution has eliminated their capacity to use something other than oxygen as an energy source, i.e. they cannot respire anaerobically.
Plants are sessile (stuck in one place), but it's only their roots that are stuck in the anoxic or salty environment of wetland substrate. A flood sensitive plant when inundated rapidly loses its oxygen supply to the roots. This shuts down aerobic metabolism thus reducing nearly all metabolic activities such as cell division and nutrient absorption. Anaerobic glycolysis (same thing your muscles do when you ask them for a "burst" of energy) works for some needs inititally, but it is inefficient and toxic endproducts accumulate. These two problems will cause death fairly rapidly, as you know if you overwater your houseplants.
Hydrophytes (flood tolerant plants) have adaptations to survive these problems.
1. Structural Adaptations
The primary plant strategy in response to flooding is the development of air spaces in the roots and stems which allow diffusion of oxygen from the aerial portions of the plant into the roots. Thus the roots don't have to depend on getting oxygen from the soil. "Regular" plants may have a porosity (% air space in roots and stems) of 2-7% of their volume, while a wetland plant may be up to 60% pore space by volume.
Water Lilies offer an example: air moves into the internal gas spaces of young leaves on the water surface and is forced down through the aerenchyma of the stem to the roots by the slight pressure caused by the heating of the leaves. The older leaves lose their capacity to support pressure gradients so gas from the roots returns out through the old leaves.
b. adventitious roots
roots above the ground (or the anoxic zone or the level of the water) which are able to function normally in an aerobic environment
ex: prop roots on mangroves have numerous small pores called lenticels above the tide level which end in long, spongy, air-filled submerged roots.
ex: pneumatophores-"air roots" on black mangroves. These stick out of the mud from the main roots and are exposed during low tides
ex: "knees"-these were traditionally thought to be important in oxygen exchange, but no one has really shown this to be the case. It's currently thought that while these may be helpful in gas exchange (and probably also support) they are not vital.
c. rapid early shoot growth
Under flooded conditions, several herbaceous and woody species exhibit this which gets the shoot above the surface of the water quickly to facilitate gas exchange
d. stem and lenticel hypertrophy
Hypertrophy is the enlargement of an organ without an increase in the number of constituent cells. An example of this is butressing or butt swell which is an increase in the diameter at the base of the stem. You immediately think of cypress trees, but tomatoes, sunflowers, and corn, can do this---experimental possibility!
The role of this seems to be to increase air space which allows for increased movement of gases. Besides that, the wide base provides extra support for shallow rooted structures on a soggy substrate.
2. Reproductive Adaptations
The ability of seeds to germinate while inundated has both advantages and disadvantages, depending on the conditions of the inundation. Species whose seeds can germinate and grow under flooded conditions experience less competition. Rice, American Elm, cottonwoods, and Black Willows can do this.
The ability of seeds to survive long periods of flooded conditions while buried in sediments in a dormant state is also helpful. Infrequent droughts would expose soil surfaces and those surviving seeds would have the opportunity to germinate. Cypress and Tupelo trees do this.
Frequent flooding can result in good seed dispersal if the seeds can float and avoid getting waterlogged. They can float until they get hung up on a dry spot, perhaps appropriate for germination.
Some plants can delay flowering or speed production during floods, or accelerate flowering during dry periods.
Vascular plants deal with salt water with specialized cells controlling import and export of salt. Mangroves, for example, almost completely exclude salt. When you squeeze their leaves you get almost pure water. Marsh grass, on the other hand, lets salts in but selectively excretes it--hence the salt crystals you find on their leaves.
Interestingly, wetland plants in saltwater have the same problem plants in arid climates have---difficulty in getting and keeping water. Because of the salt levels, water tends to leave the plant via osmosis, or at least not enter the plant. Thus you see the same fleshy structures (consider Salicornia or pickleweed), and adaptations such as the capacity to store carbon dioxide and use water more efficiently in photosynthesis which allows the plant to keep its stomata closed (pores on the leaves and stems) more during the day.
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