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Despite the negative impacts of salinity on several plant functions, many species persist in saline environments. These plants have adapted a variety of mechanisms to alleviate the negative impacts of salinity. Such mechanisms range from cellular level to whole plant reactions and are often an integrated response at multiple levels. An overview of common mechanisms of salinity tolerance is provided below, with links to mechanisms used specifically by New England salt marsh species. The mechanisms discussed are: Salt can be excluded from entering the plant through its root system or, within the plant, salt can be restricted from reaching sensitive organs (Larcher 1980). Restricted uptake in the roots, while a first line of defense, is not a very efficient mechanism and is often found in conjunction with internal exclusion mechanisms (Hagemeyer 1987). Internal exclusion mechanisms can involve such processes as sequestering salt ions in specialized tissues by “removing” them from the transport stream (Hagemeyer 1987). One way plants achieve this is by exchanging K+ ions for Na+ ions as they pass through the xylem (Hagemeyer 1987). However, the ion accumulation capacity of the xylem is limited and reduces the efficacy of this process (Hagemeyer 1987). Salt exclusion in Distichlis spicata and Spartina alterniflora (Smart and Barko 1980) Some plants simply rid their systems of salt by
excreting it back into the environment. Plants can excrete salt through
their roots, shoots, and leaves (Larcher 1980). Some plants transport
and accumulate salt to storage areas that are shed later (Larcher 1980).
Specialized structures for excretion have evolved in the epidermis of some
species (Hagemeyer 1987). Bladder hairs are structures on leaf surfaces
that consist of several “stalk” cells and a “bladder” cell. The stalk
cells transport ions into the vacuole of the bladder cell, which eventually
dies and falls off the plant (Hagemeyer 1987). Salt glands are used
by some species to excrete salt. These specialized structures transport
ions directly out of the plant through both roots and leaves (Hagemeyer
1987).
One defense against salt in plant tissues is to simply dilute the concentration of ions. Plants achieve this by increasing their storage volume by developing think, fleshy, succulent structures (Larcher 1980; Hagemeyer 1987). This mechanism is common in wet saline environments, like salt marshes, where water is not a limiting resource (Larcher 1980). Succulence is mainly a result of vacuoles of mesophyll cells filling with water and increasing in size (Hagemeyer 1987). This mechanism is limited by the dilution capacity of plant tissues (Hagemeyer 1987). Some salt tolerant plants control the accumulation of salt ions to counterbalance low water potentials created by saline soils (Hagemeyer 1987). Salt ions are compartmentalized in vacuoles to protect proteins and membranes from ion toxicity. The active transport of ions requires energy, however, and represents a trade-off where energy is allocated to tolerance rather than to growth and reproduction (Larcher 1980). Another trade-off with this mechanism is maintaining osmotic balance within the cell cytoplasm. Solute-rich vacuoles have a high osmotic potential that creates a gradient in which water moves from the cytoplasm into the vacuole. To counterbalance this gradient, plants produce osmotically active organic solutes called “compatible solutes” (Hagemeyer 1987). They are termed compatible because they do not interfere with plant physiological processes. A variety of compounds act as compatible solutes, such as amino acids and amides (e.g., proline), ammonium compounds (e.g., betaine), and soluble carbohydrates (Hagemeyer 1987). Osmotic adjustment with
compatible solutes in Spartina alterniflora (Cavalieri 1983)
Several mechanisms discussed above require specialized properties of plasma membranes, which are responsible for regulating the transport of salt ions. Membrane composition can affect membrane fluidity, permeability, and membrane protein activity (Wu et al. 1998). Each of these membrane characteristics can be tied to the effectiveness of the salinity tolerance mechanisms discussed above. However, we are just beginning to develop an understanding of the functions of plasma membrane composition in salinity tolerance (Wu et al. 1998). The response of plasma membrane composition in one New England salt marsh plant, Spartina patens, has been studied recently (Wu et al. 1998). Plasma membrane lipid composition of Spartina patens in response to salinity stress (Wu et al. 1998)
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