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Salt stress is one of the major abiotic stresses affecting plants negatively. It is a condition were excessive salts accumulated in plants causes inhibition of plant growth and plant death. When sodium and potassium exceeds the normal range within plant's cystol plant undergo salinity stress conditions. Effects of salinity stress include two ways, Osmotic effects and specific ion toxicity. Osmotic conditions change outside the root, solutes create low osmotic potential that lowers the soil water potential, which later causes reduction in leaf growth and cell death, and plant overall growth is affected. Salt toxicity effects happen when salts accumulate in leaves, resulting in leaf death and inhibition of photosynthesis in general. Enzymes are also affected and protein synthesis is inhibited. To measure the effects of salinity on plant Chlorophyll fluorescence is used. Chlorophyll fluorescence machines measures the capacity of the plant to convert light energy to biochemical energy in photosynthesis. Plants developed mechanism to limit the accumulations of salts inside the plant. By three ways, first, by salt exclusion were plants prevent excess uptake of salt by root. Second, by salt secretion which prevents the buildup of excess concentrations of salts on leaves, some plants have developed glands and salt hairs were excess salts are stored. Finally by the mechanism of salt tolerance, plants living under high salt concentrations are more efficient in delivering ions into the vacuoles to prevent excess concentrations of ions in cytoplasm.
Plants face high concentration of salts naturally around salt marshes, salt lakes, tidal swamps or natural salt scalds. On the other hand plants could encounter high salinity far inland for reasons such as: seepage of salt water from marine deposits and from the accumulation of salt from irrigation water.
Salts are very important for the growth and development of plant. Potassium is an essential and one of the most abundant monovalent cations in cells working as a co-factor in cystol that can activate more than 50 enzyme, which are very sensitive to high systolic Na+, and high Na+/K+ ratios . Normally at nonsaline conditions the cystol of plants cells contains 100-200 mM k+ and 1 to 10mM Na+, an optimal environment for enzymes to function. Therefore, when concentrations of sodium and mainly potassium exceeded the normal range in cystol, plants will go under salinity stress condition.
Salt accumulation affects plants in different ways such as osmotic effects and specific ion toxicity. Plants go through two phases of growth response to salinity, the first response happens moments after salinization, due to osmotic changes outside the root where dissolved solutes create low osmotic potential that lowers the soil water potential, cells become unable to absorb water, dehydrate and shrink, which later cause reduction in cell division and elongation that leads to slower leaf size and appearance, several minutes after that the plant gradually recover its growth rate until reaching a steady state, weeks later lateral shoot development is affected, and after few months a difference can be seen on the overall growth of plants under salt stress compared to plant under normal conditions .
The second response happens at a slower rate taking days or months, which is a result of accumulation of salts on leaves, causing salt toxicity effects in the plant, which could result in death of leaves and reduction of the total photosynthetic leaf area as well as reduction of chlorophyll content and inhibit photosynthesis in general, as a result the carbon balance necessary to growth is disturbed. The presence of Na+ and K+ at high ratios and the buildup of salts in leaves with high concentrations that exceeds the capacity of salt compartmentation in vacuoles causes toxicity in cytoplasm and unflavored condition for enzymes, where they will be inactivated and proteins synthesis inhibited.
To measure Salinity stress Chlorophyll fluorescence Measurement is used. This measurement shows the capacity of the plants to convert light energy to biochemical energy in photosynthesis. This technique is very powerful and has the advantage of preforming a rapid, non-invasive, and nondestructive method.
Salt sensitive plants depends on the ability of roots to prevent harmful ions from reaching the shoot. Plants exclude salts from meristems (in shoot) and from leaves to minimize injury, casparian strip act as barrier to water and solute movement into the xylem, for ions to move they must move from apoplast to the symplastic pathway across cell membrane which offers plant a salt resistance mechanism.
Because ions enter roots passively, roots must use energy to push out Na+ to the outside, however Cl- is excluded by negative electric potential through cell membrane. While movement of Na+ into leaves is minimized by transportation stream (xylem sap) during movement from roots to shoots and leaves. Some plants use another technique, using salt glands that exist at the surface of their leaves, where ions are sent to be crystallize and become unharmful.
Salt resistance plants such as mangrove, make an osmotic adjustment to take water from the low water potential of the external environment. Those plants adjust their water potential in response to osmotic stress by lowering their solute potential, two factors contribute to this decrease in solute potential are: the accumulation of ions in vacuole and synthesis of compatible solutes in cytosol.
A transport system to facilitate the compartmentation of Na+ into vacuoles is very critical to insure normal salt concentration inside the cell. Ca2+ and K+ both affect Na+ concentration, where at high concentration of Na+, K+ uptake trough high affinity K+ Na+ transporter, KHT1 is inhibited. The transporter work as Na+ uptake system. On the other hand Ca2+ enhances K+/Na+ selectivity thus increase salt tolerance.
The activity of two types of pumps in the plasma membrane and the tonoplast are required for the secondary transport of Sodium across plasma membrane and Tonoplast, the activity of these pumps increase by salinity. The ATPase in the plasma membrane is responsible for large âˆ†pH and membrane potential across the plasma membrane. The vacuolar H+ - ATPase generates âˆ†pH and membrane potential across the tonoplast so that cations such as Na+ could be imported into the vacuole.
In this paper the Plant whole responses to high salinity mechanism of tolerance will be discussed.
Water movement through root
Most important function of root is supply of water to plant by absorbing soil water and delivering it to parts of the plant. Water travel from the soil into plant roots passively in response to a water gradient potential, since water potential is lower in the roots than in the surrounding soil, it moves from the higher potential to lower potential, then continues within the plant, from epidermis through the cortex to the xylem, and to different parts of the plant, finally to the leaves and from there to the atmosphere.
As seen on the figure (1) nutrient follow two routes either through free space between cells, or from cell to cell because of the high concentration of solutes inside the cells the nutrients are transported across plasma membrane also following the concentration gradient, the nutrient uptake is an energy consuming process and the energy is generated by cell metabolism.
Figure (1) the two pathways of water conduction in the root tip (Taiz & Zeiger,Â 2010).
Cell membrane are permeable to water but not permeable to solutes, water will enter to the cell because of the concentrating difference by passing through the semi permeable membrane in order to lower the solute concentration incised the cell, the membrane has transmembrane proteins (glycoproteins) imbedded in the phospholipid double layer, those proteins play the role of passage pores, allowing a catalysed transport of specific ions like calcium or potassium into or out of the cells cytoplasm across the membrane and prevent free diffusion of salts s or sugars , which is an energy consuming process.
Water enters the roots by two pathways, first water enters by the apoplastic pathways, water can move into the root along the cell wall without entering the cells, as shown in figure (1) until it reaches the endordermis where it faces an apoplastic barrier called the casparian strip at that point it is the last chance for the plant to modify the composition of the solutes and exclude nonessential nutrients or toxics ions in the water before they can be transported inside the xylem.
Second pathway is the symplastic pathway, water moves from cytoplasm of one cell to cytoplasm of neighboring cell through the linking structure plasmodesmata, until it arrives at the endodermis where the water from the apoplastic pathway also end.
Water that have arrived into the endodermes by either pathways moves into the xylem by osmosis. Which is facilitated by carrier proteins that happen actively transporting salt into the xylem to lower the water potential inside. When transpiration happens water moves up the xylem by use of cohesive forces between water molecules and by root pressure of water moving into the xylem by osmosis (Ehlers & Goss,Â 2003).
Salt Stress effects
Excess concentrations of salts that are dissolved in water are very harmful to the plant by two ways. One way is by osmotic influences where high concentration of salts in the soil disturb capacity of the root to absorb water, the other way is by specific ion toxicities that could inhibit many biochemical and physiological processes of plants. These effects can reduce plant growth and development and survival. Figure 2 describes those two effects of salt stress.
Figure 2. two-phase growth response to salinity (Carillo et al.,Â 2011).
During osmotic phase, that start after concentration of salt increase around the root, There is a reduction in the rate at which the root growth because of inability of the root to extract water from soil. Plant uses a temporary tolerance strategy where the stomata of leaves close in order to lessen the ion flux to the shoot. Which soon has to open up because of the water potential difference between the atmosphere and leaf cells and the need for carbon fixation.
The growth of shoot is more sensitive to osmotic stress than root growth, due to the fact that reduction of the leaf area development relative to root growth in relation to root growth decrease the water use by plant and allowing it to conserve soil water and prevent the concentration of salt in soil. Reduction in leaf area and stunted shoot causes the reduction in shoot growth and inhibition of leaf growth. Another reason of inhibition leaf growth is because of inhibition by salt of symplastic xylem loading of Ca2+ in the root. The leaf initiation is not affected by salinity stress whereas leaf extension was affected by salt stress.
In the second phase, the salt specific toxicity, happens because of the accumulation Na+ ions in the leaf, after being deposited in the transpiration stream, and not the root. Na+ accumulation is toxic to older leaves that are non able to dilute the salt that arrives to them. Here the photosynthetic capacity of plant is unable to supply the carbohydrate requirement to the plant, resulting in increasing the rate of the death of leaves in compare to the rate of production of new leaves. Accumulation of Na+ affects photosynthetic components such as enzymes, chlorophylls, and carotenoids, and decreases the efficiency of photosynthesis. This will lead to increase production of reactive oxygen species (ROS).Salt stress can impair the normal procedure of removing these ROS by antioxidative mechanisms (Carillo et al.,Â 2011).
Loss of water due to salinity stress
Intracellular water is lost from the plant because of the high concentration of salt outside and the low water potential outside the plant. Plant produce and accumulate compatible solutes. Including glucine betain, proline, sorbitol, mannitol, pinitol and sucrose. Those solutes adjust water movement from higher water potential to lower water potential inside the cell to prevent it from being lost (Mahajan & Tuteja,Â 2005).
Effects of salt stress on photosynthesis :
Salt stress inhibits the repair of PSII, where the results of salt stress of 0.5M NaCl did not accelerate photodamage to PSII, but inhibited the repair of the photodamaged PSII. Were it was not only able to suppress the synthesis of D1 protein in PSII, but also suppressed almost all other proteins. The mechanism of inhibition is not fully understood, but several possible mechanisms were suggested as follows, first it might be possible that high concentrations of NaCl inactivates the translational machinery (or ribosomes) in vitro, therefore salt stress inhibits protein synthesis directly. Second, mechanisms is based on the studies of Tamarix jordanis (the Jordan Tamarix plant), were rubisco was inactivated by the presence of NaCl, therefore we might say that the primary target of salt stress is rubisco and the inhibition of CO2 fixation by salt stress induce the generation of ROS, which in turn inhibits protein synthesis .Third, it is possible that the increase in intracellular concentration of salts inactivates ATP synthase and decrease the intracellular lever of ATP, that is essential for protein synthesisÂ (Murata et al., 2007).
Measurements of salinity by chlorophyll Fluorescence :
Chlorophyll fluorescence is used to measure Salinity stress effects on plant, because salt stress can reduce the ability of plant to metabolize normally and causing an imbalance between absorption of light energy by chlorophyll and the use of energy in photosynthesis.
Electrons are excites in chlorophyll molecules after light energy is absorbed, and the energy will be converted to chemical form for photosynthesis. When photosynthesis is not efficient, the extra energy will damage the leaf. This energy is emitted as heat or chlorophyll fluorescence, when little energy is emitted as heat or used in photosynthesis fluorescence yield is high. Fluorescence can be measured by shining a defined wavelength of light onto a leaf and measuring the level of light emitted at longer wavelengths (Kalaji & Guo,Â 2008).
Mechanism of salt tolerance:
The mechanism of salt tolerance is divided into two main types: first, the minimization of entry of salts into the plant and the accumulation of salts on the photosynthetic tissue. Second is minimizing the concentration of these salts in the cytoplasm of the cell. This is done with two strategies; first, the salt stress avoidance, by the use of physical barriers. Second, by stress tolerance that uses some adaptive mechanisms to survive even with the plant being under salt stress (Rao et al.,Â 2006).
Limiting salt accumulations:
A complex but very efficient way to prevent excess ion uptake by the root, or to prevent salt from reaching sensitive plant organs, which is based upon lower root permeability of ions in high salt stress. Exclusion of salt rely on selective release of Na+ into the xylem and disappearing from the xylem stream.
It was proven that when the amount of external Na+ 200 mM, almost 97% of all Na+ at the root surface must be excluded, both in halophyte and glycophyte, therefore it is necessary to restrain Na+ uptake and accumulation in the shoots (Rao et al.,Â 2006).
Secretion of salts is an efficient way to prevent the buildup of excess concentrations of salt on photosynthetic tissue (Rao et al., 2006). Certain plants that have this mechanism developed specialized adaptive structures called salt glands and salt hairs on the leaves and stem, this phenomenon is seen on the surface of the leaves and shoots as a thin salt crust (Tan & Lim,Â 2010).
A key factor in salinity tolerance is the capacity of vacuolar compartmentalization that adjusts the osmotic condition of plants under salt stress, and emptying the cytosol from the presence of toxic ions.
The survival of halophytic plants living in areas of high concentration of salts is linked with their efficiency to deliver ions into vacuoles, the higher the number of vacuolated cells and tissues, and the activity of the transport system located at the tonoplast, the higher the ability of the plant to transfer ions into the vacuoles that prevents excess concentration of ions in cytoplasm (Rao et al.,Â 2006).
Salinity is a significant problem affecting plants, the visible effects on plants are slower rate of growth and limitation of plant production. The way in which salt affects plants is divided into two parts, first Osmotic effects and specific ion toxicity. In osmotic stress phase salt concentration is higher outside the root making it harder for the root to extract water from soil, and more salts are absorbed into the root. This eventually affects the growth of the shoots and leaf extension. While in salt specific toxicity sodium accumulate in the leaf and affect the photosynthetic capacity of the plant were it will inhibit the repair of PSII. Resulting in death of leaves, production of reactive oxygen species and impair the normal procedure of removing them by antioxidative mechanisms. Some plants are better adapted to salt stress than others, those plants use different mechanisms to tolerate this problem, such as avoidance, exclusion and tolerance. The understanding of how plant cell response to salt stress and how it is coordinated to maintain the perfect balance for uptake of salt and compartmentalization is important in order to understand the mechanism of tolerance of salt stress.