Physiological basis for —— Sodium toxicity (how does Na+ kill the plant?)
- Sodium (form and [ ]) in soils
- Sodium movement in soils
- Sodium uptake by roots (mechanism and pathway)
- Sodium movement within plant (vascular)
- Competition (other nutrients)
- Storage (vacuole)
Sodium form and concentration in the soil
Problem of salinity is common in the arid and semi-arid regions around the world. Salts which contribute to the salinity are most commonly the carbonates, sulfates, chlorides of sodium, magnesium, calcium and potassium (Warrence et al., 2002). These salts have their unique solubility and composition of mineral material, which determines their presence in the water. They get disassociated into anions and cations after dissolving in the soil solution (Buckman & Brady, 1960) and increase the soil pH by contributing OH- ions to the soil solution which replaces the H+ ions from the soil. Most common cations are Calcium, sodium and magnesium which dominates the cation exchange sites of the clays by replacing the hydrogen and aluminum. Salts of sodium and potassium can exist in solid state, but calcium and magnesium salts can only be found in the solution. So, when moisture content in the soil decreases due to the evaporation, drainage or plant uptake, the calcium bicarbonate decomposes to a solid precipitate, water and carbon dioxide. This process removes the calcium from the clay particles leaving behind the sodium. This makes the soil sodic and increases its pH (Handbook, 1995).
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Increasing salt concentrations in the soil solution also affects the soil physical properties. High conc. of salts in the soil lead to the flocculating effect which causes the fine soil particles to bind together. This improves the soil physical conditions in terms of improving aeration, drainage and root growth in the soil (Buckman & Brady, 1960). But interesting thing here is that sodium though being a salt has opposite effect on the soil. Sodium lead to the dispersion of soil instead of flocculating it. This is due to its charge, hydration status and relative size. Na exists as a monovalent cation in the soil solution and have less tendency than the divalent cations to bind to the exchange sites of clay. Therefore, Na exists between the clay particles rather than binding to it. When two cations have the same valence, then the cation with smaller hydrated radius will have more affinity to bind to the clay particles. Na and K have same valence, but Na have larger hydrated radius than K, therefore it is unable to bind and gets inserted into the layers of clay particles disrupting their binding forces and leading to dispersion of clay particles (Warrence et al., 2002). Dispersed clay particles block the soil pores, when this soil undergoes the alternate wetting and drying cycles it ultimately turns into a cement like layer with very little or no structure (Buckman & Brady, 1960). Moderate rate for exchangeable sodium in the soil is 0.3-0.7cmol (+)/kg (GIS).
Na+ as a functional nutrient
Arnon and stout gave the criteria of essentiality for the nutrients which determined the nutrient essential only if plant cannot complete its life cycle in absence of particular nutrient, no other nutrient can replace the function of that nutrient and nutrient is directly involved in the plant metabolism. Sodium do not follow this criterion but have some important functions in the plant such as it increases their biomass growth, improve the post-harvest quality of produce. In celery Na+ applications increased the resistance to blight and the crispiness of the tissue which increased the market value (Pardossi, Bagnoli, Malorgio, Campiotti, & Tognoni, 1999). Na+ also increased the sweetness of carrot (Lehr, 1953).
There are several other functions such as in the Calvin cycle of some C4 plants such as Amaranthus tricolor, Sodium plays an important role in formation of phosphoenolpyruvate which is involved in the flow of number of metabolites between the mesophyll and the bundle sheath cells to operate the CO2 concentration mechanism. For some C4 plants Na+ has also been reported to take part in the chlorophyll formation and involved in the nitrate reductase activity in the leaves, which increases the nitrate uptake by the roots.
Na+ can serve few functions of the K+ due to their same valence and ionic radius such as maintaining internal osmotic potential of the cells to uptake water through the roots, opening and closing of the stomata, photosynthesis and counter ion for long distance transport. These functions can also be performed by Na+ in the plant which makes it functional nutrient though not essential (Subbarao et al., 2003). Na+ not being an essential element, required by plants in trace amounts as micronutrients. This amount of Na+ can be obtained from the irrigation water or commercial fertilizers may contain it as impurity. So, there is no problem of Na+ deficiency and plants do not show any deficiency symptoms but there is a problem of Na+ toxicity which really matters. Na+ toxicity causes necrosis and scorching of the leaf tips and margins, same as the micronutrient toxicities. Saline water is the major cause of Na+ toxicity in the plants. Symptoms of toxicity appears if Na+ levels are over 50ppm in the water. Exchangeable Sodium Percentage (ESP) is also a good indicator of potential Na+ toxicity. There are 3 categories of Na+ tolerance in plants. Plants are as sensitive, semi tolerant and tolerant at 15, 15-40 and more than 40 ESP respectively (Ayers & Westcot, 1985).
How Na+ toxicity differs from Cl-
Tavakkoli et al.(2011) conducted a solution culture experiment to assess the individual contribution of Na+ and Cl- to the salt toxicity in four different varieties of Barley namely Barque73, Clipper, Tadmor and Sahara. They determined the effect of the Na+ independent of the Cl- in three ways; by using DIDS (120 mM NaCl, 0.07 mM 4,4′-diisothiocyanostilbene-2,2′-disulphonic acid), it is non-permeating amino acid which inhibits the transport of Cl- by Na+ gluconate as it is an anion which is unable to permeate the membrane; by + Hoagland solution (Lin & Kao, 2001). Chlorophyll content of the leaves were increased by 15-19% in all the varieties under Na+ treatments. But it decreased in the Cl- and NaCl treatments. So, this means that Chlorosis occurs due to the chloride injuries. In Barque73 variety the dry weight of shoot was reduced 24% in Na+, 10% in Cl-, 34% in NaCl treatments but in the variety Clipper the dry weight was reduced by 18% in Na+, 23% in Cl-, 47% in NaCl treatments. So, this states that Sodium and chlorine contribute differently to Salt toxicity. Gas exchange rates were measured to get estimated rates of photosynthesis. Photosynthesis was decreased by 20-25% in Barque73, Clipper, Tadmor and 10% in Sahara with Na+ treatment. In Cl+ soils it was 40% reduced in Tadmor, 23% in both clipper and Sahara, 10% in Barque73. But the reduction was greatest in NaCl treatments for all parameters such as photosynthesis, shoot dry weight and total water taken up by plant. This experiment was done both in the soil and soil less media. Increasing Concentrations of Na+ in the Soil system decreased the amount of K+ uptake but in the hydroponic experiment the K+ uptake was not affected by the Na+ treatments. This may be due to the replacement of K+ from the exchange sites by the Na+. High Na+ conc. also reduced the K+ levels in the stomatal and epidermal cells. Reduction in the assimilation of the carbon dioxide is due to the change in K+ levels which effect the opening and closing of the stomata (Chow et al., 1990).
Sodium uptake by roots (mechanism and pathway)
Plants have developed various means to adapt to salinity such as control over Na+uptake, Na+ loading in the xylem, Na+ removal from xylem, Na+ expulsion from the roots, compartmentation into the vacuoles and excretion. So, minimizing the Na+ uptake by the roots would be the most efficient approach to deal with Na+ toxicity in the plant then we need not to work on the other mechanisms dealing with excess Na+ (Zhang et al., 2010). Most of the research based on Na+ metabolism in plants is concerned with the initial uptake by the plant through root cell membrane (Cheeseman, 1988). Electrochemical potential across the membrane of the root cells serve as the driving force for entry of the Na+ into the cells. Electrical force is determined by the activity of Na+ concentration across the cytosolic and extra-cytosolic sides of the membrane (Amtmann & Sanders, 1998). Chemical driving force depends upon the salinity of the soil. These forces work together as if electrical potential is negative across the membrane, even the low external Na+ concentrations can develop a strong electrochemical gradient across the membrane which favors the passive transport of Na+ into the roots (Blumwald et al., 2000). Entry of Na+ is also passive at lower K+ concentrations (Cheeseman, 1982). So, selectivity for K+ over the Na+ is important for plants to survive under salinity (Pitman, 1984). But it was found that there was no decrease in the Na+ uptake upon increasing K+ concentrations in the soil (Kronzucker et al., 2006). It states that K+ uptake can be reduced by the increasing external Na+ concentrations but not vice versa. However, it is still not clear that how this selectivity is effected but it was found to be controlled by some genes in some species such as salt resistance in the wheat was linked with locus on the D genome which result in low Na+ uptake and more selectivity for K+ in presence of saline conditions (Munns et al., 2000).
Na+ also effects the cell wall and membrane stability as it removes the Ca2+ from pectin involved in crosslinking and also from the binding sites of plasma membrane (Essah, 2003). It results in the depolarization of the plasma membrane which lead to the loss of K+/Na+ selectivity. More the Na+ entering the cell, more the efflux of K+ from the cell (Horie et al., 2006).
Entry of Na+ into the plant roots occurs through the Non-Selective Cation Channels or Voltage independent channels in the cortical cells of the roots (Amtmann & Sanders, 1998). These channels have poor selectivity for K+ over the Na+ and these are the main routes for Na+ for entering into root cortical cells. Study by Tyerman et al., (1997) found the role of Non-Selective Cation Channels present in the lipid bilayers for uptake of Na+ which was measured in the root segments and protoplast of the cortical cells. There are also channels which favors the K+ uptake like Arabidopsis K+ transporter and high affinity K+ transporter. Research is needed to focus on cloning these ion transporters to induce salinity resistance in plants (Zhang, 2008)
Movement of Na+ in the plant
P. R. Stout and D. R. Hoagland (1939) conducted a study at UC Berkeley to trace the movement of salts in the plant with the help of radioactive isotopes of Sodium, potassium and phosphorus absorbed through roots. They found that after the absorption of salts by roots they enter the xylem in a short time and rapidly translocated to the leaves under the transpiration pull. Movement was not significant in the strip of the bark but there was a significant radioactivity found in the wood. There was a lateral movement of salts to the bark from the wood when in contact. These results were consistent for all three elements. So, they concluded that xylem is the path for rapid movement of the salts in the plant. There are different carriers in the plasma membrane which allows the entry of the Na+ into the cells. HKT1 is high affinity symporter for K+/Na+, NORC (non-selective outwardly rectifying channel) which do not discriminate between the Na+ and K+ and VIC (Voltage Independent Channels) which have higher Na+/K+ selectivity (Blumwald et al., 2000).
Enzymes and signaling systems are sensitive to the higher ion concentrations (Cheeseman, 2013) such as Stability of the enzymes that catalyze the synthesis of proteins in ribosomes was found to be affected by increasing concentrations of the Na+ (Brady et al., 1984). Sodium other than causing dehydrated conditions for plants when present in higher concentrations, it is also toxic to various organelles at the cellular level. Respiration in the mitochondria is lower in the plants under salt stress.Cytochrome c pathway is more affected by salt stress than the alternative oxidative pathway because former is in the intermembrane space of the mitochondria but the alternative oxidative pathway is protected by the inner membrane (Jacoby et al., 2011).
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Na+ competes with H+ for entering into the mitochondrion and results in the reduction of ATP production. if, this was the case then K+ would also pose a same competition for H+ for entrance into the mitochondria but evidence for K+ is not found. In chloroplasts salts such as KCl and NaCl uncoupled the rates of electron transport from the water to oxaloacetate which effected the oxygen evolution during hill reaction. NaCl was 85% as effective as KCl for uncoupling the electron transport rates (Marsho et al., 1980). There are some other factors than Na+ toxicity at high salinity that may affect the Growth and may lead to death of cells and tissues. Potassium is the essential element in the plants. K+ acts as dominant counter ion for the light induced influx of the H+ through the thylakoid membranes to generate pH gradient across the membrane necessary for ATP synthesis. K+ is involved in the long-distance transport due to its high mobility in the phloem and continuous circulation within the plant. Most of the Potassium is present in the vacuole for maintaining the turgor but it may be replaced by the high sodium concentrations (Kronzucker et al., 2013). Under high salinity conditions, cytoplasmic concentrations of the K+ also gets reduced which causes depolarization of plasma membrane and ROS activating the channels which allow K+ efflux in roots (Shabala, 2013). High Na+ concentrations also lead to the reduction of the NH4+, Mg2+ and Ca2+ uptake due to competition for cation exchange sites on the soils. Plant may die due to the deficiency of any of these particular nutrients. When levels of Na+ rises in the cell, it is required to be compartmentalized into the vacuole. There is an additional demand for ATP even during low photosynthesis rate to sequester Na+ in vacuoles. This requires ATP to generate electrochemical gradient across the membrane to energize Na+/H+ antiporters in the tonoplast. (Shabala, 2013). This causes the cell to run out of energy which in turn effects other active processes of the plant.
Mechanisms of Sodium Tolerance
Exclusion of Na+ from the cell is an energy consuming process because Na+ ions have to be transported against the electrochemical gradient. Exclusion in higher plants occurs with the help of plasma membrane H+ ATPase. Energy from the ATP hydrolysis is used to pump out the H+ from the cell. It generates an electrochemical gradient for the H+ across the membrane which results in the activation of plasma membrane antiporter in which H+ enters the cell along the concentration gradient and Na+ is excluded against the concentration gradient with the help of force supplied by H+ ATPase. This antiporter activity of Na+/H+ resulting in the exclusion of Na+ have been found in the various plant species (Blumwald et al., 2000).
Large amounts of Na+ enters the cell but it remains non-toxic until certain levels. This is due to the storage of Na+ in the vacuoles. This prevents the various toxic effects of Na+ if present in the cytoplasm. Storage of Na+ into the vacuoles also results in the change in osmotic potential of the cell which allows it to uptake more amount of water. Entry of the Na+ into the vacuole is also through the Na+/H+ antiporter which runs on the proton motive force generated by the H+- ATPase, H+-PPiase and H+ translocating enzymes of the vacuole. These antiporters are mostly found in the salt tolerant species and there is need to focus research on cloning them into the salt sensitive species (Blumwald et al., 2000). Tonoplast should be able to store more number of ions and not allow them to leak into the cytoplasm. Which is decided by its ion transporters and lipids in the membrane which makes it permeable (Flowers, Munns, & Colmer, 2014).
Plants have to maintain a positive turgor pressure if they want to survive in saline conditions.
In dicots most of the Ions can be accommodated only with the help of compartmentalization within the cell such as most of ions when stored in vacuoles then organic solutes adjust the osmotic potential in the cytoplasm. For sequestration of Na+ in the vacuole there must be the ion exchangers and the H+ pumps which balances the electrochemical gradient across the tonoplast. Sequestration of Na+ is more expensive in terms of energy than the Cl- because potential inside the vacuole is positive than the cytoplasm outside (Flowers & Colmer, 2008). Low leakage of Na+ from the vacuole is attributed to the high phospholipid to protein ratio of the tonoplast (Leach et al., 1990). Organic solutes such as sucrose, proline, sugar alcohols are accumulated in cytoplasm compartments instead of the whole cell to adjust the osmotic potential between the vacuole and the cytoplasm (Flowers et al., 2014).
There are some genes such as TRK1 and ENA1 which increased the tolerance of the plant cell to Na+. These are present over plasmid or the chromosome. But they are more effective when present on the chromosome. These genes code for the efficient transport systems in the cell membrane which enhances the cell selectivity for K+ over Na+. These genes aid the cell in changing the external/internal ratios of the K+ and Na+ that could be easily maintained but they could not change the toxicity levels of internal Na+ (Haro et al., 1993)
Secretion is one of the mechanisms followed by the plants to deal with salinity. Salt glands are present over the leaf surface which help to maintain the balance of the salts in the leaves and they help secreting the excess salts present in the leaves. There are number of small vacuoles present in the secretory cells of glands. They accumulate the salts in it before excreting them outside the cell. Small Vacuoles are termed as microvacuoles, they fuse with the plasma membrane along the cell wall which then releases the salt in the walls of secretory cells. Slat moves along the wall and finally passes through the pores to the outer surface. Cuticle over the mesophyll cells prevents the backflow of the salts into the cell. Movement of the salts within the cell is through the plasmodesmata present between the secretory cells (Thomson, Berry, & Liu, 1969).
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