OSMOTIC ADJUSTMENT

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OSMOTIC ADJUSTMENT:

OSMOTIC ADJUSTMENT AK CHHABRA

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1 Osmosis  is the spontaneous net movement of solvent molecules through a partially permeable membrane into a region of higher solute concentration, in the direction that tends to equalize the solute concentrations on the two sides. It may also be used to describe a physical process in which any solvent moves, without input of energy, across a semi-permeable membrane (permeable to the but not the solute) separating two solutions of different concentrations.  Although osmosis does not require input of energy, it does use kinetic energy  and can be made to do work. Osmosis

Osmotic pressure:

Osmotic pressure Osmotic pressure  is the pressure which needs to be applied to a solution to prevent the inward flow of water across a semipermeable membrane.   It is also defined as the minimum pressure needed to nullify osmosis. The phenomenon of osmotic pressure arises from the tendency of a pure solvent to move through a semi-permeable membrane and into a solution containing a solute to which the membrane is impermeable. This process is of vital importance in biology as the cell's membrane is selective toward many of the solutes found in living organisms. 2

Osmotic Adjustment:

Osmotic Adjustment 2 Osmotic adjustment is defined as a change in solute content per cell While change in water content per cell also can occur, this is not osmotic adjustment. Source of animation : http://www.authorstream.com

Osmotic Adjustment:

Osmotic Adjustment Turgor pressure on plant cells In biology, turgor pressure or turgidity is the pressure of the cell contents against the cell wall, in plant cells, determined by the water content of the vacuole, resulting from osmotic pressure.

The mechanism of osmotic adjustment:

The mechanism of osmotic adjustment Organic solutes Many plants are in response to environmental stresses of salinity and drought by accumulating organic solutes, which are known as compatible solutes and reported to be nontoxic even in relatively high concentrations (Zhang et al. 2002 a). Compatible solutes are low molecular weight and highly soluble compounds. Generally, they protect plants from stress through different courses, including contribution to cellular osmotic adjustment, detoxification of reactive oxygen species, protection of membrane integrity, and stabilization of enzymes or proteins ( Ashraf and Foolad 2007). Compatible solutes are usually referred to as important osmolytes in osmotic adjustment (Table 1).

Fig. 1. The mechanisms of osmotic adjustments resist drought and salinity stresses in plants.:

Fig. 1. The mechanisms of osmotic adjustments resist drought and salinity stresses in plants.

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Main organic osmolytes in osmotic adjustment on different plants and algae Species Stress Organic osmolytes Beta vulgaris L. (Sugar beet) Drought Glycinebetaine Distichlis spicata L. (Halophytic grass) Salt Proline Medicago truncatula and Phaseolus vulgaris (Legumes) Salt Trehalose Nicotiana tabacum L. (Tobacco) Drought Proline Oryza sativa L. (Rice) Salt Total soluble sugar(Glucose Fructose and Sucrose) , Triticum aestivum L. (Durum wheat) Drought Proline Triticum aestivum L. (Spring wheat) Salt Glycine betaine Bangiopsis subsimplex (Stylonematophyceae) Salt Sorbitol Dixoniella grisea (Rhodellophyceae) Salt Mannitol Dunaliella salina (Chlorophyta) Salt Glycerol Dunaliella tertiolecta (Chlorophyta) Salt Glycerol

ROLE OF ENZYMES:

ROLE OF ENZYMES In osmotic adjustment, some enzymes play main roles in the synthesis of osmolytes to alleviate or eliminate saline and drought environmental stresses. Researchers have found that betaine aldehyde dehydrogenase (BADH), pyrroline-5- carboxylate reductase (P5CR), and ornithine -d- aminotransferase (OAT) were enhanced in two varieties of reed (DR and HSR) (Zhu et al. 2003), whereas proline oxidase (PO) activities were inhibited, which suggested that changes in the activities of enzymes involved in osmotic adjustment might play important roles in the adaptation of reed plants to more extreme arid and saline habitats.

Glycine betaine:

Glycine betaine Betaines are quaternary ammonium compounds possessing a permanent positive charge on the fully methylated nitrogen atom (Zhang et al. 2002 a). Figure 2 shows six kinds of common structures of betaines , with glycine betaine as one of the most familiar betaines , which is widely distributed in plants (Zhang et al. 2002 a). Glycine betaine is synthesized by several plant families in response to saline or drought stress ( Munns 2002; Ashraf and Harris 2004; Su et al. 2006), whose primary effect on plant cells is to balance the osmotic potential of intracellular and extracellular ions to keep water and reduce salinity toxicity, and also function as a compatible solute to stabilize the structure of proteins to protect the major enzymes, protect membrane structures, protect photosynthetic apparatus, and protect cytoplasm and chloroplasts from adverse effects of Na+ ( Raza et al. 2007). In plants, glycine betaine is synthesized in chloroplasts

C0NTINUE:

C0NTINUE from serine via ethanolamine, choline , and betaine aldehyde . The biosynthetic pathway of glycine betaine is shown in Fig. 3 ( Ashraf and Foolad 2007). Several enzymes play important catalytic roles in the pathway of glycine betaine synthesis, such as choline monooxygenase (CMO) from beet ( Beta vulgaris L.), betaine aldehyde dehydrogenase (BADH) from spinach ( Spinacia oleracea L.), which have been introduced into different plants and enhanced salt and drought stress tolerances (Yang et al. 2008; Zhang et al. 2008).

CONTINUE:

CONTINUE Glycine betaine is an important osmolyte in some plants. In sugar beet ( Beta vulgaris L.), for example, glycine betaine is accumulated to a high level and plays the main role in osmotic adjustment under osmotic stress ( Chołuj et al. 2008). In two spring wheat ( Triticum aestivum L.) cultivars, the important determinants of salt tolerance are maintenance and acquisition of both K+ and Ca2+ according to Ashraf and Harris (2004), who postulated that K+/Na+ and Ca2+/Na+ ratios might be valid selection criteria for assessing salinity tolerance of different crop species. In both cultivars, application of glycine betaine resulted in an increased K+/Na+ and Ca2+/Na+ ratios under saline conditions, therefore it was concluded that glycine betaine played an important role in resistance to osmotic stress ( Raza et al. 2007).

Proline:

Proline Proline is an important amino acid for plant resistance to osmotic stress, and the accumulation of proline in plants was usually related to increased contents of L- glutamic acid, which is one of the possible precursors for proline biosynthesis ( Ashraf and Foolad 2007; Kishor et al. 2005). Figure 4 shows the biosynthetic pathway of proline , and it is shown that pyrroline-5-carboxylate synthetase (P5CS) and pyrroline - 5-carboxylate reductase (P5CR) are key enzymes in proline biosynthetic pathway ( Ashraf and Foolad 2007). Furthermore,

CONTINUE:

CONTINUE P5CS is a rate-limiting enzyme in this pathway. For example, over-expressing P5CS in transgenic tobacco ( Nicotiana tabacum L.) plants have shown increased concentration of proline and resistance to drought stress ( Gubis ˇ et al. 2007). In some plants under drought stress, the accumulation of proline was also related to the increased contents of other precursors for proline biosynthesis, including ornithine and arginine , and ornithine D- aminotransferase (OAT) is the key enzyme responsible for the ornithine pathway (Zhu et al. 2003; Ashraf and Foolad 2007).

CONTINUE:

CONTINUE Proline was one of the minor free amino acids in control plants, but increased proportionally more than the others in response to osmotic stress. In plants, proline accumulation could mediate osmotic adjustment, stabilize subcellular structures and scavenge free radicals (Hare and Cress 1997). In durum wheat ( Triticum aestivum L.), for example, a positive correlation was observed between proline level and osmotic potential, and it was concluded that proline is an important osmolyte in osmotic adjustment under salinity stress ( Poustini et al. 2007). In plants, the accumulation of proline normally occurs in the cytosol , and it contributes substantially to the cytoplasmic osmotic adjustment in response to drought or salinity stress ( Ashraf and Foolad 2007). For example, when cells of the halophytic grass Dis - tichlis spicata L. treated with higher salinity, there have been considerable accumulation of proline concentrations in the cytosol (Ketchum et al. 1991).

Glycerol :

Glycerol In some plants, glycerol is the main osmolyte , which is synthesized from glucose. Figure 5 shows the biosynthetic pathway of glycerol. In this pathway, glycerol-3-phosphate dehydrogenase (G3PDH) plays a major role in glycerol biosynthesis. In our previous work, it was first found that there are five isozymes of G3PDH in D. salina , and these isozymes , respectively, take effects in different salinities and may play important roles in glycerol metabolism (Chen et al. 2009). Glycerol is also often a major osmolyte in some algae subjected to salinity stress. For example, D. salina accumulates

Biosynthetic pathway of glycerol:

Biosynthetic pathway of glycerol

CONTINUE:

CONTINUE glycerol to counterbalance the osmotic pressure due to the high salinity of the surrounding medium ( Hadi et al. 2008; Mishra et al. 2008). The extracellular osmotic pressure is combated in Dunaliella by changing the intracellular glycerol content. The glycerol is synthesized rapidly when salinity increases, and glycerol transforms to starch when salinity drops (Chen and Jiang 2009). For example, the cells of another green alga Dunaliella tertiolecta (Chlorophyta) could adapt to the different concentration of saline by synthesizing

CONTINUE:

CONTINUE or eliminating the intracellular glycerol to balance the osmotic potential of intracellular and extracellular ( Goyal 2007 a, 2007b). Glycerol may be an effective osmotic element at high salinities. First, the high solubility of glycerol cannot be matched by most other compatible solutes. Second, glycerol is chemically inert and therefore non-toxic. Third, glycerol is an end-product metabolite, and therefore its accumulation is unlikely to offset major metabolic pathways. Fourth, the energetic cost of glycerol synthesis from glucose is relatively low and it does not depend on the availability of nitrogen (Chen and Jiang 2009)

Sugars:

Sugars In several plants, sugars are main osmolytes for osmotic adjustment, including sucrose, trehalose , glucose and fructose, etc. The sugar content was a very sensitive factor for salt tolerance improvement and the increase in total sugar content in plant cells was observed with NaCl treatment (Liu and van Staden 2001). Sugars diversion plays a key role in the adaptive processes linked with NaCl -tolerance, such as NaCl and Cl – translocation and (or) compartmentation , solute synthesis for interdependent mechanisms of growth and osmotic adjustment, and protein turn-over (Liu and van Staden 2001).

CONTINUE:

CONTINUE The accumulation of sugars appears to be common in some plants when they grow under osmotic stress. For example, Cha-um et al. (2009) found the total soluble sugar level in a salt-tolerant rice variety was higher than in the salt-sensitive variety, and that sugars enhance resistance to salt-induced osmotic stress in rice plants. It was also found

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CONTINUE that, in root nodules of legumes ( Medicago truncatula and Phaseolus vulgaris), the synthesis and accumulation of trehalose was enhanced as a compatible solute which was resistant to salt stress ( Lo´pez et al. 2008). In the red alga Bangiopsis subsimplex (Stylonematophyceae), sorbitol was the main low molecular weight carbohydrate and its level increased linearly with increasing salinities, indicating its important function as an osmolyte and compatible solute under high salt conditions ( Eggert et al. 2007 a). In another red alga Dixoniella grisea (Rhodellophyceae), the main low molecular weight carbohydrate is mannitol , which increased with increasing salinities, indicating its role as an osmolyte for the first time in a unicellular red alga ( Eggert et al. 2007 b).

Inorganic ions:

Inorganic ions To maintain an osmotic gradient for the uptake of water, many halophytic plants accumulate inorganic ions to a concentration equal to or greater than that of the surrounding solution (Merchant and Adams 2005). In some plants, inorganic ions play more important roles in osmotic adjustment than that of compatible solutes. Patakas et al. (2002) thought that the production of organic osmotica was more metabolically expensive than to accumulate a sufficiently high content of ions from the soil. For example, in contrast to organic solutes, inorganic ions formed the largest component contributing to osmotic adjustment in grapevines ( Vitis vinifera L., cv. Savatiano ), which seem to save energy and enable grapevines to grow in less favorable conditions ( Patakas et al. 2002). Table 2 shows the main inorganic ions in osmotic adjustment of different plants.

The effect of inorganic ions in osmotic adjustment :

The effect of inorganic ions in osmotic adjustment Sodium ion, K+, and Ca2+ are the main inorganic ions in some halophytic plants under osmotic stress, and these ions prevent plants from harm caused by drought and salinity stresses by absorbing water into cells. In Cynara cardunculus, a robust thistle widespread in arid and semi-arid regions where high salinity is frequently present, osmotic adjustment was mainly due to inorganic ions and not to other organic

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Main inorganic ions in osmotic adjustment on different plants. Species Stress Inorganic ions Arabidopsis thaliana Salt Na+ Cynara cardunculus (Cardoon) Salt Na+ Manihot esculenta (Cassava) Drought K+ Oryza sativa L. (Rice) Salt Cl – Salicornia europaea and Suaeda maritime (Halophytic plant) Salt Na+ Sesuvium portulacastrum (Sea purslane ) Salt Na+ Suaeda salsa ( Chenopodiaceae ) Salt Na+ Triticum aestivum L. (Wheat) Drought Ca2+ Vicia faba L. (Bean) Salt, Drought K+,Cl –

CONTINUE:

CONTINUE solutes. Sodium ion is usually less toxic than K+ at high concentrations in C. cardunculus thriving in salty environments. The induction of a specific water stress stimulated Na+ but not K+ uptake and translocation to the shoot, indicating, once again, that C. cardunculus plants were welladapted to the presence of moderately high Na+ concentrations, and that this plant used Na+ to counteract the low water potential in the environment ( Benlloch-Gonza´lez et al. 2005). According to Duan et al. (2007), tolerance to salinity stress in Suaeda salsa, one member of the family Chenopodiaceae , was linked through the mechanism of Na+ uptake, which is used for osmotic adjustment (Table 2). The presence of salt decreases the water potential, so plants have problems with absorption of water. Vacuolar compartmentation of ions serves a function to maintain the

CONTINUE:

CONTINUE appropriate water potential by enhancing the absorption of water. Quintero et al. (2000) have shown that the mechanism of confinement of toxic Na+ in the vacuole contributed to the maintenance of sublethal ion levels in the cytosol for osmotic adjustment. In a study by Moghaieb et al. (2004), additional Na+ in Salicornia europaea and Suaeda maritima grown at higher salt concentration accumulated in the vacuole and provided an osmotic driving force for the uptake of water in highly saline environments, which is consistent with the salt-accumulating character of the halophyte Sesuvium

CONTINUE:

CONTINUE portulacastrum and its capacity to sequester Na+ in the vacuoles for osmotic adjustment ( Messedi et al. 2004). Furthermore, Quintero et al. (2000) have also shown that uptake of ions for osmotic adjustment must reach the leaves at a rate that did not exceed the capacity of leaf cells to accumulate them in the vacuole. Thus, salt exclusion in the root and salt sequestration in the cell vacuoles have been presumed to be critical co-ordinated determinants for salt tolerance. In addition, some studies indicate that in several plants K+ or Ca2+ are regarded as the most important cationic osmolytes and accumulated to an osmotically significant degree

CONTINUE:

CONTINUE in cells (Laurie et al. 2002; Reddy and Reddy 2004). For example, three species of cassava were grown in greenhouse conditions and subjected to water deficit treatments, and it was found that the concentration of K+ increased in response to water stress, which was positively correlated with the extent of osmotic adjustment ( Alves and Setter 2004). Ma et al. (2009) have shown that Ca2+ participates in many physiological and biochemical reactions in plants as a common second messenger through coupling both cellular and intercellular

CONTINUE:

CONTINUE signal transduction networks, and the change of Ca2+ concentration and distribution in plant cells may be considered as the plant’s responses and adaptations to the outside environment. In the presence of Ca2+, for example, wheat ( Triticum aestivum) showed significantly more accumulation of osmolytes in response to water stress by osmotic adjustment ( Nayyar 2003). Besides cations introduced above, Cl – is also an important inorganic ion and might also play key roles in osmotic adjustment. For example, Shabala et al. (2000) suggested a role of the hyperosmolarity induced influx of K+ and Cl – in plant (e.g., bean) cells that could be sufficient for osmotic adjustment without additional accumulation of organic solutes.

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CASE STUDIES

Osmotic Adjustment In Barley Two Weeks After Transfer to High Salt:

Osmotic Adjustment In Barley Two Weeks After Transfer to High Salt Standard error = 0.057 MPa or less (n = 3) Barley

Contribution of NaCl to Osmotic Adjustment In Barley Two Weeks After Transfer to High Salt:

Contribution of NaCl to Osmotic Adjustment In Barley Two Weeks After Transfer to High Salt Standard error = 0.04 MPa or less (n = 3) Barley

Osmotic Adjustment In Maize Ovaries After Withholding Water:

Osmotic Adjustment In Maize Ovaries After Withholding Water Osmotic adjustment “turned on” in ovary by feeding sucrose to stem Maize

Osmotic Adjustment in Sorghum 1. Mechanisms of Diurnal Osmotic Potential Changes:

Osmotic Adjustment in Sorghum 1. Mechanisms of Diurnal Osmotic Potential Changes Osmotic adjustment, defined as a lowering of osmotic potential due to net solute accumulation in response to water stress, has been considered to be a beneficial in drought tolerance mechanism in some crop species. The objective of this experiment was to determine the relative contribution of passive versus active mechanisms involved in diurnal 'II changes in sorghum ( Sorghum bicolor L. Moench) leaf tissue in response to water stress. A single sorghum hybrid (cv ATx623 x RTx430) was grown in the field under variable water supplies. Sorghum

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and relative water content were measured diumally on expanding and the uppermost fully expanded leaves before flowering and on fully expanded leaves during the grain-filling period. Diurnal changes in total osmotic potential (A*In) in response to water stress was 1.1 megapascals before flowering and 1.4 megapascals during grain filling in comparison with 0.53 megapascal under well-watered conditions. Under water-stressed conditions, passive concentration of solutes associated with dehydration accounted for 50% (0.55 megapascal) of the diumal AI,, before flowering and 47% (0.66 megapascal) of the change during grain filling. Net solute accumulation accounted for 42% (0.46 megapascal) of the diumal A*I' before flowering and 45% (0.63 megapascal) of the change during grain filling in water-stressed leaves. The relative contribution of changes in nonosmotic volume (decreased turgid weight/dry weight) to diumal A*I' was less than 8% at either growth stages. Water stress did not affect leaf tissue elasticity or partitioning of water between the symplasm and apoplasm.

Influence of Osmotic Adjustment on Leaf Rolling and Tissue Death in Rice (Oryza sativa L.):

Influence of Osmotic Adjustment on Leaf Rolling and Tissue Death in Rice (Oryza sativa L.) Osmotic adju t, meaed by the lowering ofthe osmotic pottial at fall turg , aNd its on leaf rolilg and eaf death wa assessed in the lowland rice ( Oryza sai a L.) cultivar 1136 In both thege and field. The degre of osmotic t vaied with the degree and duration of stress, but was ususlly 0.5 to 0.6 megap (lm lly 0.8 to 0.9 megpcal ) nder severe stress c s In leaves in which osmotic adjustment wa 0.5 to 0.6 megapl leaf roiling and lafdeath occurred at lower leaf water potentis in adsted than in adjed leaves. We conclude that osmotic adjument aids in the dronght resistance of rie by delaying laf rollng thereby ggs excn , and by delaying leaf death.

CONCLUSION :

CONCLUSION Osmotic adjustment occurs in saline and dehydrating soils. Osmotic adjustment results from solute accumulating faster than it is used . Growth is inhibited first, decreasing solute use, but remaining growth is more rapid than in absence of osmotic adjustment. Solute may be obtained from inorganic salts in soil and from products of photosynthesis . Solute accumulates in vacuole and cytosol . Osmotic adjustment maintains ability to absorb water from environment thus maintaining water volume and Turgor .

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