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INTERNATIONAL JOURNAL OF ENVIRONMENTAL SCIENCES Volume 1, No 6, 2011 

© 2011 Mane A.V et al., licensee IPA- Open access - Distributed under Creative Commons Attribution License 2.0 

Research article ISSN 0976 – 4402 

A critical review on physiological changes associated with reference to 

salinity 

Mane A. V 1 ,Deshpande T.V 2 ., Wagh V.B 3 , Karadge B. A., Samant J. S. 5 

1- Department of Environmental Sciences, Fergusson College, Pune (India) 

2- Department of Chemistry, Fergusson College, Pune (India) 

3- Haffkine Institute for Training, Research and Testing, Mumbai (India) 

4- Department of Botany, Shivaji University, Kolhapur (India) 

5- Department of Environmental Sciences, Shivaji University, Kolhapur (India) 

ashishmane145@yahoo.co.in 

doi:10.6088/ijes.00106020014 

ABSTRACT 

Soil salinity is abiotic stress which adversely influences on growth, overall development and 

productivity of plants. The plant response to salinity consists of numerous morphological and 

cellular changes which function in a well coordinated way to alleviate toxicity and changes 

therefore. Adaptation of some species to elevated salt concentrations provides evidence for 

inherent potential existed in plants to survive under unfavorable conditions. It is well 

identified that tolerance and yield constancy are multifaceted genetic characters and are 

difficult to establish in crops since salt stress may occur as a disastrous. Salt stress may occur 

immediately, slowly or continually which may again differ in dose as periodically or 

gradually become severe during any stage of the life cycle of the plants. Therefore research 

strategies have to be developed to make the plants adaptable to saline environment to face 

diverse conditions at any stage of growth. Plant growth and internal changes responds to 

salinity by a way of rapid osmotic phase which inhibits growth of leaves by hampering 

photosynthesis and another slower but distraous ionic phase that accelerates senescence of 

leaves. Plants may adapt to salinity with three different types as osmotic stress tolerance, Na + 

or Cl - exclusion and by the way of tissue tolerance to sodium and chloride ions. Nowadays, 

plant physiology, cell biology and molecular genetics research are providing new insights 

into the plant response to salinity and to improve tolerance of plants relevant to food 

production and environmental sustainability. Further improvement in tolerance to salinity 

may be definitive to find out the genetic resources more easily with the understanding of 

physiological mechanisms concerned in controlling the responses to stress and also if the 

plants indicate salt tolerance at morphological or cellular level, selection becomes a suitable 

applied method. This will definitely give a hand in choosing the wonder plant species for the 

breeders and to overcome a challenging problem of salinity. Better management of soil 

resource with wise practices with the tolerant and adaptive varieties could be used 

successfully for raising crop productivity especially in the areas where salinity is consistent 

and with huge economic loss to the farmers. Therefore, an understanding of appropriate 

physiological mechanisms controlling stress tolerance so as to provide plant breeders with 

appropriate selection criteria is essential. The present review elucidates the biochemical 

changes and associated reasons of the parameters mainly growth, photosynthesis, 

polyphenols, nitrogen metanbolis, antioxidant enzymes, carbohydrates and minerals. 

Keywords: Antioxidants, carbohydrates, growth characteristics, minerals, nitrogen 

metabolism, photosynthetic pigments, polyphenols. 

Received on February, 2011 Published on March 2011 1199

A critical review on physiological changes associated with reference to salinity 

1. Introduction 

Abiotic stress is the negative impact of non-living factors on the living organisms in a 

specific environment (Mane et al., 2010). It is well established that abiotic stress is the most 

harmful factor concerning the growth and productivity of crops worldwide. Research has also 

shown that abiotic stressors are at their most harmful stage when they occur together. The 

most basic stressors include high winds, extreme temperatures, salinity, drought, flood and 

other natural disasters, such as tornado and wild fire. The lesser-known stressors like poor 

edaphic conditions like changes in pH, soil compaction, radiation and other highly specific 

conditions like rapid rehydration during seed germination generally occur on a smaller scale 

and so are less noticeable. Plants under stressful conditions adapt very differently from one 

another, even from a plant living in the same area (Munns, 2002). A plant‟s first line of 

defense against abiotic stress is in its roots. If the soil holding the plant is healthy and 

biologically diverse, the plant will have a higher chance of surviving under stressful 

conditions. Even a low concentration of the contaminants typically alter plant metabolism, 

most commonly to reduce crop yields (Flowers, 2004, Munns, 2002). Severe land 

degradation affects a significant portion of the earth's arable lands, decreasing the wealth and 

economic development of nations. Land degradation cancels out the gains advanced by 

improved crop yields and reduced population growth. Environmental pollution is one of the 

most significant problems that the world faces today. Population growth, urbanization, 

industrialization, intensive agriculture, poverty and many other developmental activities are 

the key drivers of environmental pollution. 

Soil is a valuable resource with a key productive role in agriculture and forestry, since it is 

needed to produce crops, vegetables, fruit, timber and other economically important items 

(Mane et al., 2010a). Maintenance of soil fertility, together with sustainable management of 

crop productivity, is therefore one of the fundamental issue, not only for the present but also 

for the welfare of the future generations. Soil salinity might not be as dramatic as earthquake 

or large-scale landslides, but certainly, a severe environmental hazard. Salinity is one of the 

major abiotic stresses that adversely affect crop productivity and quality (Shani and Dudley, 

2001; Ouda, 2008) with increasing impact on the socio-economic fabric and health, 

especially of the farming communities. Salinity is a general term used to describe the 

presence of elevated levels of different salts such as sodium chloride, magnesium and 

calcium sulphates and bicarbonates in soil and water. The focus of this review is to 

understand the physiological changes that occur within a plant under the influence of salt 

stress. The aim will be definitely helpful to plant breeders and researchers involved in 

relation with development of salt tolerant varieties. The newer advanced molecular 

techniques and the results obtained associates with it will really helpful in making great 

insights into the salinity research. The growth in knowledge and newer prospects in salt 

tolerance mechanism are providing us to achieve the aims of higher food productivity and 

wise use of environmental resources including water, soil and plant. 

1.1 Causes and Tolerance of Salinity 

The United States Salinity Laboratory Staff (1954) had defined a saline soil as one having 

electrical conductivity of saturation extract of soil greater than 4 mS cm -1 or equivalent to 

approximately 40 meq l -1 and an exchangeable sodium percentage less than15%. Usually, pH 

of saline soils remains below 8.5. The causal factors may be geological, climatic and 

hydrological in nature. According to Chapman (1966), saline soil owes its origin in single or 

combination of the following factors as a primary or secondary type. Primary salinity arises 

Mane A. V,Deshpande T. V., Wagh V. B, Karadge B.A., Samant J. S. 

International Journal of Environmental Sciences Volume 1 No.6, 2011 

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due to 1) weathering of rocks, 2) capillary rise from shallow brackish groundwater, 3) 

intrusion of seawater along the coast, 4) salt laden sand blown by sea winds and 5) impeded 

drainage. While, secondary salinisation is the result of human activities such as introduction 

of irrigation without proper drainage system, industrial effluents, overuse of fertilizers, 

removal of natural plant cover and flooding with salt rich waters, high water table and the use 

of poor quality groundwater for irrrigation. 

Physiologically and genetically salt tolerance is complex among the variety of plants with a 

wide range of adaptations in halophytes and less tolerant plants (Flowers, 2004, Mane et al., 

2010a). According to Fitter and Hay (1987), plants resist salinity by four different ways 1) 

phenological escape (seasonal adaptation) 2) exclusion (selective absorption) often seen in 

halophytes (Epstein, 1969) 3) amelioration (selective localization) and 4) tolerance (stress 

acceptance). High concentrations of soluble salts in the soil moisture of the root zone are 

always associated with the saline soils (Larher et al., 1993). Tolerance to high soil (Na + ) 

involves processes in many different parts of the plant and is manifested in a wide range of 

specializations at disparate levels of organization, such as gross morphology, membrane 

transport, biochemistry and gene transcription (Tester and Davenport, 2003). The results 

suggest that common physiological mechanism confers tolerance to both excess CaCl 2 and 

excess NaCl 2 , but a different mechanism to excess MgCl 2 (Kobayshi et al., 2005). In a more 

specific sense, oxidative stress due to NaCl might contribute to the deleterious effects of salt 

and significant growth reductions in plants (Hernandez et al., 1999). 

1.2 Magnitude of Salinity 

About 1.93 lakh km 2 area has been estimated to be affected by saline water of electrical 

conductivity (EC) greater than 4000 mS cm -1 . In some areas of Rajasthan and Gujarat ground 

water salinity is so high that the well water is directly used for salt manufacturing by solar 

evaporation (www.pib.nic.in). Sehgal and Abrol, (1994) report that 187.2 mha (million 

hectare) area in India is degraded, of which 162.4 mha is degraded by water and wind erosion 

and 21.7 mha by salinity and water logging. The remaining 4 mha area is affected by the 

depletion of nutrients. Statistics about the extent of salt affected areas vary according to 

authors, but estimates are in general close to one billion hectares, representing about 6% of 

the earth‟s continental extent. In addition to these naturally salt affected areas, about 77 mha 

have been salinised by human activities (Ghassemi et al., 1995) but Oldeman et al. (1991) 

estimates it to be 76 mha, with 58% of these concentrating in irrigated areas. Based on the 

Food and Agriculture (FAO, 1997) soil map of the world, the total area of saline soils is 397 

mha and of sodic soils is 434 mha at global level. Of the current 230 mha of irrigated land, 45 

mha are salt-affected soils (19.5%) and of the almost 1500 mha of dryland agriculture, 32 

mha are salt-affected soils (2.1%) to varying degrees by human-induced processes. On an 

average, 20% of the world‟s cultivated area and nearly half of the world‟s irrigated lands are 

affected by salinity, but this figure increases to more than 30% (Zhu, 2001) in countries such 

as Egypt, Iran and Argentina. When soil salinisation is assessed in economic terms, reasons 

to be worried as it become more apparent. For instance, the economic damage caused by 

secondary salinisation was estimated at 750 million US $ per year for the Colorado river 

basin in the USA, 300 million US $ per year for the Punjab and northeast frontier provinces 

of Pakistan and 208 million US $ per year for the Murry-Darling basin in Australia 

(Ghassemi et al., 1995). 

2. Alterations in Growth Characteristics of Plants 



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The causes of growth reduction differ (Munns et al., 1995), but it is not clear which 

mechanism plants employ to maintain residual growth and to what extent these mechanisms 

differ between short and long-term responses. The soluble salts in high concentration 

interfere with a balanced absorption of essential nutritional ions by the plants. The main 

effects can be seen as slow and insufficient germination of seeds, a patchy stand of the crop, 

physiologic drought, wilting, desiccation, stunted growth, reduction in leaf area, reduction in 

root and shoot lengths, retarded flowering, fewer flowers, sterility and smaller seeds etc 

(Mane et al., 2010). Membrane disorganization, reactive oxygen species, metabolic toxicity, 

inhibition of photosynthesis and attenuated nutrient acquisition are the startling factors that 

initiate more catastrophic events (Yeo, 1998). Seed germination is the most critical step in the 

life cycle of the plant but it is hampered by high salinity stress (Maghsoudi and Maghsoudi, 

2008). A high NaCl concentration causes a reduction in growth parameters (Sixto et al., 

2005; Pessarakli and Touchane, 2006; Razmjoo, et al., 2008, Turhan et al., 2008; Sohrabi, 

2008; Schuch and Kelly, 2008) such as fresh and dry weight of leaves, shoots and roots along 

with a decrease in moisture content (Parvaiz and Riffat, 2005). But decrease in growth 

characteristics varies species-wise (Jarunee et al., 2003). Several aspects of reproductive 

growth including flowering, pollination, fruit development, yield and quality are also 

influenced by salinity (Shannon et al., 1994). High salinity stress also delays the emergence 

of nodal roots, leaf and tiller (Coardoba et al., 2001) with decrease in relative growth rate 

(RGR), leaf area ratio (LAR) and specific leaf area (Sharon et al., 2005). Stomatal 

conductance, leaf-level transpiration and internal CO 2 concentrations lower at high salinity 

(Hoffman et al., 2006) along with senescence (Lutts et al., 1999). NaCl stress also inhibits 

cell expansion, however; the precise contribution of subsequent processes to inhibition of cell 

division and expansion and acceleration of cell death has not been well-elucidated (Yeo, 

1998). 

2.1 Root Length, Shoot Length and Leaf Area 

Roots might seem to be the most vulnerable part as they are directly exposed to salt or to 

drying soil, but they are surprisingly robust. Their ionic status remains relatively good and 

also the ion concentrations do not increase with time as in the leaves and they often have a 

lower Na + and Cl - concentrations than the external solution, which rarely happens in the 

leaves (Tattini et al., 1995; Munns, 2002). Shonjani (2002) observed much inhibition in root 

and in particular shoot growth with NaCl treatments for sugar beet, rice and cotton seedlings 

while a decrease in length of shoots was more pronounced at higher salt treatment (200 mM). 

The inhibition of shoot growth during water deficit is thought to contribute to solute 

accumulation and thus eventually to osmotic adjustment (Osorio et al., 1998). Gulzar et al. 

(2003) reported a decrease in the growth of Urochondra setulosa (Trin.) under the influence 

of salinity and a significant (P < 0.05) inhibition of root length, shoot length, number of tillers 

and number of leaves at NaCl salinity level above 200 mM. Number of authors in relation 

with growth characteristics had reported a significant decrease in shoot length of plants under 

the influence of salinity (Cicek and Cakirlar, 2002; Shalhevet et al., 1995; Pessarakli and 

Touchane, 2006; Mane et al., 2010a). Tattini et al. (1995) noticed a decrease in shoot to root 

ratio in olive plants while Bauci et al. (2003) also found gradual decline in shoot to ratio with 

increasing NaCl concentrations ot the rooting medium. Similar results were also reported by 

number of authors (Mathangi et al., 2006; Parida and Das, 2005). According to Munns 

(1993), the first phase of the growth response results from the effect of salt outside the plant 

which results in reduced leaf growth and to a lesser extent root growth. When salinity is 

applied to the root medium, leaf elongation is immediately inhibited for maize (Cramer, 



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1992; Munns et al., 2000), rice (Yeo et al., 1991), barley (Munns et al., 2000a), tomato 

(Tantawy et al., 2009) and Jowar (Stuart et al., 2000). Reduction in leaf area under the 

influence of salinity is also noticed in Gossypium hirsutum and Phaseolus vulgaris (Brugnoli 

and Lauteri, 1991), Maiz (Cicek and Cakirlar, 2002), Prosopis argentina and Prosopis 

alpataco (Villagra and Cavangnaro, 2005), Atriplex prostrata (Wang et al., 1997), 

Catharanthus roseus (Jaleel et al., 2008) and in wheat and chickpea (Sheldon et al., 2004). 

Salinity has both osmotic and also specific ion effects on plant growth (Dionisio-Sese and 

Tobita, 2000). Reduction in plant growth as a result of salt stress has also been reported in 

several other plant species (Ashraf and McNeilly, 1990; Ashraf and O‟leary, 1997). Salinity 

in soils restricts water availability to plants in a similar manner to water stress, which causes 

reductions in growth rate and even in production (Munns, 2002). 

3. Biomass and Moisture Content 

The reduction in shoot biomass production by the plant may be due to the chlorosis and 

necrosis of the leaves that reduce the photosynthetically active area (De Herralde et al., 1998). 

Digby and Timothy (1999) noticed that the shoot fresh weight of H. pergranulata ssp. 

pergranulata was highest for plants grown at 10 to 300 mol m -3 NaCl and declined in plants 

exposed to NaCl concentrations of 400 mol m -3 or higher. On the contrary, Ceyhan and Ali 

(2002) observed an increase in fresh weight of lettuce plant at higher levels of salinity. Jaleel 

et al. (2008) found a reduction in fresh weight (about 25%) in Catharanthus roseus while 

Cicek and Cakirlar (2002) found a decrease in fresh weight of maize under salt stress. Inal et 

al. (1997) and Tantawy et al. (2009) also noticed a decrease in fresh weight of tomato plants 

while Khosravinejad et al. (2009) noticed in two barley varieties namely Afzal and EMB82- 

12 with increasing levels of salinity. Shoot dry weights of Halosarcia pergranulata ssp. 

pergranulata was the highest for plants grown at 10 to 300 mol m -3 NaCl and both were 

declined in plants exposed to NaCl concentrations of 400 mol m -3 or higher (Digby and 

Timothy, 1999). Contrary to this, Ceyhan and Ali (2002) reported an increase in dry weight 

of lettuce plants while Jaleel et al. (2008) found 26% reduction in dry weight of Catharanthus 

roseus due to salinity. Gulzar et al. (2003) also reported a significant (P


Photosynthesis is one of the most important biochemical pathways by which plants prepare 

their own food material and grows. As a matter of fact, there has been knowledge on increase 

of chlorophylls content in saline environment depending on salt levels (Romero-Aranda et al., 

2001). Chlorophyll content in plants correlates directly to the healthiness of plant (Zhang et 

al., 2005). The total chlorophylls content decreases under NaCl salinity stress (Kaya et al., 

2002, Khodary, 2004; Sadale, 2007; Kate, 2008; Jaleel et al., 2008; Hudai and Arzu, 2008) in 

salt stressed sorghum and maize plants. Salinity stress causes changes in chloroplast 

ultrastructure (Locy et al., 1996; Keiper et al., 1998) and there is also a decrease in rate of 

photosynthesis under saline conditions (Sixto et al., 2005) based on species and clones of 

genus. Photosystem II is a relatively sensitive component of the photosynthetic system with 

respect to salt stress (Allakhverdiev et al., 2000). A considerable decrease in the efficiency of 

PS II, electron-transport chain (ETC) and assimilation rate of CO 2 occurs under the influence 

of salinity (Piotr and Grazyna, 2005). In contrast to this, Mathangi et al. (2006) observed no 

change in photosynthetic potential of the plants by lower level of salt treatment and with 

increase in time. Usually there is dominance of chlorophyll „a‟ over chlorophyll „b‟ in plants 

but their values become closer with increasing salinity (Mane et al., 2010). The decrease in 

chlorophyll content under stress is a commonly reported phenomenon and in various studies, 

this may be due to different reasons, one of them is related to membrane deterioration (Mane 

et al., 2010; Tantawy et al., 2009). The resistance of photosynthetic systems to salinity is 

associated with the capacity of the plant species to effectively compartmentalise the ions in 

the vacuole, cytoplasm and chloroplast (Reddy et al., 1992). 

3.2 Nitrogen Metabolism 

Nitrogen is a critical chemical element in both proteins and DNA. The key enzyme in 

nitrogen metabolism, nitrate reductase is very sensitive to NaCl (Gouia et al., 1994). One of 

the amino acids, glycinebetaine shows increased trend with increase in salinity in perennial 

halophytes, Atriplex griffithii (Khan et al., 2000a) and Suaeda fruticosa (Khan et al., 2000). 

Proline is an α-amino acid, one of the twenty DNA-encoded amino acids and occurs widely 

in higher plants and accumulates in large amounts as compared to all other amino acids in salt 

stressed plants (Ali et al., 1999, Mane et al., 2010). An increased level of Cl - is responsible 

for proline accumulation in plants under salinity stress (Ceyhan and Ali, 2002; Wanichananan 

et al., 2003). 

3.3 Nitrate and Nitrite Nitrogen 

An increase in nitrate nitrogen content in roots and shoots of Setaria italica due to salinity 

stress has been reported by Toro (2000). Rubinigg et al. (2003) observed that under steady 

state conditions, the negative effect of sodium chloride on the rate of net uptake of nitrate in 

the shoot of halophyte Plantago maritima at the higher levels was mainly due to the 

interaction between sodium chloride and nitrate transporters in the root plasma membrane 

and/or processes mediating the translocation of nitrogen compounds, possibly nitrate to the 

shoot. According to Da Silveira et al. (1999) the nitrate content was decreased in shoot of 

cowpea plant but it was not changed in roots under the influence of salinity. If nitrate (NO 3 - ) 

absorbed by roots is not reduced in the roots, it must be transported to shoots, where it 

stimulates shoot growth (McDonald et al., 1996). In the soils with EC (electrical 

conductivity) range of 0 to 5 dS m -1 , sharp increase of nitrate content in the lettuce represents 

the stimulating effect of EC on the nitrate accumulation and might be due to the inhibition of 

nitrate uptake by chloride and sulphate in the soil (Ceyhan and Ali, 2002). Nitrite ions also 

present a more acute toxicity toward the photosynthetic apparatus especially in their acid 



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form, nitrous acid, which can diffuse freely across membranes (Sinclair, 1987). Nitrite can 

also lead to the formation of NO (Nitrogen monoxide) which can further react with reactive 

oxygen species to produce peroxynitrite; a strong oxidant able to nitrate, tyrosine residues 

and thus to modify protein activities (Morot-Gaudry et al., 2002). 

3.4 Amino Acids and Amides 

Amino acids (alanine, arginine, glycine, serine, leucine, and valine, together with the imino 

acid,proline, and the non-protein amino acids, citrulline and ornithine) and amides (such as 

glutamine and asparagines) have been reported to accumulate in plants subjected to salt stress 

(Mansour, 2000). Amino acids represent one of the most important classes of metabolites in 

the cell and obviously due to the fact that they are the building blocks of proteins, which form 

the chemical basis necessary for life and have a variety of roles in metabolism. Free amino 

acids, especially proline can accumulate in a variety of species and serve as osmotically 

active solute (Fougere et al., 1991). Di Martino et al. (2003) found glutamate, glutamine, 

serine, alanine and aspartate as the most abundant free amino acids in spinach under the 

influence of salt stress while the pool of other amino acids like valine, leucine, isoleucine, 

lysine, tryptophan and proline was approximately 1% of the overall amount with only traces 

of glycinebetaine. Amino-acids such as proline, asparagine and aminobutyric acid, can play 

an important role in the osmotic adjustment of the plant under saline conditions (Gilbert et al., 

1998). Total free amino acids in the leaves have been reported to be higher in salt tolerant 

than in salt sensitive lines of sunflower (Ashraf and Tufail, 1995; Sayed and Gadallah, 2005), 

Bruguiera parviflora (Parida et al., 2002), safflower (Ashraf and Fatima, 1995), Eruca sativa 

(Ashraf, 1994) and in Ipomoea pes-caprae (Venkatesan and Chellappan, 1998). The impaired 

plant health results in ammonia accumulation early during the stress period and that the 

detoxification process in which excess ammonium in the cells is removed, results in the 

accumulation of nitrogen containing compounds such as glutamine, arginine and proline (Lin 

and Kao, 2001). 

Proline is an α-amino acid, one of the twenty DNA-encoded amino acids. It is unique among 

the 20 protein forming amino acids because the α-amino group is secondary. Proline 

accumulation in response to environmental stresses has been considered by number of 

authors as an adaptive trait concerned with stress tolerance and it is generally assumed that 

proline is acting as a compatible solute in osmotic adjustment (Larher et al., 1993). Its 

accumulation is caused by both the activation of its biosynthesis and inactivation of its 

degradation (Mattioni et al., 1997). Proline accumulation normally occurs in the cytosol 

where it contributes substantially to the cytoplasmic osmotic adjustment (Ketchum et al., 

1991). It is osmotically very active and contributes to membrane stability and mitigates the 

effect of NaCl on cell membrane disruption (Mansour, 1998). Maggio et al. (2002) are of the 

view that proline may act as a signaling or regulatory molecule and able to activate multiple 

responses that are component of the adaptation process. Proline occurs widely in higher 

plants and accumulates in larger amounts than other amino acids (Ali et al., 1999; Abraham 

et al., 2003), regulates the accumulation of useable nitrogen. Proline production decreases the 

osmotic potential of cells, which can increase the uptake of water (Hare et al., 1998). 

Additionally, increased level of free proline is an indicator of protein degradation (Yoshiba et 

al., 1997). However, according to the contrasting reports on the role of proline in salt 

tolerance, its use as selection criterion for salt tolerance has been questioned (Ashraf and 

Harris, 2004). The effect of salt stress on proline accumulation was reported in many plant 

species such as Oryza sativa L. and Morus alba L. cv. Khonpai (Harinasut et al., 2000), 

Phaseolus Mungo (Dash and Panda, 2001) and in wheat seedlings (El-Shintinawy and El- 



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Showbagy 2001). It is concluded that excess proline induces the expression of salt stress 

responsive proteins and improves the salt tolerance in plants (Abdel et al., 2003). Maggio et 

al. (2002) were of the view that proline may act as a signaling or regulatory molecule, able to 

activate multiple responses that are components of the whole adaptation process. 

Amides are important chemical intermediates since they can be hydrolysed to acids, 

dehydrated to nitrites and degraded to amines containing one less carbon atom. Compounds 

in which a hydrogen atom or nitrogen from ammonia or an amine is replaced by a metal 

cation are also known as amides or azanides. Glutamine and asparagine produced from 

glutamic acid and aspartic acid respectively are the two important amides produced in the 

plants. Amides are commonly formed from the reaction of a carboxylic acid with an amine. 

Toro (2000) observed that the amide content of roots of Setaria italica cv. SIC-1 was not 

much affected by salinity but that of stem and leaves was reduced. She concluded that amides 

did not play any significant role in osmotic adjustment under saline conditions. However, 

Carillo et al. (2005) reported that in Durum wheat seedlings under salinity stress, asparagine 

in particular functions as osmolyte to balance water potential within the cells, especially 

when nitrogen availability exceeds the need for growth. 

3.5 Nitrate Reductase 

Nitrate reductase (NR) is a large and complex enzyme with multiple subunits and a mass of 

~800 kDa. Molybdenum is bound to a cofactor containing a pteridine ring to form 

molybdopterin. Nitrate reductase, a metaflavo-protein, catalyzing the reduction of nitrate to 

nitrite is considered as the limiting step for conversion of nitrate-N to amino acids and so for 

protein synthesis (Ferrario et al., 1998). Sugars, cytosolic acidification and anaerobiosis are 

factors, all known to activate NR in both leaves and roots (Kaiser and Huber, 2001). 

Regulation of NR is closely coupled with photosynthesis and post translational inactivation of 

NR takes place when light intensity is suddenly decreased or the leaves are deprived of CO 2 

(Liso et al., 2004). The close coupling of NR regulation to photosynthesis may be important 

in order to avoid the accumulation of the product of the NR reaction, NO 2 - . Slow seed 

germination can be accelerated either by hormonal treatment or by nitrate (Khan, 1997). 

Regulation of nitrate reductase in roots of Hordeum vulgare L. can be carried out by nitrogen 

source and salinity (Omarov et al., 1998). It is clear that at higher levels of salinity the 

activity of this enzyme decreases resulting in damage to the photosynthesis process. 

3.6 Carbohydrates Metabolism 

Carbohydrates fill numerous roles in living things, such as the storage and transport of energy 

(starch, glycogen) and as a part of structural components (cellulose in plants, chitin in 

animals). Additionally, carbohydrates and their derivatives play major roles in the working 

process of the immune system, fertilization, pathogenesis and development (Maton et al., 

1993). Carbohydrates represent the most important compounds so far as dry matter 

production and energy relations of cells concerned. Carbohydrate changes are of particular 

importance because of their direct relationship with such physiological processes as 

photosynthesis, translocation and respiration. Among the soluble carbohydrates, sucrose and 

fructans have a potential role in adaptation to these stresses (Housley and Pollock, 1993; 

McKersie and Leshem, 1994). Sugar content increases under lower levels of salinity (Parvaiz 

and Riffat, 2005; Sadale, 2007) but in contrast to this Rathert (1983) observed that the 

sucrose and starch are the predominant carbohydrates affected by salinity, and soluble sugars 

are more sensitive to salt stress than starch. Sugars in plants generally serve mainly as source 



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of carbon and energy, osmotica, stress protectants and signal molecules. In general, the roles 

of particular soluble saccharides in plants are very difficult to distinguish from one another, 

as they are thought to be mutually tightly interconnected. Shonjani (2002) observed increase 

in glucose, fructose and maltose content (except sucrose) in maize. Al-Sobhi (2006) also 

noticed the increase in content of soluble and insoluble sugars and total carbohydrates in the 

shoot and root of the seedlings of Calotropis procera plants with increasing salinity level and 

age. Khosravinejad et al. (2009) observed the increase in soluble sugars in two varieties of 

barley namely Afzal and EMB82-12 where the increase in Afzal var. was higher than 

EMB82-12. Djanaguiraman et al. (2006) observed a decrease in starch content of rice under 

the salinity regime at 100 mM NaCl. 

3.7 Polyphenols 

Polyphenols are a group of chemical substances found in plants, characterized by the 

presence of more than one phenol units or building blocks per molecule. Polyphenols are 

generally divided into hydrolysable tannins and phenylpropanoids, such as lignins, flavonoids 

and condensed tannins. Notable sources of polyphenols include berries, tea, beer, wine, olive 

oil, cocoa, coffee, walnuts, peanuts, pomegranates and other fruits and vegetables. Very little 

attention has been paid towards the influence of salinity on the polyphenol metabolism in 

plants. Karadge (1981) observed a linear decrease in polyphenol content of the leaves of 

Portulaca oleracea with increasing concentrations of NaCl in the rooting medium. Parida et 

al. (2002) observed an accumulation of polyphenols in Bruguiera parviflora and such an 

accumulation of polyphenols played a key role in the plants towards stress. Levels of 

polyphenols also increases under increasing levels of salinity, which shows that the induction 

of secondary metabolism is one of the defence mechanisms adapted by the plants to face 

saline environment (Sadale, 2007; Kate, 2008, Mane et al., 2010). A considerable increase in 

polyphenol content of the leaves under NaCl salinities had been recorded by Chavan and 

Karadge (1986) in Elusine coracana, Karadge and Chavan (1981) in Groundnut var. TMV- 

10, Parida et al., (2004) in Aegiceros corniculatum and Singh and Kumari (2006) in Brassica 

campastris. 

3.8 Antioxidants 

Plants maintain complex systems of multiple types of antioxidants, such as glutathione, 

vitamin C and vitamin E as well as enzymes such as catalase, superoxide dismutase and 

various peroxidases. Low levels of antioxidants or inhibition of the antioxidant enzymes, 

causes oxidative stress and may damage or kill cells. Salt stress induces the increased 

activities of superoxide dismutase, ascorbate peroxidase, catalase and glutathione reductase in 

rice, representing a higher antioxidative capacity to NaCl, which is one of the salt tolerant 

mechanisms adapted by rice to cope with it (Hoai, 2005). Similarly increase in the activities 

of superoxide dismutase, catalase, peroxidase (Gao et al., 2008, Arora et al., 2008) and 

polyphenol oxidase (Agarwal and Pandey, 2004) was reported. With increasing salinity up to 

150 mM NaCl, the hydrogen peroxide content and the activity of guaiacol-specific 

peroxidase increases markedly, but in contrast, catalase activity with increasing salinity is not 

correlated with hydrogen peroxide content in mulberry (Poontariga et al., 2003). Decreased 

reactive oxygen of speceis (ROS) production contributes to the salinity associated reduction 

in maize‟s leaf elongation, acting through a mechanism which is associated with pH change 

(Andre et al., 2004). In an environment of molecular oxygen (O 2 ), all living cells are 

confronted with the reactivity and toxicity of active and partially reduced forms of oxygen 

can lead to the complete destruction of cells (Mittler et al., 2004). In plants, the links between 



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ROS production and photosynthetic metabolism are particularly important (Rossel et al., 

2002). Salt stress can lead to stomatal closure, which reduces CO 2 availability in the leaves 

and inhibits carbon fixation, exposing chloroplasts to excessive excitation energy, which in 

turn could increase the generation of Reactive Oxygen Species (ROS) and induce oxidative 

stress (Uchida et al., 2002; Parida and Das, 2005; Parvaiz and Satyawati, 2008). In plant cell, 

antioxidant enzymes such as superoxide dismutase (SOD), peroxidase (POD) and catalase 

(CAT) have been considered to act as a defensive team, whose combined purpose is to 

scavenge reactive oxygen species and protect cells from oxidative damage (Mittler, 2002; 

Mittova et al., 2003). The main sites of ROS production in the plant cells are chloroplasts, 

mitochondria and peroxisomes (Garnczarska et al., 2004). In chloroplasts, ROS can be 

generated by the direct transfer of the excitation energy from chlorophyll to produce singlet 

oxygen or by oxygen reduction in the Mehler reaction (Meloni et al., 2003). H 2 O 2 and O 2 

- 

may interact in the presence of certain metal ions and chelates to produce highly reactive OH - 

(Sudhakar et al., 2001). However, ROS are scavenged by plant antioxidant defense system, 

comprising both enzymatic and non-enzymatic components (Ashraf, 2009). 

3.9 Catalse and Peroxidase 

Catalase is a common enzyme found in nearly all living organisms, where it functions to 

catalyze the decomposition of hydrogen peroxide to water and oxygen. Catalase is a tetramer 

of four polypeptide chains, each over 500 amino acids long (Boon et al., 2007). Catalase is 

usually located in a cellular organelle called the peroxisome (Alberts et al., 2002). It contains 

four porphyrin heme (iron) groups that allow the enzyme to react with the hydrogen peroxide. 

Catalase can also oxidise different toxins, such as formaldehyde, formic acid, phenols and 

alcohols. The reaction of catalase in the decomposition of hydrogen peroxide is as follows 

(Asada, 1992; Scandalios, 1994, Mane et al., 2010). 

2 H2O2 

2 H2O + O2 

Hydrogen peroxide is a harmful byproduct of many normal metabolic processes and to 

prevent damage, it must be quickly converted into other less toxic substances and to end this, 

catalase is frequently used by cells to rapidly catalyse the decomposition of hydrogen 

peroxide into less reactive gaseous oxygen and water molecules (Gaetani et al., 1996). 

Catalase (CAT) is the most effective antioxidant enzyme preventing oxidative damage 

(Mittler, 2002). The changes in CAT activity may depend on the species, the development 

and metabolic state of the plant, as well as on the duration and intensity of the stress 

(Chaparzadeh et al., 2004). Catalase activity in Cassia angustifolia L. (Agarwal and Pandey, 

2004), maize (Azevedo et al., 2006), rice (Sudhakar et al., 2001), wheat (Sairam et al., 2002) 

and Sesamum indicum (Koca et al., 2007) was found to be differing under the influence of 

salt. Peroxidase (POD) is widely distributed in higher plants where it is involved in various 

processes, including lignification, auxin metabolism, salt tolerance and heavy metal tolerance 

(Passardi et al., 2005). Peroxidase typically catalyzes a reaction of the form: 

- + 

ROOR' + electron donor (2 e ) + 2H ROH + R'OH 

Muscolo et al. (2003) were of the opnion that the higher (POD) activity under saline 

conditions was the result of minimization of oxidative stress in kikuyu plants. The increased 

total peroxidase activity in the medium of salt adapted cells of tomato reflected the changing 

mechanical properties of the cell wall, which in turn, could be related to the salt adaptation 

process, since cell wall properties are known to be modified by salt stress (Sancho et al., 

1996). Increase in guaiacol dependant peroxidase was also noticed by Parida et al. (2004) in 

Bruguiera cylindrica under saline conditions. 

4. Changes in Minerals 



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Plants need different essential minerals to grow and survive but excessive soluble salts in the 

soil are harmful to most of the plants. There is often an interaction between macronutrients 

and micronutrients in the root medium and in plants (Marschner, 1995). It is confirmed that 

the micronutrients are generally less affected by salt stress than macronutrients (El-Fouly and 

Salama, 1999). The changes in the micronutrient concentrations in plants depend upon the 

type of crop species (Sharpley, 1992) the levels of macronutrients, the levels of salinity and 

the organs of plants (Hu and Schmidhalter, 2001). Decrease in the concentrations of root N, 

K, Ca, and Mg with no change in P in under salinity stress is reported by Loupassaki et al. 

(2002). Turhan and Eris (2005) observed the increase in Fe and Mn content with no change in 

Cu content in the aerial parts of strawberry under salinity stress. Wei and Wei (2006) reported 

that 14-3-3 proteins may be involved in the salt stress, with potassium and iron deficiencysignaling 

pathways in young tomato roots. Huang and Stevninck (1990) suggested that 

vacuolation might provide a means for accumulation of excess ions in plants. Thus, the 

composition of macroelements and microelements alters greatly under varying levels of 

salinity and depends on plant species. Under saline conditions mutual effects of ions on their 

absorption are of particular interest. It is no wonder that reports on the micronutrients in 

different species are so variable (Hu and Schmidhalter, 2001). It is well known that the 

micronutrients are generally less affected by salt stress than macronutrients (El-Fouly and 

Salama, 1999). Hu and Schmidhalter (2001) also suggested that ions in high concentration in 

the external solution (i.e. Na + or Cl - ) are taken up at high rates, which may lead to their 

excessive accumulation in the tissue. These ions may inhibit the uptake of other ions into the 

root (i.e. K + or Ca ++ ) and their transport to the shoot through the xylem, finally leading to a 

deficiency in the tissue. Deficiency of other nutrients in the soil is due to the high 

concentration of Na + that interacts with other environmental factors, such as drought, which 

exacerbate the problem to the plants (Silberbush and Ben-Asher, 2001). It is well established 

that the genotypes of plants vary widely in their ability to metabolize micronutrients 

efficiently and also, different varieties of the same species may differ in uptake efficiency of 

micronutrients as well (Marschner, 1995). 

4.1 Sodium and Chloride 

Sodium cycling through plants and the overall environment can be a critical factor 

influencing the productivity of biological systems. In spite of decades of research, the full 

role of Na (sodium) in plant metabolism remains unresolved. Much of the indecision in 

regard to sodium‟s role in plant nutrition is because it does not fit neatly into any of the 

categories used to describe the essential mineral nutrients. One of the most noteworthy 

features of Na in plant nutrition is the remarkable difference among species in their ability to 

either accumulate or exclude Na from their tissues. Despite the physical and chemical 

similarity between Na and K (potassium), many higher plants have developed a high degree 

of selectivity for the uptake of K, even in the presence of large amounts of Na. Leaves are 

more vulnerable than roots to Na + simply because Na + and Cl - (chloride) accumulate to 

higher levels in shoots than in roots (Tester and Davenport, 2003). Many of the major 

metabolic processes, such as protein synthesis, photosynthesis and glycolysis, occur within 

the cytoplasmic compartment, which places them in a protected location in respect to the 

occurrence of high concentrations of Na + found in the vacuole (Wyn Jones, 1999). 

Significantly higher concentrations of Na + in plants in relation to more tolerance might 

explain the differences found between the responses, whether due to the toxic effect that the 

ion can produce (Maas, 1993). It has been suggested that a restriction of Na + transport from 

roots to shoots take place, minimizing Na + accumulation in the leaves (Sagi et al., 1997). 



1209


Accumulation of Na + can be considered as an important indicator for salt tolerance (Yasar, 

2007; Yildiz et al., 2008). Not much is known about the function of chloride in other 

metabolic process except its role in photosynthetic light reaction. According to Martinez- 

Ballesta et al. (2004) chloride was the main anion used by pepper plants to achieve the 

osmotic adjustment under saline environment. Ashraf and Mc-Neily (1990) noticed the 

higher amount of chloride ions in roots of salt sensitive variety of Brassica as compared to 

the shoots while Kohl (1997) observed the chloride accumulation in green leaves of A. 

maritima as compared to sodium at all salt treatments. Essa (2002) observed the substantial 

differences in tolerance to chloride ions at high salinity levels among soybean cultivars. 

Similar results were reported in Vicia faba (Gadallah, 1999), soyabean (Essa, 2002), cowpea 

(Murillo-Amador et al., 2002) and in Saccharum officinarum L. (Huwyzeh et al., 2007). 

Symptoms of Cl - toxicity are frequently documented (Kurniadie and Redmann 1999; Xu et al., 

2000) but much less information is available regarding the specific effects of high Cl - is 

available. 

4.2 Calcium and Potassium 

Calcium is known to play a special role in tolerance under salinity (Sagi, 2005). The 

formations of calcium bonds in plants gives stability to a variety of cytoplasmic structures 

and enzymes and have apoplastic importance in cell membranes, cell walls and their adhesion 

(Jarunee, 2004). Na + through non-specific ion channels under salinity may cause membrane 

depolarisation that activates Ca ++ channels (Sanders et al., 1999) and thus generates Ca ++ 

oscillations and signals salt stress. Low Ca ++ is known to impair the selective root 

permeability to Na + (Loupassaki et al., 2002). Salinity had shown to inhibit root hair 

elongation via alterations in the tip focused Ca ++ gradient that regulates root hair growth 

(Halperin et al., 2003) and by a reduction in cytosolic Ca ++ (Cramer and Jones, 1996) in 

Arabidopsis roots. Apoplastic reactive oxygen speceis (ROS) can participate in the 

regulation of cell expansion through a modulation of Ca ++ channel activity (Demidchik et al., 

2003; Foreman et al., 2003). It is commonly believed that the Ca ++ status of the plant is 

important in salt tolerance (Suhayda et al., 1992). Ca ++ reduces the adverse effect of Na + by 

controlling the intake of toxic ions through the selectivity of the cell membrane (Volkmar et 

al., 1998; Munns, 2002). Adding calcium to root growth medium enhances salt tolerance in 

glycophytic plants (Lauchli, 1990). Tattini et al. (1995) noticed that calcium concentration in 

the Olea europaea leaf was unaffected by salinity, whereas it level in the root was reduced 

significantly. Potassium (K) is essential in nearly all the processes needed to sustain plant 

growth and reproduction. It plays a vital role in photosynthesis, translocation of 

photosynthates, protein synthesis, activation of plant enzymes, control of ionic balance and 

regulation of plant stomata. It is well known that plants deficient in potassium are less 

resistant to drought, diseases, excess water and varying temperatures. Marcum and Murdoch 

(1992) observed that shoots of Sporobolus virginicus were selective for K + over Na + and 

maintaining K + concentrations resulted in relatively high K + /Na + ratio at high salinity. They 

also observed excess accumulation of K + in roots, might have acted as a reservoir of K + for 

shoots at high salinity. Peng et al. (2004) noticed that alkali grass resists salt stress through 

high K + and an endodermis barrier to Na + . Khan et al. (2000) observed an increase in K + , Na + , 

Ca ++ and Cl - content in a halophyte Atriplex griffithii with increasing level of salinity. In 

contrast, the principal electrolyte for plants is K + and even in ecosystems where there is a 

predominance of Na + , plants still exhibit a strong preference for K + (Walker et al., 1996). 

4.3 Magnesium and Iron 



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Plants have use of magnesium in that chlorophylls are magnesium-centered porphyrins. Many 

enzymes require the presence of magnesium ions for their catalytic action, especially 

enzymes utilizing ATP or those that use other nucleotides to synthesise DNA and RNA. It 

stabilizes the ribosomal particles in the configuration necessary for protein synthesis. It is 

well reported that magnesium deficiency in plants causes late-season yellowing between leaf 

veins, especially in older leaves. Very little attention has been paid towards the role of Mg ++ 

in the plants in their salt tolerance. In Pongamia pinnata, Mg ++ content did not exhibit any 

definite relationship with the increase in salinity (Singh and Yadav, 1999) but in constrast to 

this Khan et al. (2000a) noticed that the leaf Mg ++ concentration decreased with increasing 

salinity in halophyte, Suaeda fruticosa L. Forssk. Similar observations were made in 

Sporobolus virginicus (Marcum and Murdoch, 1992), alfalfa (Eschie and Rodriguez, 1999), 

cowpea genotypes (Murillo-amador et al., 2002), bittler almond (Shibli et al., 2003) and in 

Pinica granatum (Naeini et al., 2004). In all living cells iron is often incorporated into the 

heme complex. Heme is an essential component of cytochrome proteins which mediate redox 

reactions and of oxygen carrier proteins such as hemoglobin, myoglobin and leghemoglobin. 

Inorganic iron also contributes to redox reactions in the iron-sulfur clusters of many enzymes 

such as nitrogenase and hydrogenase. Lazof and Bernstein (1999) found that salinity had no 

effect on leaf Fe +++ content in lettuce. In the root part of plants, Fe +++ content did not change 

with the salt applications in wheat, rice (Alpaslan et al., 1998) and zucchini (Villora et al., 

2000). However, El-Hamdaoui et al. (2003) have recorded that salinity caused to reduce iron 

content in pea plants, where it is an important nutrient for plant growth and development 

particularly symbiotic nitrogen fixation. Similar observations were made by Eschie and 

Rodriguez (1999) in alfalfa and Shibli et al. (2003) in bitter almond. 

4.4 Zinc and Copper 

It is welll known that zinc is one of the essential micronutrients required for optimum crop 

growth. Zinc controls the activity of carbonic anhydrase, an enzyme that regulates the 

conversion of carbon dioxide to reactive bicarbonate species for fixation to carbohydrates in 

these plants. Zinc is also a part of several other enzymes such as superoxide dismutase and 

catalase, which prevents oxidative stress in plant cells. In addition, zinc is involved in 

production of auxin, regulation of starch, root development, formation of chlorophyll „a‟, 

withstanding to lower air temperatures, biosynthesis of cytochromes, maintenance of plasma 

membrane integrity and synthesis of leaf cuticle. Frans (2006) suggested that there might be 

an important link between ionic aspects of salinity stress and transition metal homeostasis 

and it appears that uptake of transition metals like Fe ++ , Zn ++ and Cu ++ is reduced during 

salinity stress whereas their active extrusion is promoted. The physiological relevance of this 

is unclear, since little is known about interactions between salinity stress and the requirement 

for micronutrients such as Fe ++ , Cu ++ and Zn ++ . Zn concentration has been found to be 

increased in shoots of salt-stressed grains of rice (Verma and Neue, 1984) but decreased in 

shoots of mesquite (Jarrell and Verginia, 1990). Very little attention has been given towards 

copper as an essential micronutrient inrelation with salinity which may be due to its less 

contribution in ionic balance and osmoregulation in plants. It plays a vital role in 

reproductive growth and its requirement is well known in photosynthesis. In addition to its 

enzymatic role in cytochrome oxidase and superoxide dismutase, copper is used for 

biological electron transport. It reacts with amino acids, proteins and other biopolymers 

producing stable complexes. Copper content did not change with salt applications in aerial 

parts of the plants in both varieties of strawberry „Camarosa‟ and „Osogrande‟. With salt 

applications, however, Cu ++ content was increased in root parts of the plant in Camarosa 

(Kaya et al., 2002). Increase in copper content was also recorded by Shibli et al. (2003) in 



1211


bitter almond. Similar results were obtained in different plants like rice and wheat (Alpaslan 

et al., 1998), zucchini (Villora et al., 2000) and alfalfa (Esechie and Rodriguez, 1999) while 

on the other hand a decrease in copper content has been reported by Tozlu et al. (2000) in 

Pancirus trifoliata under saline stress conditions. 

4.5 Manganese and Nickel 

Manganese is essential for chlorophyll production and photosynthesis, aids nitrogen and 

carbohydrate metabolism, oxidation-reduction and activation of several enzymes. Chloroplast 

is the final destination of Mn ++ during transport in plant cells and Mn ++ deficiency may cause 

a reduction in photosynthesis, which in turn may be responsible for inhibition of growth by 

salinity (Cramer et al., 1990). Mn ++ can replace Mg ++ in plants at low Mg ++ concentration in 

the growth medium (Hu and Schmidhalter, 2001) while decrease in Ca ++ and Mn ++ uptake 

into and internal concentration in the leaves of plants might be because of reduction in ionic 

activity in external solution (Shennan et al., 1990) or might be due to an interference with 

their uptake by the major saline cations, Na + and K + (Cramer et al., 1991). Accumulation of 

Mn ++ under saline conditions has been recorded in Ipomea pes-carprae Sweet (Venkateson et 

al., 1998) and broad bean (Dahdoh and Hassan, 1997). Nickel is an essential element for 

higher plants nutrition (Brown et al., 1993) since it is a component of urease enzyme (Dixon 

et al., 2004) required for nitrogen metabolism in higher plants (Brown et al., 1993). Nickel 

deficiency depressed urease enzyme activity (Eskew et al., 1983) and other enzymes 

responsible for nitrate reduction (Brown et al., 1993). One of the carbon monoxide 

dehydrogenase enzymes consists of a Fe-Ni-S cluster (Jaouen, 2006). Other nickel containing 

enzymes include a class of superoxide dismutase (Szilagyi et al., 2004) and a glyoxalase 

(Thornalley, 2003). The effect of salinity on nickel content in plants has rarely been studied. 

Ni uptake was clearly depressed by salinity in Plantago coronopus, from 633 mg Kg -1 in the 

no-salt treatment to 305 mg kg -1 at 9 dSm -1 and 152 mg kg -1 at 18 dS m -1 (Zurayk, 2001). 

5. Conclusions 

The time of exposure to salinity and the severity of the salt treatment determine the 

physiological and molecular changes. A high salt concentration in soil induces changes 

predominantly associated morphology however, a low salt treatment may not result in 

morphological changes but can definitely have changes in physiological mechanisms. Plant 

responses to salinity occur as a rapid, osmotic phase that inhibits growth of young leaves and 

another slower, ionic phase that accelerates senescence of mature leaves. Plant adaptations to 

salinity may be related to osmotic stress tolerance related to Na + exclusion and tolerance of 

tissue to accumulated Na + and Cl − . Osmotic tolerance and tissue tolerance both increase the 

ability to maintain growth under the influence of Na + in the leaf tissue. Increased osmotic 

tolerance is evident mainly by the increased ability to produce newer leaves whereas tissue 

tolerance is evident mainly by survival of older leaves. Membrane disorganization, reactive 

oxygen species, metabolic toxicity, inhibition of photosynthesis and attenuated nutrient 

acquisition are the startling factors that initiate more catastrophic events under the influence 

of salt stress. Complex physiological changes such as cell wall extensibility and osmotic 

adjustment are involved in the early inhibition of growth. Water stress developed under stress 

causes reductions in growth rate and even in production. The reduction in shoot biomass 

production by the plant is associated with chlorosis and necrosis of the leaves while the 

resistance of photosynthetic systems to salinity is associated with the capacity of the plant 

species to effectively compartmentalize the ions in the vacuole, cytoplasm and chloroplast. 

Amides and amino acids such as proline, asparagine and aminobutyric acid, can play an 



1212


important role in the osmotic adjustment of the plant under saline conditions. Increased 

sugars in plants generally serve mainly as source of carbon and energy, osmotica, stress 

protectants and signal molecules while increased levels of polyphenols under salinity stress 

shows the induction of secondary metabolism as one of the defense mechanisms adapted by 

the plants to face saline environment. In plant cell, antioxidant enzymes such as superoxide 

dismutase, peroxidase and catalase have been considered to act as a defensive team, whose 

combined purpose is to scavenge reactive oxygen species and protect cells from oxidative 

damage. It is confirmed that the micronutrients are generally less affected by salt stress than 

macronutrients and such changes in the micronutrient concentrations in plants depend upon 

the type of crop species, previous levels of macronutrients, salinity stress applied and organs 

of plants. It is also clear that vacuolation might provide a means for accumulation of excess 

ions in plants. 

5.1 Future Aspects 

It is well recognized that salt stress causes huge losses of agriculture productivity worldwide. 

In view of the increasing pressure of population throughout the world, particularly in 

developing countries, plant breeders, molecular biologist and geneticist are obliged to look 

for ways to yield from the deserted or barren lands. Nowadays, together with conventional 

plant physiology, genetics and biochemical approaches in studying plant responses to abiotic 

stresses have foundation for fruitful outcomes in relation with salinity research. In future 

research arenas on salinity, relevant information on biochemical indicators at cellular level 

may serve as one of the important selection criteria for tolerance of salts in plants. It is also 

well identified that plant abiotic stress tolerance especially salinity is a complex trait that 

involves multiple physiological and biochemical mechanisms and numerous genes. There is 

an urgent need to pursue the breeding plans effectively with the new dimensions of 

understanding of further improvements in salt tolerance with the help of outstanding 

investigations and close exchanges involving physiological, molecular and genetic bases of 

stress tolerance. Genetic modifications carried out to low uptake and high tolerance in the 

plants should be aimed at production of combined varieties with breeding programs. 

Moreover, future work should include more emphasis on the engineering of metabolic 

pathways into plants and transgenic plants. The use of transgenic plants would be a step 

forward in salinity research with the combination of physiological basis in controlling 

responses to salinity stress. It is also important to consider the focus of research under long 

term saline conditions based on the ecological relevance and life cycle of the selected species 

while short times of salt exposure may be useful for signaling studies. Nowadays, molecular 

processes that control Na + compartmentalization in vacuoles have received much attention, 

but other essential processes in tissue tolerance of Na + and Cl − and osmotic adjustment 

remain relatively unknown. The largest gains from plant diversity could be made by selecting 

for specific traits and recombining these from a series of donor parents. These newer 

approaches and the insights will definitely open exciting avenues in salinity research. 

Coverage of plants exposed to salinity stress outlined in this review is subject to revision as 

knowledge based on the topic expands. 

Acknowledgements 

Authors are thankful to Mr. M. A. Mhavale, Vice president of „Sahyadri Environmental 

Awareness Organisation‟, Kolhapur, Maharashtra and Dr. P. A. Banne of „Saitech Research 

and Development Organisation‟, Pune, Maharashtra for encouragement and helpful 

discussions during writing phase of present review. 



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