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Salinity is one of the serious environmental problems that cause osmotic stress and reduction in plant growth and crop productivity in irrigated areas of arid and semiarid regions. Salinity is a significant limiting factor in the agricultural productivity (Hasegawa et al., 1986).
Soil salinity is one among the several environmental stresses causing drastic changes in the growth, physiology and metabolism of plants and threatening the cultivation of plants around the globe. Salt accumulation in irrigated soils is one of the main factors that diminish crop productivity, since most of the plants are not holophytic. (Hoshida et al., 2000).
Salt stress induces various biochemical and physiological responses in plants and affects almost all plant processes (Nemoto and Sasakuma, 2002).
Salinity stress is known to affect various growth processes including photosynthesis, ion regulation, water relations etc. (Ashraf, 2004).
Salinity reduces productivity of agricultural soils in large areas of the world. Salt stress affects plant physiology at both whole plant and cellular levels through osmotic and Ionic stress (Hasegawa et al., 2000; Muranaka et al., 2002).
Salinity retarded seed germination and growth of higher plants (Botella et al., 1997).
Approximately, 7 % of the world's land area, 20 % of the world's cultivated land, and nearly half of the irrigated land is affected with high salt contents (Rhoades and Loveday, 1990; Szabolcs, 1994; Zhu, 2001).
In addition to that priming agents i.e., CaCl2, KCl, and NaCl are effective in alleviating the adverse effects of salt stress on wheat plants (Iqbal et al., 2006).
The addition of supplemental Ca2+ to the root zone has been suggested as a mean of enhancing the plant tolerance to salt stress (Rengel, 1992).
Supplementing the medium with Ca2+ alleviates growth inhibition by Salt in glycophyte plants (Imlay, 2003).
The interaction of Na+ and Ca2+ on plant growth and ion relations is well established (Jaleel, 2007).
Besides general stunting of plant growth, salinity causes several specific structural changes that disturb the plant water balance or status. The shape and size of plant organs and cells may change in response to salt stress. This includes increased leaf succulence, decreased leaf size and leaf number, reduced numbers of stomata, thickening of the leaf cuticle, and deteriorated or undeveloped xylem (Shannon et al., 1981).
Salt stress causes various effects on plant physiology such as increased respiration rate, ion toxicity, changes in plant growth, mineral distribution, and membrane instability resulting from calcium displacement by sodium (Marschner, 1986),
Wheat (Triticum aestivum L.) is the major staple food of Pakistan. It is grown from meeting the food demand of the overgrowing population of the country. But its per hectare yield is below than its yield potential, which might be due to several reasons like poor nutrient management, shortage of good quality water, poor weeds and pest management, drought, salinity and water logging (Lamond and Whitney, 1992).
Wheat is an essential commodity to food security in NWFP (Pakistan). It was cultivated on about 8.6 million hectares, the largest area allocated to any crop in Pakistan, with total production of 23.3 million tonnes (Minfal, 2007).
The wheat crop is mainly cultivated under rain fed conditions where precipitation is less than 900 mm annually. Wheat is grown both as spring and winter crop. Winter crop is more extensively grown than spring (Munns 1988).
In wheat, growth is affected by water stress which can reduce the final number of tillers per plant by reduced tiller production and (or) increased tiller mortality (Fischer, I973).
The number of kernels per spike of wheat has been reported to be most severely reduced by water stress during the 15 days prior to anthesis (Baier and Robertson, 7969).
Salinity is a serious threat to wheat production in Pakistan. Reclamation methods of salt affected soils include proper drainage system, irrigation water application, residues incorporation, leaching, plantation of tolerant crops, and chemical amendments (gypsum addition) to sodic or saline-sodic soils (Lamond & Whitney, 1992).
Maize is an annual plant belonging to the grass family (Gramineae or Poaceae). As a result of genetic improvement, the potential yield of corn has been increased by approximately 50% during this century in the United States alone (Frey, 1984).
Maize (Zea mays L.) is a main food and economical crop. It is one of the most important crops throughout the world so, it is urgent to increase maize yields even under the unfavorable conditions (Fang- Gong- Sui et al. 2006).
After wheat and rice, maize (Zea mays L.) is the third most important cereal crop grown all over the world in a wide range of climatic condition. Maize, being highly cross pollinated, has become highly polymorphic through the course of natural and domesticated evolution and thus contains enormous variability (Paternian, 1990).
The area of maize production in the world was 130 million ha and total world production was 507 million tons in 1995 (Pomeranz, 1987).
Although, maize (Zea mays L.) is widely grown in many regions of the world where soil salinity is one of the major agricultural threats to its productivity. While comparing different crops for their response to salinity stress this crop has been categorized as moderately salt-sensitive (Maas and Hoffman, 1977),
In Pakistan, maize is the third most important cereal after wheat and rice. Being an important "Kharif" crop, maize is grown on about one million hectares with a total yield of about 2 million tones and an average yield of 1882 kg ha-1 (Govt. Of Pakistan, 2005).
Salt sensitivity of maize plants has been found to be due to high accumulation of Na+ in the leaves (Munns, 1993; Fortmeier and Schubert, 1995).
Pearl Millet (Pennisetum americanum L.) is a member of the Gramineae family and it is the staple food and fodder crop (Fao and Icrisat, 1996).
Salt stress affects the germination percentage, germination rate and seedling growth in different ways depending on plant species (Ungar, 2005; Gul et. al., 1999).
Salinity has three potential effects on plants: Lowering of water potential, specific ion toxicity (sodium and chloride) and interfere with the uptake of essential nutrients. The latter may not be considered because it has no immediate effect due to mobile reserve nutrients present in plants (Flowers, 2005).
Excessive sodium inhibits the growth of many salts-sensitive plants, which includes most crop plants. The typical first response of all plants to salt stress is osmotic adjustment. Compatible solute accumulation in the cytoplasm is considered a mechanism to impart salt tolerance (Hare et al., 1998; Jaleel et al., 2007b).
Salinity is the growth medium cause significant reduction in plant growth parameters like leaf length, diameter of leave and dry weight (Hamada and Al-Hakimi 2001; Ashrafuzzamana et al., 2002)
Salinity induces water deficit even in well watered soils by decreasing the osmotic potential of soil solutes thus making it difficult for roots extract water from their surrounding media (Sairam et al., 2002)
Salinity is one of the most important problems in irrigated soils of the arid and semi-arid areas in the world. Currently, there are about 275 million hectares of irrigated land of which about 20% is salt affected (Ghassemi et al., 1995).
Salinization is the scourge of intensive agriculture. High concentration of salts has detrimental effects on germination of seeds and plant growth. Many investigators have reported retardation of germination and growth of seedlings at high salinity. However plant species differ in their sensitivity or tolerance to salt. Soil salinity affects plant growth in a variety of ways, reducing water uptake, causing toxic accumulation of sodium and chloride, and reducing nutrient availability. Salinity also induces water deficit even in well-watered soils by decreasing the osmotic potential of soil solutes, thus making it difficult for roots extract water from their surrounding media (Jaleel et al., 2007).
Katerji et al., (1994) study the effect of salinity on the emergence and early seedling growth of sunflower and maize, experiments were conducted in pots filled with sandy clay and sandy loam. The emergence of sunflower and maize were affected by salinity. Soil texture did not affect the emergence. During early seedling growth, salinity and soil texture affected the development of the seedlings that showed symptoms of water stress in the form of lower leaf water potential, stomatal conductance, and evapotranspiration. The higher the salinity, the lower the leaf area and the dry matter production. Leaf, stem, and root showed an almost similar growth reduction due to salinity. Seedlings on sandy loam were more affected by water stress than those on sandy clay.
Jamil et al., (2006) conducted in an experiment in which four vegetable species were treated with different concentration of salt solution to study the salt effect. Results indicated that salinity caused significant reduction in germination percentage, germination rate, and root and shoot lengths and fresh root and shoot weights. Liner relation was developed to find relation between salt stress and plant growth and also between germination and rest of plant characters.
Cheruth Abdul Jaleel et al., (2008) studied the effect of calcium chloride (CaCl2) as an ameliorating agent on sodium chloride (NaCl) stress was in Dioscorea rotundata plants. Plants were raised in pots and exposed to salinity stress (80 mM NaCl) with or without 5 mM CaCl2. NaCl-stressed plants showed decreased protein and total sugars, and increased free amino acid and Proline content. When NaCl treatment was combined with CaCl2, overall plant metabolism was altered, with increased antioxidant enzyme activity, paving the way for partial amelioration of oxidative stress caused by salinity.
Mujeeb-ur-Rahman et al (2008) investigated the response of four cultivars of wheat (Triticum aestivum L.) to NaCl salinity at germination and early seedling growth. There was a decrease in water uptake and germination of all cultivars. Increase salt concentration also affected the early seedling growth. Among the cultivars under investigation zarlasht cultivar appeared to be more sensitive at germination stage. However, it performed quite satisfactorily at seedling stage.
A. Farsiani, and M. E. Ghobadi (2009) studied the effect of PEG and NaCl stress on germination and early seedling stages on two cultivar of corn. The principal aim of the current study was to compare the two varieties of maize in relative to the stress conditions. Results indicated that significant decrease was observed in the percentage of germination, germination rate, length of radicle and plumule and radicle and plumule dry matter. On the basis of the results, NaCl as compared with PEG had more effect on germination and early seedling stage and sweet corn had more resistant than flint corn in both stress conditions.
Datta et al (2009) conducted the impact of salt stress under different salinity level (0, 25, 50, 75,100,125,150 mM NaCl ) on five varieties of Wheat viz., HOW-234, HD-2689, RAJ-4101, RAJ-4123, and HD-2045. The data showed that different level of salinity significantly affected the growth attributes by reducing root and shoot length for salinity below 125mM. Fresh weight and dry weight of root and shoot were reduced significantly with subsequent treatment. Regarding germination maximum germination was found in a variety HD2689 in all the treatments and maximum inhibition was found to be in case of HOW234 variety at 150mM salinity level. Regarding biochemical analysis the sugar, Proline content increased with increasing salinity level whereas protein content decreased in the physiologically active leaves of different treatments for all the varieties of wheat.
Gobinathan et al., (2009) studied the interactive effects of Calcium Chloride on
Salinity-Induced Oxidative Stress in Pennisetum typoidies in terms of lipid per oxidation (TBARS content), H2O2 content and antioxidant enzyme activities. The study revealed that NaCl-stressed plants showed increased TBARS, H2O2, when compared to control. The antioxidant enzymes superoxide dismutase (SOD), peroxidase (POX) and catalases (CAT) were increased under salinity and further enhanced due to CaCl2 treatment.
Birol and Haluk (2009) applied salt stress to RX 770 hybrid maize plant and evaluated the effects of an additional supply of CA, K, and Mg salts to plants under salt stress using membrane permeability, relative water content and Proline concentrations as stress indicators. Furthermore, effects of the treatments on growth and on macro and micro nutrient concentrations in shoots and roots were determined. It was observed that supplemental Ca, Mg, and K had positive effects on plant performance, by decreasing membrane permeability and enhancing relative water contents (RWC) under salt stress.
Emine et al., (2010) subjected six cultivars of maize (Zea mays L.) (Ada-523, Bora, C-955, PR 3394, Progen-1550 and Trebbia) to 0 and 100 mM NaCl and their response to salt stress were determined by growths related to relative shoot growth weight (RSGR), shoot and root dry weight and stress tolerance index by biochemical parameters associated with total chlorophyll and proline contents and by mineral element contents such as Na+ and K+ contents and K+\Na+ ratio. The results indicate that salinity decreased RSGR, shoot and dry weight, stress tolerance index, total chlorophyll and K+ contents and K+\Na+ ratio, but increased Proline and Na+ accumulations. Especially, Proline accumulation appears to react to salt stress damage rather than a plant response associated with salt tolerance.
Amirjani., (2010) exposed rice plants to increasing concentrations (0, 25, 50, 100 and 200 mM) of NaCl. Fresh weight, dry weight of the treated seedlings decreased. Decreasing of plant growth depended on lowering of available CO2 which was caused by stomatal closure and also on the additive effects of leaf water and osmotic potential, relative leaf water content, and biochemical constituents such as photosynthetic pigments, soluble carbohydrates, and proteins. Increasing concentrations of NaCl resulted in increase and decrease of Na+ and K+ ions respectively. NaCl salinity caused for increasing both peroxide content and lipid peroxidation.
Hussain et al., (2010) conducted Experiments for the study of alleviation of salt stress effects by exogenous applications of salicylic acid (SA) in pearl millet (Pennisetum glaucum (L.) R. Br.) NaCl significantly reduced the plant and root lengths, plant fresh and dry weights. In contrast, NaCl did not show any adverse effect on plants treated with NaCl plus SA. Salicylic acid treated pearl millet plants under NaCl salinity strongly reduced accumulations of Na+, K+, Ca2+ and Clâˆ’ and glycinebetaine (GB) and total soluble carbohydrates (TSC) as compared to NaCl treatments. Higher N and relative water contents (RWC) was noted in T2 (NaCl + SA) but it reduced in T1 (NaCl) as compared to control. It was concluded that SA could be used as a potential growth regulator to improve salt tolerance in plants.
Mohammad Hosein et al., (2011) tested the effects of different salinity levels on germination and early growth of Spinach seedlings. The result showed that the percentage and speed of germination, plumule length, radicle length and heaviest wet and dry seedling weights were higher in the control treatment. At 150 mM and more concentration, germination decreased significantly. This reduction in germination indicates this plant's extreme insensitivity to salinity, so it isn't advisable to cultivate it in saline soil.
R. Johnsi., (2011) studied that salt stress as a major adverse factor can lower leaf water potential, leading to reduced torgor and some other responses, and ultimately lower crop productivity in arid and semi arid zone. Salt stress reduces the ability of plants to take up water and this quickly causes reductions in growth rates. The initial reduction in shoot growth is probably due to salt effects. If excessive amounts of salt enter into the plant, salt will eventually rise to toxic levels and reduce the photosynthetic leaf area of the plant that cannot sustain growth. In order to understand the processes that give rise to tolerance of salt and to identify the salt stress proteins in the salt stress effect of on plant growth was studied using different salt solutions like Copper sulphate, Cadmium chloride and zinc sulphate with different concentrations like 200Î¼M, 150Î¼M, 100Î¼M.
Houman.,(2011) identified salt tolerance genotype, an experiment with five rapeseed cultivars under different levels of salt stress treatments (0, -2, -4, -8 and -10 bars) has been undertaken using a factorial experiment. There was a decrease in water uptake and germination of all cultivars. Increase salt concentration also affected the early seedling growth. Among the cultivars under investigation, Seimareh cultivar appeared to be more sensitive at germination stage. However, it performed quite satisfactorily at seedling stage.
Yousaf Jamal et al., (2011) determined the effects of seed priming with 30 mM NaCl on various growth and biochemical characters of 6 wheat varieties. Statistical analysis of the data revealed that varieties, salinity and seed priming had significantly (P â‰¤ 0.05) affected germination (%), days to emergence, tillers plant-1, leaves plant-1 and shoot chlorophyll-b contents (mg g-1 fresh weight). However, the effects of varieties and salinity were significant (P â‰¤ 0.05) and of seed priming was nonsignificant (P > 0.05) on plant height (cm), root length (cm) and shoot chlorophyll-a contents. Maximum germination percentage (88.75), tillers plant-1 (3.63), leaves plant-1 (16.01), plant height (27.86 cm), root length (18.57 cm), shoot chlorophyll-a contents (1.15 mg g-1 fresh weight) and shoot chlorophyll-b contents (0.84 mg g-1 fresh weight) were recorded for Bakhtawar-92. Maximum days to emergence (10.04) were recorded for Pirsabaq-2004.