The study will be conducted in order to identify physio-morphological, attributes which can be used for characterization of salinity tolerance in onion (Allium cepa) in a pots culture. The experiment will be conducted in vegetable area, Institute of Horticulture Sciences, University of Agriculture, Faisalabad. Twelve genotypes will be grown in plastic pots, using fine sand as growth medium. One month after transplanted plants will be treated with 0, 2, 4, 6 and 8 decimal/cm saline water and Hoagland solution will be used as nutrient solution. The experiment will be laid out in Completely Randomized Design (CRD) with six replications and each replication contains three pots. Data pertaining to various physio-morphological and parameters will be collected and analyzed statistically using standard statistical procedures.
IV. NEED FOR THE PROJECT:
Salinity has been a threat to agriculture in some parts of the world for over 3000 years; in recent times, the threat has grown". As the world population continues to increase, more food needs to be grown to feed the people. This can be achieved by an increase in cultivated land and by an increase in crop productivity per area. The former has brought agriculture to marginal, salt-affected lands. Moreover, the salinity problem has been aggravated by the requirement of irrigation for crop production in arid and semiarid environments (Anonymous, 1999)
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Salinization commonly occurs as an outcome of agricultural practices, either associated with irrigation or due to long-term changes in water flow in the landscape that can follow land clearance or changed water management. Salinization associated with agriculture occurs when salts build up in the root zone, either because the soil is intrinsically saline, or because the drainage of water from the sub-soil is not sufficient to prevent saline waters rising into the root zone. It therefore tends to be common in arid and semi arid regions where leaching of salt is poor due to low rainfall; where there are strongly saline sub-soils formed from marine deposits or where irrigation changes water tables and salt flow. The ionic balance differs according to the salts in the sub-soils or water (Kanber et al.,1992). However, Salinization also can be a part of natural landscapes and saline pockets may occur where poor drainage and high soil salinity come together. In these areas salt tolerant species have become established that may have other uses. For example, the salt tolerant grass species Diplachne fusca, used as cut fodder in Pakistan, grows naturally in subtropical saline areas in Australia from where it is believed to have been introduced to Pakistan (Anonymous, 2002).
The total global area of salt-affected soils has recently been estimated to be approximately 830 million hectares (Martinez-Beltran and Manzur, 2005). The different types of soil salinity that impact agricultural productivity, i.e. irrigation-induced salinity and 'transient' dry-land salinity have been characterized in detail by Rengasamy (2006). Clearly, soil salinity is one of the major environmental stresses that limit agricultural productivity worldwide.
Salinity is a major constraint to crop production which is often observed under natural environmental conditions. Increased salinization of arable land is expected to have devasting global effects, resulting in 30% land loss within next 25 years and upto 50% by the middle of 21thÂ century. It is a problem of great importance, because many agricultural areas previously fertile became saline due to irrigation with unfit water. Salinization of underground water resource is another major problem affecting the agricultural productivity. It is very important to sustain the soil fertility and quality of water resources to fulfill the food, feed and fiber demand of ever-growing population of the world. Salinity limits the production of nearly over 6% of the world's landÂ and 20% of the irrigated land (15% of total cultivated areas) and negatively impacts agricultural yield throughout the world (Cramer and Nowak, 1992). Â In Pakistan, a loss of about 20 billion rupees (350.88 million US$) annually has been estimated from the salt-affected irrigated areas of the Indus Basin on account of decrease in crop yield. Salinization of good arable land in Pakistan is creating a problem with immense socio-economic losses. The loss of excellent natural resource is another problem, because the population depends for its livelihood on these lands, which are gradually dwindling through the spread of salinity (Anonymous, 2005).
Salt stress causes multifarious drastic effects in plants and among these factors production of reactive oxygen species (ROS) is a common phenomenon. These ROS are highly reactive because they can interact with a number of cellular molecules and metabolites and ultimately leads to cellular damage. In saline environment, plant growth is affected by complex interaction of hormones, osmotic effects, specific ion effects and nutritional imbalances, probably all occur simultaneously (Dubey, 1999). In pea, it affects the leaf growth, photosynthesis, mineral nutrition, stomatal conductance, transpiration, water and ion transport and increases sugars, amino acids and different ions along with acute effects on yield and quality. Salinity induced symptoms such as nonspecific chlorosis, stunted leaf size and impaired shoot growth.
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They however, under salt stress adopt different mechanisms to adjust the osmotic and ionic stresses caused by salinity. These mechanisms include osmotic adjustment by accumulation of compatible solutes such as Proline, Glycine-betaine and Polyols. Proline accumulation is known as the adaptation of salinity and Glycine-betaine is one of the most abundant quaternary ammonium compounds produced in higher plants due to salinity (Munns and Tester, 2008).
Plants have various mechanisms to reduce the drastic effects of salt and these mechanisms take place at three levels of organization i-e. whole plant, cellular and molecular level. Various scientists have described the physiological mechanisms at whole plant level. Salt tolerance of plant depends upon its ability to check the movement of salt at five sites i-e. selectivity of uptake by root cells, loading of xylem, removal of salt from xylem in the upper part of the roots, stem and petiole, loading of the phloem and excretion through salt glands (Shafqat et al.,1998).
All halophytes have well developed mechanisms to monitor the uptake, transport and excretion of salt. Glycophytes use first three mechanisms. As pea is not a halophyte so the above mechanisms are not well established in it. There are many other features that work to maintain low rates of salt accumulation in leaves. High shoot/root ratios and high intrinsic growth rates and absence of apoplastic pathway in rootsÂ all serve to reduce the rate at which salt enters the transpiration stream and deposit in the shoot. As these features are not efficient in salt sensitive plants like pea so there is a need to enhance their salt tolerance by exogenous application of beneficial compounds and growth regulators. These compounds comprise of proline, glycinebetaine, salicylic acid, brassinosteroids, silicates etc. These chemicals play a vital role in osmotic adjustment under saline condition and create the hindrance in the way of ion toxicity (Malcolm, 1993).
The onion crop is of great importance in Egypt and is one of the main exported fresh vegetables. In cultivated onion, the development of new and improved crop genotypes is of vital importance for its growing in various ecological areas. The techniques of plant tissue culture may play a key role in the development of new cultivars. Aseptic culture technique offers a potential for selection of salt-tolerant lines of onion. The present work was planned to study the influence of different salt levels on growth and chemical contents of onion tissue cultures and in vitro selection for salt stress tolerant genotypes (Rhoades el al., 1992).
Vegetable species, in general, differ greatly in their ability to tolerate drought conditions depending on their genetic make up and evolutionary adaptations. Basic plant structure and development also contribute to drought tolerance among species. Since onion is a shallow rooted crop, a severe impact ofdrought on growth and physiological processes are expected. Continued transpiration under declining soil moisture status will result in severe water stress in plants during their development. Among the growth processes, leaf expansion is perceived highly sensitive to water deficit (Kriedmann, 1986). Continued inhibition of leaf expansion reduces net photosynthetic capacity of the plants, leading to eduction in biomass production (Kriedmann, 1986). A decrease in root zone water potential would result in partial or complete closure of stomata (Kaufmann, 1981).
Review of Literature:
Ahmad and Riffat, (2005) evaluated the effect of salinity on some physio-biochemical parameters in pea ( Pisum sativum L. cv.), under four salt treatments, 50, 100, 150 and 200Â mM NaCl, for 30 days in sand culture experiments and reported that high NaCl concentrations caused a great reduction in fresh and dry weight of leaves and roots, but insignificantly influenced the number of leaves. They concluded that all these changes were associated with a decrease in the relative water contents and the K+ uptake. The proline and sugar contents were increased while nitrate reductase activity and chlorophyll contents were decreased.
Gad, (2005) studied the interactive effect of salinity and cobalt on growth and mineral composition of two tomato varieties namely Moneymaker (as salinity sensitive) and Edcawy (as salinity tolerant). A pot experiment was carried out using acid washed sand and 10 kg capacity plastic pots. It was reported that leaf chlorophyll contents were decreased in response to salt stress, on the other hand salt stress significantly enhanced the leaf water potential and proline contents.
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Gamze, (2006) investigated the effects of salt and drought stresses at the water potentials of -2, -4, -6 and -8 bars induced by NaCl and PEG 6000(polyethylene glycol 6000) at germination and early seedling growth, of three pea cultivars. Electrical conductivity (EC) values of the NaCl solutions were 4.5, 8.8, 12.7 and 16.3 dS m-1. It was reported that both NaCl and PEG inhibited germination and seedling growth in all cultivars, but the effects of NaCl compared to PEG were less on germination and seedling growth. It was concluded that inhibition in germination at equivalent water potentials of NaCl and PEG was mainly due to an osmotic effect rather than salt toxicity.
Sato et al., (2006) examined the effect of NaCl on taste grading and chemical composition of fruit of hydroponically grown tomato plants and reported that NaCl treatment increased sweetness, acidity, umami (i.e. the taste of deliciousness) and overall preference. Hexose concentration of the fruit grown on NaCl treated plants significantly increased and at the same time, chloric ion, organic and amino acids in general had higher concentrations in NaCl treated plants than the control. It was concluded that consumer grading of the tomato fruit was influenced not only by sugar contents but also by the amino acids and increased concentration of soluble solids in the fruit of NaCl treated plants was not the result of simple overall condensation due to the reduction of water transport.
Shibli et al., (2007) studied the physiological and biochemical responses of open-pollinated 'Roma' and dwarf F1 hybrid 'Patio' tomato (Lycopersicon esculentum Mill.) cultivars to salinity. Growth, leaf cell sap osmolarity, leaf tissue viability and shoot soluble protein contents were generally depressed with elevated salinity treatments, whereas, electrolyte leakage, membrane injury, raffinose and total sugars were concomitantly increased. Incorporating ethylene inhibitors CoCl2 or NiCl2 at 5.0 or 10.0Â mg/l into media supplemented with 100Â mM NaCl significantly reduced ethylene accumulation in the headspace and prevented epinasty, but did not eliminate the negative impacts on growth and other physiological parameters caused by salinity treatment in either cultivar. It was reported that the increase in ethylene under salinity stress is not the primary factor contributing to salinity's deleterious effect on tomato plant growth and physiology.
Khan et al., (2009) studied the effect of seed priming with salicylic acid (SA) and acetylsalicylic acid (ASA) in improving seed vigor in salt tolerance of chillies seedlings. Concentrations over 1.0 mM of ASA or SA showed adverse effects on seed emergence. Seeds primed with SA (0.8 mM) and ASA (0.2 mM) were sown in medium at different salinity levels [0, 3, 6 and 9 dS m-1]. Both, SA and ASA treatments showed significantly better results over the control by improvement in time taken to 50% emergence, final emergence percentage, root and shoot length, seedling fresh and dry weight and seedling vigor. Overall, acetylsalicylic acid exhibited superiority over salicylic acid. They reported that hormonal priming, especially with acetylsalicylic acid, can be a good treatment for chillies to enhance uniformity of emergence and seedling establishment under normal as well as saline conditions.
Maiti et al., (2010) used a novel semi-hydroponic technique and evaluated that highly signifcant differences were found among all genotypes with respect to seedling parameters studied. In all vegetable crops, in general, signifcant differences were found among genotype, NaCl concentration and interaction between genotypes & NaCl concentration with respect to emergence (%), emergence index, shoot length & root length. High r2 and low CV (%) indicates the reliability of the technique. Emergence was highly correlated with shoot length and root length showing the contribution of shoot length and root length to salinity tolerance in different vegetable crops. With increasing salinity, there was increase in root length in salinity tolerant lines but there was corresponding decrease in root length in susceptible ones. Salinity tolerant genotypes selected in okra at 0.1 M NaCl were: 7025, 7033 and 703. Salinity tolerant genotypes selected in tomato at 0.1 M NaCl were: 132, 113, 125, 126 and 12, also in chilli at 0.1 M NaCl.
The vegetative growth components and bulb yield were measured in the tested cultivars. Salinity retarded onion vegetative growth. At the highest salinity level bulb fresh weight was reduced by 72.8 and 81.5% while bulb diameter was reduced by 50.2 and 51% in the first and second experiments, respectively. Contessa, Texas Grano 502 and Dorado gave the highest bulb yield in both seasons. No interactive effect between cultivars and salinity levels was observed on the growh and yield.
Vi) Materials and Methods
The proposed study will be carried out at vegetable area, Institute of Horticultural Scieces, University of Agriculture, Faisalabad. The following genotypes will be used as planting material.
Giza no. 6
Swat no 1
The experiment will be laid out in Completely Randomized Design (CRD) with three replications.
The experiment will comprised of following treatments.
T1 = Control (No. salt treatment)
T2 = 25 mM NaCl
T3 = 50 mM NaCl
T4 = 75 mM NaCl
T5 = 100mM NaCl
T6 = 125mM NaCl
Seeds will be sown in 9L, bottom perforated, plastic pots containing sand rinsed with distilled water. After emergence of first true leaves (15 days after germination), the number of seedlings per pot will be adjusted to three and they will be irrigated according to seedlings need. After twenty days of sowing, half strength (0.5) Hoagland's nutrient solution will be given to plants. Salt treatment will be started one month after sowing. To avoid the osmotic shock NaCl concentrations will be adjusted, by gradually increasing 25 mM every two days until desired concentration reached. Each pot (three plants) will be considered as one replicate and there will be three pots per treatment.
Parameters to be studied
i) Morphological traits
No of leaf per plants
Weight of fresh bulb per plants
Bulb dry weight per plants
Bulb moisture contents
Bulb dry matter
ii) Physiological traits