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The world having a decline in food production per capita and the increasing global demand for food make it necessity to produce solution for maximum utilization of available resources and improve crops to overcome this problem. In many arid and semi arid regions, good soils are scarce with their overall productivity declining because of soil degradation and lack of proper soil and water management practices. Salt-affected soils, which are widespread in arid, semi-arid and coastal regions of sub-humid areas, have low productivity. There are 380 million hectares of saline soils on earth’s land surface, and of these 140 million hectares are highly saline and have higher electrical conductivity (EC). Food production in many parts of the world is severely affected by high salt contents in soils. In southern Asia and the Near East, for example, several million hectares of agricultural area are affected by salinity (e.g. 6.3 million ha in Pakistan, 2.5 million ha in India) causing losses in food production, excessive runoff due to compaction of saline soils and progressive desertification. It is estimated that nearly 10 % of the total land of the world used for crop production is adversely affected by soil salinity.
The major solutes comprising the dissolved mineral salts that affect soil fertility are the cations Na+, K+, Ca++, Mg++ and anions Cl-, SO4--, HCO33-and SIO32-. Normally, salt-affected soils often occur under natural conditions. Salinity problems of greatest importance in agricultural areas arise when previously fertile, productive soils become salinized as a result of irrigation.
Salinity and crop production
Salinity is increasingly important constraint to crop production worldwide (Ghassemi et al., 1995) regardless of the cause (ion toxicity, water deficit and nutritional imbalance) high salinity in the root zone severely impeded normal plant growth and development, resulting in reduced crop productivity or crop failure. The main effect of salinity on plant growth and crop production are
• Slow and insufficient germination of seeds.
• Physiological drought, wilting, desiccation of plant.
• Stunted growth, reduce branching.
• Retarded flowering, fewer flowers, sterility and small seed.
• Low yield of seed and other plant parts.
Prevention and reclamation of soil salinity
Different measures are taken to reclaim the saline land which includes physical, chemical and biological.
Physical methods of land reclamation:
The proper solution of salinity and water-logging is through engineering technology i.e. proper drainage system of all agricultural land. This technology has been used in Pakistan at national level to control salinity by draining the soil salt through a network of surface and subsurface drain and tube wells.
Chemical Method of Land reclamation:
Although reclamation of salt-affected soils by chemical means is an established technology, traditional reclamation methods have been proved to be difficult (Rafiq, 1990), inadequate (Qureshi, et al., 1992), expensive (Qureshi, 1993; Qureshi and Barrett-Lenard, 1998), and uneconomical (Rafiq, 1975) on highly impermeable dense saline-sodic soils in Pakistan. Further, under the existing circumstances not only the scope of this approach is limited, its sustainability is also questionable (Qureshi, 1993; Qureshi and Barrett-Lenard, 1998).
Biological method of salt land reclamation
This approach is based on growing salt tolerant plant species and use of saline waters to utilize salt-affected soils has been explored to a lesser extent (Qureshi and Barrett-Lenard, 1998). However, an understanding of the plant responses to various stresses and the mechanisms that make some species/genotype more tolerant than other is essential.
Mechanism of salt tolerance in plants
Plants survive under saline conditions by adapting some special physiological, biochemical and anatomical mechanisms, which enable them to grow under salt stress. In general, plants avoid toxic concentration of salts either by restricting ion uptake or by compromising with high salt concentration through osmotic adjustment. Some important mechanisms for salt tolerance are:
Histological changes under salt stress
Plant transport salts to shoots (even in halophytes) the amount of salts in excess is required for turgor maintenance. Excretion of salts through special glands i.e. salt glands is one of the most important mechanisms for salt tolerance (Gorham, 1996).
Salt gland controls the salts content of leaves. The quantitative contribution of salt glands to regulation of salt concentration in leaves has been studied in relatively few species. However, a substantial portion of salt entering a leaf of Leptochloa fusca can be excreted through salt glands (Gorham, 1996). The structural details of various kinds of salt glands have also been reviewed by Thomson et. al. (1988) and Fahn (1988). They may be multicellular organs of highly specialized cells, for example Avicenna marina, or are simple type glands comprising of only two cells, e.g in Leptochloa fusca (Wieneke et.al. 1987).
Salinity causes several specific structural changes that disturb plant water balance (Robinson et. al. 1983). These structural change include fewer and smaller leaves, less number of stomata per unit leaf area, thickening of leaf cuticle and wax deposition of leaf surface, reduced differentiation and vascular tissues, increased development of tyloses, earlier lignification of roots, low chlorophyll content, higher elasticity of cell walls, fully developed water storing tissues and increased succulence (Yeo and Flowers 1984). These responses vary with plant species and the type of salinity (Aslam et. al. 1993).
Physiological mechanism under salinity
Plant has the ability to regulate the influx of salt, which determine salt tolerance. In the pathway from the rhizodermis (the point of initial entry of salts in to roots) to the xylem, the movement of ions could be controlled by exchange processes in the cortex (Staples and Toenniesson, 1984) enforced passage through membranes (and hence selectivity) at the endodermis and by selective xylem loading (Gorham, 1996). Some species recirculate sodium in the phloem, although this is mainly a feature of salt-sensitive species such as beans and lupines (Jeschke et. al. 1987). Young expanding leaves are supplied with a potassium-rich inorganic solute supply via phloem, while sodium accumulates in older leaves, often replacing potassium accumulated previously. The potassium in older leaves is thus available for recirculation via phloem to sink tissues (Gorham, 1996). All the plants are salt excluders with varying degrees of exclusion. Some important physiological mechanisms for salt tolerance are:
1. Osmotic adjustment
2. Exclusion/inclusion of ions
3. Potassium-sodium selectivity and
Tomato (Lycopersicon esculentum L.) is one of the major vegetable crops of the world especially of the most of countries like America, Japan, Pakistan, India, Bangladesh, and China. It is grown commercially in 161 countries in 2004 with a combined production of over 115 million metric tonnes. The leading countries in tomato production are the United States of America, Italy, Egypt, Mexico and Spain. The United States of America accounts for about one fifth of the world’s production (Moresi and Liverotti, 1982).
It is a member of nightshade family (Solanaceae) along with pepper, eggplant and potato. Botanically it is classified as fruit, since it is developed from ovary, although it is commercially recognized and treated as vegetable. It include the genus with several Known wild forms of tomato, i.e., Lycopersiocon pimpinellifolium, L. hirsutum, L. peruvianum, that have been useful in breeding programs for biotic and abiotic stresses.
Tomato thrive at many latitude under a wide range of soil types, temperature and method of cultivation provided with adequate nutrients, cultivated tomatoes can be produced anywhere. Tomato production in the tropics tend to be more successful in mountain region or in low land during the cool season (Ruben, 1980).
Production of Tomato
Tomato (Lycopersicon esculentum) is the second most important vegetable crop next to potato. Present world production is about 100 million tons fresh fruit produced on 3.7 million hectares.
In Pakistan tomato production has been increased since 90s.During 2001-2002 tomato crops was grown on average area of 29-30 thousand hectares with annual production of 294 thousand tons and the average yield is 13.8 tons per hectare (Anonymous, 2001).
Table 1.2 Area, yield and production of tomato in Pakistan during 2001-06
Year Area (Million hect) Tomato yield (kgs/hectare) Production
2001-02 1.33 1501 1.99
2002-03 1.37 1673 2.30
2003-04 1.52 1659 2.52
2004-05 1.55 1639 2.55
2005-06 1.65 1761 2.92
Tomato can be consumed either cooked or raw. It is used in a variety of ways i.e. ketchup, beverages, salad, sauces and various other products. After processing oil can be extracted from the seed and the residual seed cake used for animal feed. Tomatoes have a very high nutritive value. It contains protein, thiamine, riboflavin, vitamin C, -carotene Ca, Fe and carbohydrates. It is the cheapest and richest source of vitamin C and A (Kanahama, 1980). The acids present in it are citric acid, malic acid, aminobutyric acid, cis-aconitic acid and formic acid along with fair amount of histidine, lysine and certain minerals (Loh and Woodroof, 1975).
Table 1.4 Food value per 100 grams of Tomato contains
Nutritional Value Per 100 g
vitamin C 27 mg
Thiamine 0.12 mg
Riboflavin 0.06 mg
-carotene 35 g
Ca 48 mg
Fe 0.4 mg
Source: http://en.wikipedia.org/wiki/References _Daily _Intake
Factors affecting tomato production
Diseases and pathogens are important among biotic stresses. Some diseases reach epidemic proportion and causes serious crop losses which others causes only negligible crop losses. Numerous disease of tomato, caused by fungi, bacteria, viruses and nematode.
Tomato is subjected to various abiotic stresses, which are unfavorable soil, temperature and water conditions, which cause very extensive losses to the yield of tomato. Similarly salinity, drought, cold, acidity, iron toxicity and submergence under water adversely affect tomato production. It is one of the most important vegetable crop where yield and quality are significantly decreased by soil salinity (cuartero and Fernandez-munoz, 1999) among soil factors affecting tomato productivity, salinity was reported to decrease absorption of essential cation like calcium (lopz and Satti, 1996). A survey reports on the kitchen crops showed that 1.88 kg tomato was consumed by per capita and it is expected to reach more then 2.89 kg per head for about 188.5 million population of Pakistan by the year 2010 (Survay report 1997). It is therefore need of the day to enhance the tomato production on the areas affected by salinity that can be achieved by manipulating tomato gene to improve the salt tolerance in tomato crop.
Objectives / Aims of thesis
In Pakistan lots of work had been done on trees and cereal crops with regards to salt tolerance but very little work had been reported on vegetables. Present investigation was aimed to study the effect of salt on the physiology of tomato genotypes and to transfer the salt tolerant gene in the selected tomato genotype. To achieve this goal, twelve tomato genotypes viz. Avinash, NTH242, Nagina, Checo, Roma, BARI, Lyp No 1, 88572, Pant Bahar, Gol, Pakit and Riogrande were selected for screening the mechanism of salt tolerance with respect to plant growth, ionic content, water content of plant and the production of ABA- the stress hormones. The genetic diversity among these genotypes was studied by Randomly Amplified Polymorphic DNA (RAPD). The tolerant gene was transferred to selected tomat genotype through Agrobacterium mediated gene transformation.