The plants are element of this universe that has to stay on the one place for whole life so they have to adopt it according to the climatic changes that may be beneficial or harmful for plant growth and development (Gill et al., 2002). The environmental changes that affect adversely to plant growth and development is consider as abiotic stress such as drought, salinity, heat, frost and lodging etc ultimately affect the crop quality and productivity (Van Breusegem et al., 2001). Salt stress is one of the most significant abiotic stress and considers as a sign of disaster for the crop plants (Eker et al., 2006) and is increasing day by day throughout the globe (Flowers, 2004 and Foolad, 2004). In fact, soil salinity is a continuous and unnecessary accumulation of salts in soil which affects morphological, physiological and biochemical traits of plants (Amirjani, 2010; Siringam et al., 2011). Salinity is a global problem and it has impacts on the socio economic conditions of the grower. The land under cultivation is decreasing while the saline land is increasing which ultimately affecting the food production. Soil salinity is commonly limited to semiarid and arid regions so salts affected area throughout the globe is 800 M ha, from which 437 M ha sodicity affected area (FAO, 2005). In South Asia 87.6 M ha land deteriorated due to the salinity (Khan et al., 2003).
Pakistan is an agricultural country so it directly influence upon its economy (Ali 2000). It fulfills their food requirements and earns a big share of foreign exchange usually it consider that the major crops have large contribution for GDP but minor crops have the maximum growth potential especially for vegetables, fruits, and flowers. FAO (2010) reported that in developing countries, there should be 2/3rd increase in crop production through high yielding and salt tolerant varieties. According to Economic Survey of Pakistan 2006-2007, area under cultivation is 23.13 M ha which developed by just 2.3 % during the last ten years. The large area of land in Pakistan is salt affected that is almost 6 M Ha (Pakistan Economic Survey, 2007- 2008).
The unit which is used to meaure the salinity is electrical conductivity (EC). Therefore, on the basis of EC soils are classified into sodic and saline soil. Saline soils are those that have EC above 4 dS m-1. Salinity is mainly categorized into two types that are primary salinity and secondary salinity (Rengasamy 2006). Primary salinity occurs due to the natural factors such as salty areas i-e. Salt lakes and salt pans etc. Secondary salinity is the result of human interference in the natural ecosystem. It may be irrigation salinity, dry land salinity, sea water intrusion and point source. (Munns and Tester 2008) two types of stresses i.e. osmotic stress and ionic stressinduce due to salinity. Osmotic stress causes the low water potential in soil due to high sodium ion concentrations in the soil while ionic stress results in the accumulation of high toxic ions concentrations in the plant root zone.
The factors that enhancing soil salinity are inadequate water, low precipitations, high temperature, high surface evaporation, weathering of rocks, irrigation with saline water or poor quality water and poor cultural practices that are adopted by the farmers ( Abed Alrahman et al., 2005). In Pakistan like other agronomic crops, salinity is the major obstruction for good quality and yield of vegetable crops because the brackish water use for the irrigation which contains exchangeable salt especially NaCl which accumulates in the root zone and reduce the vegetable growth and yield. Shah (2007) reported that at farm level about 50 percent of the irrigation water use from groundwater, whose quality is lower than canal water. In Pakistan there are about 562,000 private and 16,000 public tubewells, in them 70 % are pumping brackish water that is not fit for irrigation for vegetable crops (PCST, 2003). In saline soil, when seed is sown very few seedlings emerge (Jamil et al., 2006). Salt stress commonly changes the morphological characters of a plant (Zadeh and Naeini, 2007). Morphological symptoms are more visible for grower to see the effects of salinity like in leaf reduction elongation and growth (Beartiz, 2001). It reduces plant growth by affecting its shoot and root growth, decreases the root and plant height that ultimate lower root and shoot fresh weight (Parida and Das, 2005; Hajer et al., 2006). Photosynthesis is the process which prepares food for the plants and this process is carried out in the leaves under saline conditions the H ion concentration of plant decrease and apoplastic pH of leaves increase which results in the lowering the leaf growth (Fercha, 2011) leaf fresh weight and its dry weight but it does not affect on leaf succulence.
Ionic toxicity occurred due to uptake of Na+ and Cl– ions through roots and then these ions transport throughout the plant system but they accumulated in leaves at injurious level that created different ionic imbalance in plant (Tejera et al., 2006). Ionic stress occurs due to salinity in which Na+ and Cl¯ ions accumulates in leaves which leads to leaf senescence and reduces photosynthetic area of plant (Munns, 2002). In response of high salinity the Na+ and Cl– accumulates above its optimum level causes the deficiency of essential nutrients K+ and Ca+2 which are necessary for plant growth (Tejera et al., 2006). Wei et al., (2003) reported that optimum level of K+ ions is important for the maintenance of cellular structures and for cellular functions. Salt stress inhibits the growth and metabolic processes of plants by osmotic stress, ionic stress and oxidative stress (Okhovatian-Ardakani et al., 2010). (Ashraf, 2004) Furthermore, cell membrane stability, chlorophyll content synthesis and photosynthetic activity are significantly hampered under saline conditions. Hajiboland and Hasani, (2007) salinity affects the net photosynthesis rate by reduces the diffusion of CO2 in respiration process through stomata. Excessve accumulation of Na+and Cl−ions might lead major variation in the protein structure and amino acids, while turgor pressure reduces due to osmotic stress (Errabii et al., 2007).Water potential in leaves decreases in response of high NaCl concentrations which leads to reduction of plant growth (Wahome, 2003; Neocleous and Vasilakakis, 2007). Ahmad and Jabeen, (2009) documented that as salt concentration increases in soil it directly reduces the osmotic potential of sunflower and wheat leaves.Nitrate Reductase Activity (NRA) starts to decreases when the concentration of NaCl increases in comparison of control (Bybordi and Elnaz, 2011).
Vegetables for a consumer, consider as a best source for getting minerals but for farmer cultivation of these short duration vegetables as a source of their profit (AVRDC, 2006). Tomato (Lycopersicon esculentum) is winter vegetable, consumed worldwide (Matoo et al., 2008). The tomato is originated to South America. It is an important vegetable of Solanaceae family the other most common vegetables of this family are eggplant, peppers and potato. It is somewhat short duration crop and consider as a cash crop for the small farm holders Tomato is beneficial for human health because it contain Vitamin A and C, minerals, sugars, dietary fibers and carotenoids. It grows well and produce good quality crop in fairly cool and dry climate. It requires optimum temperature between 21 and 24 °C. Light affects its fruit set and fruit colour. Tomato grows best in soils of pH 5.5 – 6.8 with sufficient nutrient availability.
It is usually consumed as fresh, cooked or after processing by canning, making into juice, pulp, paste, or as a variety of sauces. It is cultivated to an extensive range of climates vary from the tropics to few extent of Arctic Circle. Similar to other vegetables, salinity also restricting the tomato productivity worldwide and particularly within the Pakistan. Total tomato cultivated area in the world is 4.41 M ha with the production of 151.69 M tonnes while in Pakistan its cultivated area is 0.49 M ha with the production of 0.47 M tonnes (FAO, 2010). Reclamation of salt affected soil is a laborious and very expensive practice. While, appropriate alleviation of salt affected soils and their improvement with the use of different chemicals and organic amendments. However, we are unable to do these all amendments due to certain difficulty such as deficiency of good quality irrigation water and cost of amendments, we are unable to meet the requirements. Therefore, we have to learn to live with salinity and develop such varieties that can tolerate salinity. Both transgenic application and molecular approach are of vital importance in producing salt tolerant plants. But these will generate fruitful outcomes after long span. Therefore, some simple and quick outcome giving approaches like salt tolerance induction by application of various growth regulators and identification of tolerant genotypes among the available germplasm should be employed to enhance the growth and productivity of crops (Horticultural/ Agronomic) from salt affected areas. In this way these identified salt tolerant genotypes can be cultivated on marginal saline soils of Pakistan.
Screening in all commercial vegetable should be done because through screening we can identify salt resistance vegetables genotypes the conventional breeding is also an approach that can be used for generating salt tolerant genotypes but it has low advantage because it deals with mostly morphological attributes but the main attributes which consider to be effective for salt tolerance are plants physiological, biochemical and genetic makeup (Flowers 2004). Salinity is that extreme environmental stress that affect detrimental to each specific stage of plant if we develop salt tolerance against any specific plant stage like during germination then it does not mean that it does not affect during its developmental stages (Foolad, 2004). So screening is a very effective method through which we might be able to identify the salt tolerant vegetable genotypes. Screening in field should not be done because the salt level is different in field soil so it is necessary to provide the same level of salinity to plants that use for screening purpose so plant should grow with uniform salinity (Cuartero and Fernandez-Munoz, 1999).
Foolad (2004) evaluated that tomato during seed germination salt tolerance may be improved by their phenotypic selection. There are various potential indicators of salinity on the basis of which screening can be done (Ashraf and Harris, 2004; Okhovatian-Ardakani et al., 2010). But plants morphological (plant biomass, leaf area, number of leaves, shoot fresh weight, root fresh weight, shoot fresh weight, shoot dry weight etc. physiological and some ionic contents (Na+ and Cl–) are of vital importance for early stage screening (Ashraf and Harris, 2004). The findings of this research project are of great importance for the plant breeders working on various breeding programmes related to salt tolerance in tomato. On the other hand, the findings of project will also guide the tomato growers about salt tolerance level of tested genotypes. Since, the identified salt tolerant tomato genotypes can be grown on marginal saline soils therefore, the investigation will be very fruitful by adding a share in poverty alleviation, in the form of high yield which leads to better economic condition of farmer community.
Dr. Rashad Mukhtar Balal1,2, Dr. Muhammad Anjum Aqueel1,3, Dr. Muhammad Adnan Shahid1,2, Hira Tariq1 and Ayesha Siddiqua1
1 University College of Agriculture, University of Sargodha, Pakistan
2 Cornell University, NY, USA
3 Imperial College London, Silwood Park Campus, Ascot, SL5 7PY, U.K.