Abiotic stresses like heat, cold, light, drought and salinity are serious threats for plant growth and productivity but among these stresses, salinity is increasing day by day, consequently causing decline in the crop quality and quantity. It not only negatively affects the agronomic crops but horticultural crops as well. Among the horticultural crops, vegetables are more susceptible to salt stress. Tomato (Lycopersicon esculentum Mill.) is a common vegetable of significant importance, but salinity limits its productivity by negatively affecting various physiological and biochemical aspects. Keeping in view the drastic effects of salinity and importance of tomato in our daily life, the present study was planned. This study was carried out in two phases, (i) screening of tested tomato genotypes for salt tolerance and (ii) characterization of salt tolerant and salt sensitive genotypes under saline conditions. In screening phase, ten tomato genotypes having varied salinity tolerance potential were submitted to four saline regimes i.e. 0, 25, 50 and 75 mM and germination percentage and plant height was evaluated. On the basis of these attributes, the tested genotypes were categorized into salt-tolerant and salt-sensitive groups. However, in second phase, we utilized two salt tolerant and two salt sensitive genotypes, identified in screening experiment to compare the main differences in morphological, physiological and biochemical attributes of salt-tolerant and salt-sensitive genotypes on exposure to salt stress. It was observed that salt stress induced more deleterious effects on the morpho-physiological and biochemical aspects of salt-sensitive plants as compared to salt-tolerant. It was also observed that salt tolerant potential of tested genotypes is positively linked with the concentration of osmolytes like proline and glycinebetaine.
A saline soil is generally defined as one having the electrical conductivity (Ec) of the saturation extract in the root zone above 4 dSm-1 at 250C and has exchangeable sodium (Foolad, 2004). It is reported that total available land on earth is 14 billion ha and 6.5 billion ha of this land is comprised of arid and semi arid regions while 1 billion ha of this arid and semi arid region is salt affected (Foolad, 2004). Furthermore, it is estimated that about 20% of cultivated lands and 33% of irrigated agricultural lands worldwide are deteriorated by high salinity (Szabolcs, 1994, Ghassemi et al., 1995, Foolad, 2004).In agricultural land water-logging and salt accumulations adversely affect plant growth and reduce the potential crop production due to which plants can be killed in the early growth stages leaving the land unusable. The salinization of agricultural land at extensive scale causes massive economic loss at the global level. The annual global income losses due to salinization of agricultural land could reach 11.4 billion US$ in irrigated land and 1.2 billion US$ in non-irrigated areas (Ghassemi et al., 1995).
In Pakistan, salinity is one of the country’s most serious environmental problems, caused by human-induced soil erosion and long-term mismanagement of various agricultural practices (World Bank, 2006). Not only the soils but also the irrigation water itself is one of the major source of salinity in the arid region (Rus et al., 2002). Among the 25% of all irrigated land affected by salinity, approximately 1.4 Mha of land has now been abandoned (World Bank, 2006). Salinity is categorized as an environmental agent, which confines the vegetative growth and productivity by elevating the toxic ion concentration in rhizosphere, osmotic stress and water shortage by reducing the water absorbing capacity of the plant. The salt stress may be due to excess of many salts like Na2SO4, NaCl, MgCl2 etc but NaCl is considered as dominating agent in this regard. High concentration of soluble salts in growing media causes osmotic stress and limits the availability of water to plant and this condition creates a drought condition regarded as physiological drought. Salt stress disturb the water relations and various metabolic processes within the plant tissue. (Porgali, 2001).
Salinity is a global issue so great effort has been dedicated to explore the physiological aspects of salt tolerance in plants, as a basis for plant breeders to develop salinity-tolerant genotypes. In spite of this great effort, there is a dire need to identify the salt tolerant cultivars because only a little worth has been focused on this approach. (Cuartero et al., 2006).
The importance of tomato (Lycopersicon esculentum Mill.), as a vegetable crop, is manifested by its large-scale cultivation around the globe. The area of Pakistan under tomato cultivation in 2010 was 0.0049 Mha and production was 0.476828 M tons ( FAO, 2010 ). Tomato has high nutritional value and consumed in our daily life in the form of raw, cooked or processed forms. In Pakistan tomato is used as an important component in making curry, chutneys, salads and in pickles almost everyday at homes. It contains certain vitamins like vitamin A, C and E and very important antioxidant lycopene. But, we are not getting the satisfactory production under saline conditions instead of other factors like seed, fertilizers, insect-pest and diseases, salinity also mainly contribute in minimizing the tomato production. Tomato is moderately sensitive to salt tolerance level proved from various experiments performed by Lyon (1941), Hayward and Long (1943), Bierhtjizen and Ploegman (1967), Peralta et al.,( 2005). But the response to a single salinity level also depends on the variety of tomato used (Byari and Al-Maghrabi, 1991). In tomato plants stomatal density is reduced by salinity (Romero-Aranda et al., 2001). Many researchers verified through their studies that due to salinity the photosynthetic capacity of plants also concealed (Dubey, 1997; Kao et al., 2001; Ashraf, 2001; Romero-Aranda et al., 2001). Most of the crops which are stressed by salinity showed that it affects seed germination and early seedling growth which are certainly the most responsive stages. (Sivritepe et al., 2003; Ashraf and Foolad, 2005). Salinity negatively affects seeds and seedlings and obviously significantly contributes in establishment of poor stand and ultimately causing poor production of vegetable crops (Grassbaugh and Bennett, 1998).
Since salinity is a global issue that limiting the vegetable growth and yield, and tomato cannot be ignored in this scenario. As discussed above that there are various causes of salinity but addition of factories effluents and use of underground salty water are the dominant salinity causing agents. These agents adds and enhance the toxic ions particularly the Na+ and Cl– in the root zone, So these ions are transmitted to the internal plant body where they cause malfunctioning of various tissues and disturbances in metabolic mechanisms. Therefore, this study is planned to compare the various physio-chemical differences in ten commercially grown cultivars and on the basis of these findings we will categorized the tested cultivars into salt tolerant and non-tolerant ones. The outcomes of present investigation will be highly useful to the tomato growers. Secondly, the findings of current investigation will also be helpful for the tomato breeders in strengthening the tomato breeding programs related to salt stress.
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.