1. Saline soils: the soils which have electrical conductivity (EC) >4 dS m-1; exchangeable sodium percentage (ESP) <15%; pH below 8.5 and sodium adsorption ratio (SAR) of 0-12.
2. Saline-sodic soils: these have EC >4 dS m-1, pH below 8.5, ESP >15%, SAR >12. and
3. Sodic soils: have EC less than 4 dS m-1, pH above 8.5, ESP is >15% and SAR above 12. These types of soils may cause abiotic stress in plants known as saline stress. There are various abiotic stress factors that affect growth and productivity of plants in earth planet. On regular basis, plants are in continuous exposure to pressure conditions such as water shortage, saltiness, higher temperature, heavy rainfall and heavy metal toxic levels. Salt stress characterized by higher accumulation of salts is a serious threat and a main restraint to growth and productivity of crop plants. Salinity incurs drastic changes in plants mechanisms through ionic stress, osmotic stress and nutritional imbalance, which alter proteins formation, photosynthesis, lipids and energy status of the plant. Salinity causes both toxic (ion accumulation) and osmotic (cell dehydration) stresses in the whole plant, thus interrupting ion homeostasis, restricting production, nitrogen fixation and essential mineral uptake. In a current situation several studies have been done to check the response of various crops to salinity stress (Parida and Das, 2005).
The high salt concentration in the cell sap actually prevents uptake of water and essential nutrients by plant roots. Salinity is not only a matter of higher absorption of sodium and chloride in plant tissues but it also disturbs various physiological processes in plants (Ashraf and O’Leary, 1996). These include disturbances in the functioning of various glycolytic enzymes and photosynthetic pigments. Generally salinity imposes bad impact on plant growth and development and creates malfunctioning of photosynthetic apparatus and enzyme activities (Jitesh et al., 2005). To rectify the salinity problem in many plant species several techniques have been used. Among the nutrients, potassium and silicon are recognized for their critical role in combating salt stress in various plant species (Liang et al., 2007). Particularly, increased potassium level in crop plants under saline conditions have been investigated by Cerda et al. (1995). Thus, higher K+/Na+ ratios are distinctive in the tissues of salt-tolerant crop varieties and frequently help in screening and identification of salt stress-tolerant crop cultivars (Mittler, 2002).
Among others, anthropogenic actions have accentuated the obtainable stress factors. Among these stresses, salt stress is a severe threat in whole world agricultural lands; it restricts plants growth, development and production. Irrigation with saline water affects severely physiological processes in plants counting ionic stress, nutritional imbalance and osmotic stress. Salinity reduces the uptake of essential nutrients which are required by the plants for their optimum growth. In current situation, there has been a growing tendency toward the examination of better and efficient antioxidant and efficient ingredient from naturally diet resources like oilseeds, vegetables, fruits, herbs, grains and cereals. The process of lipid oxidation is retarded by most of these antioxidants by playing a potential role in reacting with catalytic metals, free radicals and chelating mechanisms (Rababbah et al., 2005). The anti-oxidative effect of vegetables and fruit is primarily because of the incidence of phenolics compound, such as tannins and phenolics diterpenes, flavonoids, phenolic acid, (Rababah et al., 2004).
In plants salt stress response is initiated when plants recognizes stress at the cellular level, Stress recognition activates signal transduction pathways that transmit information within the individual cell and throughout the plant , the changes in gene expression may also modify their growth and development and even influence reproductive capabilities. As a result of all these plant adopt an avoidance mechanism that prevents them from exposure to stress. Plant may adopt a tolerance mechanism that permit plant to withstand under stress conditions, furthermore, plants may acclimatize them by altering their physiology in response to stress conditions (wahid et al., 2007).
During the stage of Stress escapers plants remain dormant or die, but are active during good/normal times (e.g. desert annuals – ephemeral plants etc.) and as stress avoiders the entire growth process are slow down or stopped to avoid themselves from harmful effects of a stress by restricting water loss from tissues/cells through stomatal closure or changes in leaf morphology or orientation. The salt stress tolerant plants attain an equilibrium with a stress via acclimation (when physiological modifications occur over short-time (e.g. a complete season or even for the whole life of an organism). It is non-heritable phenomenon and is adopted by plant for a short period of time. While, as a stress resistant plant modify their genetics due to mutation and get adaptation against stress, such kind of adoption is heritable (genetic) traits that increase fitness of plant against stress. The plant fail to adopt these mechanisms will die under stress conditions.
Muhammad Wajid Khan1, Dr. Rashad Mukhtar Balal1, 2, Dr. Mansoor Javid, Dr. Muhammad Adnan Shahid1 and Muhammad Zubair
Faculty of Agriculture, University of Sargodha-40100, Pakistan; Harper Adams University-TF10 8NB, UK. (Submitted: 25-01-2015)