The productivity of food is greatly affected with the salinity since fruitful soils are turned into nonbearing. According to Ghassemi, Jakeman and Nix (1995), the process may also lead to the decline of biodiversity through the degradation of habitats. Soils can get salt affected in two different ways. One of them is a natural process that is referred to as primary salinization. It implies weathering, which results in accumulation of ions. The other way is secondary salimnization caused by the activities of people. FAO (2005) reported that there are approximately 831 Mha of the salt affected lands. What is more, devastating human activities result in increased salinity and transformation of fertile soils into infertile. Thus, according to the estimations of Ghassemi et al. (1995), salinity leads to degradation of 1.5 Mha of lands every year.
Researchers claim that about thirty percent of the food in the world are produced on the irrigated areas that cover 230 Mha (Munns 2005; Munns & Tester 2008). About one fifth of the irrigated lands is salt affected. According to Flowers and Flowers (2005), this percentage is much higher, and it amounts up to 50 %. The problem of salinity is a burning one. If the agricultural areas are salt affected to a great extent, they are abandoned. The annual estimation of such abandoned lands is 10 Mha as reported by FAO (1995).
The sources of salinity can be classified the following way.
- Deforestation or desertification. It is considered to be a prime cause of alkalization and salinization of soils. The main reason of this process is the migration of salt in all layers of soils. Different types of stress, marsh formation, natural salinity, and human activity contribute greatly in the desertification.
- Increase of the amount of water or air-borne salts in soil. Industrial wastes and chemical emissions store in soils and lead to the accumulation of salt, particularly in the soil upper layers. Bond (1998) reported that the upper layers of soil may be similarly contaminated with the waste water that can cause alkanization and salinization. It is explained by the high concentration of salt in such water.
- Irrigation with saline water. Such type of irrigation is used in intensive and greenhouse farming. Alcanity or salinity is a result of accumulation of salt and chemicals in greenhouse (semi-closed) or closed farming systems. According to Pessarakli (1991), it is a common problem for such countries as Holland or Japan which are known for the intensity of agriculture.
- Overgrazing. The process may result in desertification since one of its consequences is a decrease of vegetation. So, salinization develops very rapidly, and the pasture areas diminish considerably. The process of overgrazing is typical for semi-arid and arid areas. There are insignificant natural soil areas, so the animal husbandry requirements are not satisfied.
Mechanisms of Growth Inhibition
Plants are affected with salinity as well. According to Ayers and Hayward (1948) and Ayers (1951), this effect can be explained by specific ion impact, also known as chloride and sodium toxic effect, and indirect saline media osmotic factors. Sodium in high concentration interferes with other nutrient elements in lower concentrations and causes special ion effects that can be toxic to the plants. American Society of Civil Engineers (1990) specified three stages of plant development, in particular, germination, vegetative growth, and reproductive growth. The degree of plant sensitivity or salt tolerance depends on the specific stage of development. The rate of germination, yield, and seedling establishment can be reduced with the salinity stress as a consequence of the effect of NaCl.
Non-tolerant plants get toxic due to high concentration of salt and accumulation of excessive ions in the cells. Ion concentrationdetermines the extent of toxicity while the reaction of the plant is subject to the species, variety, or individual peculiarities of every plant. Fitter and Hay (1987) reported that an excessive amount of salt affects the growth of plants and interferes with certain physiological functions and metabolic processes. The negative effect of ions can be produced differently. Ions can act as catalysers of metabolite decomposition or even anti metabolites and enzyme inhibitors. They can combine with membranes of cells, therefore, changing the permeability. Some electrochemical elements replaced with ions no longer perform their functions.
Salt generates negative impact on a complex membrane system of the cells and interferes with the process of cell exchange with the surroundings. A special type of membrane called plasma membrane or plasmalemma is a characteristic only of the cells of higher plants. The breakdown of membrane structure is caused by direct influence of salt. Salt ruins the structure of membranes since Ca++ that ensures the membrane integrity is replaced with Na+. As explained by Leopold and Willing (1984), salt effect can be seen in the lesions on the plasma membranes, change of their structure, and leakage of solutes. Since Ca++ facilitates the growth of plants, it can be stipulated that the increase of the Ca++ concentration will contribute in better salt tolerance of plants and keeping an adequate ration of K+/ Na+. Numerous researchers indicated that the toxic effect of high concentration of salt and membrane damage may be shown when K+ leaks to some external solution.
According to Taiz and Zeiger (1991), enzymes can function at the optimal level in non-saline conditions with higher K+ and lower Na+ concentration in the higher plant cell cytosol. On the contrary, if the salt concentration increases, there is a decrease of the internal K+ concentration. Bohra and Doerffing (1993), Cerda et al. (1995) and a number of other researchers emphasized the impact of K+ as an essential macronutrient. If it is replaced with Na+, metabolic functions fail (Flowers & Lauchli 1983; Leigh & Storey 1991), though such substitution can be successful in some cases.
According to the division done by Marschner (1995), plants can be subdivided in relation to how they react to substitution of potassium with sodium, meaning their tolerance to Na+. The first group is distinguished with no effect of Na+ that replaces K+ on the growth and development. The second group allows a less significant amount of K+ substituted without negative consequences. The plants in the third group can grow only with limited substitution. In the fourth group, such kind of replacement is impossible. Thus, high cytoplasm ratio of K+ to Na+ ions ensures successful growth in saline conditions.
Osmotic effect is the most essential one. Plants take less water from the saline soil with low water potential and can even get some kind of drought stress. If the concentration of salt is at least moderate or even low, plants adjust and can take water adequately. Plants do not suffer from water deficiency. Plants lose water in a saline medium because the cell water potential of plants is decreased. Thus, as Jacoby (1994) and Poljakoff-Mayber and Lerner (1994) claim, the osmotic potential is reduced the same way; thus, the turgor pressure, which is of great importance to the plants, diminishes. O’Leary (1973) and Karen (1994) refer to this effect as to the physiological drought as salt solution causes reduced uptake of water and is similar to drought. Water deficiency is inevitable even if there is sufficient concentration of water in such kind of soil.
The Effect of Salinity on Germination
High ion concentration facilitates the adverse effect of salinity on germination of seeds. Sen, Kasera and Mohammed (2002) pointed out the reduction, postponing, or inhibition of seed germination under the influence of salinity. Khan et al. (2003) even observed entering a dormant state by seeds under the conditions of salinity. Provided that the concentration of NaCl is low, the germination rate and development of seeds can even increase depending on the species. This was documented by Zadeh and Naeini in 2007. They proved that the effect is not stable and may vary between non-halophytes and halophytes. Sen, Kasera and Mohammed (2002) determined other factors of the salinity effect. They are concentration of salt and type of salt in the substrate. Other researchers, including Khan, Gul, Weber, Gulzar and Ungar, added temperature to the list of factors. Besides, some halophytes like H. salicornicum or Haloxylon recurvum adapt and germinate rapidly at the period of time when salinity of the soil is the lowest, this is the precipitation season.
Salt stress effects germination of seeds via ion effect or osmotic stress. Disproportionate salt accumulation produces adverse effect on seeds while osmotic stress reduces osmolitic potentials of soil. Thus, plants can take less water.
Non-germinated seeds react on distilled water after toxic effect and water deficiency in different ways. The origin of non-germination can also be traced if seeds are compared in NaCl and mannitol or PEG. Non-germinated seeds of rock samphire recovered after they were transferred from 100 mM NaCl to distilled water. That was documented by Atia et al. in 2006. Osmotic effect, in its turn, resulted in inhibition of water uptake and germination.
Sunflower seeds germination was studied under the effect of PEG and NaCl. Germination inhibition in PEG was complete while in NaCl, it was 98.6%. However, those seeds after PEG conditions after osmotic stress germinated in the deionized water.They did not get any toxic effect. The explanation of Mujeeb-ur-Rahman et al. (2008) provided for the osmotic effect after which the wheat cultivar seeds recovered in distilled water. Excessive amounts of Cl and Na injured the embryo and germination failed. Ramoliya, Patel and Pandey proved that the seeds of a mustard tree lose their viability under the salinity conditions higher than 12 and 14.9 dS m-1. Moreover, the chemists detected a relationship between the level of salinity and Cl content of the seed. According to Othman et al. (2006), if the level of salinity is high, the seeds cannot germinate in the distilled water after their development was inhibited and effected with ion toxicity. The fact that the seeds fail to germinate in distilled water after being in saline environment was explained by Mujeeb-ur-Rahman et al. (2008) with the fact that the embryo is ruined by Cl- and Na+. The chemist carried out the investigation of the recovery of wheat cultivars (Zarghoon, Zardan, Raskoh & Zarlasht) and made his conclusions on its basis.
In one of the incestigations, germination of A. Nilotica was significantly delayed and slowed down with the increase of salt concentration. Ramoliya and Pandey (2002) pointed out that at 14.3 dS m-1, the inhibition of germination was complete. The germination of red quebracho was reduced at 200 mM NaCl and completely inhibited at 300 mM.
Researchers found a negative correlation between germination percentage of various species of plants and the salt level. Some of the examples are A. nilotica, grey-leaved cordial, A. catechu, A. mellifera, false senna (Patel and Pandey 2007), two wheat cultivars, A. tortilisand others. Rehman et al. (1996) reported the negative effect of NaCl on germination using A. Saligna and some other Acacia species. The level of complete inhibition of Pinus mariana germination was determined as Na2SO4 and 250 mM NaCl by Croser et al. in 2001.
Other scientists worked on determining the causes of postponement of germination and emergence. One of the numerous instances is the postponed emergence of red river gum and lemon-scented eucalyptus under the conditions of salinity and imposure of NaCl.