عنوان مقاله [English]
In saline environments, plant growth and crop production are greatly reduced. Salinity, induces oxidative stress in the plants resulting in the production of reactive oxygen species (ROS), subsequently, cell membranes, proteins and nucleic acids are destroyed by ROS. Calcium plays a key role in processes that preserve the structural and functional integrity of plant cell membranes, stabilizes cell wall structures, regulates ion transport and selectivity, and controls ion-exchange behavior as well as cell wall enzyme activities. High concentration of Ca2+, stimulates its entry to the cell through ion channels. These channels are also permeable to sodium. Studies have shown that increasing Ca2+ concentration, decreases plasma membrane permeability to Na+ and changes the cell wall properties resulting in reduced Na+ transport by passive transport and decreased Na+ accumulation in the cell. The main objective is to identify the interaction effects of Na+/Ca2+ ions on morpho physiological characteristics of Bean plant and investigation the ameliorative effects of Ca2+ on salinity-induced damages.
Material and Methods
In order to evaluate the effects of different concentrations of Na+ (NaCl) including: 0, 50, 100 and 150 mM NaCl and Ca2+ (CaCl2) including: 0, 5, 10 and 15 mM CaCl2 on morph physiological characteristics of Bean an experiment was arranged as a factorial, based on completely random design. The plastic pots containing seeds were transferred to growth chamber with 600 µmol.m2.s-1 light intensity, 16/8h light and dark period, respectively. The pots were irrigated with water (without NaCl) for 14 days until emergence, then different concentrations of Ca2+ and Na+ were applied. In 6th week after sowing, plants were harvested and morphological and physiological characteristics were evaluated. The amount of some elements in roots and leaves were determined. Data were analyzed using MSTAT-C software.
Results and Discussion
The interaction of Na+ and Ca2+ on all morphological traits except root dry weight was significant. Toxicity and drought stress are the result of plant exposure to high concentrations of sodium. As water enters the cell, the turgor pressure increases causing the cell walls to extend irreversibly. The rate at which a cell expands is a function of its turgor pressure and cell wall properties. In all salinity levels, the use of 5 and 10 mM Ca2 + significantly increased plant height compared to control. In high salinity levels (100 and 150 mM NaCl), the role of calcium in increasing plant height decreased. In severity stress (150 mM NaCl), application of 10 mM Ca2 +, significantly increased shoot dry weight and leaf area compared to control. Results for root dry weight showed that, with increasing salinity, root dry weight at all levels of Ca2+, decreased. The highest root dry weight and total root length were attributed to the application of 5 mg Ca2 + in saline-free medium. Application of Ca2 + (mainly at 5 and 10 mM) moderated the negative effects of salinity on morphological traits. The elevated Ca2 + in the medium containing Na + ions, inhibits the binding of Na + to cell walls and the plasma membrane probably. In this way electrolyte leakage in the membrane may be reduced. Calcium improves the ability to synthesize and repair of cell walls with a more efficient function by participating in cell wall construction. In low and medium salinity, the use of 10 mM Ca2+ protected cell membranes from adverse effect of Na+, when compared to the control. Supplemental of 5 and 10 mM Ca2+ in all salinity levels, almost improved the leaf relative water content when compared to the control (non-applied Ca2+). Promotion in hydraulic conductivity, more stability and efficient membranes for selective absorption are the other features that were affected by Ca2+ supplemental. In high salinity, the use of 5 mM Ca2+ reduced the negative effects of salinity on total chlorophyll contain when compared to the control. At all salinity levels, application of 5 and 10 mM CaCl2 significantly reduced leaf proline content compared to control. In this regard, the effect of 5 mM Ca2+ was greater than 10 mM Ca2+. Addition of Ca2+ to the medium of plant exposed to salt stress, reduced proline concentration by increasing proline oxidase and following that reduction in glutamyl kinase activity and finally glutamine is used to synthesize more chlorophyll. At all salinity levels, the use of 5 and 10 mM Ca2 + significantly increased the activity of poly phenol oxidase compared to the control. Calcium promoted the synthesis and activity of many enzymes involved in defense mechanism and reduces the rate of proteolytic degradation. In this way Calcium modulates oxidative stress by altering plant metabolism. Results showed that the use of 5 and 10 mM Ca2 + significantly (P≤ 0.05) reduced the amount of Na+ in the plants (leaves + roots) when compared to control. This result for K+ accumulation was adverse. The obtained results go in line with the findings of other scientists. Wu and Wang, 2012 reported, Ca2+ decreased roots Na+ accumulation, increased shoots K+ accumulation, and enhanced the selective absorption and transport capacity for K+ over Na+ in the plant.
Salinity stress significantly reduced plant morphological characteristics but other traits such as proline and polyphenol oxidase increased. Membrane stability index, leaf relative water content, total chlorophyll content and leaf and root potassium content were significantly decreased with applying salinity stress. The use of Ca2+ ions, especially 5 and 10 mM, greatly reduced the negative effects of salinity. It seems that the use of calcium application can be considered as a simple and low cost method for reducing the adverse effects of salinity stress.