Salinity, defined as “the presence of salts (such as sodium chloride, magnesium and calcium sulfates) and bicarbonates, in soil and water” is a major environmental factor that limits plant growth and development (Silva & Uchida, 2000). The most common salt are made of “sodium (Na+), potassium (K+), magnesium (Mg2+), and calcium (Ca2+) and the anions chloride (Cl–), sulfate (SO42–), and carbonate in the form of bicarbonate (HCO3–)” (Silva & Uchida, 2000). It is a critical problem in many arid, or semi arid regions of the world. 40% of the world is a part of this arid/semi arid region and this includes major countries such as Saudi Arabic whose economy is dependent on agriculture (Qados, 2011). High salinity is one of the biggest obstacle that stands in the way of farmers in Saudi Arabia. This is caused by high concentration of salt (NaCl) running through irrigation systems of Saudi Arabia and the high temperature of Saudi Arabia which speeds up evaporation causing land to be dry and contain many traces of salt left in the soil (Qados, 2011). These arid/semi arid would contain a high amount of salt, deeply affecting the future crops that is growing in the area. High salinity can also be due to the use of sodium chloride to defrost icy roads during a snowy winter. States such as Massachusetts, Michigan and Wisconsin uses sodium chloride to melt roads since it is inexpensive and easy to access and buy from a store (Day-Brewer et al.,2011). But when spring comes along and the ice melts, the sodium chloride used to defrost roads will have already mix with and become a part of surrounding soil. This then would affect future crops growing in that area (Day-Brewer et al.,2011). In addition, salinity could also affect tropical areas such as Hawaii. The coastlines of Hawaii often have salinity issues due to sea water intruding and affect groundwater supplies. This might be caused by floods and spraying of sea water onto land with groundwater in the area. Other source of salinity in Hawaii could be caused by using salt affected soil as landfill, leaving a lasting impact and limits on the land filled. Salinity is a difficult problem that challenges many all around the world. The amount of salinity in an area could cause serious problems and affect the physiological and biochemical features of plant growing in the area.
A recent study done by Brewer, Larsen, Morey and Wickham of Portland Community College shows that salinity does deeply affect plants. Brewer, Larsen, Morey and Wickham observed how sodium chloride could affect the growth of Lycopersicon esculentum. The experiment was tested on four groups including a control group that received only water. The other three group would receive 23%, 11.5% and 5.75% concentration of sodium chloride once a week. Every seven days, the height of the plant would be measured from the base to the apical meristem (Day-Brewer et al.,2011). At the end of the experiment, the control group had a greater amount of growth compared to the other three experiment group. The group watered with 5.75% concentration of solution show a bit growth throughout the duration of the experiment but the other two experimental group “were flaccid at day two, and were unable to recover to a turgid state with subsequent watering” (Day-Brewer et al.,2011). “Chinnusamy, et al. (2005) suggest the turgor lost of these plants is caused by osmotic stress that is a result of a decrease in water potential due to soil salinity” (Day-Brewer et al.,2011). The death of the 11.5% and 23% experimental group may be because of “a salt affected soil, creating a high concentrations of ions, particularly Na+ resulting in cytoplasmic toxicity” (Day-Brewer et al.,2011). This meant the two experimental group that died most likely had a higher concentration of water inside the cell meaning it was under a hypertonic condition. With a higher concentration of water inside the cell, the water would leave the cell causing the cell’s turgor pressure to decrease and lose water. The lost of water would then cause the cell to be unable to function, and eventually cause the plant to die. Base on the experiment done by Brewer, Larsen, Morey and Wickham one can conclude that high salinity does have an effect on plants, and could potentially cause a lost of turgor pressure in plants which could lead to death.
Another study done by Amira M.S Abdul Qados of King Saud University on the Effect of salt stress on plant growth and metabolism of bean plant Vicia faba have similar results. The experiment was carried out in a greenhouse at Nura Int Abdul-Rahman University and seeds were obtained from a local market. 10 homogeneous plants were grown in each plastic pot. The plants were then separated into four groups, each watered with either 0, 60, 120, and 240mM of concentration of sodium chloride. Control plants would only receive water and nutrient supply (Qados, 2011). In addition, plants in experimental groups started off with a lower concentration of sodium chloride and gradually increased on a daily basis to avoid osmotic shock. From then on, plants received nutrient solutions every three day, and an addition of sodium chloride every two weeks (Qados, 2011). Measurements were taken “10 days after the start of treatment, and when 40% of the fourth group plants on highest concentration had died.” which was day 90 (Qados, 2011). The length of shoot system, number of plant leaves, leaf area and fresh and dry weights was measured. Like the results of Brewer et al. experiment, Qados found a decrease of plant height with higher concentrations. The control group, 60mM and 120mM group’s plant height grew increased at a pretty decent rate while the group with 240mM was growing slower and less than the other groups. Results also showed that the 240mM group also had a lower amount of leaves and leaf areas than the other groups. Unlike the other measurements, salinity has caused an increase in both fresh and dry weight compared to the control group. The increase of fresh weight though “may be due to the ability of the plant to increase the size of its sap vacuoles, which allows for the collection of a lot of water, and this in turn dissolves salt ions that have accumulated and leads to subsequent increase in fresh weight” (Qados, 2011). Qados also concluded that increasing salinity caused the osmotic potential of the plant to decrease. The change is considered to be a defensive mean by which plants tolerate stress and increase its ability to absorb water (Qados, 2011). Furthermore, Qados had found that chlorophyll a, chlorophyll b and carotenoids content has decreased when the concentration of sodium chloride is increased. With the chlorophylls and carotenoids decreasing, this would affect photosynthesis, a process that depends on chlorophylls and carotenoids absorbing light energy so it can store energy in organic compound (Qados, 2011). Less light energy would be absorbed causing the rate of photosynthesis and sister process cellular respiration to be affected.
Besides causing plants to lose turgor pressure and affecting the rate of photosynthesis/cellular respiration, salinity can also cause an excessive or inadequate amount of certain elements. A study on the Effect of Soil Salinity on Growth, Water Status and Nutrient Accumulation in Seedlings of Ziziphus mauritiana (Rhamnaceae) done by Bhatt et al. shows that salinity causes an imbalance in ions and nutrient uptake. Soil salinity caused either an excessive or inadequate amount of Sodium, Potassium, Nitrogen, Phosphorus, Calcium and Magnesium in plant cells (Bhatt et al., 2007). “Potassium and sodium content significantly increased in leaves, stems, tap roots and lateral root tissues” as soil salinity increased. “The cation K+ is essential for cell expansion, osmoregulation and cellular and whole plant homeostasis” (Bhatt et al., 2007). A large amount of K+ is required for photosynthesis so therefore an increase of K+ could potentially be beneficial for the plant. (Bhatt et al., 2007) On the other hand though, a large amount of Na could poison the plant and cause cytoplasmic toxicity. (Day-Brewer et al.,2011). Other elements such as “Nitrogen, phosphorus, calcium and magnesium content significantly decreased in leaves, stems, tap roots and lateral root in response to increase in soil salinity.” (Bhatt et al., 2007). With a decrease in many of these essential elements, it could greatly affect plant growth. Many of these elements are essential for plants to function and support the plants (Homayoun, 2011). But the other elements such as Phosphorus, Calcium and Magnesium decreased due to salinity affecting the soil. Phosphorus helps strengthen the plant especially with root development (Homayoun, 2011). Calcium and Magnesium also promote plant growth and seed development (Homayoun, 2011). A decrease in these elements could cause the plant to have weaker roots, grow slower and overall affecting the growth of the plant.
In conclusion, a high salinity can cause a huge change to the development and growth of a plant. First, it could based on the Brewer et al. experiment, a high salinity caused the plant height and growth of the plant to decrease and grow at a slower rate compared to the control plant. The plant grew slower due to less turgor pressure, which can lead to a death of plant. In addition, another study done by Qados concluded that a high salinity could cause the osmotic potential of a plant to decrease which causes plant cells to shrink and lose water. He also concluded that there was a lost of chlorophylls and carotenoids which could affect photosynthesis since chlorophylls are necessary since it absorbs light energy from the sun used in photosynthesis. Lastly, another study done by Bhatt et al. noted that salinity can cause an increase and decrease in essential elements. Elements such as sodium, phosphorus, calcium, and nitrogen decreased. These are essential elements that strengthen plants and help development so a decrease can harm the plant and affect plant growth. Despite sodium, phosphorus, calcium and nitrogen decreasing, the cation K+ increased. K+ ions helps with cell expansion and homeostasis and overall cell growth and development. Thus, a large amount of salinity can cause plant cells to lose turgor pressure and possibly death, a decrease in plant height, affect osmotic potential of a cell, a loss of chlorophylls and carotenoids, and cause an insufficient amount of essential elements such as phosphorus, calcium and sodium in plants.
Reference
Bhatt, M. J., Bhatti, P. M., Patel, A. D., & Pandey, A. N. (2007). Effect of Soil Salinity on Growth, Water Status, and Nutrient Accumulation in seedlings of Ziziphus Mauritiana (Rhamnaceae). Journal of Fruit and Ornamental Plant Research, 16, 383–401.
Day-Brewer, S., Larsen, S., Morey, A., & Wickham, D. (2011, ). The Effects of Sodium Chloride (NaCl) on the Growth of Tomato Plants (lycopersicon esculentum). Retrieved January 27, 2017, from http://www.pcc.edu/library/wp-content/uploads/ sodium-chloride.pdf
Homayoun, H. (2011). Effect of NaCl salinity on wheat (Triticum aestivum L.) cultivars at germination stage. Advances in Environmental Biology, 1716+. Retrieved from http:// libraries.state.ma.us/login?gwurl=http://go.galegroup.com/ps/i.do?p=PPES&sw=w&u=
mlin_s_thomas&v=2.1&it=r&id=GALE%7CA263659406&asid=fdf33c4dd 16239d30766a428b2591cb6
Qados, A. M. (2011, January). Effect of salt stress on plant growth and metabolism of bean plant Vicia faba (L.). Retrieved January 28, 2017, from http://www.sciencedirect.com/science/ article/pii/S1658077X10000032
Silva, J. A., & Uchida, R. S. (2000). Plant Nutrient Management in Hawaii’s soils: Approaches for Tropical and Subtropical Agriculture.