Plants are constantly influenced by a range of both living and non-living environmental factors. For a plant to grow to its fullest potential, it requires a specific amount of each abiotic environmental factor. Salinity stress is considered the most damaging abiotic stress to agricultural productivity for many crops. It can have destructive effects on plant growth and development. Reactive oxygen species (ROS) are produced when plants experience salinity stress, leading to oxidative stress. Plants have special ways to protect themselves from environmental stress. These include antioxidant systems, both enzymatic and non- enzymatic, that help get rid of oxidative stress. The Brassicaceae family vegetables are rich in bioactive phytochemicals, making them an excellent source of antioxidants. Brassica species are widely consumed as a staple food and contribute approximately 15% of global oil production. Brassica napus (canola) is a member of this family. This study examined the defense system of canola when exposed to varying concentrations of K2SO4 and Na2SO4 salts. After disinfection with 1% bleach water, the canola seeds were washed. The Petri dishes and filter paper were placed in bleach water and then rinsed with distilled water. Three replicates were used for each treatment including 40, 80, and 120 mM Na2SO4 and/ or K2SO4. To germinate, the 40 seeds were put into petri dish and placed in an oven at 30 oC. To calculate the germination percentage and rate, we counted the number of germinated seed every day. We collected after five days. The seedling's growth (fresh and dry weight of shoots and roots) was evaluated. The levels of sodium and potassium, the content of hydrogen peroxidase (H2O2), malondialdehyde (MDA), and DPPH radical scavenging activity, as well as the activity of oxidative enzymes including peroxidase (POD), total superoxide dismutase (SOD), manganese superoxide dismutase (Mn-SOD) and copper/zinc superoxide dismutase (Cu/Zn-SOD), polyphenol oxidase (PPO), phenylalanine ammonia-lyase (PAL) enzyme, and lipase enzyme was assayed. According to the results, increasing concentrations of K2SO4 and Na2SO4 led to a decrease in the fresh and dry weight of both shoots and roots. However, this reduction was higher in high concentrations and Na2SO4 showed adverse effect more than K2SO4, which can attribute to more toxicity of Na2SO4. The changes of K2SO4 and Na2SO4 content were in parallel with the changes of decrease in the germination percent and rate. In such a way that, it was observed the germination percent and rate were decreased in the high concentration of these two salts more than low concentration. Nevertheless, maybe because of more increased in the osmotic pressure under Na2SO4, this salt had been high toxic effect on germination percent and rate. Similarly, the activity of lipase was dramatically decreased in canola exposed to high concentration of K2SO4 and Na2SO4 salinity especially in plant exposed to Na2SO4 salt. In seedlings exposed to K2SO4 salt, the content of K+ was increased with the increase in K2SO4 salt concentration. On the other hand, the content of Na+ and K+ was enhanced in seedlings exposed to Na2SO4 salt. According to the current study, increase in the levels of K2SO4 and Na2SO4 treatment was resulted in an increase in both MDA and H2O2 and this increase was concentration depend manner. Also, Na2SO4 was found to have a greater effect compared to K2SO4. Result indicated that a decrease in DPPH-radical scavenging activity in response to various levels of K2SO4 and Na2SO4 treatment, but more reduction in response to Na2SO4 salt. The results of the present study showed that the activity of total SOD, Mn-SOD and Cu/Zn-SOD changed under the effect of different treatments and this change was accompanied by an increase in the activity of these enzymes. However, increase in the activity of total SOD activity was more due to Mn-SOD activity. The activity of POD was substantially decreased in canola exposed to high concentration of K2SO4 and Na2SO4 salinity especially in plant exposed to Na2SO4 salt. The production of H2O2 has increased in both iso-osmotic salts, which may be due to a significant decrease activity of POD under Na2SO4 salt. However, both K2SO4 and Na2SO4 cause the increase in PAL activity, but Na2SO4 indicated a more positive impact on the activity of PAL compared to K2SO4 and these changes were very significant in high concentration camper to the low concentration. In addition, the activity of PPO was substantially enhanced with increase in the content of K2SO4 and Na2SO4. Therefore, our results revealed that high concentration of K2SO4 and Na2SO4 was resulted in an increase in the activity of antioxidant enzymes and the content of MDA and H2O2 in canola plants. However, it was found that Na2SO4 had a greater effect compared to K2SO4 salt. In addition, the results indicated that germination percentage and rate and seedlings growth were decreased with higher ROS levels following high content of both salts. These data also strengthen the role of antioxidant enzymes as an important scavenger during K2SO4 and Na2SO4 salts stress.
