Effect of potassium nitrate on antioxidant enzymes activity of aged milk thistle (Silybum marianum) seeds

Seed aging increases activity of reactive oxygen molecules and antioxidant enzymes have a key role against increased oxidative stress which is caused by free radicals. Effect of seed priming on increasing of these enzymes activity and its impacts on seed germination characteristics of deteriorated milk thistle seeds was studied in University of Mohaghegh Ardabili in 2013. Factoral experiment based on Complete Randomized Design (CRD) with three replications. The accelerated seed aging was to increase free radicals activates and seed priming with potassium nitrate was to decrease. Priming treatment was done in five levels (0, 15, 30, 45 and 60 mg per liter potassium nitrate) and seed aging at four levels (non-aging, 48, 96 and 144 hours aging). The results showed that enhancing seed aging reduced the activity of antioxidant enzymes superoxide dismutase (62%), catalase (34%), peroxidase (43%), ascorbate peroxidase (57%), glutathione peroxidase (61%) and glutathione reductase (58%), which eventually led to a decrease in germination percentage (25%) and germination rate (15%). Effect of seed priming reduced aging effect and increased the activity of antioxidant enzymes, which eventually led to an increase in germination. The concentration of 30- 40 mgL-1 nitrate showed the greatest impact. Regression results showed that the enzyme activity follows a linear relationship with the percentage and rate of seed germination. According to path analysis results, the antioxidant enzymes had direct and indirect effects on the germination rate and percent respectively, during changes in free radicals activity, so that the peroxidase activity had the greatest impact on the germination percent (0.56) and rate (0.73), respectively.