Effect of salicylic acid application methods on seed germination and biochemical characteristics of quinoa seedlings (Chenopodium quinoa Willd.) under cadmium stress

Introduction Considering that heavy metals are one of the most important environmental stresses in today's agricultural ecosystems, which lead to a decrease in germination, seedling growth, and ultimately plant production. Therefore, it is recommended to use several compounds such as an addition to the culture medium or seed priming to increase the tolerance of plants to environmental stresses. Considering the role of salicylic acid in reducing the effects of heavy metal stress, including cadmium, on germination and seedling growth, this research was conducted to investigate the effect of salicylic acid application methods on seed germination and biochemical characteristics of quinoa seedlings under cadmium stress. Materials and methods To investigate the effect of salicylic acid application methods on seed germination and biochemical characteristics of quinoa seedlings under cadmium stress, a factorial experiment based on completely randomized design (CRD) was conducted with four replications at University of Mohaghegh Ardabili. The treatments included the application methods of salicylic acid (seed priming and adding to the culture medium) and cadmium stress at five levels (0, 50, 100, 150, and 200 mg l-1). In this study, the measured traits include the germination percentage, germination rate, germination synchronicity index, mean daily germination (MDG), seedling length, and seedling dry weight, vigor index (VI), mean germination time (MGT), D50, the activity of catalase, peroxidase and polyphenol oxidase enzymes of quinoa seedlings were investigated. All tests were performed in 3 replications and the mean comparison was based Duncan's Multiple Range test at a 5% probability level. The statistical analysis was carried out using SAS 9.4 and Excel software.  Results and discussion The results of this research showed that the germination percentage, germination rate, germination synchronicity index, mean daily germination, seedling length, seedling dry weight and, vigor index decreased, and the mean germination time, D50, the activity of catalase, peroxidase and, polyphenol oxidase enzymes increased with the increasing cadmium concentration. Among the salicylic acid application methods (priming and adding to the culture medium), seed priming method had a greater effect on improving the germination percentage, germination rate, germination simultaneity index, mean daily germination, seedling length, seedling dry weight, and increasing vigor index, activity of catalase, peroxidase and polyphenol oxidase enzymes. Also, the mean germination time and D50  were reduced by seed priming methoed. Under cadmium stress, Both methods salicylic acid application, there was a significant increase in the activity of antioxidant enzymes in seedlings. Under 200 mg l-1 of cadmium concentration, seed priming with salicylic acid caused an increase in the catalase activity (11.27%), peroxidase activity by 6.51 to 9.69%, and polyphenol oxidase activity by 68.39%.  Conclusion Overall, the results of this study showed that with increasing cadmium stress concentrations, the traits of germination percentage, germination rate, seedling length, seedling dry weight, germination synchrony index, mean daily germination, and vigor index decreased, while the traits of mean germination time, time to 50% germination, and the activities of catalase, peroxidase, and polyphenol oxidase enzymes increased. Cadmium stress causes ionic imbalance that stimulates the production of reactive oxygen species (ROS), which leads to disruption of cell membranes, osmotic regulation, and the production of secondary metabolites. Considering the negative effect of this stress on germination and growth of the obtained seedlings, among the application methods (priming and adding to the culture medium), seed priming had a greater effect on improving germination, seedling growth and increasing the quinoa seed vigor index, as well as increasing the activity of antioxidant enzymes compared to the method of adding salicylic acid to the planting medium. Priming of quinoa seeds with salicylic acid by shortening the necessary germination time causes primed seeds to germinate faster than control seeds under normal and cadmium-stressed conditions, and by reducing the mean germination time and D50, it reduces cadmium damage and protects the germination process against the toxicity of this heavy metal.