| The aim of this research was to investigate the effects of exogenous nitric oxide on malondialdehyde (MDA), osmoregulation compounds, and antioxidase activity of alfalfa seed germination under polyethylene glycol (PEG) stress. Three alfalfa varieties were selected, Algonquin, Golden empress, and Sanditi. The seeds were placed in Petri dishes with filter paper soaked with PEG for germination. We measured concentrations of malondialdehyde and osmoregulation compounds, and antioxidase activity. Our results showed that SNP+PEG treatment significantly decreased MDA content of the seeds compared to PEG treatment. MDA content in the seeds at the sixth day of germination decreased by 23.11% in Algonquin， 21.81% in Golden empress, and 30.38% in Sanditi. Compared to PEG stress, exogenous application of SNP+PEG increased proline by 13.71% at the fourth day, soluble sugar by 18.57% at the second day, and soluble protein content by 9.23% at the fourth day in Algonquin seeds and they reached the peak values at those time. Those 3 compounds in Golden empress peaked at the second day by 20.96%，16.84%, and 5.48%, respectively and peaked in Sanditi seeds at fourth day by 17.87%, 17.52 %, at the second day, and 13.40% at the second day, respectively. Furthermore, the activities of 4 antioxidases, superoxide dismutase (SOD), guaiacol peroxidase (POD), catalase (CAT), and ascorbate peroxidase (APX), increased first and then decreased and peaked at the fourth day. Compared to PEG stress, we noticed that SOD and POD activities of Sanditi seeds were 2.76 and 1.21 times of that of Algonquin, 4.10 and 1.21 times of that of the golden empress, respectively. Our results suggested that, under osmotic stress generated by PEG, the enhancement of exogenous SNP on alfalfa was mainly embodied in the osmoregulation substances and antioxidase activity. The sensitivity of Sanditi to NO impact was greater than that of Golden empress and Algonquin. |