Studies On Effects Of Exogenous Nitric Oxide On Growth And The Mechanisms In Ryegrass Seedlings Under Salt Stress

Perennial ryegrass (Lolium perenne L.) is a kind of important gramineous herbgrass and turfgrass, which is widely planted in our country at present. In this study, Pinnacle of perennial ryegrass as an experiment material was used and effects of exogenous nitric oxide (NO) on growth and the potential mechanisms in grass seedlings under salt stress were investigated.Ryegrass seedlings, which had grown for 10 d after germination, were treated with 1/2 Hoagland solutions suppled with different concentrations of NaCl. It was found that salt stress obviously inhibited the growth of the seedlings. After treatments of the ryegrass seedlings with salt for 10 d, the increased height, fresh weight and dry weight of seedlings were lower than those of the control.Furthermore, the inhibition of the seedling growth by salt was more significant with the increase in NaCl concentration. When the treatment concentration of NaCl reached 100 mmol·L^-1,50 mmol·L^-1,150 mmol·L^-1 respectively, differences in the increased height, fresh weight and dry weight of seedlings between the treated and the control plants were significant. The three parameters of the treated seedlings were respective 59%,56% and 67% those of the control when the NaCl concentration became 200 mmol·L^-1. Moreover, exposure of the seedlings to 200 mmol·L^-1 NaCl, the parameters above had highly significant difference between the treated and the control plants. We therefore selected 200 mmol·L^-1 as the treatment concentration of NaCl in the following experiments.In order to determine roles of NO, we investigated effects of sodium nitroprusside (SNP, an NO donor) on growth of ryegrass seedlings under salt stress. Treatments with 50-200μmol·L^-1 SNP and salt resulted in higher enhanced height, fresh weight and dry weight of the seedlings than those of plants treated by NaCl alone, suggesting that 50-200μmol·L^-1 SNP alleviate the inhibition of seedling growth resulted from salt stress. In addition, SNP at concentration of 100μmol·L^-1 revealed the strongest alleviation effects, and the three parameters mentioned above of seedlings treated by SNP combined NaCl increased by about 63%,69% and 44% in comparison with those of plants treated only by NaCl; and the difference of these data between the two treatments was significant. However, SNP at concentration over 250μmol·L^-1 had no marked alleviation effects, and SNP higher than 400μmol·L^-1 suppressed the growth of ryegrass seedlings.To elucidate the mechanisms for exogenous NO affecting growth of ryegrass under salt stress, we studied effects of SNP on changes in salt resistance related characteristics, levels of reactive oxygen species (ROS) and activities of several antioxidant enzymes in leaves of ryegrass seedlings. The results indicated that 100μmol·L^-1 SNP obviously abolished the salt-induced enhancement of relative electrolyte leakage of leaves from ryegrass seedlings, increased the proline content of leaves, decreased the uptake of Na+ and enhanced the uptake of K+, thus obviously improved K⁺ to Na⁺ ratio. Moreover, SNP treatments led to potent alleviation of increases in malondialdehyde content and levels of H₂O₂ and O₂^·- induced by salt in leaves of ryegrass seedlings in comparison with the controls. In addition, SNP treatments marked enhanced the activities of the main antioxidant enzymes-superoxide dismutase, catalase, ascorbate peroxidase and peroxidase in leaves of the seedlings. However, different antioxidant enzymes revealed diverse responses to the SNP treatments, and the same kind of antioxidant enzyme had different activity when the treatment time was changed. The findings imply that these antioxidant enzymes may have different alleviation roles in plant response to salt stress and NO treatments, and the alleviated effects of SNP depend on the treatment time. Taken together, these results indicate that exogenous NO may alleviate the damage caused by salt in ryegrass seedlings via decreasing the content of Na+, increasing the contents of K⁺ and proline, and activating the activities of the antioxidant protest enzymes, thus improves salt resistance of the plants.