Effects Of Exogenous Nitric Oxide On Physiological Characteristics Of Ryegrass Under Joint Action Of Lead And Benzopyrene

With the rapid development of economy,environmental pollution is becoming more and more serious.Polycyclic aromatic hydrocarbons?PAHs?and heavy metals are the typical representatives of many environmental pollutants.They often enter the environmental media at the same time or successively to form complex pollution,causing serious pollution to the environment.Therefore,the study of complex pollution has become one of the important directions of ecological development.The combination of inorganic pollution and organic pollution is the main type of compound pollution.Heavy metals and polycyclic aromatic hydrocarbons are the representative pollutants of inorganic pollution and organic pollution.Therefore,the study of combined pollution of heavy metals and polycyclic aromatic hydrocarbons is of great significance for environmental and ecological safety.Nitric oxide?NO?is a ubiquitous gaseous active substance in plants,which is water-soluble and lipid-soluble and can shuttle freely between cell membranes.As a second messenger in plants,NO mediates many processes of plant growth,development and physiological metabolism,such as seed germination,root and pollen tube growth,stomatal closure,flowering,maintenance of iron homeostasis and programmed cell death.NO can also regulate almost all signaling pathways through Snitrosation,and interdepend and interact with reactive oxygen species?ROS?to participate in plant defense and regulation of abiotic stress responses such as high temperature,low temperature,water,UV-B radiation,heavy metals and salinity.In regulating plant environmental stress resistance,exogenous NO has a positive mitigation effect on stress.Studying the mitigation effect of exogenous NO on oxidative damage of plants under combined stress of heavy metals and PAHs has a positive significance for improving plant stress resistance.In this paper,the changes of germination rate,germination potential,germination index,germination length,radicle length,net photosynthetic rate?P_n?,intercellular CO₂ concentration?C_i?,stomatal conductance?G_s?and transpiration rate?T_r?of ryegrass seeds were studied under joint action of different concentrations of Pb(0,50,150,250,400?mol·L^-1)and B[a]P(0,5,15,25,40?mol·L^-1).The optimum concentration of inhibiting effect on growth and development of ryegrass laid a foundation for further study on the effects of exogenous NO on physiological characteristics of ryegrass under joint action of Pb and B[a]P.The physiological indexes and antioxidant enzymes of ryegrass under joint action of Pb and B[a]P at different concentrations(50,150,250,400?mol·L^-1)were studied by selecting 400?mol·L^-1 Pb and 40?mol·L^-1 B[a]P as the optimum inhibitory concentration,using ryegrass as materials and Sodium Nitroprusside?SNP?as exogenous NO donors.The effects of chlorophyll a,chlorophyllb,carotenoids,and antioxidant enzymes of ryegrass treated with SNP at different concentrations(50,150,250,400?mol·L^-1)were Total chlorophyll,soluble protein,soluble sugar,free proline?Fpro?,reduced glutathione?GSH?,antioxidant enzymes such as peroxidase dismutase?SOD?,peroxidase?POD?,catalase?CAT?and so on were used to investigate the physiological regulation of exogenous NO in ryegrass growth under joint action of Pb and B[a]P.The results show that:?1?Under the combined action of Pb and B[a]P,the germination process of ryegrass seeds showed inhibition effect,and the inhibition rate of each index was the smallest in low concentration K1 group and the largest in high concentration K4group.The inhibition rates of germination rate,germination potential and germination index of ryegrass seeds were 24.82%-34.75%,32%-45.82%,23.91%-50.45%,26.18%-46.96%and 58.49%-80.88%respectively.Under the combined action of Pb and B[a]P,the photosynthesis of ryegrass seedlings was also inhibited.The inhibition rates of P_n,G_s,C_i and T_r were the lowest in low concentration K1 group and the highest in high concentration K4 group.The inhibition rates of P_n,G_s,C_i and T_r were5.74%-23.96%,45.88%-60.57%,33.71%-47.25%,24.52%-456.88%.Therefore,the combined effect of Pb and B[a]P on ryegrass seed germination and growth stage showed inhibition effect.With the increase of the concentration of Pb and B[a]P,the inhibition effect increased,and the concentration of K4 group showed the maximum inhibition concentration.Therefore,K4 group was selected as the group with the highest inhibition degree,i.e.the best inhibition effect.?2?Under the joint action of Pb and B[a]P,250?mol·L^-1 SNP was the best mitigation concentration in T4 group.The chlorophyll a,chlorophyll b,carotenoids and total chlorophyll of T4 group increased by 20.28%,51.81%,14.93%and 48.70%respectively compared with the control group T1,while they were lower than CK group by 3.55%,8.91%,12.54%and 3.81%.The soluble protein and soluble sugar increased by 41.89%,64.40%,and decreased by 4.03%and 8.67%compared with CK;the free proline increased by 213.33%compared with T1,and by 3.81%compared with CK group.Compared with CK group,the activity of SOD,POD and CAT increased by 19.86%,46.17%,86.11%and 29.66%,35.76%and 167.29%respectively,while the activity of reduced glutathione increased by 64.44%and214.89%respectively.The results showed that exogenous NO could alleviate the physiological indexes of ryegrass under the stress of Pb and B[a]P.With the increase of SNP concentration,the degree of alleviation increased gradually.The maximum alleviation effect was achieved at 250?mol·L^-1 SNP in T4 group,and then the degree of alleviation decreased with the increase of SNP concentration.?3?In the linear regression analysis of the physiological indexes of ryegrass and the concentration of exogenous NO,the four most correlated items were CAT,SOD,POD and free proline,and their determinant coefficients were 0.9991,0.9958,0.9872and 0.9872,respectively.The correlation between CAT and exogenous NO was the highest,indicating that the correlation between antioxidant enzymes and exogenous NO was the closest;the three least correlated items were chlorophyll b,total chlorophyll and reductive valley.The determination coefficients of cysteine were0.8855,0.9182 and 0.9607,respectively.The lowest correlation coefficient between chlorophyll b and exogenous NO was 0.8855.