Regulatory Role And Mechanism Of NO On Oxidative Damage And The Accumulation Of Cd In Alfalfa Seedlings Under The Cd Stress

Cadmium(Cd) is a heavy metal element existing widely in nature, in a certain range of concentration, toxicity of cadmium on plants is greater than other heavy metals. Nitric oxide(NO) is signal molecules which widely exists in plants, it not only regulate physiological process of plants such as seed germination, growth and aging, but also as a key messenger in defense responses, involved in the response of plants to environmental stress, such as drought stress, thermal stress, salt stress, ozone stress, heavy metal stress, mechanical damage and plant disease resistance. Alfalfa seedlings with four true leaves were treated with Cd Cl2, L-NAME(N’-nitro-L-arginine-methyl ester hydrochloride), Na N3, SNP and c PTIO(2-4-carboxyphenyl-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide) to analyze response of Alfalfa to Cd stress, role of Nitric oxide synthase(NOS), Nitrate reductase(NR) enzyme in responses of Alfalfa to Cd stress, effects of NO on membrane lipid peroxidation, antioxidant enzyme system, osmotic regulation substances and Cd accumulation in alfalfa, to explore the role of NO in plant adversity stress response and its mechanism. The results indicated:(1)NOS, NR, carotenoids(Car) and nitrite(NO2-) plays an important role in alfalfa resistance to Cd damage. A certain concentration of Cd stress could increase activities of NOS and NR, contents of Car and NO2-, when the concentration of Cd in the range of 0.25mmol/L-0.5mmol/L, activity of NOS and NR in leaf, stem and root reached the highest value. The response of enzymes and osmotic regulation substances in different parts of Alfalfa to Cd stress was different, the activity of NOS was root > stem > leaf, the activity of NR was leaf > stem > root, the content of Car was leaves > stems, the content of NO2- was leaf > stem > root. In leaves, NR, Car and NO2- play an important role in generating of NO; in stems and roots, NOS is the main way to generate NO.(2)100μmol/L SNP significantly enhanced activities of peroxidase(POD) in leaves, superoxide dismutase(SOD) and glutathione reductase(GR) in leaves and stems. 50 μmol/L SNP significantly enhanced activities of ascorbic acid oxidase(APX) in leaves, stems and roots. 400 μmol/L SNP significantly enhanced activities of peroxidase(POD) in stems and roots. 200 μmol/L SNP remarkably reduced cadmium accumulation in leaves and stems, 100 μmol/L SNP remarkably reduced cadmium accumulation in roots. It was shown that appropriate concentration of exogenous NO can increase the contents of osmotic regulation substances and regulate activities of antioxidant enzymes to effectively protect the membrane system stability and alleviate the harm of Cd stress on alfalfa seedlings.(3)Endogenous NO is a signal molecule regulating antioxidant enzyme activity in the seedlings of Alfalfa, NOS inhibitor L-NAME and NR inhibitor Na N3 can reduce the activity of NR and NOS in the leaves, stems and roots of alfalfa, resulting in NO content decreased, REC increased, the activity of antioxidant enzymes reduced, the membrane lipid peroxidation aggravated; the NO scavenger c PTIO, can decrease the content of NO more obviously; While applying the appropriate concentration of SNP, decreased the REC and degree of membrane lipid peroxidation of Alfalfa. These results indicated that endogenous NO plays an important role in the response of Alfalfa to Cd stress.(4)With the addition of exogenous SNP, accumulation of Cd in leaf, stem and root of Alfalfa reduced significantly, resistance of Alfalfa to Cd stress increased, however, with the increasing of SNP concentration, the accumulation of Cd will not decrease continually, on the contrary it will rise. These indicated that the effect of NO on the antioxidant enzyme activity, the accumulation of Cd in Alfalfa is limited, which may be related to a negative effect of high concentration of NO on the growth of alfalfa. Research shows that the distribution of Cd accumulation in alfalfa is: root > stem > leaf, as same as most of the plants, accumulation of Cd in the roots of Alfalfa was greater than other parts.