Effects Of Lead Stress On Wheat Seed Germination And Seedlings Physiological And Biochemical Properties

Heavy metal pollution was focused because of it's inhibitory effect on plant growth and it's safety problem on agriculture product. Lead is one of the most serious and stronger dangerous elements in heavy metal pollution, which shows strong mobility, high biology toxicity and easy absorbtion in plants. It has been indicated that lead stress leads to the biopolymer modification, electrolyte leakage, enzyme activity deficient and membrane lipid peroxidation, finally interfering with the growth and development of plant as well as normal physiological metabolism of plant. In addition, lead threatens human beings by entering food chain, leading to the injure of hematopoietic function, immune function and endocrine system. So the damage and resistance mechanism of plant in response to lead stress has become the hot spot issues. Meanwhile, it has been given attention at present that how do plant protect themselves against lead poisoning and then improve yield and quality of agriculture crops.Nitric oxide, which is commonly found as a signaling molecule, is involved in series of the regulation of plant processes, including seed germination, growth and development of roots and stems, cell apoptosis, plant defense and resistance responses to biotic and abiotic stresses, therefore the effects of this molecular on seed germination and plant physiochemical characteristics were focused, especially on alleviating abiotic stresses including salt, drought and higher temperature. However, there were few reports about exogenous NO on seed germination and physiological characteristics of wheat seedlings under Pb²⁺. In the present study, two wheat cultivars (Xihan 2 and Ningchun 4) were used to investigate the changes of seed germination and seedlings growth in response to Pb²⁺ or SNP treatment alone and Pb²⁺ + SNP treatment, which was to indicate the phyotoxicity of lead on wheat and to provide theoretical basis for knowing and understanding the mechanism of physiochemical responses of plant to heavy metal pollution, even to improve the tolerance of plant against heavy metals by regulating NO levels and provide the basis on alleviating Pb²⁺ toxicity in agricultural production. The main results and conlusions were as follows:1. There were no changes of chlorophyll content and superoxide dismutase (SOD) activity in two wheat seedlings exposed to lower Pb(NO₃)₂ concentration, while the amount of chlorophyll a, chlorophyll b and total chlorophyll was decreased and SOD activity was significant increased when wheat seedlings were subjected to higher Pb(NO₃)₂ concentration. The destroy to chlorophyll of Ning Chun was more serious than that of Xi Han in responses to the same Pb(NO₃)₂ stress. Different Pb(NO₃)₂ concentration induced enhanced activities of catalase, peroxidase and ascorbate peroxidase, but no significant changes in malondialdehyde (MDA) content. In addition, the amount of proline was increased by Pb(NO₃)₂ in two wheat seedlings in a concentration-dependent manner, but no change in soluble sugar content. The results suggested that chlorophyll content was destroyed and that the activities of antioxidases and the amount of proline were enhanced in wheat seedlings under the stress of Pb(NO₃)₂, thus the stronger effects of antioxidanted role and osmotic regulations were existed and enhanced wheat seedlings tolerance to Pb²⁺ stress. Therefore there was no significant difference in MDA content between the control and Pb(NO₃)₂-stressed wheat seedlings.2. Lower SNP treatment had no significant effects on germination rate, energy, root and shoot length, but induced enhanced the activities of CAT and POD. Higher SNP concentration significantly increased seed germination rate and energy, inhibited the growth of root and shoot length, as well as decreased the content of proline. In addition, different SNP concentrations induced synthesis of soluble protein, but no significant changes in the contents of malondialdehyde and souble sugar. These results suggested that higher SNP treatment promoted seed germination, and inhibited the seedling growth, while lower SNP treatment enhanced the activities of antioxidant enzymes and osmotic regulations so there was no oxidative damage on cell membrane.3. The results showed that Pb²⁺ treatment resulted in significant decreases in seed germination and seedling growth of wheat, 25μmol·L^-1 SNP could alleviate the inhibiton of Pb²⁺ on these indexes. Under Pb²⁺ stress, the levels of chlorophyll a, chlorophyll b and chlorophyll fluorescence decreased in wheat seedlings, whereas no significant changes in the activities of CAT and POD were observed in comparison with the control. The application of SNP abolished decreased rates of chlorophyll content and chlorophyll fluorescence, and induced increase of CAT activity. In addition, SNP led to increased soluble protein in wheat seedlings under Pb²⁺ treatment. Exogenous NO promoted wheat seed germination and seedling growth, improved the content of chlorophyll and soluble protein, increased the activities CAT so as to alleviate the poison of Pb²⁺ and enhance the adaptation of wheat seedlings to Pb²⁺ stress.