Metabolism of antioxidants and nitrosothiols in barley (Hordeum vulgare L.) seeds during germination and seedling growth

Ma, Zhenguo (2016) Metabolism of antioxidants and nitrosothiols in barley (Hordeum vulgare L.) seeds during germination and seedling growth. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

In the process of seed germination, antioxidant metabolism in barley seeds is intensified. Production of reactive oxygen and nitrogen species, and activities of enzymes scavenging these reactive species such as ascorbate peroxidase, monodehydroascorbate reductase, dehydroascorbate reductase, glutathione reductase, catalase, superoxide dismutase and S-nitrosoglutathione reductase, increased in germinating barley seeds instead of non-germinated more dormant Sundre seeds. Meanwhile the content of total ascorbate and glutathione and the rate of fermentation dropped in germinating seeds. Sharp decrease in glutathione content in embryos of barley seeds led to a dramatic increase in reduction potential of glutathione disulfide/glutathione couple. High levels of adenosine triphosphate and adenosine diphosphate were observed during the first few hours of germination but later their content decreased with a concomitant increase in their ratio in germinating seeds. Expression of the gene encoding alcohol dehydrogenase III declined in Harrington seeds from the onset of germination, while expression of the gene encoding hemoglobin was elevated within the first few hours and later declined, and the expression of the both genes did not change significantly in Sundre seeds. In the process of further transition from imbibition of dry Harrington seeds to seedling growth, glyoxylate cycle and citric acid cycle in scutellum became more active in the first few days, as evidenced by the increased activities of isocitrate lyase and malate synthase in glyoxylate cycle and succinate dehydrogenase and fumarase in citric acid cycle. During this period, the content of malic acid, succinic acid, and overall pH value declined in both scutellum and endosperm of Harrington seeds. In the endosperm, the content of succinic acid was much lower than malic acid and citric acid. The level of citric acid in scutellum was high and maintained stable while its level in endosperm was low. The increase of H₂O₂ production was observed during the process of seed germination and early period of seedling growth. In summary, the antioxidant metabolism, metabolism of reactive oxygen and nitrogen species, glyoxylate cycle and citric acid cycle were found to be more active during seed germination and early period of seedling growth.

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