Glutathione redox state, tocochromanols, fatty acids, antioxidant enzymes and protein carbonylation in sunflower seed embryos associated with after-ripening and ageing
Résumé
Background and Aims Loss of seed viability has been associated with deteriorative processes that are partly caused by oxidative damage. The breaking of dormancy, a seed trait that prevents germination in unfavourable seasons, has also been associated with oxidative processes. It is neither clear how much overlap exists between these mechanisms nor is the specific roles played by oxygen and reactive oxygen species. Methods Antioxidant profiles were studied in fresh (dormant) or after-ripened (non-dormant) sunflower (Helianthus annuus) embryos subjected to controlled deterioration at 40 degrees C and 75 % relative humidity under ambient (21 %) or high O-2 (75 %). Changes in seed vigour and viability, dormancy, protein carbonylation and fatty acid composition were also studied. Key Results After-ripening of embryonic axes was accompanied by a shift in the thiol-based cellular redox environment towards more oxidizing conditions. Controlled deterioration under high O-2 led to a faster loss of seed dormancy and significant decreases in glutathione reductase and glutathione peroxidase activities, but viability was lost at the same rate as under ambient O-2. Irrespective of O-2 concentration, the overall thiol-based cellular redox state increased significantly over 21 d of controlled deterioration to strongly oxidizing conditions and then plateaued, while viability continued to decrease. Viability loss was accompanied by a rapid decrease in glucose-6-phosphate-dehydrogenase, which provides NADPH for reductive processes such as required by glutathione reductase. Protein carbonylation, a marker of protein oxidation, increased strongly in deteriorating seeds. The lipid-soluble tocochromanols, dominated by alpha-tocopherol, and fatty acid profiles remained stable. Conclusions After-ripening, dormancy-breaking during ageing and viability loss appeared to be associated with oxidative changes of the cytosolic environment and proteins in the embryonic axis rather than the lipid environment. High O-2 concentrations accelerated dormancy alleviation but, surprisingly, did not accelerate the rate of viability loss.