ACTIVITIES OF FREE-RADICAL PROCESSING ENZYMES IN DRY SUNFLOWER SEEDS

Changes in the lipid peroxidation and the enzymatic activities of the oxygen radical detoxification were studied in dry seeds of sunflower (Helianthus annuus L.) and related to their germinability. There was a positive relationship between the total dehydrogenase activity extracted from whole seeds and germination at both 25 degrees C and 10 degrees C. Catalase and superoxide dismutase activities in embryonic axes and germination at 10 degrees C were negatively correlated. Glucose-6-phosphate dehydrogenase and total peroxidase activities were higher in seeds showing high germination capacity. A high malondialdehyde content and a high total glutathione content, were found in cotyledons of dry seeds exhibiting no germination capacity. A net decrease (20 %) in the activities of catalase and glucose-6-phosphate dehydrogenase was found in these cotyledon fragments. Glutathione reductase and glutathione peroxidase activities were increased by 20 and 50 %, respectively. Kinetic properties of glucose-6-phosphate dehydrogenase were also affected; the apparent K-m for NAD(+) was lower in seeds unable to germinate than in seeds with a high germination ability. Oxidative stress appeared to affect seed quality by lowering antioxidant defence capacity; the collapse of the oxygen radical detoxification system appeared to be the result of the ineffectiveness of the glucose-6-phosphate dehydrogenase activity; its potential role in oxidative stress tolerance and seed germination ability is discussed.