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Changes in the lipid peroxidation and the enzymatic activities of the oxygen radieal 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 aetivity extracted from whole seeds and germination at both 25 °C and 10 °C. Catalase and superoxide dismutase activities in embryonic axes and germination at 10 °C were negatively correlated. Glueose-6-phosphate dehydrogenase and total peroxidase activities were higher in seeds showing high germination capacity. A high malondialdehyde content and a high total gkitathione 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 glueose-6-phosphate dehydrogenase were also affected; the apparent K^ 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 aetivity; its potential role in oxidative stress tolerance and seed germination ability is discussed.

French bean (Phaseolus vulgaris L. cv. Contender) leaf extracts catalyse the reduction of oxaloacetate to malate in the presence of NADH and NADPH. Under the experimental conditions used, the optimum pH values are 8 and 6 respectively. After chromatography on diethylaminoethyl cellulose, two principal forms of NADH‐MDH (L‐malate: NAD+ oxidoreductase, E.C. 1.1.1.37) upon which NADPH activities are superposed, can be characterized. This result is confirmed by electrophoresis on polyacrylamide gel. On the other hand, after filtration on Ultrogel 34, NADH‐MDH is eluted as a single peak; once again, NADPH activity is associated with it. When PtCl2−4, a powerful inhibitor of MDH, is added to the reaction medium, the degree of inhibition is the same irrespective of the cofactor employed. When root extracts are submitted to chromatography on diethylaminoethyl cellulose, activity profiles are identical to those obtained with leaves. These results suggest that the NAD dependent enzymes can also utilize NADP to reduce oxaloacetate. After addition of dithiothreitol, another NADPH‐MDH activity manifests itself in the leaf extracts; it differs from the foregoing ones in its optimum pH, its chromatographic properties and its response to PtCl2−4 action. Root extracts do not exhibit this activity thus showing a specific localization of this enzyme in the green part of the plant.

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