A comparative study of glutathione and ascorbate metabolism during germination of Pinus pinea L. seeds

The ascorbate and glutathione systems have been studied during the first stages of germination in orthodox seeds of the gymnosperm Pinus pinea L. (pine). The results indicate that remarkable changes in the content and redox balance of these metabolites occur in both the embryo and endosperm; even if with different patterns for the two redox pairs. Dry seeds are devoid of the ascorbate reduced form (ASC) and contain only dehydroascorbic acid (DHA). By contrast, glutathione is present both in the reduced (GSH) and in the oxidized (GSSG) forms. During imbibition the increase in ASC seems to be mainly caused by the reactivation of its biosynthesis. On the other hand, the GSH rise occurring during the first 24 h seems to be largely due to GSSG reduction, even if GSH biosynthesis is still active in the seeds. The enzymes of the ascorbate-glutathione cycle also change during germination, but in different ways. ASC peroxidase (EC 1.11.1.11) and glutathione reductase (EC 1.6.4.2) activities progressively rise both in the embryo and in endosperm. These changes are probably required for counteracting production of reactive oxygen species caused by recovery of oxidative metabolism. The two enzymes involved in the ascorbate recycling, ascorbate free radical (AFR) reductase (EC 1.6.5.4) and DHA reductase (EC 1.8.5.1), show different behaviour: the DHA reductase activity decreases, while that of AFR reductase remains unchanged. The relationship between ascorbate and glutathione metabolism and their relevance in the germination of orthodox seeds are also discussed.