duvilla, C. J. 1996. Functioti of the ascorbate-glutathione cycle in aged sunflower seeds. -Physiol, Plant. 96: 543-550,The function of the ascorbate-glutathione (AsA/GSH) cycle was analyzed in seeds of sunflower (Helianthus annuus L. cv. Peredovik) subjected to accelerated ageing at 43°C and 75% relative humidity for 1 to 11 days. The study was performed using dry seeds and seeds hydrated by imbibition in distilled water for 4 h at 25°C. Lipid peroxidation was also determined by measuring the malondialdehyde (MDA) level. As the ageing period increased, a progi'essive loss of seed viability became increasingly evident. Even though high levels of MDA were detected, the MDA level did not change during accelerated ageing, suggesting that lipid peroxidation might occur to some extent, the study of the ascorbate/glutathione {.4sA/GSH) cycle revealed that the GSH system is the major detoxifying mechanism in both dry and imbibed sunflower seeds. The GSH system is mainly located in the embryo, and its protective role is mediated by reactions that consume the GSH pool and, thereby, minimize the increase of the oxidized form (GSSG). Seed imbibition activates cellular metabolism and allows some antioxidant enzymes like glutathione reductase (EC 1.6.4.2) to^ act upon toxic agents. These reactions provide a reducing status, so that repair of damage becomes possible. However, prolonged ageing conditions (II days) restilt in an irreversible damage, as evidenced by tlie appem'ance of dead seeds when the germination period ended. Multiple regression analysis revealed the effectiveness of the GSH system in aged seeds, especially upon imbibition and until the AsA/GSH cycle became completely functional.

Ageing-induced changes in glutathione system of sunflower seeds. -Physiol. Plant. 101: 807-814.The glutathione system is thought to be involved in defence mechanisms present in plant tissues. The efficacy of this system was evaluated in large seeds of sunflower {Helianthus annuus L. cv. Peredovik) in response to accelerated ageing (43°C/75% relative humidity from 1 to 11 days). Differences between the embryo axis and cotyledons in relation to the glutathione system were also investigated. Additionally, lipid peroxidation was determined by measuring the malondialdehyde (MDA) content. All assays were performed using dry seeds and seeds subsequently hydrated by imbibition in distilled water for 12 h at 25°C. Accelerated ageing caused a marked decrease in seed viability, accompanied by an increase in mean germination time. There were no changes in total glutathione in dry seeds. However, the distribution in its reduced (GSH) and oxidized (GSSG) forms revealed that ageing produced a slow conversion from GSH to GSSG. As the ageing period increased, this effect was accompanied by a decrease in glutathione reductase (GR, EC 1.6.4.2) activity. The results also indicated that the GSH system exerts a different response in the embryo axis as compared with the cotyledon: (1) the GSH levels decreased less in the cotyledons than in axes of aged seeds, and (2) the GSSG level in cotyledons was independent of ageing, while its amount increased in aged embryo axes. These different responses, in conjunction with the lower MDA levels in large as compared with small seeds, indicate a possible protective role of the reserve lipids. The efficacy of the GSH system in aged seeds was associated with seed viability, as revealed by multiple regression analysis. Upon imbibition, aged seeds were able to restore their GSH levels, reaching values approximating those of unaged seeds.