FUNCTION OF THE ASCORBATE-GLUTATHIONE CYCLE IN AGED SUNFLOWER SEEDS

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 degrees 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 degrees C. Lipid perox idation was also determined by measuring the malondialdehyde (MDA) lev el. As the ageing period increased, a progressive loss of seed viabili ty became increasingly evident. Even though high levels of MDA were de tected, the MDA level did not change during accelerated ageing, sugges ting that lipid peroxidation might occur to some extent. The study of the ascorbate/glutathione (AsA/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 protect ive role is mediated by reactions that consume the GSH pool and, there by, minimize the increase of the oxidized form (GSSG). Seed imbibition activates cellular metabolism and allows some antioxidant enzymes lik e glutathione reductase (EC 1.6.4.2) to act upon toxic agents. These r eactions provide a reducing status, so that repair of damage becomes p ossible. However, prolonged ageing conditions (11 days) result in an i rreversible damage, as evidenced by the appearance of dead seeds when the germination period ended. Multiple regression analysis revealed th e effectiveness of the GSH system in aged seeds, especially upon imbib ition and until the AsA/GSH cycle became completely functional.