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The responses of certain antioxidants in detached leaves of two cultivars of spinach ( Spinacia oleracea  L.) differing in their senescence rates were assessed during storage in order to explore the significance of these antioxidants in senescence regulation and dynamics. To identify spinach cultivars differing in their senescence rates, 10 cultivars were grown in field plots, harvested at maturity, and their leaves detached and stored at 10 °C in the dark. At the point of harvest (d 0) and on d 5, 8, 12, and 15, samples were analyzed for lipid peroxidation (MDA), chlorophyll loss, and electrolyte leakage. The cultivars were also grown in laboratory growth chambers to corroborate field results. Two cultivars that were consistently identified as having relatively high (Spokane F1) and low (BJ 412 Sponsor) senescence rates were grown in growth chambers for 45 d, harvested at maturity, and their leaves detached and stored as above. At the point of harvest (d 0) and on d 4, 8, 12, 16, and 20, samples were analyzed for (i) activities of ascorbate peroxidase (ASPX; EC 1.11.1.11), catalase (CAT; EC 1.11.1.6), and superoxide dismutase (SOD; EC 1.15.1.1), and (ii) concentrations of MDA, total ascorbate, reduced ascorbate (AsA), oxidized ascorbate (DAsA), total glutathione, reduced glutathione (GSH) and oxidized glutathione (GSSG). Although MDA accumulated in leaves of both cultivars concomitant with time after detachment, levels became significantly higher in Spokane. Activities of ASPX declined in Spokane leaves following detachment but activities of SOD and levels of glutathione increased in this cultivar. GSH/GSSG increased in `Sponsor', but dramatically more so in `Spokane'. Ascorbate concentrations did not diminish in leaves of `Spokane' to the degree that they did in `Sponsor' tissue. DAsA/AsA values did not decrease in `Spokane' leaves following detachment, though they did in those of `Sponsor'. It is argued that declining activities of ASPX and levels of ascorbate and increasing activities of SOD manifested in accumulation of hydrogen peroxide in Spokane, leading to a greater potential for lipid peroxidation in this variety than for Sponsor. SOD activities and glutathione levels may have increased as a result of elevated oxidative stress in Spokane. Increased hydrogen peroxide accumulation in `Spokane' relative to `Sponsor' may have contributed to an increased rate of senescence in the leaves of this cultivar.

126 Antioxidant Responses in Detached Leaves of Two Cultivars of Spinach (Spinacia oleracea L.) Differing in Their Senescence Rates