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The antioxidant status of potato (Solanum tuberosum L.) tubers of two genotypes. cv. Désirée and clone 10337de40 was investigated in relation to susceptibility to internal rust spot (IRS), a Ca2+-related physiological disorder. Concentrations of total calcium within the perimedulla tissue of tubers, grown with a restricted (1 mM CaCI2) Ca2+ supply, were similar in cv. Désirée (IRS resistant) and clone 10337de40 (IRS susceptible). A range of antioxidants was assayed in order to assess antioxidant status in both genotypes under the two Ca2+ treatments. Although no appreciable differences were detected between low Ca2+ and control treatments, certain antioxidants were present at significantly higher levels in the IRS resistant genotype. cv. Désirée. These included dehydroascorbate reductase (EC 1.8.5.1) activity (more than 100% higher), total glutathione content (ca 40% higher), glutathione reductase (EC 1.6.4.2) activity (almost 50% higher). peroxidase (EC 1.11.1.7) activity (ca 60% higher) and superoxide dismutase (EC 1.15.1.1) activity (almost 80% higher). There was no difference in ascorbate content. ascorbate free radical reductase activity (EC 1.6.5.4), α-tocopherol levels and catalase activity (EC 1.11.1.6) between the two genotypes. The possible relationship between resistance to IRS and a superior antioxidant status, found in cv. Désirée. is discussed. [ABSTRACT FROM AUTHOR]