Antioxidant Isozyme Variability in Different Genotypes of Citrus and Kumquat

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An understanding of antioxidant variability in citrus genotypes and kumquat is needed to develop future plants with greater resistance to oxidative stress. We evaluated part of the basal, enzymatic anti-oxidative metabolism in developing leaves (40–50% expanded) of commercially important citrus: grapefruit, sweet orange ‘Hamlin’, and kumquat. Young leaves of kumquat showed lower rates of lipid peroxidation and H2O2 generation in comparison to grapefruit and sweet orange ‘Hamlin’. Total superoxide dismutase (SOD) activity, which catalyzes the transmutation of superoxide ion to H2O2, was two times higher in kumquat than in grapefruit and sweet orange. Kumquat also had more superoxide dismutase isoforms. Despite the higher superoxide dismutase activity in kumquat, it had substantially lower H2O2 than grapefruit and sweet orange ‘Hamlin’. Catalase (CAT), guaiacol peroxidase (POD), and ascorbate peroxidase (APOD) catabolyze H2O2 to non-toxic by-products. Total catalase activity was about 50% higher in kumquat than in grapefruit and sweet orange ‘Hamlin’. Four CAT isoforms were observed in kumquat and sweet orange ‘Hamlin’, whereas grapefruit had only one. POD activity was about two times higher in grapefruit than kumquat. Kumquat had only one POD isoform, whereas grapefruit and sweet orange ‘Hamlin’ had four. APOD activity was about two times higher in grapefruit than kumquat, with sweet orange ‘Hamlin’ intermediate. Kumquat had two, grapefruit had one, and sweet orange ‘Hamlin’ had two APOD isoforms. Kumquat had greater potential to remove reactive oxygen species, principally through higher SOD and CAT activities, compared to grapefruit and sweet orange ‘Hamlin’. This variation in antioxidant enzyme profile among citrus genotypes can be used in citrus breeding programs and in production of transgenic plants with better oxidative stress amelioration capacities.