Evaluation of antioxidant metabolism in commercially grown citrus genotypes in Florida
Generation and excess accumulation of hydrogen peroxide (H2O2) is toxic to plants. To combat the ill effect of H2O2, plants have developed an array of enzymatic and non-enzymatic antioxidants. In the present investigation, H2O2 metabolism was studied in young and developing leaves of ‘Nagami’ kumquat [Fortunella margarita (Lour.) Swingle], grapefruit (Citrus paradisi Macfad.), and sweet orange (Citrus sinensis Pers.). Lower H2O2 levels were detected in kumquat and appeared to be associated with higher catalase (CAT) activity than sweet orange and grapefruit. However, higher guaiacol peroxidase (POD) and ascorbate peroxidase (APOD) activity was observed in grapefruit and sweet orange than kumquat. Besides having higher POD and APOD activity, H2O2 levels were higher in grapefruit and sweet orange, which indicates inefficient removal of H2O2 in these genotypes. This might be the cause of poor availability of co-substrates (phenolics and ascorbate) required for POD and APOD activity. On the other hand, kumquat leaves with higher redox insensitive CAT activity effi ciently removed H2O2. Thus, developing kumquat leaves were better protected from the ill effects of oxidative stress than grapefruit and sweet orange.