A Quantitative and Qualitative Analysis of Antioxidant Enzymes in Relation to Susceptibility of Apples to Superficial Scald

ADDITIONAL INDEX WORDS. postharvest physiology, storage, physiological disorder, Malus ABSTRACT. The activities and isoenzyme patterns of guaiacol-dependent peroxidase (POX), ascorbate peroxidase (APX), superoxide dismutase (SOD) and catalase (CAT) were studied in yellow- and red-fruited crab apple (Malus (L.) Mill. ) selections from a 'White Angel∑ x 'Rome Beauty∑ cross that show differential susceptibility to the physiological storage disorder, superficial scald. There were no consistent relationships between total enzyme activities and scald incidence, high activities of the enzymes being detected in selections with both high and low susceptibilities to scald. However, ad- ditional individual isoforms of some antioxidant enzymes were detected in the scald-resistant selections when compared with scald-susceptible selections. In a native gel system, four guaiacol-dependent POX isoenzymes were detected in both yellow and red scald-resistant selections compared with only two in scald-susceptible selections. Similarly, for anodic acidic POX assayed using benzidine, six isoenzymes were detected in both yellow and red scald-resistant selections compared with five in yellow and four in red susceptible selections. Ten SOD isozymes were detected in scald-resistant yellow-fruited selections compared with only five faint bands in scald-susceptible selections, but similar patterns were not detectable for red-fruited selections. Differences in the presence of various isoenzymes for CAT and APX were also detected among the selections, but associations with scald susceptibility were also affected by fruit color or were inconsistent. The presence or absence of individual isoenzymes may be a better indication of scald resistance or susceptibility than the total enzyme activities. Isoenzyme analyses, especially of POX, could be useful to breeders for the early detection of scald resistance/ susceptibility in apples. The reactive oxygen species (ROS), hydrogen peroxide (H 2 O 2 ), singlet oxygen, superoxide (O 2 -), and hydroxyl (.OH) radicals, are recognized as important features of plant metabolism and in plant responses to stress (Halliwell and Gutteridge, 1989). While ROS are present in all plants in varying degrees as a result of normal metabolism, their levels in vivo depend on the balance between ROS production and the capacity of the cell to scavenge them. ROS, especially . OH, can react with all biological macromolecules causing lipid peroxidation as well as modification to the structure and function of proteins and nucleic acids (Bailly et al., 1996; Hal- liwell and Gutteridge, 1989). ROS induce the antioxidant defense system of low molecular weight antioxidants (Rao et al., 1996), and enzymes such as peroxidase (POX), catalase (CAT), ascorbate peroxidase (APX), superoxide dismutase (SOD) and glutathione reductase (GR) (Alscher et al., 2002; Bowler et al., 1992; Shigeoka et al., 2002). SOD dismutates O 2 - to H 2 O 2 and O 2 in a reaction that is spontaneous and extremely rapid (Van Camp et al., 1994), thus protecting the cells from damage by O 2 - reaction products. The product H 2 O 2 , which is also a potentially toxic compound, is then reduced to H 2 O by a number of enzymes such as POX, CAT, APX, and GR (Criessen et al., 1994). Although toxic in high con- centrations, ROS are also involved as early messenger molecules