Apoplastic Antioxidant Enzymes in the Leaves of Two Strawberry Cultivars and Their Relationship to Cold-Hardiness

In this study, apoplastic antioxidant enzymes in the leaves of two strawberry cultivars ('Aromas' and 'Diamante') and their relationship to cold-hardiness were investigated. Fully expanded, uniformly sized leaves from 1-year-old field plants of the cultivars were collected at the hardening (late autumn, in November and winter, in January) and de-hardening (summer, in July) stages. Leaf samples were exposed to low temperatures of 5, -5, -10, -20 and -30 degrees C for 12 h to determine their cold-hardiness (LT50; lethal temperature, where 50% of the plants were killed). Cold-acclimation produced a remarkable increase in cold-hardiness. It was found that Diamante had higher cold-hardiness than 'Aromas'. Moreover, malondialdehyde and total carotenoid content increased during the hardening stage and decreased during the de-hardening stage. The activities of catalase, peroxidase and ascorbate peroxidase in the leaf apoplast and nicotinamide adenine dinucleotide phosphate oxidase activity in the leaf tissue were correlated with changes in cold-hardiness. The activities of these enzymes were higher in the hardening stage than in the de-hardening stage. The activities of apoplastic catalase, peroxidase and ascorbate peroxidase varied significantly depending on the cold-acclimation stage and the cold-hardiness level of the cultivars. This study indicates that elevated apoplastic antioxidative enzymes may be determinants of cold-hardiness in the strawberry plant. The lower malondialdehyde content and higher total carotenoid and apoplastic enzyme