Seasonal Variation in the Antioxidant System of Eastern White Pine Needles

Abstract

Antioxidant metabolites in eastern white pine (Pinus strobus L.) needles increased two- to fourfold from the summer to the winter season. Antioxidant enzymes in needle tissue increased between 2- and 122-fold during this same period. These seasonal changes were determined by monitoring ascorbate and glutathi- one concentrations and the activity of ascorbate peroxidase, gluiathione reductase (GR), and superoxide dismutase. Levels of antioxidant metabolites and enzymes were observed always to be lowest during the summer, or active growing season, and highest during the winter, or dormant season. These data corre- lated well with the thermal kinetic window for purified GR obtained from summer needles. The minimum, apparent K.H.NADPH for two isoforms of GR (GRA and GRB) occurred at 5 and 10°C, respec- tively. The upper limit of the thermal kinetic window (200 of the minimum Km) for GRA and GRB was 20 and 25°C, respectively, indicating that needle temperatures exceeding 25°C may result in impairment of antioxidant metabolism. The needle content and kinetic properties of GR, the increased activities of other en- zymes, and the high substrate concentrations observed during the winter are consistent with the protective function this pathway may provide against photooxidative, winter injury.