Antioxidant response of overwintering conifers suggest species specific strategies for coping with excess excitation energy

Background/Question/Methods Evergreen plants, growing in seasonally cold environments, must cope with a severe imbalance between light absorption and its utilization, as low temperatures during winter inhibit photosynthetic carbon reduction. In such conditions photoprotective mechanisms are critical, including the leaf antioxidant systems of plants. In a previous study examining Taxus cuspidata, we found significant increases in the enzyme glutathione reductase (GR) and the metabolite glutathione (GSH) during winter, but no increases in the enzymes superoxide dismutase, ascorbate peroxidase (APX) or the metabolite ascorbate (ASC). These results led to the hypothesis that GR and GSH serve an important role during winter stress that was separate form their role in the ascorbate glutathione cycle. The goal of this study was to examine the seasonal response of several species of conifers in order to determine if the differences in the antioxidant response observed in Taxus would occur generally in conifers during winter, or if there are species-specific effects. We collected needle tissue in summer and winter from three conifer species: eastern white pine (Pinus strobus L.), balsam fir [Abies balsamea (L.) P. Mill], and white cedar (Thuga occidentalis L.). We analyzed the activity of both GR and APX and the amount of GSH and ASC.

Results/Conclusions Our results demonstrated significant increases in the antioxidant enzyme GR in all species in response to winter stress. The antioxidant enzyme APX showed significant winter increases in two species (pine and cedar) but not the third (fir). Preliminary results of the antioxidant metabolites glutathione and ascorbate show similar results. Our data suggest that the different species examined utilize different strategies for coping with excess excitation energy during winter. Pine and cedar apparently boost their overall antioxidant enzyme capacity and their utilization of the ascorbate glutathione cycle. Balsam fir shows a similar response to Taxus cuspidata, where glutathione reductase activity is significantly increased but ascorbate peroxidase activity is not. These results suggest a role for reduced glutathione in winter-stressed fir that is independent of its role in the ascorbate glutathione cycle.