Photoprotective Strategies of Overwintering Evergreens

High levels of solar radiation (direct and reflected off snow) can be absorbed by green leaves in the winter. However, the energy that is absorbed by these leaves often cannot be used through photosynthesis under winter conditions. Most colored objects would simply bleach if faced with the same conditions, because light-absorbing pigments pass energy on to oxygen, forming reactive oxygen species that destroy the pigments. Two key means by which overwintering leaves prevent reactive oxygen-mediated damage have been identified. One, used by all leaves in the winter, involves the employment of xanthophyll carotenoids in a photoprotective process whereby the light energy absorbed by chlorophyll is converted to heat and dissipated harmlessly. The other, found in some evergreen species but not in overwintering annual or biennial mesophytes, involves the degradation of proteins responsible for generating high-energy electrons, thereby suppressing transfer of these electrons to oxygen.