Photoprotective mechanisms against winter stresses in the needles of Abies mariesii grown at the tree line on Mt. Norikura in Central Japan

Evergreen fir Abies mariesii growing at the tree line (near 2 500 m altitude) on Mt. Norikura (36°61'N, 137°33'E, 3 026 m altitude) in Central Japan is exposed to harsh winter stresses. To protect against these stresses, the deep-oxidation state of the xanthophyll cycle pigments increased, because the needles contained large amounts of zeaxanthin, which resulted in an increase of non-radiative thermal dissipation from the antenna system. Not only the antenna system but also the inactivated photosystem (PS) 2 reaction centre (RC) might contribute to the heat dissipation of absorbed excess photon energy. In addition, a decrease in the PS2 activity during winter was derived from the degradation of the PS2 RCs. Thus the needles acclimated to the strong sunlight during the harsh winter. Under such conditions, only the abaxial side of A. mariesii needles occasionally changed colour from green to reddish-brown in early spring. Since this needle damage was only observed in shoots that protruded from the snow surface, this phenomenon might be caused by the interaction between the strong sunlight reflected from the snow surface and the long period of sub-zero temperatures. We also examined how the photoprotective functions of A. mariesii growing at the tree line of a temperate zone mitigate the interactive stresses of high photon flux density and sub-zero temperature in harsh winter.