Abstract
During winter, the light-harvesting complexes of evergreen plants change function from energy-harvesting to energy-dissipating centers. The goal of our study was to monitor changes in the composition of the photosynthetic apparatus that accompany these functional changes. Seasonal changes in chlorophyll fluorescence, pigment concentration, and abundance and phosphorylation status of photosynthetic proteins in Pinus strobus L. (sun-exposed trees) and Abies balsamea (L.) P. Mill. (sun-exposed and shaded trees) were examined in the cold winter climate of Minnesota. Results indicated typical seasonal changes in chlorophyll fluorescence and pigment concentration, with sustained reduced photosystem II (PSII) efficiency during winter, accompanied by retention of zeaxanthin and antheraxanthin, and winter increases in the pool of xanthophyll cycle pigments and lutein. In sun-exposed trees, all photosynthetic proteins that were monitored decreased in relative abundance during winter, although two light-harvesting chlorophyll a/b binding proteins (Lhcb2 and Lhcb5), and the PsbS protein, were enriched in non-summer months, suggesting a role for these proteins in winter acclimation. In contrast, shaded trees maintained most of their protein throughout winter, with reductions occurring in spring. Thylakoid protein phosphorylation data suggest winter increases in the phosphorylation of a PSII core protein, PsbH, in sun-exposed trees, and increases in phosphorylation of all PSII core proteins in shaded trees.