Close relationship between the state of the xanthophyll cycle pigments and photosystem II efficiency during recovery from winter stress

The potential involvement of the xanthophyll cycle in photoprotection of overwintering evergreen plants was investigated. Leaves from five evergreen species. Pseudotsuga menziesii, Pinus panderosa, Euonyums kiautschovicus. Mahonia repens and Malva neglecta, were collected from the field predawn during winter and transferred to the laboratory where chlorophyll fluorescence emission as well as the chlorophyll and carotenoid composition were ascertained periodically for 4.5 days. Leaves and needles from all species were found to have retained large amounts of the xanthophyll cycle pigments zeaxanthin and antheraxanthin, and they exhibited sustained low values of the intrinsic efficiency of photosystem II (PSII; measured as the ratio of variable to maximal fluorescence, F_v/F_m) upon collection. The increase in PSII efficiency was biphasic, with a rapid phase (requiring several hours) and a slow phase (requiring several days). Changes in the conversion state of the xanthophyll cycle were found to correlate with increases in PSII efficiency in both phases, with the latter phase involving large increases in both F_m (maximal fluorescence) and F_o (minimal fluorescence) throughout the period of recovery. The relationship between F_m quenching (expressed as nonphotochemical or Stern-Volmer quenching NPQ] of F_m, i.e. F_m/ F_m–1) and F_o quenching (F_o/F_o–1) was linear, as expected for changes in xanthophyll cycle-dependent energy dissipation in the antenna complexes. Furthermore, the relationship between F_v/F_m and NPQ during recovery followed the theoretical relationship predicted for changes in the rate constant for energy dissipation in the antenna complexes. This fit between the theoretical relationship and the actual data indicates that all changes in NPQ or F_v/F_m can be accounted for by changes in this rate constant. The results suggest a role for the photoprotective xanthophyll cycle-dependent dissipation process in the lowered efficiency of PSII observed in coldstressed evergreen plants in the field.