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 e vergreen species, Pseudotsuga menziesii, Pinus ponderosa, Euonymus kia utschovicus, Mahonia repens and Malva neglecta, were collected from th e field predawn during winter and transferred to the laboratory where chlorophyll fluorescence emission as well as the chlorophyll and carot enoid composition were ascertained periodically for 4.5 days. Leaves a nd needles from all species were found to have retained large amounts of the xanthophyll cycle pigments zeaxanthin and antheraxanthin, and t hey exhibited sustained low values of the intrinsic efficiency of phot osystem II (PSII; measured as the ratio of variable to maximal fluores cence, F-v/F-m) upon collection. The increase in PSII efficiency was b iphasic, with a rapid phase (requiring several hours) and a slow phase (requiring several days). Changes in the conversion state of the xant hophyll cycle were found to correlate with increases in PSII efficienc y in both phases, with the latter phase involving large increases in b oth F-m (maximal fluorescence) and F-o (minimal fluorescence) througho ut the period of recovery. The relationship between F-m quenching (exp ressed 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 N PQ during recovery followed the theoretical relationship predicted for changes in the rate constant for energy dissipation in the antenna co mplexes. 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 cold-stressed evergreen plants in the field.