Sustained downregulation of photosystem II in mistletoes during winter depression of photosynthesis.

Cold acclimation by sustained downregulation of PSII was studied in intact leaves of an Australian mistletoe Amyema miquelii (Lehm. ex Miq.) Tiegh. and its host Eucalyptus. The trends were followed from autumn to spring on leaves that had developed in summer and were exposed to different microclimates with progress of the seasons. In sun leaves of mistletoe, efficiency of excitation energy transfer from light-harvesting pigments to Chl a molecules in PSII core complexes was markedly reduced in winter. Concomitantly, a band in 77K fluorescence emission spectra emerged at 715 nm, in the same way as the cold-hard band found in overwintering snow gum seedlings (Gilmore and Ball 2000, Proceedings of the National Academy of Sciences USA , 11 098-11 101). Further, a distinct band, which presumably involves Chl-b-containing antennae complexes, appeared at 705 nm in -2°C fluorescence emission spectra with decreasing intensity of the PSII band. Much shorter PSII fluorescence lifetimes measured in sun leaves of mistletoe that were exhibiting sustained downregulation of PSII indicated enhanced thermal dissipation of excitation energy. Winter acclimation symptoms of the photosynthetic apparatus were more striking in mistletoe sun leaves compared with eucalypt sun leaves. Because the light and temperature environments of sun leaves are similar for the parasite and host, we primarily attribute the enhanced light-acclimation symptoms to the limited photosynthetic capacity of A. miquelii in winter.