Excitation energy flow at 77 K in the photosynthetic apparatus of overwintering evergreens

The flow of excitation energy from the antennae to photosynthetic reaction centre complexes at 77 K was studied in leaves of two evergreen species, namely, snow gum (Eucalyptus pauciflora Sieb. ex Spreng.) and a hemiparasitic mistletoe (Amyema miquelii, Lehm. ex Miq.). The leaves that were naturally acclimated to winter conditions of freezing temperatures and high irradiance displayed the recently discovered cold-hard-band or CHB feature of the chlorophyll a fluorescence spectra (Gilmore & Ball, Proc. Nat. Acad. Sci. USA 97:11098-11101, 2000). A streak-camera-spectrograph was used and the double convolution integral method for global analysis was applied to simultaneously acquire and simulate, respectively, the time- and wave-length-dependence of all major chlorophyll a components (Gilmore et al. Phil. Trans. Roy. Soc. B-London 355:1371-1384, 2000). The CHB coincided with changed amplitudes and decreased excited state lifetimes for the main F685 nm and F695 nm emission bands from the photosystem II (PSII) core-inner-antenna. The CHB dissipates energy as heat separate from PSII while also reducing the PSII quantum yield by competing for both photon absorption and antenna excitation. The CHB did not correlate with changes in the decay kinetics of the PSI antenna F740 nm band. The spectral-kinetic features of the altered energy flow were similar in the unrelated evergreen species. These results are consistent with a functional association between the CHB, PSII energy dissipation and protective storage of chlorophyll in overwintering evergreens.