Protection against photoinhibition in the alpine plant Geum montanum

Geum montanum L. is an alpine plant usually found at altitudes between 1700 and 2600 m. Its wintergreen leaves can be subjected to very low temperatur es and at the same time receive high photon flux densities at the beginning of the growth season when the snow melts. We report results of a study, pe rformed with classical methods of biophysics, showing that leaves of G, mon tanum were remarkably tolerant to sunlight even at low temperatures. This t olerance results from the interplay of photorespiration and CO2 photosassim ilation. When temperatures approach 0 degrees C, responses include stomatal opening and CO2 uptake even under desiccation stress. This permits linear electron transport that is sufficient to avoid the excessive reduction of t he electron transport chain which is known to lead to photodamage. In addit ion, excitation energy was shifted from photo system (PS)II to PSI which is a very efficient energy quencher. Sensitivity of P700 in PSI to oxidation by far-red light was decreased and rates of dark reduction of photooxidized P700 were increased by actinic illumination, suggesting activation of cycl ic electron transport. Consistent with this, far-red light was able to decr ease the quantum yield of PSII (measured by the F-v/F-m ratio of chlorophyl l fluorescence). We suggest that cyclic electron transport decreases the lu menal pH under strong light. In the presence of zeaxanthin, this increases energy dissipation at the PSII level. At low temperatures, P700 remained st rongly oxidized under high irradiation while the primary electron acceptor of PSII, Q(A), was largely reduced. This shows efficient control of electro n transport presumably at the level of the cytochrome b/f complex and sugge sts formation of a protective transthylakoid proton gradient even when line ar electron transport is much reduced in the cold. Thus, several mechanisms cooperate to effectively protect the photosynthetic apparatus of G. montan um from photodamage. We see no indication of destructive "photo-stress" in this species during the growth season under alpine low-temperature and drou ght conditions.