XVI International Botanical Congess

Photosynthetic efficiency of green leaves submitted to low temperature and high light was assessed in three species of the genus Geum. Resistance to photoinhibition, as revealed by PSII chlorophyll flurescence analysis, was directly related to the altitudinal distribution range of these species. In the alpine Geum montanum, tolerance was attributed to the interplay of photorespiration and CO2 photosassimilation at normal and elevated temperatures, and to stomatal opening even under water stress when temperatures approahced 0oC. In addition, excitation energy was shifted from PSII to PSI. Simultaneously, sensitivity of P700 in PSI to oxidation by far-red light was decreased and rates of dark reduction of photooxidized P700 were increased suggesting activation of cyclic electron transport. As far-red light was able to decrease the quantum yield of PSII, we suggest that the cyclic electron flow might be responsible for a build up of a low lumkenal pH which would increase thermal dissipation at the PSII level.