S35-020 Light utilization in photosynthesis after cold acclimation of plants representing two different seasonal growth strategies LV Savitch 2,4 , ED Leonardos 3 , M Krol 2 , S Jansson 1 , B Grodzinski 3 , NPA Huner 2 , G Öquist 1 1 Umeå Plant Science Center, Department of Plant Physiology, Umeå University, S-901 87 Umeå, Sweden, Stefan.Jansson@plantphys.umu.se; Gunnar.Oquist@plantphys.umu.se; Fax: (+46) 90 786 66 76 2 Department of Plant Sciences, The University of Western Ontario, London, ON, N6A 5B7 Canada, mkrol@uwo.ca; nhuner@uwo.ca; Fax: (519) 661-3935 3 Department of Plant Agriculture, University of Guelph, Guelph, ON, N1G 2W1 Canada, dleonard@uoguelph.ca; bgrodzin@uoguelph.ca; Fax: (519) 767-0755 4 Agriculture and Agri-Food Canada, Eastern Cereal and Oilseed Center, Ottawa, ON, K1A 0C6 Canada, savitchl@em.agr.ca; Fax: (613) 759-6566 Key-words: cold acclimation; dormancy; photosynthesis; PsbS; xanthophyll cycle. Introduction Photosynthesis provides the energy required for cold acclimation of plants during the autumn, making them resistant to subfreezing winter temperatures (Levitt 1980). However, the combined exposure of plants to low temperatures and light increases the probability that plants will succumb to partial inactivation of photosynthesis due to photoinhibition (Öquist et al. 2001). In fact, evergreen species such as Scots pine may suffer substantial photoinhibition of photosynthesis during the autumn months (Ottander et al. 1995). In contrast, however, herbaceous winter annuals such as winter cereals are much more resistant to low temperature induced photoinhibition of photosynthesis (Öquist and Huner 1993). We hypothesized (Huner et al. 1993) that these different autumnal responses of photosynthesis in conifers and cereals is related to their different growth and developmental strategies. In a comparative study of Lodgepole pine and winter wheat, we tested this hypothesis and demonstrate that the different growth and developmental strategies exhibited by these two species during cold acclimation is indeed correlated with two totally different strategies to utilise the photosynthetically absorbed light. Materials and methods Winter wheat (Triticum aestivum L., cv. Monopol) plants were grown at a photon flux density of 250 µmol m -2 s -1 and a 16 h photoperiod under a temperature regime of 20/16°C (day/night), for control, and 5/5°C for cold acclimated plants. Cold acclimated and dark adapted one-year-old seedlings of Lodgepole pine (Pinus contorta L.) were transferred to the temperature regime of 25/15°C (day/night), a PFD of 250 µmol m -2 s -1 and a 17 h photoperiod, to initiate the second year growth. After a period of six weeks at 25°C/250 PFD, the second-year needles were fully developed and considered as “summer” pine. At this stage, plants were transferred to a temperature regime of 15/10°C (day/night), a PFD of 250 µmol m -2 s -1 and an 8 h photoperiod. After a period