As shown before [C. Ottander et al. (1995) Planta 197:176-183], there is a severe inhibition of the photosystem (PS) II photochemical efficiency of Scots pine (Pinus sylvestris L.) during the winter. In contrast, the in vivo PSI photochemistry is less inhibited during winter as shown by in vivo measurements of deltaA820/A820 (P700+). There was also an enhanced cyclic electron transfer around PSI in winter-stressed needles as indicated by 4-fold faster reduction kinetics of P700+. The differential functional stability of PSII and PSI was accompanied by a 3.7-fold higher intersystem electron pool size, and a 5-fold increase in the stromal electron pool available for P700+ reduction. There was also a strong reduction of the QB band in the thermoluminescence glow curve and markedly slower Q-A re-oxidation in needles of winter pine, indicating an inhibition of electron transfer between QA and QB. The data presented indicate that the plastoquinone pool is largely reduced in winter pine, and that this reduced state is likely to be of metabolic rather than photochemical origin. The retention of PSI photochemistry, and the suggested metabolic reduction of the plastoquinone pool in winter stressed needles of Scots pine are discussed in terms of the need for enhanced photoprotection of the needles during the winter and the role of metabolically supplied energy for the recovery of photosynthesis from winter stress in evergreens.

(u.s.A. SUMMARYClass II spinach chloroplasts were fragmented by passage through the French pressure cell (French press), and the fragments were separated. by fractional centrifugation. Fragments sedimenting between lOQO X g and 10,000 X g (10K) .have a lower chl a/chl b ratio and lower P-700 content than whole chloroplasts.· Fragments -sedimenting between 4o.·ooo x g and 160,000 x g (160K) have a much higher chl a/chl b ratio (6.0) and amuch higher P-700 content (1 P-700/105 chlorophylls) than whole chloroplasts. The chlorophyll and cytochrome contents of the Frenc::hJ>ress fractions are similar to those found in fractions iso-,.lated after digitonin disruption.The 160K fraction performs photosystem 1 but not photosystem 2 reactions. The lOK fraction contains both photosystems. Electrophoresis of sodium dodecylsulfate solubilized lOK and 160K fractions gives further evidence for this distribution of photosystems.

Exposure of control (non-hardened) Arabidopsis leaves for 2 h at high irradiance at 5°C resulted in a 55% decrease in photosystem II (PSII) photochemical efficiency as indicated by F v /F m . In contrast, cold-acclimated leaves exposed to the same conditions showed only a 22% decrease in F v /F m . Thermoluminescence was used to assess the possible role(s) of PSII recombination events in this differential resistance to photoinhibition. Thermoluminescence measurements of PSII revealed that S 2 Q A Ϫ recombination was shifted to higher temperatures, whereas the characteristic temperature of the S 2 Q B Ϫ recombination was shifted to lower temperatures in cold-acclimated plants. These shifts in recombination temperatures indicate higher activation energy for the S 2 Q A Ϫ redox pair and lower activation energy for the S 2 Q B Ϫ redox pair. This results in an increase in the free-energy gap between P680 ϩ Q A Ϫ and P680 ϩ Pheo Ϫ and a narrowing of the free energy gap between primary and secondary electron-accepting quinones in PSII electron acceptors. We propose that these effects result in an increased population of reduced primary electron-accepting quinone in PSII, facilitating non-radiative P680 ϩ Q A Ϫ radical pair recombination. Enhanced reaction center quenching was confirmed using in vivo chlorophyll fluorescence-quenching analysis. The enhanced dissipation of excess light energy within the reaction center of PSII, in part, accounts for the observed increase in resistance to high-light stress in cold-acclimated Arabidopsis plants.It has been shown previously in winter cereals (Ö quist and Huner, 1993) and Arabidopsis (Strand et al., 1997(Strand et al., , 1999) that cold acclimation results in an increased capacity for photosynthesis at suboptimal temperatures. This recovery in photosynthetic capacity is closely associated with the posttranslational activation and the selective increase in the expression of enzymes involved in Suc synthesis, with changes in expression and activity of Calvin cycle enzymes (Strand et al., 1997(Strand et al., , 1999(Strand et al., , 2003 Hurry et al., 2000; Stitt and Hurry, 2002), and with changes in the lipid composition and the content of unsaturated fatty acids of chloroplast membranes (Raison et al., 1982; Hugly and Somerville, 1992; Moon et al., 1995; Routaboul et al., 2000). These changes in photosynthetic capacity and in chloroplast membrane composition protect the photosynthetic apparatus against photoinhibition at low temperatures by allowing increased turnover of the photosynthetic electron transport chain (Hurry et al., 1993(Hurry et al., , 1995 Huner et al., 1998).However, results obtained with Scots pine (Pinus sylvestris) indicate that cold acclimation can increase the level of photosystem II (PSII) resistance to excessive light directly without any increase in photosynthetic capacity (Krivosheeva et al., 1996). The acquisition of increased tolerance to photoinhibition in cold-acclimated plants has also been ascribed to growth and development under high...