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.

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The potential of photosynthesis to recover from winter stress was studied by following the thermoluminescence (TL) and chlorophyll fluorescence changes of winter pine needles during the exposure to room temperature (20 degrees C) and an irradiance of 100 micromol m(-2) s(-1). TL measurements of photosystem II (PSII) revealed that the S(2)Q(B)(-) charge recombinations (the B-band) were shifted to lower temperatures in winter pine needles, while the S(2)Q(A)(-) recombinations (the Q-band) remained close to 0 degrees C. This was accompanied by a drastically reduced (65%) PSII photochemical efficiency measured as F(v)/ F(m,) and a 20-fold faster rate of the fluorescence transient from F(o) to F(m) as compared to summer pine. A strong positive correlation between the increase in the photochemical efficiency of PSII and the increase in the relative contribution of the B-band was found during the time course of the recovery process. The seasonal dynamics of TL in Scots pine needles studied under field conditions revealed that between November and April, the contribution of the Q- and B-bands to the overall TL emission was very low (less than 5%). During spring, the relative contribution of the Q- and B-bands, corresponding to charge recombination events between the acceptor and donor sides of PSII, rapidly increased, reaching maximal values in late July. A sharp decline of the B-band was observed in late summer, followed by a gradual decrease, reaching minimal values in November. Possible mechanisms of the seasonally induced changes in the redox properties of S(2)/S(3)Q(B)(-) recombinations are discussed. It is proposed that the lowered redox potential of Q(B) in winter needles increases the population of Q(A)(-), thus enhancing the probability for non-radiative P680(+)Q(A)(-) recombination. This is suggested to enhance the radiationless dissipation of excess light within the PSII reaction center during cold acclimation and during cold winter periods.

Analysis of the partitioning of absorbed light energy within PSII into fractions utilized by PSII photochemistry (Ø PSII ), thermally dissipated via ∆pH-and zeaxanthin-dependent energy quenching (Ø NPQ ) and constitutive non-photochemical energy losses (Ø NO ) was performed in wild type and F2 mutant of barley. The estimated energy partitioning of absorbed light to various pathways indicated that the fraction of Ø PSII was slightly higher, while the proportion of thermally dissipated energy through Ø NPQ was 38% lower in F2 mutant than in WT. In contrast, Ø NO , i.e. the fraction of absorbed light energy dissipated by additional quenching mechanism(s) was 34% higher in F2 mutant. The increased proportion of Ø NO correlated with narrowing the temperature gap (∆T M ) between S 2/3 Q B -and S 2 Q A -charge recombinations in F2 mutant as revealed by thermoluminescence measurements. We suggest that this would result in increased probability for an alternative non-radiative P680 + Q A -radical pair recombination pathway for energy dissipation within the reaction centre of PSII (reaction center quenching) and that this additional quenching mechanism might play an important role in photoprotection when the capacity for the primary, zeaxanthin-dependent non-photochemical quenching ( Research ArticleChanges in irradiance, temperature, nutrient and water availability result in imbalances between the light energy absorbed through photochemistry and energy utilization through photosynthetic electron transport coupled to carbon, nitrogen and sulphur reduction. Such an imbalance caused by changes in irradiance and/or temperature, nutrient and water availability leads to photoinhibition of photosynthesis that may result in photodamage to the D1 reaction centre polypeptide of PSII (Krause, 1988;Aro et al., 1993). The major mechanism for thermal de-excitation of excess light energy in higher plants is currently considered to be the ∆pH-and xanthophyll-cycle dependent nonphotochemical quenching (NPQ) occurring within LHCII antenna pigment bed of PSII (Demmig-Adams and Adams, 1992;Horton et al., 1996). The role of pH-and zeaxanthin-dependent shifts in the oligomerization state of LHCII in developing the rapidly relaxing energy dependent component (qE) of NPQ is well established with qE representing the major protective mechanism against photoinhibitory damage of PSII (Horton et al., 1996; Niyogi, 1999). It has been demonstrated that trimers of LHCII exhibit the optimum level of non-photochemical energy dissipation by modulating the development of the quenched state of the complex (Wentworth et al., 2004).The redox-dependent reversible phosphorylation of light-harvesting Chl a/b-binding proteins (LHCII) is the