Seasonal responses of photosynthetic electron transport in scots pine (Pinus sylvestris L.) studied by thermoluminescence

Abstract

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°C) and an irradiance of 100 µmol m^-2 s^-1. TL measurements of photosystem II (PSII) revealed that the S₂Q_B^- charge recombinations (the B-band) were shifted to lower temperatures in winter pine needles, while the S₂Q_A^- recombinations (the Q-band) remained close to 0°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 S₂/S₃Q_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.