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

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 S2QB- charge recombinations (the B-band) were shifted to lower temperatures in winter pine needles, while the S2QA- recombinations (the Q-band) remained close to 0°C. This was accompanied by a drastically reduced (65%) PSII photochemical efficiency measured as Fv/Fm, and a 20-fold faster rate of the fluorescence transient from Fo to Fm 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/S3QB- recombinations are discussed. It is proposed that the lowered redox potential of QB in winter needles increases the population of QA-, thus enhancing the probability for non-radiative P680+QA- 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

Study of the background in the measuring station at the n_TOF facility at CERN: sources and solutions

A background roughly two orders of magnitude higher than tolerable was found in the n_TOF facility at CERN during the first measurements [1]. This note describes a series of additional measurements performed in the n_TOF experimental area to study the origin and the characteristics of the background. The program of these measurements was determined taking into account the results from the simulations carried out by the EET group [2]. A first phase of measurements confirmed what was expected from the simulations, namely that the dominant source of background was due to neutrons generated by negative muon capture. Actions to reduce the background were taken according to the results from both measurements and simulations. An iron shielding wall 3.2 m thick was then placed in between the sweeping magnet and the second collimator, with the purpose of stopping most of the muons. In a second phase of measurements, results showed that the additional shielding reduced the main component of the background by about a factor of 30