Seasonal Changes in the Photosynthetic Efficiency of Thuja occidentalis (L.) and Chamaecyparis lawsonia (A. Murray bis.)

Abstract: Seasonal changes in the efficiency of charge separation in PSII were studied in Thuja occidentalis (L.) and Chamaecyparis lawsonia (A. Murray bis.). Maximum light-dependent charge separation decreased with decreasing temperatures in early winter in both species, but this was less drastic in Chamaecyparis than in Thuja. No positive relationship was seen between photoinhibition and irradiance. Rather, photoinhibition increased as photon flux densities decreased towards midwinter, and it decreased as photon flux densities increased towards spring. However, the decrease in maximum light-dependent charge separation was much stronger on the light-exposed upper surface of the twigs, where in Thuja visible browning occurred, than on the underside of the twigs. During spring, recovery of the photosynthetic efficiency and regreening were observed as both mean temperatures and irradiance increased. Transfer in midwinter of strongly photo-inhibited twigs of Thuja to temperatures close to 20 °C resulted in considerable recovery of PSII activity within several days when low light was also present. Recovery did not occur at temperatures close to freezing or at room temperature in darkness. An analysis of fluorescence quenching suggested photoprotective dissipation of excess radiation not only in the light harvesting antennae of PSII but also in the reaction centres. Reaction centre quenching appeared to be stronger in Thuja than in Chamaecyparis. PSI was fully active in twigs whether or not PSII was photoinhibited. The antioxidant ascorbate was almost fully reduced even in midwinter.