Seasonal changes in the photosynthetic efficiency of Thuja occidentalis (L.) and Chamaecyparis lawsonia (A. Murray bis.)

Seasonal changes in the efficiency of charge separation in PSII were studie d in Thuja occidentalis (L.) and Chamaecyparis lawsonia (A. Murray bis.). M aximum light-dependent charge separation decreased with decreasing temperat ures in early winter in both species, but this was less drastic in Chamoecy paris than in Thuja. No positive relationship was seen between photoinhibit ion and irradiance. Rather, photoinhibition increased as photon flux densit ies 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 t he twigs, where in Thuja visible browning occurred, than on the underside o f the twigs. During spring, recovery of the photosynthetic efficiency and r egreening were observed as both mean temperatures and irradiance increased. Transfer in midwinter of strongly photo-inhibited twigs of Thuja to temper atures close to 20 degreesC resulted in considerable recovery of PSII activ ity within several days when low light was also present. Recovery did not o ccur 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 stro nger 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.