AbstractKinetics of fluorescence at room temperature, electron transport and photooxidation of P700 and cytochromefhave been studied in chloroplasts isolated from active and winter stressedPinus silvestris. The winter stress induced block in the electron transport chain between the two photosystems is close to the site of plastoquinone, since winter stress and DCMU caused the same type of inhibition of the reoxidation of the primary electron acceptor Q of photosystem II. No winter inhibition of the electron transport between cytochromefand P700 was observed.Time course studies of P700 photooxidation in chloroplasts of active and winter stressed pine have shown that the photosynthetic unit size must be about equal in the two types of chloroplasts. An apparent increase of the photosynthetic unit size was induced by winter stress, as revealed by the high chlorophyll/P700 ratio of winter stressed pine. The phenomenon is explained by the formation of photosynthetically inactive chlorophyll.Low‐temperature fluorescence emission spectra were recorded when either chlorophylla(433 nm) or chlorophyllb(477 nm) were preferentially excited. Winter stress induced the formation of a chlorophyllafraction emitting at 673 nm. This chlorophyll is most likely derived from the chlorophyllaantennae of the two photosystems, and it probably contributes to the photosynthetically inactive pool of chlorophyll in winter stressed pine. The light harvesting chlorophylla/bcomplex is relatively resistant to winter stres