Comparison of 18 trees from three different stands of damage classes 0 ( healthy) to 3 (severely damaged) showed that die fimction of tfae photosynthetic apparatus measured either as the cUorophyll fluorescence or as the whole chain electron transport capacity remained staÚe. Only in springtime we could detect a decrease of photosynthetic activities in the most damaged trees. Nevertheless, flie synthesis of the D-1 reaction centre polypeptide increased with the degree of damage when measured as incorporation of i^C-leucine. Comparison with the content of Úie D-1 protein determined by immunoblotting showed that also the degradation of this protein was stimulated. This led to a strongly variable content of tfae D-1 protein in tfae damaged spruce trees. The content of tfae D-1 protein in faealthy trees was more or less constant. During springtime the degradation seemed to outbalance tfae syntfaesis in the damaged trees and the content of tfae D-1 protein was decreasing togetfaer with the photosynthetic activities. Hence the repair of the photosynthetic apparatus is enhanced in the damaged trees to keep the photosynthetic activities constant.