Recovery of photosynthesis in winter‐stressed Scots pine

Abstract

Abstract. . Winter‐induced inhibition of photosynthesis in Scots pine ( Pinns sylvestris L.) is caused by the combined effects of light and freezing temperatures; light causes photoinhibition of photosystem II (Strand & Oquist, 1985b, Physiologic Plantarum , 65 , 117–123), whereas frost causes inhibition of enzymatic steps of photosynthesis (Strand & Öquist, 1988, Plant, Cell & Environment, 11 , 231–238). To reveal limiting steps during recovery from winter stress, the potential of photosynthesis to recover and the actual recovery outdoors during spring, were studied in Scots pine. Studies of light dependent O 2 ‐evolution under saturating CO 2 and recordings of room temperature fluorescence induction kinetics were used. When branches of pine, in February and March, were brought into the laboratory and kept at 18°Cand 100μmol m −2 s −1 , light saturated rates and apparent quantum yields of photo‐synthetic O 2 ‐evolution recovered fully within approximately 48h. The photochemical efficiency of photosystem II, as measured by Fv/Fm ratios, recovered fully within 24h after an initial lag‐phase of 2‐3 h. Under natural winter conditions, the Fv/Fm ratio decreased more in exposed than in shaded pine, whereas the efficiency of photosynthesis was similarly inhibited in exposed and shadedpine. However, when recovery from winter stress occurred during spring, the Fv/Fm ratios of both shaded and exposed pine recovered well before photosynthesis. It is concluded that the light‐induced photoinhibition component of winter stress in photosynthesis of pine recovers well before the frost induced component(s) of winter stress. In this context, reversible photoinhibition of photosynthesis in evergreen conifers is considered as a dynamic down‐regulation of photosystem II to prevent more severe photodynamic damage of the thylakoid membrane when photosynthesis is inhibited by frost.