RECOVERY OF PHOTOSYNTHESIS IN WINTER-STRESSED SCOTS PINE

Winter-induced inhibition of photosynthesis in Scots pine (Pinus sylvestris L.) is caused by the combined effects of light and freezing temperatures; light causes photoinhibition of photosystem 11 (Strand & Oquist, 1985b, Physiologia Plantarum, 65, 117-123), whereas frost causes inhibition of enzymatic steps of photosynthesis (Strand & Oquist, 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 O2-evolution under saturating CO2 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-degrees-C and 100-mu-mol m-2 s-1, light saturated rates and apparent quantum yields of photosynthetic O2-evolution recovered fully within approximately 48 h. 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 shaded pine. 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.