Photosynthetic electron transport adjustments in overwintering Scots pine (Pinus sylvestris L.)

As shown before [C. Ottander et al. (1995) Planta 197:176-183], there is a severe inhibition of the photosystem (PS) II photochemical efficiency of Sc ots pine (Pinus sylvestris L.) during the winter. In contrast, the in vivo PSI photochemistry is less inhibited during winter as shown by in vivo meas urements of DeltaA(820)/Delta (820) (P700(+)). There was also an enhanced c yclic electron transfer around PSI in winter-stressed needles as indicated by 4-fold faster reduction kinetics of P700(+). The differential functional stability of PSII and PSI was accompanied by a 3.7-fold higher intersystem electron pool size, and a 5-fold increase in the stromal electron pool ava ilable for P700(+) reduction. There was also a strong reduction of the QB b and in the thermoluminescence glow curve and markedly slower Q-A re-oxidati on in needles of winter pine, indicating an inhibition of electron transfer between QA and QB. The data presented indicate that the plastoquinone pool is largely reduced in winter pine, and that this reduced state is likely t o be of metabolic rather than photochemical origin. The retention of PSI ph otochemistry, and the suggested metabolic reduction of the plastoquinone po ol in winter stressed needles of Scots pine are discussed in terms of the n eed for enhanced photoprotection of the needles during the winter and the r ole of metabolically supplied energy for the recovery of photosynthesis fro m winter stress in evergreens.