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Keywords:

  • CO;
  • elevation and additional soil-nitrogen supply;
  • chlorophyll fluorescence;
  • photosynthesis;
  • branch-in-bag experiment;
  • Finns sylvestris

SUMMARY

Applying the branch-in-bag method, naturally seeded Scots pine (Finns sylvestris L.) trees, 25–30 yr old, were subjected to two CO, concentrations (350 and 700/tmol mol') and two soil-nitrogen-supply regimes for three growing seasons (1994–96). Gas exchange and chlorophyll a fluorescence in detached shoots were measured simultaneously in a diffuse radiation field. Elevated CO, did not lead to a significant ‘downward regulation’ in the light-saturated rate of net photosynthesis (Pn,max), the maximum apparent quantum yield (aA,max) or the maximum photochemical efficiency (Fv/Fm) of photosyscem II (PS II). However, the elevated CO2 significantly decreased the light-saturated stoniatal conductance and increased the sensitivity of stoniatal conductance to change in low photon-flux densities. The high soil-nitrogen supply significantly increased photosvnthetic capacity, as manifested by increases in Pn,mBS, £tAjMBlFJFm, and the effective photochemical efficiency (dLF/F'm) at low photon-flux densities, did not, on the other hand, enhance the magnitude of photosynthetic response to elevated CO2concentration. In addition, the treatment-induced modifications in fluorescence parameters are discussed in detail.