Inhibition of photosynthesis by freezing temperatures and high light Levels in cold‐acclimated seedlings of Scots pine (Pinus sylvestris). ‐ I. Effects on the light‐limited and light‐saturated rates of CO2assimilation

Shoots of cold‐acclimated seedlings ofPinus sylvestrisL. were subjected to low temperatures either in darkness or at a photon flux density of 1300 umol m−2s−1. CO2and water vapour exchange of the seedlings were measured at 12°C before and after treatment. Quantum yield at 12°C decreased both with decreasing temperatures below 0°C during, exposure to high light, and with increasing duration of light exposure. Quantum yield was also inhibited after treatment in darkness when needles were exposed to temperatures below their freezing point. Exposure of the pine shoots to a temperature of ‐7 to ‐8°C for 3 h in high light or darkness decreased quantum yield after thawing by ca 50 and 20%, respectively. Furthermore, light or dark exposure of the pine shoots at ‐7°C for 3 h decreased both the initial slope of the response of CO2assimilation rate to intercellular partial pressure of CO2, and the CO2assimilation rate at high partial pressures of CO2. It is concluded that excess light both at low temperatures, and freezing temperatures, cause inhibitions at the metabolic level, although the mechanisms may be different.