Shoots of cold‐acclimated seedlings of Pinus sylvestris L. were subjected to low temperatures either in darkness or at a photon flux density of 1300 umol m−2 s−1. CO2 and 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 CO2 assimilation rate to intercellular partial pressure of CO2, and the CO2 assimilation 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.

Abstract. The kinetics of in vivo chlorophyll fluorescence of photosystem II (PS II) was measured at room temperature and 77 K during frost hardening of seedlings of Scots pine (Pinus sylvestris L.), and after exposure of frost‐hardened shoots to sub‐freezing temperatures. A more pronounced decrease in variable fluorescence yield for the upper exposed than for the lower shaded surface of the needles suggested that some photoinhibition occurred during prolonged frost hardening at 50 μmol photons m−2 s−1 and 4°C. Reversible inhibition of photosynthesis after exposure to sub‐freezing temperatures was initially manifested as an increase of steady‐state energy‐dependent fluorescence quenching (qE) and a reduction in the rate of O2 evolution. Further inhibition after treatment at still lower temperatures caused a progressive decline of steady‐state photochemical quenching (qQ) and the rate of O2 evolution, whereas qE remained high. This implies an inactivation of enzymes in the photosynthetic carbon reduction cycle decreasing the consumption of ATP and NADPH, which is likely to cause an increase of membrane energization and a reduction of the primary electron acceptor (QA) of PS II. Alternatively, the changes in qQ and qE might be attributed to an inhibition of photophosphorylation. Severe, irreversible damage to photosynthesis resulted in a suppression of qE and of variable fluorescence yield, probably because the photochemical efficiency of PS II was impaired. Changes in the fast fluorescence kinetics at room temperature after severe freezing damage were interpreted as an inhibition of the electron flow from QA to the plastoquinone pool. It is suggested that irreversible freezing injury to needles of frost‐hardened P. sylvestris causes damage to the QB,‐protein.

Seasonal courses of light-saturated rate of net photosynthesis (A360) and stomatal conductance (gs) were examined in detached 1-year-old needles of Scots pine (Pinus sylvestris L.) from early April to mid-November. To evaluate the effects of soil frost and low soil temperatures on gas exchange, the extent and duration of soil frost, as well as the onset of soil warming, were manipulated in the field. During spring, early summer and autumn, the patterns of A360 and gs in needles from the control and warm-soil plots were generally strongly related to daily mean air temperatures and the frequency of severe frost. The warm-soil treatment had little effect on gas exchange, although mean soil temperature in the warm-soil plot was 3.8 degrees C higher than in the control plot during spring and summer, indicating that A360 and gs in needles from control trees were not limited by low soil temperature alone. In contrast, prolonged exposure to soil temperatures slightly above 0 degrees C severely restricted recovery of A360 and especially gs in needles from the cold-soil treatment during spring and early summer; however, full recovery of both A360 and gs occurred in late summer. We conclude that inhibition of A360 by low soil temperatures is related to both stomatal closure and effects on the biochemistry of photosynthesis, the relative importance of which appeared to vary during spring and early summer. During the autumn, soil temperatures as low as 8 degrees C did not affect either A360 or gs.

The seasonal variation in maximum photochemical efficiency of photosystem II (F v /F m ) and the relationship between F v /F m and climatic factors such as irradiance, frost-nights and daily mean temperature was studied in young Norway spruce trees for 4 years in northern Sweden. As a result of night frost, the F v /F m -ratio gradually decreased during the autumn. There was between-year variation in the pattern of F v /F m in fully exposed shoots during autumn and spring, largely as an effect of differing temperature conditions. During spring, there was a strong apparent relationship between daily mean temperature and F v /F m within the temperature range -3 to 12°C. The light regime to which the needles were exposed during winter affected F v /F m , and moderately shaded shoots from the bottom of the canopy generally had a higher F v /F m -ratio than fully exposed shoots from the top of the canopy.