• Picea sitchensis;
  • SO2+ NO2 mixtures;
  • low temperatures;
  • interactions;
  • water relations

Picea sitchensis (Bong.) Carr. seedlings were exposed to SO2, NO2 and SO2+ NO2 during dormancy in controlled environments, and were taken to night temperatures of 4, 0, −5, −10 and −15 °C in a freezer. Conditions in the freezer were carefully monitored during the low–temperature treatments. In two experiments, different photoenvironments and temperature regimes were imposed prior to the cold treatments, and different effects were observed. In the first, only limited frost hardiness was achieved and night temperatures of −15 °C were lethal. Temperatures of −5 and − 10 °C led to poor survival of lateral buds, particularly in plants exposed to 45 ppb SO2. The poor bud break in plants exposed to SO2 and to − 5 °C resulted in a loss of the effectiveness of this temperature as a chill requirement. Pressure-volume analysis showed that the shoots of plants exposed to NO2 had greater elasticity (lower elastic moduli, e), so that loss of turgor occurred at lower relative water contents. In contrast, a hardening period (2 weeks in night/day temperatures of 3/10 °C and 8 h days at 50 μmol m−2 s−1 PAR) gave decreased elasticity and lower solute potentials of spruce shoots. In the second experiment, exposure to 30 ppb SO2 and SO2+ NO2 led to slight, but consistent, increases in frost injury to the needles of plants frozen to − 5 and − 10 °C. The results suggest that the main interaction of low temperatures and winter pollutants may be on bud survival rather than on needle damage, but that effects are subtle, only occurring with certain combinations of pollutant dose and cold treatment.