Sulphate and ammonium in mist impair the frost hardening of red spruce seedlings.

Two-year-old seedlings of red spruce [Picea rubens Sarg. syn. P. rubra (Du Roi) Link] were grown in open-top chambers supplied with charcoal-filtered air near Edinburgh, Scotland. Between May and November 1988, plants were exposed to mists containing NH₄^-, SO₄^2- and NO₃^- ions at concentrations of 1.6 mol m^-3 a and H⁺ at 3.2 mol m^-3, in pairwise combination and all together. The mists were applied twice weekly at a rate of 2 mm precipitation equivalent per application. Treatment with H₂SO₄ severely damaged the planta after 3 wk; the concentration for this treatment was therefore reduced from 1.6 mol m^-3 to 0.5 mol m^-3 a and new planta were substituted, in June 1988. Frost hardiness was assessed by freeze-testing detached shoots, then measuring rates of electrolyte leakage. No effects of the mist treatments were detected during the early stages of frost hardening, bur on 31 October, shoots that had received sulphate and ammonium ions were less hardy than 'control' shoots treated with deionized water. The temperature which killed 20 % of shoots was 14 °C higher, and the temperature which killed 50% of shoots was 7 °C higher than for 'control' shoots. Acidity per se had no effect on frost hardiness: nitrate ions had no effect and may have mitigated the effects of sulphate when applied together. The results confirm earliar data, and indicate that uptake of NH₄⁺ and SO₄^2- ions can disturb the frosthardening process. There are important implications for pollution control strategies in the northeastern United States, if damaging interactions between frost hardiness of red spruce and pollutant depoaition are linked to ammonium and sulphate rather than nitrate ions. [ABSTRACT FROM AUTHOR]