Influence of acidic mist on frost hardiness and nutrient concentrations in red spruce seedlings: 1. Exposure of the foliage and the rooting environment.

Two-year-old red spruce (Picea rubens Sarg.) was grown in replicated open-top chambers supplied with charcoal-filtered air near Edinburgh, Scotland. Between May and November 1989, plants were exposed to four mist treatments, three containing sulphuric acid and ammonium nitrate in equimolar concentrations at 0.005 mol m ^-3 (pH 5) or 1.0 mol m ^-3 (pH 27), and a fourth treatment with sulphuric acid alone at 1.0 mol m ³ (equivalent to 2 mm precipitation). Two dose rates were used for the pH 2.7 treatment equivalent to 2 and 8 mm of rain per week. Three subtreatments (soil surface exposed to mist, addition of extra sulphuric acid to the soil surface, exclusion of mist from the soil) were included in each chamber. Frost hardiness was assessed by measuring rates of electrolyte leakage after controlled freezing of detached shoots. At the end of October, frost hardiness, expressed as the lethal temperature for 50% of shoots (LT ₅₀ ), was decreased by 8 °C in the 8 mm wk ^-1 treatment at pH 27, compared to pH 5. The 2 mm wk ^-1 treatment at pH 2.7 had no effect on frost hardiness either when ammonium nitrate was present or absent (i.e. sulphuric acid only). Excluding mist from the soil, and adding extra sulphuric acid, both increased frost hardiness by about 3 °C when compared with uncovered soil. Excluding mist from the soil increased the amount of foliage initiated and produced inside the chambers but neither subtreatment, excluding the mist nor providing additional sulphuric acid to the soil affected foliar nutrient concentrations. Mist of pH 27 as sulphuric acid alone and in combination with ammonium nitrate both enhanced N uptake. Several observations concerning the effect of acidic mist on frost hardiness were confirmed by this study: (i) preventing mist from reaching the soil/roots, improving conditions for root growth can ameliorate the effects of acidic mist on shoot growth and frost hardiness; (ii) the effect was determined by the ion dose but not by the ion concentration in the mist; (iii) the effect was primarily mediated through foliar absorption; (iv) the presence of high foliar nitrogen concentrations did not increase frost hardiness when foliar sulphur concentrations were also high; (v) low N concentrations were more important for frost hardiness than high foliar N concentrations.